Field Rotation


M Hambrick
 

Hi Brian

I am not sure what to think. As near as I can tell, the total drift that I measured over the 5 hour continuously guided session was constant. If I assume this to be the case, then I think I am correct to conclude there was also drift occurring while I was taking my images which could result in elongated stars.

I measured 7.91" drift in RA and 25.5" in Declination over the 5 hours (304 minutes) of imaging. This calculates to an average drift rate of 0.026" per minute in RA and 0.084" per minute in Declination. For a 10-minute image this would be a drift of 0.026" in RA and 0.83" in Dec. Keep in mind that this RA drift rate is in seconds of RA. To convert to seconds of degrees you have to multiply this number by 15.

Is my system sensitive enough for this to show up as elongated stars ? I don't know. Many of my images contain very slightly elongated stars across the image. Is this due to drift ? The image scale with my scope / camera combination is 1.04" per pixel. On this particular night, the best focus I was able to get was a FWHM between 2" and 2.5".

Mike


Ray Gralak
 

Hi Roland,

 

I get what you are saying, but your demonstration is not like the user’s setup.

 

For instance, the original poster used two telescopes. To approximate the poster’s setup, you would have to create a Dec Arc model with data points identified with a guide scope, then capture images on a different, larger main scope.

 

Also,  completely unrelated, but SkyX accounts for refraction, so unless refraction is broken in SkyX, there must be another reason for the positional discrepancy in your example:

 

 

 

-Ray

 

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Roland Christen via groups.io
Sent: Friday, June 3, 2022 11:50 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Field Rotation

 

 

Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera. The keypad reads out the RA and Dec trim rate that is applied in the CP5 control in order to follow the star's motion over time. At the beginning of the run, at 7:52pm, the star was shown on the crosshair of the camera (right blue crosshair in MaximDL), while at the same time SkyX recorded the cursor position to be exactly aligned with the star. The trim rate being applied in RA was 61 arc sec per hour - this being the drift rate of the star at that 50 degree altitude, due mostly to atmospheric refraction and some slight polar alignment error.

 

After approximately an hour, the star is still on the blue crosshairs (mount's tracking was being adjusted continuously by the model). Pixel scale of this setup was 1.1 arc sec per pixel. You can see in the second image below that the actual reported position has moved by approximately 61 arc seconds in RA with respect to the planetarium position. If the mount had continued tracking this star for 4 or more hours into the West, the actual reported position would probably have been several hundred arc seconds. There is no field rotation involved, just simple and pure atmospheric refraction that causes a star to not be in the theoretical RA/Dec position that a planetarium program shows. Doing a Recal at any time will of course bring both the star and the cursor back together.

 

 

 


--
Roland Christen
Astro-Physics


Roland Christen
 

I usually let it drift 10 minutes for each point, then center the star and press Enter.
Focal length was 870mm, pixel size 3.76 microns, plate scale 0.89 arc sec/pixel.
Seeing was average, later on deteriorated somewhat.

Roland

-----Original Message-----
From: Dean Jacobsen <deanjacobsen@...>
To: main@ap-gto.groups.io
Sent: Fri, Jun 3, 2022 5:27 pm
Subject: Re: [ap-gto] Field Rotation

On Fri, Jun 3, 2022 at 02:48 PM, Roland Christen wrote:
Actually it was 4 points, not 3, and approximately 45 - 50 minutes between each point using convenient stars near or on the 20 degree Dec line. The model was run near sunset time, before any imaging cold be done.
 
I forgot to ask...

1.  For this example, do you remember how long you let it drift?

2.  What was the focal length for this example?
 
--
Dean Jacobsen
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/

--
Roland Christen
Astro-Physics


Dean Jacobsen
 

On Fri, Jun 3, 2022 at 02:48 PM, Roland Christen wrote:
Actually it was 4 points, not 3, and approximately 45 - 50 minutes between each point using convenient stars near or on the 20 degree Dec line. The model was run near sunset time, before any imaging cold be done.
 
I forgot to ask...

