Date   

Re: Unguided imaging and atmospheric refraction

Ray Gralak
 

Clearly this is a complicated issue with no “perfect” answer.
There will always be a variable tracking rate depending on sky position, so the best solution is polar aligning to the non-refracted pole.

This works for both unguided and guided setups, as it minimizes field rotation over a long period during which images are collected.

-Ray Gralak
Author of PEMPro
Author of APCC (Astro-Physics Command Center): https://www.astro-physics.com/apcc-pro
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver


-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of davidcfinch9 via groups.io
Sent: Sunday, September 20, 2020 12:04 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

Clearly this is a complicated issue with no “perfect” answer. The difference in refraction rates with varying
atmospheric conditions adds another level of complexity to this discussion. Without fear of “polishing cannonballs,” I
don’t think you can go far wrong by polar aligning using your best method and then by spending the time to develop
accurate modeling for your system. You will still be subject to the whims of our atmosphere. The proof will always be
in your images.

David


Re: APCC Temperature Sensor Input

Marcelo Figueroa
 

BTW, you can still manually enter the pressure on the Pointing Model tab, if you are using APCC Pro.
How do I do that exactly? 
 
In the Ponting Model tab I can't find where to enter that information. And if I go to Settings > Environmental Settings the text boxes do not allow you to enter data manually.

Thanks,


Re: Unguided imaging and atmospheric refraction

davidcfinch9
 

Clearly this is a complicated issue with no “perfect” answer. The difference in refraction rates with varying atmospheric conditions adds another level of complexity to this discussion. Without fear of “polishing cannonballs,” I don’t think you can go far wrong by polar aligning using your best method and then by spending the time to develop accurate modeling for your system. You will still be subject to the whims of our atmosphere. The proof will always be in your images.

David


A couple of questions about modeling #APCC #Mach2GTO

Marcelo Figueroa
 

Since a few days ago I've been testing my Mach2 (fantastic machine, by the way) and I have a couple of doubts about the modeling:
 
1) How important is the focus, does it need to be precise or is something reasonably good enough?
 
2) What's the difference between a Medium Model Map and a Large Model Map? How much does tracking improve, how much longer can the exposures be with more points (without guiding of course)?

Thanks,


Re: Low temp version of the #Mach2GTO

Terry Martin
 

Can I make the assumption that the Low Temp encoders must be used in those frigid conditions?  ...or anything approaching those frigid conditions?

Terry


Re: Unguided imaging and atmospheric refraction

Dean Jacobsen
 

Thank you.
--
Dean Jacobsen
http://astrophoto.net/wp/
Image Gallery - http://astrophoto.net/wp/image-gallery/
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/ 
Amateur Radio Call Sign - W6DBJ


Re: Unguided imaging and atmospheric refraction

Roland Christen
 


You CAN mitigate the EFFECT of atmospheric refraction.
Yes, by modeling. I wasn't shooting at the pole, rather I was shooting about 10 degrees north of the zenith. Atmospheric refraction was quite large until I got within 1 hour of the meridian. There's no getting away from RA drift elsewhere - not for 1 hour exposures and even for 10 minute exposures at 1000+mm focal length.

Rolando


-----Original Message-----
From: davidcfinch9 via groups.io <DF121819@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 12:43 pm
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

You CAN mitigate the EFFECT of atmospheric refraction. Atmospheric refraction does not go away, but as I said, if you are shooting stars NEAR THE POLE, you are not shooting near the zenith (unless you are at the North Pole). Stars NEAR THE POLE, all experience ALMOST the same atmospheric refraction (their altitude only varies with the SINE[angular distance from the pole]) . For example, if you imaging right at the refracted pole, that refracted pole will not move at all if you are aligned to that refracted pole. There will be NO RA drift.
Your modelling will also be different depending on where you are polar aligned.
 
David
 
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Sunday, September 20, 2020 1:02 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction
 
 
The problem, as I see it, is to which “pole” are you aligning your scope.
You can pick either one, but it will not solve the problem of atmospheric refraction when you image away from the zenith. The RA will always drift at the rates that I indicated below. You cannot escape RA drift by changing the polar alignment position because atmospheric refraction does not go away. The only place where the RA drift will be zero using the sidereal rate is at the meridian.
 
Rolando
 
 
-----Original Message-----
From: davidcfinch9 via groups.io <DF121819@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 9:43 am
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction
The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.
Regards,
David C. Finch
 
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction
 
Hi Astronuts,
 
During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.
 
