Field Rotation


Ray Gralak
 

Hi Roland,

In Mike's case he was autoguiding and not using a model. Since he said autoguiding was good and he was using a separate guidescope, drift in the main telescope had to come from something else. Differential flexure and/or field rotation are the best possibilities. It is also possible what Brian mentioned - that the camera rotated in the main scope either by slipping or from the tortional influence of the camera's body ii any part of the camera sticks out (although the amount of rotation from torsion should be tiny unless the focuser tube is weak),

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Roland Christen via groups.io
Sent: Thursday, June 2, 2022 8:06 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation


Seriously though, besides tweaking the polar alignment are there any other tricks that will minimize this
drift ? I see lots and lots of images on this forum with little or no apparent drift.

I do a lot of testing of modeling using both APCC-APPM and the keypad models. Even with perfect polar
alignment there is drift in RA and Dec over most portions of the sky as you get away from the zenith. That drift is
compensated by the modeling to a high degree as the model will program the mount to add slight changes to both
RA and Dec drive rates. The amount of drift that I see can range from 10 arc seconds per hour to over 100 as you
go from near the meridian to down below 40 degrees elevation.



Last night I was following a path in Leo that started about 45 degrees elevation, where the RA and Dec drift was
39 and 15 arc sec per hour. By the time the scope was pointing near the meridian the drift had changed to 16 and
5 arc sec per hour respectively. These drift rates are strictly due to atmospheric refraction and themselves are
slightly variable due to differences in temperature and pressure from night to night.


For more information, you can consult these:

https://britastro.org/2019/atmospheric-refraction
http://wise-obs.tau.ac.il/~eran/Wise/Util/Refraction.html
http://www.geoastro.de/refract/

From Brayebrook observatory:

Notes on Polar Alignment Methods - Observatory Class Telescopes


The type & accuracy of polar alignment depends on the work to be carried out and the type of drive. If you intend
to use the telescope primarily for astrophotography the polar axis should be elevated above the true pole by 60.4
cosec Ø at temperate latitudes. This will compensate for Dec drift due to refraction for ±2h either side of the
meridian and between -10º & +30º Dec. If the RA drive is fixed speed, it can be set to the King rate (1436.46 mins
per rev ), with minimal tracking correction required.

Rolando




-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 5:20 am
Subject: Re: [ap-gto] Field Rotation


Hmm. It sounds like I am going to have to move somewhere where the sky is more linear. I wonder if I can
convince my wife that we have to move to Hawaii where the sky is more linear :>)

Seriously though, besides tweaking the polar alignment are there any other tricks that will minimize this drift ? I
see lots and lots of images on this forum with little or no apparent drift.

Mike

--
Roland Christen
Astro-Physics


Roland Christen
 


drift in the main telescope had to come from something else.
Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on your planetarium program as a slow shift. Whether autoguiding or using a model, there is no difference as far as the mount is concerned. It is being told to move slightly in both axes every few seconds in order to keep tracking the star. The sky is warped due to atmospheric refraction as shown in these links:
The planetarium program is not warped and shows the stars where they would be if no atmosphere was above. As the scope tracks the actual star motion it begins to differ from the theoretical planetarium path and thus shows up as a slightly different RA/Dec coordinate as time passes. I see this all the time when I monitor the position of the object versus the position of the scope in SkyX. It only takes some few minutes for the two to part ways when you highly magnify the sky chart. And it is real, it happens even if there is zero flexure. It is a natural thing that occurs due to atmospheric refraction. As an extreme example, if you followed the Sun at the solar rate from the meridian to where it sets, your scope would be pointed ~1/2 degree (1800 arc sec) below the actual position of the Sun at sunset.

