Date   

Re: New (to me) Mach2 poor tracking

ROBERT WYNNE
 

Reminds me of the first day reporting as an apprentice machinist with what I thought was the minimum set of tools. I had bought a wooden rule which I thought was consistent with the rule requirement for a basic set of tools. The shop foreman came over to check the completeness of my set and found the wooden rule. He picked it up and tossed it in the garbage can at the same time said, "What do you think we are here, carpenters"? -Best, Robert

On 03/30/2023 1:37 AM Roland Christen via groups.io <chris1011@...> wrote:


Regardless of which axis is showing drift, the mount itself is not producing drift. It is either polar misalignment or atmospheric refraction drift, neither of which is caused by the mount's tracking. The Mach2 especially cannot drift, it has high resolution encoders maintaining a steady sidereal motion to the RA axis. The Mach 2 mount maintains a very rigid and accurate tracking rate within 1/4 of a arc second, and that's hour after hour. No cheap guidescope can measure this level of accuracy, even for a few minutes.

One problem a lot of setups have is a small guide scope held in place by flimsy 3-point screws. This is then attached to the rings which hold the main scope. As the main scope flexes slightly inside the rings, the rings then impart a small motion to the guide scope. I've seen people trying to guide a 140mm refractor with a 30mm F5 guidescope. This is a total disaster if you want to guide accurately. Even measuring the basic guidestar position is nuts - it's like trying to measure the thickness of gold leaf with a wooden yardstick. Then trying to explain the the mount is doing the job correctly is a fool's errand. Nobody believes the manufacturer these days. 

Roland


-----Original Message-----
From: Andrea Lucchetti <andlucchett@...>
To: main@ap-gto.groups.io
Sent: Wed, Mar 29, 2023 9:49 pm
Subject: Re: [ap-gto] New (to me) Mach2 poor tracking

Hi Marc,
Probably you mentioned before,
Are you 100% sure that Ra & Dec are mapped on x&y?
Basically, are you sure the drift is in RA and not in DEC? 
You know, sometimes we need a third eye to spot the simplest issues because we are so convinced.
In case just move the mount and check real  displacement.

Andrea


Il giorno gio 30 mar 2023 alle 09:28 Roland Christen via groups.io <chris1011=aol.com@groups.io> ha scritto:
RA drift is strictly a function of polar alignment and where in the sky you are shooting. The mount's primary tracking rate is sidereal. The stars move at sidereal only at the zenith. Everywhere else in the sky they will move anywhere from 5 to 150 arcsec per hour. That is fundamental for stars moving across the sky. If you have perfect polar alignment the sidereal tracking rate will work fine within a circle of about 15 degrees around the zenith for typical 5 - 10 minute exposures. This is also fundamental. 

However, perfect polar alignment will NOT eliminate RA drift over the whole sky, especially East, West and South of the zenith. This is also a well-known fact and many papers have been written that allow calculation of the drift rate at various parts of the sky. Look up Publications of Brayebrook Observatory, which has very detailed calculations for figuring out star drift across the sky.

If you want no RA and no Dec drift, you will need to model.

Roland Christen
Astro-Physics Inc.

-----Original Message-----
From: Marc Blank <marc.blank@...>
To: main@ap-gto.groups.io
Sent: Wed, Mar 29, 2023 8:43 pm
Subject: Re: [ap-gto] New (to me) Mach2 poor tracking

Thanks for the response, Roland.  Is the pronounced RA drift expected given the encoders?   That’s what I don’t understand.  
Marc


--
Roland Christen
Astro-Physics



--
Roland Christen
Astro-Physics


Re: Last image from Hawaii

M Hambrick
 

So how long did you get to stay in Hawaii Roland ? It seems like you have been sending a pot of images lately.

Mike


Re: Do you need to redo a APPM model after collimation?

Nick Iversen
 

I haven't used a model so I have no experience. I was wondering if doing a sync or recal after a secondary mirror adjustment would fix up the minor positioning error.


Re: Last image from Hawaii

Roland Christen
 

APS is no problem

Roland


-----Original Message-----
From: Michael Freeberg <mdfreeb@...>
To: main@ap-gto.groups.io
Sent: Thu, Mar 30, 2023 10:58 am
Subject: Re: [ap-gto] Last image from Hawaii

How sensitive is tilt with a APC-S size chip.