I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.
 
So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.
 
I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:
 
At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.
 
For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.
 
The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).
 
I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.
 
As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.
 
Rolando
 
 


Re: Unguided imaging and atmospheric refraction

Ray Gralak
 

Hi David,

Your modelling will also be different depending on where you are polar aligned.
In regards to tracking rates, it doesn't matter to which pole you align. The model will correct the tracking rate appropriately, including refraction.

However, the preferred "pole" is the unrefracted pole because it minimizes field rotation. PEMPro and APPM report polar alignment to the unrefracted pole.

-Ray Gralak
Author of PEMPro
Author of APCC (Astro-Physics Command Center): https://www.astro-physics.com/apcc-pro
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver


-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of davidcfinch9 via groups.io
Sent: Sunday, September 20, 2020 10:44 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

You CAN mitigate the EFFECT of atmospheric refraction. Atmospheric refraction does not go away, but as I said, if
you are shooting stars NEAR THE POLE, you are not shooting near the zenith (unless you are at the North Pole).
Stars NEAR THE POLE, all experience ALMOST the same atmospheric refraction (their altitude only varies with the
SINE[angular distance from the pole]) . For example, if you imaging right at the refracted pole, that refracted pole
will not move at all if you are aligned to that refracted pole. There will be NO RA drift.

Your modelling will also be different depending on where you are polar aligned.



David



From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via
groups.io
Sent: Sunday, September 20, 2020 1:02 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction





The problem, as I see it, is to which “pole” are you aligning your scope.

You can pick either one, but it will not solve the problem of atmospheric refraction when you image away from the
zenith. The RA will always drift at the rates that I indicated below. You cannot escape RA drift by changing the polar
alignment position because atmospheric refraction does not go away. The only place where the RA drift will be zero
using the sidereal rate is at the meridian.



Rolando





-----Original Message-----
From: davidcfinch9 via groups.io <DF121819@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 9:43 am
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your
polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about
that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should
be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not
perfect) between the two poles that best fits most situations.

Regards,

David C. Finch



From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via
groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction



Hi Astronuts,



During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is
actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The
scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.



I have read on other user sites many posts by people who don't seem to understand that the stars do not move
evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even
20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in
both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software
etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started
some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking,
that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than
CCDs today.



So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting
NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above
the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5
to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second
exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.



I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635.
The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at
around midnight. The data shows the following:



At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours
from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith
the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from
around 11/hr down to zero and back up on the other side.



For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-
all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late
1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by
simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand
calculations.



The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate
for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned
mount and matches quite closely to what the King rate produces in RA (only in RA of course).



I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a
small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.



As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at
1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale
is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees
Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA
foreshortening as you approach the pole.



Rolando






Re: Unguided imaging and atmospheric refraction

davidcfinch9
 

You CAN mitigate the EFFECT of atmospheric refraction. Atmospheric refraction does not go away, but as I said, if you are shooting stars NEAR THE POLE, you are not shooting near the zenith (unless you are at the North Pole). Stars NEAR THE POLE, all experience ALMOST the same atmospheric refraction (their altitude only varies with the SINE[angular distance from the pole]) . For example, if you imaging right at the refracted pole, that refracted pole will not move at all if you are aligned to that refracted pole. There will be NO RA drift.

Your modelling will also be different depending on where you are polar aligned.

 

David

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Sunday, September 20, 2020 1:02 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

 

 

The problem, as I see it, is to which “pole” are you aligning your scope.

You can pick either one, but it will not solve the problem of atmospheric refraction when you image away from the zenith. The RA will always drift at the rates that I indicated below. You cannot escape RA drift by changing the polar alignment position because atmospheric refraction does not go away. The only place where the RA drift will be zero using the sidereal rate is at the meridian.

 

Rolando

 

 

-----Original Message-----
From: davidcfinch9 via groups.io <DF121819@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 9:43 am
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.

Regards,

David C. Finch

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction

 

Hi Astronuts,

 

During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.

 

I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.

 

So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.

 

I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:

 

At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.

 

For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.

 

The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).

 

I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.

 

As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.

 

Rolando

 

 


Re: Low temp version of the #Mach2GTO

Roland Christen
 

The gearbox won't freeze because there is no gearbox - the Mach2 is belt driven.

The worm grease may stiffen up a bit, but the lubricant is rated to -40C and even if it stiffens up a bit, that will not impede the actions of the worm. Re-lubing the worm is easy - there is a cover plate that you remove which then exposes the worm teeth.