Roland

-----Original Message-----
From: Ray Gralak <iogroups@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 10:31 am
Subject: Re: [ap-gto] Field Rotation

Hi Roland,

In Mike's case he was autoguiding and not using a model. Since he said autoguiding was good and he was using a separate guidescope, drift in the main telescope had to come from something else. Differential flexure and/or field rotation are the best possibilities. It is also possible what Brian mentioned - that the camera rotated in the main scope either by slipping or from the tortional influence of the camera's body ii any part of the camera sticks out (although the amount of rotation from torsion should be tiny unless the focuser tube is weak),

-Ray

> -----Original Message-----
> From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Roland Christen via groups.io
> Sent: Thursday, June 2, 2022 8:06 AM
> To: main@ap-gto.groups.io
> Subject: Re: [ap-gto] Field Rotation
>
>
>     Seriously though, besides tweaking the polar alignment are there any other tricks that will minimize this
> drift ? I see lots and lots of images on this forum with little or no apparent drift.
>
> I do a lot of testing of modeling using both APCC-APPM and the keypad models. Even with perfect polar
> alignment there is drift in RA and Dec over most portions of the sky as you get away from the zenith. That drift is
> compensated by the modeling to a high degree as the model will program the mount to add slight changes to both
> RA and Dec drive rates. The amount of drift that I see can range from 10 arc seconds per hour to over 100 as you
> go from near the meridian to down below 40 degrees elevation.
>
>
>
> Last night I was following a path in Leo that started about 45 degrees elevation, where the RA and Dec drift was
> 39 and 15 arc sec per hour. By the time the scope was pointing near the meridian the drift had changed to 16 and
> 5 arc sec per hour respectively. These drift rates are strictly due to atmospheric refraction and themselves are
> slightly variable due to differences in temperature and pressure from night to night.
>
>
> For more information, you can consult these:
>
> https://britastro.org/2019/atmospheric-refraction
> http://wise-obs.tau.ac.il/~eran/Wise/Util/Refraction.html
> http://www.geoastro.de/refract/
>
> From Brayebrook observatory:
>
> Notes on Polar Alignment Methods - Observatory Class Telescopes
>
>
> The type & accuracy of polar alignment depends on the work to be carried out and the type of drive. If you intend
> to use the telescope primarily for astrophotography the polar axis should be elevated above the true pole by 60.4
> cosec Ø at temperate latitudes. This will compensate for Dec drift due to refraction for ±2h either side of the
> meridian and between -10º & +30º Dec. If the RA drive is fixed speed, it can be set to the King rate (1436.46 mins
> per rev ), with minimal tracking correction required.
>
> Rolando
>
>
>
>
> -----Original Message-----
> From: M Hambrick <mhambrick563@...>
> To: main@ap-gto.groups.io
> Sent: Thu, Jun 2, 2022 5:20 am
> Subject: Re: [ap-gto] Field Rotation
>
>
> Hmm. It sounds like I am going to have to move somewhere where the sky is more linear. I wonder if I can
> convince my wife that we have to move to Hawaii where the sky is more linear :>)
>
> Seriously though, besides tweaking the polar alignment are there any other tricks that will minimize this drift ? I
> see lots and lots of images on this forum with little or no apparent drift.
>
> Mike
>
> --
> Roland Christen
> Astro-Physics
>







--
Roland Christen
Astro-Physics


Howard Ritter
 

That seems counterintuitive. The mount should be insensitive to leveling error, since the process of polar alignment with each new setting up accounts for any direction of out-of-level. And in any case, the process of leveling would seem to me to be pretty coarse by comparison, and the error in leveling far greater, than the error in aligning. Or am I misunderstanding something?

—howard

On Jun 2, 2022, at 10:39 AM, Andrew Burwell via groups.io <andrew_burwell@...> wrote:

I get slight field rotation if I'm not fully leveled. Especially if I rely on a bubble level. It's better to take the mount head off and properly level the tripod with something more reliable. I've also noticed on light weight tripods and mounts that the weight of the load can shift things slightly (flexure) on each side of the meridian causing a slight bit of rotation in the images once stacked. But in both cases it can easily be cropped from a final image without losing much data. But when properly leveled I still see a very tiny amount and still need to crop just a tad.
-Andrew


Roland Christen
 

Leveling the mount has no effect whatsoever on field rotation. A mount does not need to be level, the polar axis just needs to be pointed at the pole. That axis doesn't care where its support structure is pointing - it could be completely upside down or sideways, and it would make zero difference in the mount's tracking or pointing ability.