Thanks Mike 

--
Roland Christen
Astro-Physics


Re: Last image from Hawaii

Michael Freeberg
 

How sensitive is tilt with a APC-S size chip.

Thanks Mike 


Re: Sky 6 versus Sky X

Mike Dodd
 

On 3/30/2023 3:59 PM, Mike Shade via groups.io wrote:
Thanks...the reason I am using The Sky 6 is for a visual representation of where things are pointed. There are (rare) times when I need to tell the system where to point for whatever reason.  I could eliminate it altogether and have ACP mastermind everything as it were as the system in question is an imaging system entirely.
You can have SkyX connected to the mount while ACP is connected and running an imaging plan (I've done it). Then SkyX will show you where the scope is pointing under ACP control. I think this is what you want.

--- Mike


Re: Sky 6 versus Sky X

Roland Christen
 

That's how I do it now. I use Sky X to get me to the object, then disconnect it from the mount so it doesn't use up precious memory that is needed for image work in MaximDL. SkyX is a memory hog, and even though I have a very powerful laptop, it does sometimes run out of memory when doing LRGB with the 6200 chip.

Roland


-----Original Message-----
From: ap@... <ap@...>
To: main@ap-gto.groups.io
Sent: Thu, Mar 30, 2023 10:03 am
Subject: Re: [ap-gto] Sky 6 versus Sky X

On Thu, Mar 30, 2023 at 03:59 PM, Mike Shade wrote:
Thanks...the reason I am using The Sky 6 is for a visual representation of where things are pointed.  There are (rare) times when I need to tell the system where to point for whatever reason.
I know in Stellarium you can do this so I suspect you can in TSX, but you can SEPARATELY hook a planetarium to the ASCOM AP driver, so that the planetarium shows the location and/or can be used for pointing the telescope, while allowing the session manager (ACP I guess in this case) to also drive the scope. Mounts (ASCOM "telescope") acts as a hub, so multiple clients can connect. You can then open the planetarium only when you need it, it does not have to be on the critical control path.

If you have an urge to reduce your dependence on TSX.

Linwood

--
Roland Christen
Astro-Physics


Re: AP Portable Mount

Chris White
 

I'd be all over a 400gto AE. 


Re: Last image from Hawaii

Roland Christen
 

50mm filter mounted in a 40mm mounting. Too small to pass light fully into the corners. I need to replace with 50mm unmounted filters.

Roland


-----Original Message-----
From: Bob Enouen <renouen@...>
To: main@ap-gto.groups.io
Cc: main@ap-ug.groups.io
Sent: Thu, Mar 30, 2023 3:33 am
Subject: Re: [ap-gto] Last image from Hawaii

Roland,

What size filter/wheel are you using with the 110?  Thanks so much for the info share and transparency you provide.  It’s so edifying to be able to learn so much at my age (65).

Thanks, Bob


Robert J. Enouen
Cell 513-504-4410

On Mar 30, 2023, at 3:12 AM, Roland Christen via groups.io <chris1011@...> wrote:


The clouds moved in so my imaging session here is pretty much over, The last session I imaged the Seagull Nebula thru holes in the clouds. Seeing was very good, but because of clouds I was able to get only 8 usable 5 minute subs over a 3 hour period. 

https://www.astrobin.com/qveff8/

I want to comment on the 110 as a visual telescope and astrograph with wide field capability. 

As is the case of this object, it spans 4 degrees of sky, so it needs a short focal length instrument. There are a few refractors that can cover this field while at the same time provide resolution on par with much larger apertures. In the case of this image, the sky was very steady with resolution of 1.3 pixels (1.8 arc sec), which is pretty much the limit for this short focal length and pixel size. At the F5 focal ratio, the scope puts 100% of the light over the entire format, which is something almost no reflective system does. Therefore the deep illumination and high resolution extends to the edge of the circle overage of 44mm. In my case the filters cut off light in the corners, not something I'm that concerned about. For a lot of the images posted below, I was able to get lots of very deep sky detail using very few subs, thanks to the fast F5 optics and the 100% illumination.

As a visual instrument it is limited by the aperture, both for resolution and for showing faint deep sky objects. It is 110mm, and was designed to fit the most aperture into a carry-on case. It will not show detail on the planets like a scope of twice or 3 times this aperture. It has a very hefty focuser and tube mechanics, so it isn't a light-weight shorty grab&go. It was made for astro-photography but at the same time meets a high level of correction visually. 