1.  For this example, do you remember how long you let it drift?

2.  What was the focal length for this example?
 
--
Dean Jacobsen
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/


Dean Jacobsen
 

On Fri, Jun 3, 2022 at 02:48 PM, Roland Christen wrote:
Actually it was 4 points, not 3, and approximately 45 - 50 minutes between each point using convenient stars near or on the 20 degree Dec line. The model was run near sunset time, before any imaging cold be done.
 
Thanks for the clarification. 

After two years of using APCC and APPM to do unguided imaging I actually bought a keypad for my Mach2 last month so that I can try out that feature.  :-)
 
--
Dean Jacobsen
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/


Roland Christen
 

Actually it was 4 points, not 3, and approximately 45 - 50 minutes between each point using convenient stars near or on the 20 degree Dec line. The model was run near sunset time, before any imaging cold be done.

Rolando

-----Original Message-----
From: Dean Jacobsen <deanjacobsen@...>
To: main@ap-gto.groups.io
Sent: Fri, Jun 3, 2022 2:19 pm
Subject: Re: [ap-gto] Field Rotation

On Fri, Jun 3, 2022 at 11:50 AM, Roland Christen wrote:
Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera.
Awesome, thanks for this example.

Your note says it was a 3 point model.  Was this an hour separation between sample points?
 
--
Dean Jacobsen
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/

--
Roland Christen
Astro-Physics


 

>> It makes absolutely no difference if you guide or model.

yep i got that. I was just clarifying what was going on thanks

On Fri, Jun 3, 2022 at 12:21 PM Roland Christen via groups.io <chris1011=aol.com@groups.io> wrote:

Just to clarify, it sounds like you did not use guiding but did use the modeling for accurate tracking, correct?
It makes absolutely no difference if you guide or model. The drift rate of the star is exactly the same in either case.

Roland

-----Original Message-----
From: Brian Valente <bvalente@...>
To: main@ap-gto.groups.io
Sent: Fri, Jun 3, 2022 2:02 pm
Subject: Re: [ap-gto] Field Rotation

Roland that is very illuminating

Just to clarify, it sounds like you did not use guiding but did use the modeling for accurate tracking, correct?



On Fri, Jun 3, 2022 at 11:50 AM Roland Christen via groups.io <chris1011=aol.com@groups.io> wrote:

Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera. The keypad reads out the RA and Dec trim rate that is applied in the CP5 control in order to follow the star's motion over time. At the beginning of the run, at 7:52pm, the star was shown on the crosshair of the camera (right blue crosshair in MaximDL), while at the same time SkyX recorded the cursor position to be exactly aligned with the star. The trim rate being applied in RA was 61 arc sec per hour - this being the drift rate of the star at that 50 degree altitude, due mostly to atmospheric refraction and some slight polar alignment error.

After approximately an hour, the star is still on the blue crosshairs (mount's tracking was being adjusted continuously by the model). Pixel scale of this setup was 1.1 arc sec per pixel. You can see in the second image below that the actual reported position has moved by approximately 61 arc seconds in RA with respect to the planetarium position. If the mount had continued tracking this star for 4 or more hours into the West, the actual reported position would probably have been several hundred arc seconds. There is no field rotation involved, just simple and pure atmospheric refraction that causes a star to not be in the theoretical RA/Dec position that a planetarium program shows. Doing a Recal at any time will of course bring both the star and the cursor back together.




--
Roland Christen
Astro-Physics


--

--
Roland Christen
Astro-Physics




Dean Jacobsen
 

On Fri, Jun 3, 2022 at 11:50 AM, Roland Christen wrote:
Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera.
Awesome, thanks for this example.

Your note says it was a 3 point model.  Was this an hour separation between sample points?
 
--
Dean Jacobsen
Astrobin Image Gallery - https://www.astrobin.com/users/deanjacobsen/


Roland Christen
 


Just to clarify, it sounds like you did not use guiding but did use the modeling for accurate tracking, correct?
It makes absolutely no difference if you guide or model. The drift rate of the star is exactly the same in either case.

Roland

-----Original Message-----
From: Brian Valente <bvalente@...>
To: main@ap-gto.groups.io
Sent: Fri, Jun 3, 2022 2:02 pm
Subject: Re: [ap-gto] Field Rotation

Roland that is very illuminating

Just to clarify, it sounds like you did not use guiding but did use the modeling for accurate tracking, correct?