Rolando

-----Original Message-----
From: Howard Ritter via groups.io <howard.ritter@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 10:59 am
Subject: Re: [ap-gto] Field Rotation

That seems counterintuitive. The mount should be insensitive to leveling error, since the process of polar alignment with each new setting up accounts for any direction of out-of-level. And in any case, the process of leveling would seem to me to be pretty coarse by comparison, and the error in leveling far greater, than the error in aligning. Or am I misunderstanding something?

—howard

On Jun 2, 2022, at 10:39 AM, Andrew Burwell via groups.io <andrew_burwell@...> wrote:

I get slight field rotation if I'm not fully leveled. Especially if I rely on a bubble level. It's better to take the mount head off and properly level the tripod with something more reliable. I've also noticed on light weight tripods and mounts that the weight of the load can shift things slightly (flexure) on each side of the meridian causing a slight bit of rotation in the images once stacked. But in both cases it can easily be cropped from a final image without losing much data. But when properly leveled I still see a very tiny amount and still need to crop just a tad.
-Andrew


--
Roland Christen
Astro-Physics


Steve Reilly
 

Just for clarity what is considered suitable imaging alignment? How close do you want the PA to be? I had always thought that having it too close was harder for guiding as it would be rocking on both sides to correct. Maybe that’s outdated thinking…From what I see and Ray informed me of I’m think my PA is OK but….

A screenshot of a computer

Description automatically generated with medium confidenceGraphical user interface

Description automatically generated with low confidence

 

Should I refine this? It’s Greek to me….

 

-Steve

 

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Roland Christen via groups.io
Sent: Thursday, June 2, 2022 12:17 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

Leveling the mount has no effect whatsoever on field rotation. A mount does not need to be level, the polar axis just needs to be pointed at the pole. That axis doesn't care where its support structure is pointing - it could be completely upside down or sideways, and it would make zero difference in the mount's tracking or pointing ability.

 

Rolando

 

-----Original Message-----
From: Howard Ritter via groups.io <
howard.ritter@...>
To:
main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 10:59 am
Subject: Re: [ap-gto] Field Rotation

That seems counterintuitive. The mount should be insensitive to leveling error, since the process of polar alignment with each new setting up accounts for any direction of out-of-level. And in any case, the process of leveling would seem to me to be pretty coarse by comparison, and the error in leveling far greater, than the error in aligning. Or am I misunderstanding something?

 

—howard



On Jun 2, 2022, at 10:39 AM, Andrew Burwell via groups.io <andrew_burwell@...> wrote:

 

I get slight field rotation if I'm not fully leveled. Especially if I rely on a bubble level. It's better to take the mount head off and properly level the tripod with something more reliable. I've also noticed on light weight tripods and mounts that the weight of the load can shift things slightly (flexure) on each side of the meridian causing a slight bit of rotation in the images once stacked. But in both cases it can easily be cropped from a final image without losing much data. But when properly leveled I still see a very tiny amount and still need to crop just a tad.
-Andrew

 


--
Roland Christen
Astro-Physics


Steve Reilly
 

I might should have maybe mentioned I image with a 12.5” RC and AP1200GTO with a image scale of 0.62 arc seconds.

 

-Steve

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Steve Reilly via groups.io
Sent: Thursday, June 2, 2022 1:13 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

Just for clarity what is considered suitable imaging alignment? How close do you want the PA to be? I had always thought that having it too close was harder for guiding as it would be rocking on both sides to correct. Maybe that’s outdated thinking…From what I see and Ray informed me of I’m think my PA is OK but….

A screenshot of a computer

Description automatically generated with medium confidenceGraphical user interface

Description automatically generated with low confidence

 

Should I refine this? It’s Greek to me….

 

-Steve

 

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Roland Christen via groups.io
Sent: Thursday, June 2, 2022 12:17 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

Leveling the mount has no effect whatsoever on field rotation. A mount does not need to be level, the polar axis just needs to be pointed at the pole. That axis doesn't care where its support structure is pointing - it could be completely upside down or sideways, and it would make zero difference in the mount's tracking or pointing ability.