Anyone who has unrealistic expectations for visual performance is encouraged to read more about the capability of optics with respect to aperture size and focal length. The 110GTX will do visually what a 110 aperture is capable of - nothing less and nothing more.

Imaging-wise it will produce nice high resolution images over a very wide field, which was the goal. It will require a very collimated chip with less than 5 microns of tilt corner to corner. Here is a list of objects that I was able to image at home and in Hawaii over the last few weeks. My skies here have been clear and good about 20% of the time, so I was forced to limit the number of subs for each object:

https://www.astrobin.com/1i0hhs/
https://www.astrobin.com/3rav9j/
https://www.astrobin.com/6r3q2h/
https://www.astrobin.com/ui9ra6/
https://www.astrobin.com/bfyydi/, https://www.astrobin.com/2dm87i/
https://www.astrobin.com/5idkm9/C/
https://www.astrobin.com/qztb8k/
https://www.astrobin.com/knokxy/
https://www.astrobin.com/rhaysr/
https://www.astrobin.com/43ybv3/
https://www.astrobin.com/oc6jwm/
https://www.astrobin.com/h8xhw8/
https://www.astrobin.com/irp1zf/
https://www.astrobin.com/qveff8/

Roland Christen
Astro-Physics Inc.









--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics


Re: Thor's Helmet Unguided

Roland Christen
 


unless you're saying the USB3 signal downgrades to USB2 speeds for you.
No, I don't downgrade the USB3 signal.

Roland


-----Original Message-----
From: Terri Zittritsch <theresamarie11@...>
To: main@ap-gto.groups.io
Sent: Thu, Mar 30, 2023 3:28 am
Subject: Re: [ap-gto] Thor's Helmet Unguided

Thanks Chris!! 

Roland,
On the internal cabling as I mentioned in an e-mail, with a newer mini computer (Mele quieter 3) the USB3 is working with a 6' non-active cable to the mount.   All else was the same in the setup and I used the highest USB throughput setting on the ASI6200!   This is good news and I'll be putting the active cable away.   Thanks for suggesting I take another look!   I think USB2 uses different signals in the cable unless you're saying the USB3 signal downgrades to USB2 speeds for you.  All of my USB2 gear worked.    With the ASI1600 that would not work for me, I'd just get failures and lockups as I recall and today can't test it further because I've sold the 1600 and replaced it with the 6200.   I suspect it may just be related to how much drive, or which USB3 chipset, a computer uses or who's IP is used to design the chipset.    Some may work well and others like the T460 might be tighter to the spec with little wiggle room.   


Terri

--
Roland Christen
Astro-Physics


Re: Sky 6 versus Sky X

ap@CaptivePhotons.com
 

On Thu, Mar 30, 2023 at 03:59 PM, Mike Shade wrote:
Thanks...the reason I am using The Sky 6 is for a visual representation of where things are pointed.  There are (rare) times when I need to tell the system where to point for whatever reason.

I know in Stellarium you can do this so I suspect you can in TSX, but you can SEPARATELY hook a planetarium to the ASCOM AP driver, so that the planetarium shows the location and/or can be used for pointing the telescope, while allowing the session manager (ACP I guess in this case) to also drive the scope. Mounts (ASCOM "telescope") acts as a hub, so multiple clients can connect. You can then open the planetarium only when you need it, it does not have to be on the critical control path.

If you have an urge to reduce your dependence on TSX.

Linwood


Re: Sky 6 versus Sky X

Mike Shade
 

Thanks.  I know it is "not" needed, but there are very infrequent times when I need to know where things are pointing.

 

Mike J. Shade

Mike J. Shade Photography:

mshadephotography.com

 

In War: Resolution

In Defeat: Defiance

In Victory: Magnanimity

In Peace: Goodwill

Sir Winston Churchill

Already, in the gathering dusk, a few of the stars are turning on their lights.

Vega, the brightest one, is now dropping towards the west.  Can it be half

a year since I watched her April rising in the east?  Low in the southwest

Antares blinks a sad farwell to fall...

Leslie Peltier, Starlight Nights

 

International Dark Sky Association: www.darksky.org

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Dale Ghent
Sent: Thursday, March 30, 2023 8:12 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Sky 6 versus Sky X

 

If you're using TSX only for mount control, it seems like you could cut it out of the workflow entirely.

