Re: Field Rotation


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

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