 

Rolando

 

-----Original Message-----
From: Howard Ritter via groups.io <
howard.ritter@...>
To:
main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 10:59 am
Subject: Re: [ap-gto] Field Rotation

That seems counterintuitive. The mount should be insensitive to leveling error, since the process of polar alignment with each new setting up accounts for any direction of out-of-level. And in any case, the process of leveling would seem to me to be pretty coarse by comparison, and the error in leveling far greater, than the error in aligning. Or am I misunderstanding something?

 

—howard

 

On Jun 2, 2022, at 10:39 AM, Andrew Burwell via groups.io <andrew_burwell@...> wrote:

 

I get slight field rotation if I'm not fully leveled. Especially if I rely on a bubble level. It's better to take the mount head off and properly level the tripod with something more reliable. I've also noticed on light weight tripods and mounts that the weight of the load can shift things slightly (flexure) on each side of the meridian causing a slight bit of rotation in the images once stacked. But in both cases it can easily be cropped from a final image without losing much data. But when properly leveled I still see a very tiny amount and still need to crop just a tad.
-Andrew

 


--
Roland Christen
Astro-Physics


Roland Christen
 


I had always thought that having it too close was harder for guiding as it would be rocking on both sides to correct.
That may apply to cheap mounts that have no spring loaded worms. Does not apply here.

Roland

-----Original Message-----
From: Steve Reilly <sreilly24590@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 12:13 pm
Subject: Re: [ap-gto] Field Rotation

Just for clarity what is considered suitable imaging alignment? How close do you want the PA to be? I had always thought that having it too close was harder for guiding as it would be rocking on both sides to correct. Maybe that’s outdated thinking…From what I see and Ray informed me of I’m think my PA is OK but….
A screenshot of a computerDescription automatically generated with medium confidenceGraphical user interfaceDescription automatically generated with low confidence
 
Should I refine this? It’s Greek to me….
 
-Steve
 
 
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Roland Christen via groups.io
Sent: Thursday, June 2, 2022 12:17 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation
 
Leveling the mount has no effect whatsoever on field rotation. A mount does not need to be level, the polar axis just needs to be pointed at the pole. That axis doesn't care where its support structure is pointing - it could be completely upside down or sideways, and it would make zero difference in the mount's tracking or pointing ability.
 
Rolando
 
-----Original Message-----
From: Howard Ritter via groups.io <
howard.ritter@...>
To:
main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 10:59 am
Subject: Re: [ap-gto] Field Rotation
That seems counterintuitive. The mount should be insensitive to leveling error, since the process of polar alignment with each new setting up accounts for any direction of out-of-level. And in any case, the process of leveling would seem to me to be pretty coarse by comparison, and the error in leveling far greater, than the error in aligning. Or am I misunderstanding something?
 
—howard


On Jun 2, 2022, at 10:39 AM, Andrew Burwell via groups.io <andrew_burwell@...> wrote:
 
I get slight field rotation if I'm not fully leveled. Especially if I rely on a bubble level. It's better to take the mount head off and properly level the tripod with something more reliable. I've also noticed on light weight tripods and mounts that the weight of the load can shift things slightly (flexure) on each side of the meridian causing a slight bit of rotation in the images once stacked. But in both cases it can easily be cropped from a final image without losing much data. But when properly leveled I still see a very tiny amount and still need to crop just a tad.
-Andrew
 

--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics


Roland Christen
 

Polar align you mount as best you can. It keeps your drift to a minimum and makes it easier on your guiding software.

Roland

-----Original Message-----
From: Steve Reilly <sreilly24590@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 12:14 pm
Subject: Re: [ap-gto] Field Rotation

I might should have maybe mentioned I image with a 12.5” RC and AP1200GTO with a image scale of 0.62 arc seconds.
 
-Steve
 


--
Roland Christen
Astro-Physics


Ray Gralak
 

Hi Roland,

Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
your planetarium program as a slow shift.
Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray


 

>>> Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)

On Thu, Jun 2, 2022 at 12:19 PM Ray Gralak <iogroups@...> wrote:
Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray









Steve Reilly
 

And dithered guiding would also contribute no?