The main shaft bearings may get a bit stiff, but there is nothing you can do about re-lubing them since they are sealed.

I have run the mount in a deep freezer at -29C (mount was covered in ice after several days) and also outside in the observatory in some very frigid conditions. The mount worked fine and slewed easily at 1800x. You can set the slew rate to a lower level for really cold conditions.

Rolando



-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 4:49 am
Subject: [ap-gto] Low temp version of the #Mach2GTO

Hi AP,

Just wondering if the "L" version of the mount is only about the encoders or if there is more to it:
- lubricant
- electronics
- motors, etc.

Arctic winds can bring temps down to -33C at my location. And it can get as hot as +30C in summer (90% + humidity), so I'd like some advice on how far down my soon-to-order Mach2(-L) is rated to work and if I'm better regreasing between seasons (hopefully not). Above figures are the extremes happening about 20-25 days a year (but seamingly more frequent from year to year) as for most part, we get -18 to -23C  in winter and +22 to +28C during summer.

Thanks and clear skies!

Sébastien


Re: [ap-ug] Unguided imaging and atmospheric refraction

Roland Christen
 

We have a lot of mass in the earth and no real friction, so the turning rate is very smooth. Consider it a Cadillac ride in the cosmos ;^))

Rolando


-----Original Message-----
From: ROBERT WYNNE <robert-wynne@...>
To: main@ap-ug.groups.io; Roland Christen via groups.io <chris1011@...>; main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Sun, Sep 20, 2020 12:08 pm
Subject: Re: [ap-ug] Unguided imaging and atmospheric refraction

The following website sheds some "light' on the matter. I find it somewhat amazing anyone can get stellar objects into sharp focus for long durations.


Best, Robert
On 09/19/2020 4:51 PM Roland Christen via groups.io <chris1011@...> wrote:


I don't think I can answer any of that.  ;^))

Rolando


-----Original Message-----
From: ROBERT WYNNE <robert-wynne@...>
To: main@ap-ug.groups.io; Roland Christen via groups.io <chris1011@...>; main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Sat, Sep 19, 2020 6:42 pm
Subject: Re: [ap-ug] Unguided imaging and atmospheric refraction

Thank-you for the very instructional post. How well does a Mach2 accommodate for both the rotation of the earth and the rotation of the earth around the sun for these very long exposures? If left alone to perform the calculations to accommodate both I would be days - months - behind the time I intended to photograph. Though I suppose a spreadsheet could be setup to account for both rotations for a particular spot on earth.

The solar system is moving through space quite fast as well. Does the movement of the solar system have any effect on interstellar imaging?  -Best, Robert
On 09/19/2020 4:20 PM Roland Christen via groups.io <chris1011@...> wrote:


Hi Astronuts,

During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.

I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.

So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.

I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:

At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.

For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.

The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).

I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.

As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.

Rolando




Re: Unguided imaging and atmospheric refraction

Roland Christen
 


The problem, as I see it, is to which “pole” are you aligning your scope.
You can pick either one, but it will not solve the problem of atmospheric refraction when you image away from the zenith. The RA will always drift at the rates that I indicated below. You cannot escape RA drift by changing the polar alignment position because atmospheric refraction does not go away. The only place where the RA drift will be zero using the sidereal rate is at the meridian.

Rolando


-----Original Message-----
From: davidcfinch9 via groups.io <DF121819@...>
To: main@ap-gto.groups.io
Sent: Sun, Sep 20, 2020 9:43 am
Subject: Re: [ap-gto] Unguided imaging and atmospheric refraction

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.
Regards,
David C. Finch
 
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction
 
Hi Astronuts,
 
During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.
 
I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.
 
So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.
 
I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:
 
At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.
 
For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.
 
The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).
 
I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.
 
As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.
 
Rolando
 
 


Re: Unguided imaging and atmospheric refraction

christian viladrich
 

Here is an interesting link on tracking rate :

https://canburytech.net/DriftAlign/DriftAlign_3.html

I also remember an old S&T issue (late 70s ?) with a figure showing the RA tracking rate depending on hour angle and declination.

In the old times of large Schmidt telescope (with 14"silver plates), they used to tune the polar elevation depending on the imaged area to reduce field rotation. But this is another story.

Best regards

Christian


Le 20/09/2020 à 14:43, davidcfinch9 via groups.io a écrit :

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.

Regards,

David C. Finch

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction

 

Hi Astronuts,

 

During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.