If your sequencing is done via MaximDL (at the direction of ACP?), just have MaximDL connect directly to the Astro-Physics ASCOM driver. Adding TSX in as a middleman between MDL or ACP and the A-P ASCOM Driver/APCC is unnecessary.


Re: aRE: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

Nathan Myhrvold
 

On 10 meter professional telescopes they do use laser alignment, and precision metrology.   Indeed, some like the Keck have segmented mirrors that also need to be aligned and so forth.  But for an amateur scope that would be a lot of overhead.

 

Lasers are great but it is just one kind of light source.   They have some nice properties, but it turns out we already have perfectly good light sources to measure our tilt  – the stars.  Measuring the diameter of the stars on an image is an easy image processing task, and you can generally measure diameter and center (centroid) to within a small fraction of a pixel, so easily within 1 micron.

 

That is the input data for Hocus Focus or other autofocus and tilt adjustment applications.   If we can measure diameter to much less than one pixel, we have more than enough precision to estimate the tilt of the sensor to within the tolerance needed to make a good astrophoto.  Which is a key point – we only need to measure the tilt well enough to get a good photo, which means that lack of precision due to the diffraction limit for the OTA, or the pixel size or the sensor flatness pretty much cancel out.   If the tilt produces a variation in the size of the stars smaller than we can measure with image processing, then by the same token you’ll never see it in a photo.

 

I don’t think that a laser is fundamentally any better – although it would have the advantage of being able to work during the day.

 

150 years ago Foucault made parabolic mirrors figured to within a fraction of a wavelength of light using a candle and a knife edge.   We can do this more easily today with laser light sources for the measurements, but it’s a good reminder that optics by its very nature is a precision measurement game.

 

Nathan

 

 

 

From: ROBERT WYNNE <robert-wynne@...>
Sent: Thursday, March 30, 2023 12:37 PM
To: main@ap-gto.groups.io; Nathan Myhrvold <nathanm@...>
Subject: Re: aRE: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

 

Nathan- I am working through this mornings e-mails and am about half way through. I responded first to your earlier message only to find other messages had followed. I find this one so far as I've read the most succinct analysis of the problem and various rabbit holes one can get lost.

 

I believe a laser leveling system coupled to a digital DAQ system w/ display ought to be in the works for this level of precision guidance. Mechanical devices are simply not up to the task at hand. Our laser lab had digital micrometers accurate to .00001 and that was a few years ago. Matters must have improved since.

 

Best,

Robert

On 03/29/2023 9:35 PM Nathan Myhrvold <nathanm@...> wrote:

 

 

It is simple geometry, so the correlation is essentially perfect.   But you are correct that it is not a simple multiple.

 

--------------------------

The key assumption is that the sensor is a flat plane.   But even if it isn’t this will work as well as one can hope.    HF effectively figures out the equation of the plane in 3-dimensional space that makes as much of the frame as possible sharp.  The equation of the plane is expressed as an offset at four points.  Each point is a corner of the frame.  

 

If we call the center of the frame coordinate (0, 0), then for a full frame sensor, one corner is at (18, 12) – in this case the dimensions are in millimeters.  The others corners are at (18,-12), (-18, -12) and (18, -12).  But HF averages over 1/9 of the sensor.   So the actual corner measurement for the corner which is nominally (18, 24) is actually computed at (12, 8), and so forth for the other points.  

 

If we call that the (x,y) coordinates, then the HF values gives you  (x,y,z) – each of the 4 positions gets a z-value.

 

That defines a plane – actually you only need 3 of the points to define a plane – but anyway you have the equation of a plane   a*x + b*y+ c*z + d = 0 for numbers a, b, c, d.   It’s a simple formula to go from offset numbers to the coefficients a, b, c, d.

 

So now you need to measure where your screws are in the same coordinate system.   I don’t have the exact numbers in front of me, but here is a sample – for my adapter the corners would be at (37.7, 37.7) mm and then the same pattern of those numbers with (+,-), (-,-), (-,+) for the other points.

 

To get the exact offset at each of those positions you plug in the x, y of each screw into the plane equation and then solve for the z value. 

 

That will be exact – meaning that much offset at each screw will give you the correct z value at the x,y of each sensor section.

------------------------

That is the basic story, but here are some details.

 

The dial indicators are not exactly at the screw positions – they are as close as a practically could get them.  So really what I will calculate is not the coordinates of the screw positions – I will calculate at the coordinates of the dial indicator feet.  But that is a very tiny difference.