 

Steve

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Brian Valente
Sent: Thursday, June 2, 2022 3:31 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

>>> Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

 

What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)

 

On Thu, Jun 2, 2022 at 12:19 PM Ray Gralak <iogroups@...> wrote:

Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray






 

--


 

that's another possibility, especially if it's spiral dither in both axis


On Thu, Jun 2, 2022 at 12:49 PM Steve Reilly <sreilly24590@...> wrote:

And dithered guiding would also contribute no?

 

Steve

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Brian Valente
Sent: Thursday, June 2, 2022 3:31 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

>>> Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

 

What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)

 

On Thu, Jun 2, 2022 at 12:19 PM Ray Gralak <iogroups@...> wrote:

Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray






 

--




M Hambrick
 

Thanks everyone for the comments.

To Ray's comment, my guide scope and main scope are not perfectly aligned, but they are within a dozen or so arc-minutes of pointing to the same spot. I am not particularly picky about the guide star as long as it is within the center 1/2 of the guide scope field of view. For any given image, the guide star I choose is probably within a few dozen arc-minutes of the center of the main image.

The main scope is a 180 EDT with a 3.5-inch focuser and Quad TCC with a SBIG STXL16200. The guide scope is a Tele-Vue Pronto with a SBIG ST2000 camera. The main and guide scopes are in a side-by side arrangement on an 1100 GTO (non AE) mount. It is pretty well balanced in RA and Declination, and I always tie off my cables at the pier to minimize the length of hanging cables. 

I use a RAPAS for polar alignment, but since I have a portable setup I have never bothered to set up a model. I will have to try it though to see if I can see a difference per Roland's comments.

Per Brian's comment. I didn't stack my images but plate solved the first and last image of the night to calculate the shift in the center of the image which I mentioned above to be 7.91" in RA and 25.5" in Declination over the 5 hour period. If I look at the images throughout the evening you can see that this drift is continuous and pretty consistent. I do not see any sudden shifts that would indicate a shift in the imaging train.

I have two (maybe three) takeaways from this thread to see if I can improve this:

  1. Try off-axis guiding rather than using a separate guide scope & camera.
  2. Try building a pointing model in APCC to see if there is a reduction in the drift.
  3. Move to Hawaii where the sky must be more linear. I'll bet there are still lots of vacant lots in Kohala Ranch :>)

Mike

Mike


Roland Christen
 


But this does not mean that the actual sky RA/Dec of the guide star has changed
Yes, the star's theoretical RA/Dec is constant. However, if you plate solve that star when it's overhead and call that the theoretical RA/Dec, then when you plate solve it at another point in the sky the RA/Dec of where that star ends up will be different.

Roland

-----Original Message-----
From: Ray Gralak <iogroups@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 2:19 pm
Subject: Re: [ap-gto] Field Rotation

Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray







--
Roland Christen
Astro-Physics


Roland Christen
 


What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)
I do this routinely every time I am out testing scopes and mounts. A star's position will change in the sky from beginning to end. The RA/Dec will change, so that the reported coordinates after a plate solve will come out different. I see this every night that I am imaging. And it has zero to do with any cable snags or other mechanical issues. It is pure and simple atmospheric refraction that is bending the tracking path of an object.

If you want, I am out tonight testing and imaging, and I will show you on the planetarium program how far the guide star moves during an imaging run, even though it is bang on the crosshairs during all this time.

Roland

-----Original Message-----
From: Brian Valente <bvalente@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 2:30 pm
Subject: Re: [ap-gto] Field Rotation

>>> Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)

On Thu, Jun 2, 2022 at 12:19 PM Ray Gralak <iogroups@...> wrote:
Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray








--

--
Roland Christen
Astro-Physics


Roland Christen
 

You are chasing a red herring down a blind alley.

Rolando

-----Original Message-----
From: Steve Reilly <sreilly24590@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 2:49 pm
Subject: Re: [ap-gto] Field Rotation

And dithered guiding would also contribute no?
 
Steve
 
From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Brian Valente
Sent: Thursday, June 2, 2022 3:31 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation
 
>>> Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.
 
What is unclear to me is if the images inbetween start and end show a slow change in position on the sensor, or if there are one or more sudden jumps (which would indicate mechanical issues: cable snag, flexure, etc.)
 