 

I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.

 

So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.

 

I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:

 

At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.

 

For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.

 

The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).

 

I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.

 

As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.

 

Rolando

 

 


Re: Unguided imaging and atmospheric refraction

davidcfinch9
 

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.

Regards,

David C. Finch


Re: Unguided imaging and atmospheric refraction

davidcfinch9
 

The problem, as I see it, is to which “pole” are you aligning your scope. If you are shooting stars near the pole, your polar alignment should be on the refracted pole for the best tracking. In that situation, your mount will rotate about that refracted pole. If you are shooting stars that are near the celestial equator, for best tracking, your mount should be aligned to the true axis of the earth which is the “real pole.” Clearly there is an optimal position (though not perfect) between the two poles that best fits most  situations.

Regards,

David C. Finch

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: Saturday, September 19, 2020 7:20 PM
To: main@ap-gto.groups.io; main@ap-ug.groups.io
Subject: [ap-gto] Unguided imaging and atmospheric refraction

 

Hi Astronuts,

 

During my recent imaging sessions I have been gathering a lot of data to better understand how the mount is actually tracking an object over a long time period. I can do this with the Mach2 thanks to the built-in encoders. The scope I'm using is the 160 EDF which yields approximately 1 arc second per pixel on my QSI camera sensor.

 

I have read on other user sites many posts by people who don't seem to understand that the stars do not move evenly across the sky at the sidereal rate. Some claim that with their inexpensive mounts they can do 5, 10 or even 20 minute unguided imaging. This is with mounts that have no modeling capabilities or even just custom rates in both axes. So that excites others who are having problems with guide scopes, off-axis guiders, guiding software etc., to believe that perhaps there is an easy way to set up a mount for unguided imaging. Back when I first started some umpteen years ago, it was well understood by all that running a mount, even with superb sidereal tracking, that it was not going to guarantee 1 hour exposures on our film negatives. And film is 10 times more forgiving than CCDs today.

 

So, as that for a background, I would like to share a few observations. These last couple of nights I was shooting NGC7635, the Bubble Nebula, along with star sprinkled M57 nearby. This object is circumpolar, so it is well above the horizon even when 8 hours away from the meridian line overhead. I observed that when the object is between 5 to 8 hours in the east, the RA tracking rate has to be slowed down considerably to avoid trailing, even for 60 second exposures at the focal length of my refractor. Compensating for that drift is exactly what modeling will do.

 

I spent some time yesterday before sunset getting good drift data and creating a model for the path of NGC7635. The model spans about 5 hours of RA motion, which is about the time it takes for the object to reach the meridian at around midnight. The data shows the following:

 

At 5 hrs east, the RA un-compensated drift rate was 73 arc-sec/hour, or about 1.2 arc sec per minute. At 3 hours from the zenith the drift rate is 54 arc-sec/hr. At 1 hour from the zenith the rate drops to 13 arc-sec/hr. At the zenith the drift rate drops to zero and begins climbing back up on the other side of the meridian. The Dec rate runs from around 11/hr down to zero and back up on the other side.

 

For each object the drift rate will be different and depends on the declination that it sits on. There is no one-size-fits-all tracking rate. The rates are variable depending where you are in the sky. People knew that back in the late 1800's. Astronomers at large observatories were actually able to take unguided images up to 1 hour and more by simply adjusting their mechanical clock drives according to charts that they painstakingly created using hand calculations.

 

The King rate will compensate the RA drive rate to some degree over a large swath of sky, but cannot compensate for Dec drift, nor can it compensate for small amounts of polar misalignment. The above data was for a well aligned mount and matches quite closely to what the King rate produces in RA (only in RA of course).

 

I did some experiments with small polar misalignments and the RA drift rate can easily double if you are off only a small amount. A decent model will easily compensate for that, along with the inevitable flexure of a large imaging rig.

 

As far as doing unguided imaging with a mount that runs sidereal rate and no model? Well if the object is drifting at 1.2 arc sec per minute, and you take a 5 minute exposure, I suppose that you can get round stars if your pixel scale is around 6 arc sec per pixel. Anything less than that and you will see trailing. If you are shooting at +60 Degrees Dec, I suppose you can get away with a 3 arc sec per pixel scale and achieve round stars - because of RA foreshortening as you approach the pole.

 

Rolando

 

 


GTO4 wifi password

Peter Bresler
 

Last night I damaged my Ethernet cable, which is what I usually use to connect and was trying to connect with wifi. I could not connect. The PW I used was three numbers of the box SN + "admin12345". That did not work. What is the PW for the wifi?


