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Field Rotation
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Am I correct in my thinking that field rotation is inevitable even on a perfectly polar aligned mount ? I seem to recall reading that somewhere on this forum.
Regardless, are there techniques besides good polar alignment that will minimize the amount of field rotation that occurs during a long series of guided exposures ? A couple possible things come to mind:
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Roland Christen
Guiding of any kind does not affect field rotation. If you are actually experiencing field rotation, the only way to counter it is rotation of the camera in the opposite direction. This is typically done with Alt-Az mounts where field rotation is 100%.
In an equatorial mount field rotation is normally encountered only when using very short focal length, very wide field imaging equipment. It's not going to be a factor in normal astro-photography.
Rolando
-----Original Message-----
From: M Hambrick <mhambrick563@...> To: main@ap-gto.groups.io Sent: Sat, May 28, 2022 11:57 am Subject: [ap-gto] Field Rotation Am I correct in my thinking that field rotation is inevitable even on a perfectly polar aligned mount ? I seem to recall reading that somewhere on this forum.
Regardless, are there techniques besides good polar alignment that will minimize the amount of field rotation that occurs during a long series of guided exposures ? A couple possible things come to mind:
-- Roland Christen Astro-Physics |
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Howard Ritter
Field rotation occurs whenever, and only when, motion occurs around two axes during tracking. Ideally, this wouldn’t happen with an equatorial mount except when the polar axis is imperfectly aligned. In practice, it can happen when atmospheric refraction moves the object higher or lower as its elevation changes and a correction in Dec is needed, but I think this is important only for long exposures at low elevations.
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Field rotation due to an imperfectly aligned polar axis necessitating corrections in Dec is a consequence of spherical trigonometry and is not affected by what star in the field is chosen; only the axis of rotation depends on that, since the rotation will be centered on the guide star, or on the average position of the stars used in multiple-star guiding. Absolute encoders don’t affect it. They only tell the control computer exactly where the telescope is pointed, factoring out things like slop and backlash in the mount between the motor shafts and the axis shafts. —howard
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Thanks for the comments Roland and Howard.
What do you think I am seeing when the position of my images is shifting over an extended period ? Friday night I was taking images of M51 over a period of about 5 hours. I took the first image at about 10:30 pm when it was .just on the west side of the meridian. The last image was at 3:30 am. I plate solved the first and last images and measured the difference in the coordinates at the center of the two images. The RA had shifted 7.91" and the Dec had shifted 25.5" Can this be explained by atmospheric refraction ? Mike |
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christian viladrich
Hello, Years ago, when professionals used large Schmidt telescopes with large photographic glass plates, they change the polar elevation of the mount according the photographic field in order to minimize field rotation. I think I have the calculation somewhere in a book. But not sure this is relevant now, even for 24x36 mm CCD sensor and 30 min integration time😉 Christian Viladrich
Le 28/05/2022 à 16:57, M Hambrick a
écrit :
Am I correct in my thinking that field rotation is inevitable even on a perfectly polar aligned mount ? I seem to recall reading that somewhere on this forum. |
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ap@CaptivePhotons.com
On Sun, May 29, 2022 at 10:18 AM, christian viladrich wrote:
Years ago, when professionals used large Schmidt telescopes with large photographic glass plates, they change the polar elevation of the mount according the photographic field in order to minimize field rotation.TSX with tPoint gives you a choice of places to point for polar alignment based on what kind of drift/rotation you want to minimize. Below is the last polar alignment I ran with it. The little table gave different adjustments (in arc seconds) to make for different purposes. My answer is guiding. :) Linwood 
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Howard Ritter
That doesn’t seem quite plausible to me. Atmospheric refraction causes a difference of about 55 arcsec in apparent elevation of an object when it’s at an elevation of 45º compared to when it’s at the zenith. I don’t know what your latitude is, but at mine in Ohio, M51 is very close to just these positions at 2230 and 0330 currently. It looks like, at a latitude of 45º, the elevation vector is almost parallel to RA at the location of M51 during this interval. This means that the better part of that 55” would be projected onto the RA coordinate. And then, because of the fact that the lines of RA, like those of longitude, converge as they approach the pole, a line segment (say) 40" in length cutting across the lines of RA nearly perpendicularly would extend across more than 40” of RA – I believe it would be 40”/cos(Dec). The Dec of M51 is close to 45º, whose cos is ~0.7, so maybe a shift in RA of 60” or so, not the 8" or so that you saw. The effect on Dec would be first to increase it, then to decrease it, as the orientation of M51’s elevation vector relative to celestial coordinates changes, probably with little net effect, whereas you saw a shift of 25". That said, I’m not an expert, of which there must be some in this group. It would be interesting to hear from them. —howard
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Ray Gralak
What do you think I am seeing when the position of my images is shifting over an extended period ? Friday night IMike, were you autoguiding or not? Also, were you using an APCC model or not? If you were using a model, were you using Dec Arc tracking rate correction? -Ray |
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Hy Murveit
Mike, That movement of the image sounds like differential flexure to me (flex between your guide scope/camera vs your main scope/camera). Hy On Sun, May 29, 2022 at 6:06 AM M Hambrick <mhambrick563@...> wrote: Thanks for the comments Roland and Howard. |
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Hi Ray and Hy
I was continuously guiding (via MaxIm DL) for the entire 5 hours that I was taking these images, but I am not using a model. The RMS guiding errors were averaging 0.4" to 0.5" in RA and 0.5" to 0.6" in Dec. I am using a separate guide scope in a side-by-side arrangement. Everything is well balanced. I suspect I have some flexure going on, but I didn't think that differential flexure will cause a progressive drift like I am seeing ? I suppose it is possible. M51 was just past the meridian with the scope pointing almost vertically when I started taking images. Five hours later it is only 25 degrees above the horizon. I suppose that during this time the forces that cause differential flexure are continuously changing and likely getting worse as the orientation of the scope changes from vertical to horizontal. I think I am going to have to switch to an off-axis guiding configuration to resolve this. Mike |
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Edward Beshore
Hi Mike
What is the focal length of your setup? If its over 1000mm, I would definitely consider an off-axis guider. As an acquaintance of mine says (who knows) "When you are dealing with an arc sec, everything is made of rubber." Ed |
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Hi Ed
It's 1186 mm, although I have seen similar behavior with a much shorter (650 mm) focal length. Mike |
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Roland Christen
The RA had shifted 7.91" and the Dec had shifted 25.5" The answer is yes.
