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Field Rotation

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
Brian Valente
 

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
 


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/

Brian Valente
 

>> 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



--

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
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/

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/

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
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

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
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