Re: Do NOT use ASCOM Platform v. 6.5 with APCC Pro #APCC #ASCOM_V2_Driver

George LaBelle
 
Edited

So, what is wrong with ASCOM 6.5 - or is there something in SGP/APPM/PlateSolve/???

This has been known about for some time. Is it being worked on?

--
George
Prineville, Oregon


Re: Updating Cabling

Dale Ghent
 

Honestly any small low power router that can operate at a voltage you can supply from your power source will work.  As for the operating temp it gets only to -10c here where I live at its coldest but I’ve had this operating in -8. Still far from -25 but below freezing nonetheless. It keeps itself warm.  

On Sep 19, 2020, at 23:17, sebastiendore1@... wrote:


Thanks for the reference and additional info. At first glance the model you use seems to be spec'ed for 0-50C but I'll take a look deeper into it to see if they might have an option with operating temp down to -25C or so.

Another alternative I know is RuggedScience. The have some fanless models (https://www.ruggedscience.com/industrial-computers/ultra-compact-computers/ars-2000-din-rail-compact-computer) but I think they are a bit pricey for the computing power you  get... Not sure if they have an external wifi antenna option either, though I might not require that.

Interesting idea the Airport Express as a mobile router! I might try that ! I also think I've read somewhere that the CP4 has its own hotspot function, but don't quote me on that. Someone can probably confirm...



De : main@ap-gto.groups.io <main@ap-gto.groups.io> de la part de Dale Ghent <daleg@...>
Envoyé : 19 septembre 2020 12:32
À : main@ap-gto.groups.io <main@ap-gto.groups.io>
Objet : Re: [ap-gto] Updating Cabling

I use an industrial fanless mini PC from OnLogic, in their ML100 series, specifically the ML100G-51 model:
https://www.onlogic.com/computers/industrial/fanless/ml100-series/

The Whiskey Lake CPUs have a 15W TDP and so are very powerful but also very kind on battry-powered setups. However, I also run the Whiskey Lake CPUs because I run a 61mp QHY600 camera, and lower-powered Celerons (J series, N series) would have a tough time with that during autofocus and other image analysis operations. But if you went with a Celeron-based model, you would see even lower-powered usage.

The body of the case is a heat sink so it keeps itself clear of dew, and it doesn't have a fan to blow humid air through the inside, either. It also has external wifi antennas (vs. an intel NUC, which is internal) so when setting up at home, it has the reach to get good speeds on my home wireless network. When in the field I use a low power wifi router (repurposed Apple Airport Express) to make a small wireless network on the spot that the unit will auto-connect to. I am starting to experiment with the reliability of Windows' Mobile Hotspot/Hotspot On Demand feature, and if that works reliably I can remove the small wifi router from the setup. It's a shame that Windows 10 discontinued wireless ad-hoc mode, which was a bit more foolproof.

The only similar alternative to the OnLogic fanless models I found were SimplyNUC's Por-cool-pine line of fanless systems:
http://simplynuc.com/nuc-products/#fanless





Low temp version of the #Mach2GTO

Sébastien Doré
 

Hi AP,

Just wondering if the "L" version of the mount is only about the encoders or if there is more to it:
- lubricant
- electronics
- motors, etc.

Arctic winds can bring temps down to -33C at my location. And it can get as hot as +30C in summer (90% + humidity), so I'd like some advice on how far down my soon-to-order Mach2(-L) is rated to work and if I'm better regreasing between seasons (hopefully not). Above figures are the extremes happening about 20-25 days a year (but seamingly more frequent from year to year) as for most part, we get -18 to -23C  in winter and +22 to +28C during summer.

Thanks and clear skies!

Sébastien


Re: How planets would look if they were as close as the Moon

Roland Christen
 

Yes, unfortunately. Confused

Rolando



-----Original Message-----
From: Tony Benjamin <tonybenjamin@...>
To: main@ap-gto.groups.io
Sent: Sat, Sep 19, 2020 11:51 pm
Subject: Re: [ap-gto] How planets would look if they were as close as the Moon

They forgot Pluto!!!!!!!!!!!!!!!!!!!!!!!!!!! J
 
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of uncarollo2 <chris1011@...> via groups.io
Sent: September 19, 2020 9:42 PM
To: main@ap-ug.groups.io; main@ap-gto.groups.io
Subject: [ap-gto] How planets would look if they were as close as the Moon
 

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