 

Some adapters let you shift the sensor in x, y.    That would change the coordinates of the screws.  But you can center and then measure the coordinates.

 

Some adapters let you rotate the sensor in the same place you do tilt and centering.  That would also change the coordinates of the screws relative to the sensor.  I am not using that degree of freedom because I have a rotator on the other side of the adapter.

 

But if you did have centering and rotation, there is a way to easily figure that out.  George (the author of Hocus Focus) is planning on putting that into HF.  Basically, what you do is take a measurement, then deliberately move two of the screws (a long a diagonal) a bunch, then measure again – you can recover the parameters you need from the difference.

 

This means you need to measure where your screws are, or have the vendor give you a diagram.  But either one is straightforward.  Since there are not that many tilt adapters out there, HF will just have a list that knows the dimensions – it’s just another configuration setting.

 

An optical detail is that the field is never flat, but the approach that HF takes is to find the best fitting plane, which is all you can hope to adjust with a tilt adapter.   What you will get from HF is the tilt plane that optimizes sharpness across the corners of the sensor – which might conceivably not be what you want for your application and that is fine. 

------------------------

Note that this assumes that HF is giving you the correct numbers.   

 

As per other email we’ve exchanged, one might say HF is not giving you the best tilt.   Which is fair enough, but it is a separate point.  The screw calculation is this - the offsets measured from the focus sweep give you the equation of the plane.  That equation tells you the exact offset at the screw positions.  

 

As you may have seen on a thread on CN, I am not a fan of averaging tilt over 1/9 of a frame - it is fundamentally less accurate.   The HF “sensor model” mode is theoretically better in some ways but it has some robustness issues.   But George has a fantastic attitude and is currently working on several ways to improve what HF outputs.  

------------------------

 

The next obvious evolution is to make this be motorized, just like we focus is motorized.  In that case rather than me fiddling with an allen wrench while looking at the dial indicator one would have a stepper motor on the screw, which is either calibrated, or where you have a sensor that tracks the offset.    This might make sense for a remote observatory.  I thought that I would try the digital display first.

 

Nathan

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Chris White via groups.io
Sent: Wednesday, March 29, 2023 7:26 PM
To: main@ap-gto.groups.io
Subject: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

 

[Edited Message Follows]

Nice work, Nathan!  It's cool idea to quantify the adjustments. 

HF determines the micron adjustment recommendation by deriving the delta between the optimal focus position of a given corner with an average of the optimal focus position of all the corners. This is essentially a step measurement between the optimal focus positions, converted to microns.  So the recommendation is not likely to correlate with how much to adjust the screw.  I have found there to be relationship though as long as you are close to optimal backspacing.  With my Epsilon system if I turned the 200tpi screw a half a turn (for a 64 micron adjustment) a Hocus Focus run would yield about 15 microns in measured movement of that corner.  Keep us posted on how your test works out.

 


Re: Sky 6 versus Sky X

Mike Shade
 

Thanks...the reason I am using The Sky 6 is for a visual representation of where things are pointed.  There are (rare) times when I need to tell the system where to point for whatever reason.  I could eliminate it altogether and have ACP mastermind everything as it were as the system in question is an imaging system entirely.

 

Mike J. Shade

Mike J. Shade Photography:

mshadephotography.com

 

In War: Resolution

In Defeat: Defiance

In Victory: Magnanimity

In Peace: Goodwill

Sir Winston Churchill

Already, in the gathering dusk, a few of the stars are turning on their lights.

Vega, the brightest one, is now dropping towards the west.  Can it be half

a year since I watched her April rising in the east?  Low in the southwest

Antares blinks a sad farwell to fall...

Leslie Peltier, Starlight Nights

 

International Dark Sky Association: www.darksky.org

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Stuart
Sent: Thursday, March 30, 2023 8:05 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Sky 6 versus Sky X

 

Mike, I wasn't impugning your current software suite, just reminding that TSX brings two features into play that do what two of your current programs do so ... you have those as options. I don't think you have any bad choices, just choices.

 

 

 

On Thu, 30 Mar 2023 at 09:49, Mike Shade via groups.io <mshade=q.com@groups.io> wrote:

Thanks,

 

I don't intend to use TSX for camera control, just in part mount control.  The current system is Sky 6/tele-api bridge/AP ASCOM driver/Maxin 6.28/Focusmax 5.xx/ACP.  This works like a champ.  The current computer system is 8 years old, that is pushing it in computer land.  And as I am very rural a computer failure would take a long time to rectify, so ordered new systems custom built.