Rolando
-----Original Message-----
From: M Hambrick <mhambrick563@...> To: main@ap-gto.groups.io Sent: Sun, May 29, 2022 8:06 am Subject: Re: [ap-gto] Field Rotation Thanks for the comments Roland and Howard.
What do you think I am seeing when the position of my images is shifting over an extended period ? Friday night I was taking images of M51 over a period of about 5 hours. I took the first image at about 10:30 pm when it was .just on the west side of the meridian. The last image was at 3:30 am. I plate solved the first and last images and measured the difference in the coordinates at the center of the two images. The RA had shifted 7.91" and the Dec had shifted 25.5" Can this be explained by atmospheric refraction ? Mike -- Roland Christen Astro-Physics |
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Roland Christen
I think I am going to have to switch to an off-axis guiding configuration to resolve this. This will do nothing to resolve what you are seeing. The motion of the object is affected by both atmospheric refraction and slight polar misalignment. No matter what you do, the object will follow that path. It is not field rotation and the change in RA/Dec values are caused by the fact that the sky is not linear.
Rolando
-----Original Message-----
From: M Hambrick <mhambrick563@...> To: main@ap-gto.groups.io Sent: Mon, May 30, 2022 9:11 am Subject: Re: [ap-gto] Field Rotation Hi Ray and Hy
I was continuously guiding (via MaxIm DL) for the entire 5 hours that I was taking these images, but I am not using a model. The RMS guiding errors were averaging 0.4" to 0.5" in RA and 0.5" to 0.6" in Dec. I am using a separate guide scope in a side-by-side arrangement. Everything is well balanced. I suspect I have some flexure going on, but I didn't think that differential flexure will cause a progressive drift like I am seeing ? I suppose it is possible. M51 was just past the meridian with the scope pointing almost vertically when I started taking images. Five hours later it is only 25 degrees above the horizon. I suppose that during this time the forces that cause differential flexure are continuously changing and likely getting worse as the orientation of the scope changes from vertical to horizontal. I think I am going to have to switch to an off-axis guiding configuration to resolve this. Mike -- Roland Christen Astro-Physics |
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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 |
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Ray Gralak
Hi Mike,
Hmm. It sounds like I am going to have to move somewhere where the sky is more linear. I wonder if I canI have a different opinion on the cause of the drift. Since you were autoguiding with a separate guide scope, you are likely observing a combination of differential flexure and field rotation. This assumes that the guide scope is roughly pointing to the same area of the sky as the main scope, in which case the effects of refraction almost equally affect the guide scope and main scope and would not cause the amount of drift you observed. Field rotation from polar misalignment may not be noticeable in individual images but may accumulate over hours and contribute to drift. However, the amount of drift depends on how far the guide scope's guide star is from the center point of your main scope's camera view. -Ray |
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Howard Ritter
You want a linear sky? Talk to Elon about a ride to LEO. Modeling might be tricky.
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—howard
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>>>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 imo i'm still not sure it's clear where this drift comes from. Did you stack them and see how much rotation happened through your night? Another test you can do is run your subs as a short video (1 frame = 1 video frame) and watch the movement. In addition to flexure and a few other things people mentioned, it's possible it can come from shifting equipment, cabling, etc. I had a similar issue and eventually tracked it down to a focuser tube that shifted at a specific sky position (not AP scope). A lot of things can contribute to this, but tracking down the root cause(s) is the first step
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Brian Brian Valente astro portfolio https://www.brianvalentephotography.com/astrophotography/ portfolio brianvalentephotography.com |
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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 |
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Roland Christen
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 TelescopesThe 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 |
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