On Thu, Jun 2, 2022 at 12:19 PM Ray Gralak <iogroups@...> wrote:
Hi Roland,

> Even if you are autoguiding, the tiny corrections from the autoguider add up over time and will even show up on
> your planetarium program as a slow shift.

Yes, because in the act of guiding the mount's RA and Dec can change, which is what is reflected in the planetarium program. But this does not mean that the actual sky RA/Dec of the guide star has changed

Mike said he plate-solved the starting and ending images and the coordinates were off by a few tens of arc-seconds. This is an actual shift in stars and sky position independent of any change in RA/Dec mount coordinates.

-Ray






 
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--
Roland Christen
Astro-Physics


Roland Christen
 


  1. Try off-axis guiding rather than using a separate guide scope & camera.
  2. Try building a pointing model in APCC to see if there is a reduction in the drift.
  3. Move to Hawaii where the sky must be more linear. I'll bet there are still lots of vacant lots in Kohala Ranch :>)
None of what you posted above will do anything to change what happens. I tried to explain what is happening (atmospheric refraction bends the path), but if you guys really must chase this red herring, then I am at a loss as to how to explain a simple natural phenomenon.

Rolando

-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 2, 2022 3:02 pm
Subject: Re: [ap-gto] Field Rotation

Thanks everyone for the comments.

To Ray's comment, my guide scope and main scope are not perfectly aligned, but they are within a dozen or so arc-minutes of pointing to the same spot. I am not particularly picky about the guide star as long as it is within the center 1/2 of the guide scope field of view. For any given image, the guide star I choose is probably within a few dozen arc-minutes of the center of the main image.

The main scope is a 180 EDT with a 3.5-inch focuser and Quad TCC with a SBIG STXL16200. The guide scope is a Tele-Vue Pronto with a SBIG ST2000 camera. The main and guide scopes are in a side-by side arrangement on an 1100 GTO (non AE) mount. It is pretty well balanced in RA and Declination, and I always tie off my cables at the pier to minimize the length of hanging cables. 

I use a RAPAS for polar alignment, but since I have a portable setup I have never bothered to set up a model. I will have to try it though to see if I can see a difference per Roland's comments.

Per Brian's comment. I didn't stack my images but plate solved the first and last image of the night to calculate the shift in the center of the image which I mentioned above to be 7.91" in RA and 25.5" in Declination over the 5 hour period. If I look at the images throughout the evening you can see that this drift is continuous and pretty consistent. I do not see any sudden shifts that would indicate a shift in the imaging train.

I have two (maybe three) takeaways from this thread to see if I can improve this:

  1. Try off-axis guiding rather than using a separate guide scope & camera.
  2. Try building a pointing model in APCC to see if there is a reduction in the drift.
  3. Move to Hawaii where the sky must be more linear. I'll bet there are still lots of vacant lots in Kohala Ranch :>)

Mike

Mike

--
Roland Christen
Astro-Physics


M Hambrick
 

OK I guess I misunderstood what you said about modeling. I was thinking that it would help.

So, what is the secret to taking guided 5-to-10-minute images with perfectly round stars ? 

Mike


 

Mike are your stars not round? I was under the impression you just had some shift in your images


On Thu, Jun 2, 2022 at 7:37 PM M Hambrick <mhambrick563@...> wrote:
OK I guess I misunderstood what you said about modeling. I was thinking that it would help.

So, what is the secret to taking guided 5-to-10-minute images with perfectly round stars ? 

Mike




Ray Gralak
 

I do this routinely every time I am out testing scopes and mounts. A star's position will change in the sky from
beginning to end. The RA/Dec will change, so that the reported coordinates after a plate solve will come out
different. I see this every night that I am imaging. And it has zero to do with any cable snags or other mechanical
issues. It is pure and simple atmospheric refraction that is bending the tracking path of an object.
Roland, I sent you a private email, but when auto-guiding, the guide star is usually not far from the area being imaged, so the guide star is *also* refracted. Thus the only refraction effect is the difference between the refraction seen at the guide scope and the main scope, which is usually pretty small.

-Ray