 

Mike J. Shade

Mike J. Shade Photography:

mshadephotography.com

 

In War: Resolution

In Defeat: Defiance

In Victory: Magnanimity

In Peace: Goodwill

Sir Winston Churchill

Already, in the gathering dusk, a few of the stars are turning on their lights.

Vega, the brightest one, is now dropping towards the west.  Can it be half

a year since I watched her April rising in the east?  Low in the southwest

Antares blinks a sad farwell to fall...

Leslie Peltier, Starlight Nights

 

International Dark Sky Association: www.darksky.org

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Stuart
Sent: Wednesday, March 29, 2023 5:08 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Sky 6 versus Sky X

 

Mike, surprised you haven't been buried in replies already so I'll kick it off. TSX works fine with AP mounts. You just want to be sure not to use the native TSX driver but rather the ASCOM driver. I used TSX with my AP900GTO CP4 without any issues. I would guess that if you move to TSX from TS6 you'll have to decide whether to use TSX for camera control as well or not. There will be significant overlap with Maxim when you move up to TSX.

 

BTW - I use TSX with both my rigs and I use @focus2 with one of them and Focusmax 5 with the other. Focusmax is a better focusing program for sure but @focus2 works pretty well and is again, one less thing in the mix if you give it a try. (or @focus3 which I don't use but was added probably to make TSX more competitive with Focusmax re filter dependent focus parameters).

 

 

 

On Wed, 29 Mar 2023 at 18:23, Mike Shade via groups.io <mshade=q.com@groups.io> wrote:

After many years I am having to upgrade the observatory computer, it will have W10 pro on it.  I have been using The Sky 6 with the tele API and AP driver to run the AP mounts.  Does the Sky X work with AP mounts, or is tele API still needed?  I use Maxim/ACP/Focusmax for imaging.

Thanks, 


Re: AP Portable Mount

Samir Patel
 

Brian,

Think I saw one thread, missed the other one, will go through it. Apologies if I created another one where it already has been addressed.

Virus-free.www.avg.com


On Thu, Mar 30, 2023 at 3:29 PM Worsel via groups.io <bryancashion=yahoo.com@groups.io> wrote:
Samir

Perhaps you have seen the following threads already

https://ap-gto.groups.io/g/main/topic/93976552
https://ap-gto.groups.io/g/main/topic/87969818

Bryan


Re: aRE: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

ROBERT WYNNE
 

Nathan- I am working through this mornings e-mails and am about half way through. I responded first to your earlier message only to find other messages had followed. I find this one so far as I've read the most succinct analysis of the problem and various rabbit holes one can get lost.

I believe a laser leveling system coupled to a digital DAQ system w/ display ought to be in the works for this level of precision guidance. Mechanical devices are simply not up to the task at hand. Our laser lab had digital micrometers accurate to .00001 and that was a few years ago. Matters must have improved since.

Best,
Robert

On 03/29/2023 9:35 PM Nathan Myhrvold <nathanm@...> wrote:


It is simple geometry, so the correlation is essentially perfect.   But you are correct that it is not a simple multiple.


--------------------------

The key assumption is that the sensor is a flat plane.   But even if it isn’t this will work as well as one can hope.    HF effectively figures out the equation of the plane in 3-dimensional space that makes as much of the frame as possible sharp.  The equation of the plane is expressed as an offset at four points.  Each point is a corner of the frame.  


If we call the center of the frame coordinate (0, 0), then for a full frame sensor, one corner is at (18, 12) – in this case the dimensions are in millimeters.  The others corners are at (18,-12), (-18, -12) and (18, -12).  But HF averages over 1/9 of the sensor.   So the actual corner measurement for the corner which is nominally (18, 24) is actually computed at (12, 8), and so forth for the other points.  


If we call that the (x,y) coordinates, then the HF values gives you  (x,y,z) – each of the 4 positions gets a z-value.


That defines a plane – actually you only need 3 of the points to define a plane – but anyway you have the equation of a plane   a*x + b*y+ c*z + d = 0 for numbers a, b, c, d.   It’s a simple formula to go from offset numbers to the coefficients a, b, c, d.