On Fri, Jun 3, 2022 at 11:50 AM Roland Christen via groups.io <chris1011=aol.com@groups.io> wrote:

Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera. The keypad reads out the RA and Dec trim rate that is applied in the CP5 control in order to follow the star's motion over time. At the beginning of the run, at 7:52pm, the star was shown on the crosshair of the camera (right blue crosshair in MaximDL), while at the same time SkyX recorded the cursor position to be exactly aligned with the star. The trim rate being applied in RA was 61 arc sec per hour - this being the drift rate of the star at that 50 degree altitude, due mostly to atmospheric refraction and some slight polar alignment error.

After approximately an hour, the star is still on the blue crosshairs (mount's tracking was being adjusted continuously by the model). Pixel scale of this setup was 1.1 arc sec per pixel. You can see in the second image below that the actual reported position has moved by approximately 61 arc seconds in RA with respect to the planetarium position. If the mount had continued tracking this star for 4 or more hours into the West, the actual reported position would probably have been several hundred arc seconds. There is no field rotation involved, just simple and pure atmospheric refraction that causes a star to not be in the theoretical RA/Dec position that a planetarium program shows. Doing a Recal at any time will of course bring both the star and the cursor back together.




--
Roland Christen
Astro-Physics


--

--
Roland Christen
Astro-Physics


 

Roland that is very illuminating

Just to clarify, it sounds like you did not use guiding but did use the modeling for accurate tracking, correct?



On Fri, Jun 3, 2022 at 11:50 AM Roland Christen via groups.io <chris1011=aol.com@groups.io> wrote:

Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera. The keypad reads out the RA and Dec trim rate that is applied in the CP5 control in order to follow the star's motion over time. At the beginning of the run, at 7:52pm, the star was shown on the crosshair of the camera (right blue crosshair in MaximDL), while at the same time SkyX recorded the cursor position to be exactly aligned with the star. The trim rate being applied in RA was 61 arc sec per hour - this being the drift rate of the star at that 50 degree altitude, due mostly to atmospheric refraction and some slight polar alignment error.

After approximately an hour, the star is still on the blue crosshairs (mount's tracking was being adjusted continuously by the model). Pixel scale of this setup was 1.1 arc sec per pixel. You can see in the second image below that the actual reported position has moved by approximately 61 arc seconds in RA with respect to the planetarium position. If the mount had continued tracking this star for 4 or more hours into the West, the actual reported position would probably have been several hundred arc seconds. There is no field rotation involved, just simple and pure atmospheric refraction that causes a star to not be in the theoretical RA/Dec position that a planetarium program shows. Doing a Recal at any time will of course bring both the star and the cursor back together.




--
Roland Christen
Astro-Physics




Roland Christen
 


Below shows a short session where I used a simple Keypad Dec arc model to keep the star (Arcturus) on the crosshairs of my imaging camera. The keypad reads out the RA and Dec trim rate that is applied in the CP5 control in order to follow the star's motion over time. At the beginning of the run, at 7:52pm, the star was shown on the crosshair of the camera (right blue crosshair in MaximDL), while at the same time SkyX recorded the cursor position to be exactly aligned with the star. The trim rate being applied in RA was 61 arc sec per hour - this being the drift rate of the star at that 50 degree altitude, due mostly to atmospheric refraction and some slight polar alignment error.

After approximately an hour, the star is still on the blue crosshairs (mount's tracking was being adjusted continuously by the model). Pixel scale of this setup was 1.1 arc sec per pixel. You can see in the second image below that the actual reported position has moved by approximately 61 arc seconds in RA with respect to the planetarium position. If the mount had continued tracking this star for 4 or more hours into the West, the actual reported position would probably have been several hundred arc seconds. There is no field rotation involved, just simple and pure atmospheric refraction that causes a star to not be in the theoretical RA/Dec position that a planetarium program shows. Doing a Recal at any time will of course bring both the star and the cursor back together.




--
Roland Christen
Astro-Physics


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


 

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




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


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


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






 
--

--
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
 


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


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


 

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






 

--