So now you need to measure where your screws are in the same coordinate system.   I don’t have the exact numbers in front of me, but here is a sample – for my adapter the corners would be at (37.7, 37.7) mm and then the same pattern of those numbers with (+,-), (-,-), (-,+) for the other points.


To get the exact offset at each of those positions you plug in the x, y of each screw into the plane equation and then solve for the z value. 


That will be exact – meaning that much offset at each screw will give you the correct z value at the x,y of each sensor section.

------------------------

That is the basic story, but here are some details.


The dial indicators are not exactly at the screw positions – they are as close as a practically could get them.  So really what I will calculate is not the coordinates of the screw positions – I will calculate at the coordinates of the dial indicator feet.  But that is a very tiny difference.


Some adapters let you shift the sensor in x, y.    That would change the coordinates of the screws.  But you can center and then measure the coordinates.


Some adapters let you rotate the sensor in the same place you do tilt and centering.  That would also change the coordinates of the screws relative to the sensor.  I am not using that degree of freedom because I have a rotator on the other side of the adapter.


But if you did have centering and rotation, there is a way to easily figure that out.  George (the author of Hocus Focus) is planning on putting that into HF.  Basically, what you do is take a measurement, then deliberately move two of the screws (a long a diagonal) a bunch, then measure again – you can recover the parameters you need from the difference.


This means you need to measure where your screws are, or have the vendor give you a diagram.  But either one is straightforward.  Since there are not that many tilt adapters out there, HF will just have a list that knows the dimensions – it’s just another configuration setting.


An optical detail is that the field is never flat, but the approach that HF takes is to find the best fitting plane, which is all you can hope to adjust with a tilt adapter.   What you will get from HF is the tilt plane that optimizes sharpness across the corners of the sensor – which might conceivably not be what you want for your application and that is fine. 

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Note that this assumes that HF is giving you the correct numbers.   


As per other email we’ve exchanged, one might say HF is not giving you the best tilt.   Which is fair enough, but it is a separate point.  The screw calculation is this - the offsets measured from the focus sweep give you the equation of the plane.  That equation tells you the exact offset at the screw positions.  


As you may have seen on a thread on CN, I am not a fan of averaging tilt over 1/9 of a frame - it is fundamentally less accurate.   The HF “sensor model” mode is theoretically better in some ways but it has some robustness issues.   But George has a fantastic attitude and is currently working on several ways to improve what HF outputs.  

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The next obvious evolution is to make this be motorized, just like we focus is motorized.  In that case rather than me fiddling with an allen wrench while looking at the dial indicator one would have a stepper motor on the screw, which is either calibrated, or where you have a sensor that tracks the offset.    This might make sense for a remote observatory.  I thought that I would try the digital display first.


Nathan


From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Chris White via groups.io
Sent: Wednesday, March 29, 2023 7:26 PM
To: main@ap-gto.groups.io
Subject: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout


[Edited Message Follows]

Nice work, Nathan!  It's cool idea to quantify the adjustments. 

HF determines the micron adjustment recommendation by deriving the delta between the optimal focus position of a given corner with an average of the optimal focus position of all the corners. This is essentially a step measurement between the optimal focus positions, converted to microns.  So the recommendation is not likely to correlate with how much to adjust the screw.  I have found there to be relationship though as long as you are close to optimal backspacing.  With my Epsilon system if I turned the 200tpi screw a half a turn (for a 64 micron adjustment) a Hocus Focus run would yield about 15 microns in measured movement of that corner.  Keep us posted on how your test works out.



Re: Dec Arc Model Failure

Terry Martin
 

I will Brian.

~Terry


AP Portable Mount

Samir Patel
 

Good afternoon all. I know this has been discussed before but is AP interested in coming out with a portable mount say smaller than Mach1, feel there is an absolute need for a portable mount in the market and if AP fills that gap that would be great. The new harmonic mounts which have come out recently seem to be a nice addition, I've only seen AM5 in person, looking forward to check Pegasus and Rainbow mounts at NEAF however would love if AP can also compete in that category and give the AP community an option in that range. 


Re: Dec Arc Model Failure

midmoastro
 

I am on the latest updates for APCC/Ascom/NINA and I ran DecArc the other night on my Mach2 with success.  Hopefully this resolves the issue for you
Todd


Re: Dec Arc Model Failure

 

Terry please let us know how it goes with the updates - thanks

Brian