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Matching Guide camera to imaging camera

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#1 tloebl

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Posted 16 July 2020 - 08:36 AM

I tried finding a thread on this but have not come across any.  Never thought much about imager / guide camera parameters.

 

I have been using a ccd camera with 9x9 micron pixels on a 1000mm scope (approx 1.85 microns/pixel) using a guide camera with 13 x 15 micron pixels on a 400mm piggyback scope (7 arcsec/pixel).  With this setup I have been getting nice round stars and good tracking for years.  I recently upgraded to an asi1600mm and though the guider movement appeared nominal, the stars in the subs were not round any more. I looked at balance changes and other issues to pin down the problem with no success.

 

Last night I crunched some numbers.

 

I think larger pixels of the guide camera were hiding errors that now show up with  the 3.75 micron pixels of the new imaging camera. I ordered an asi120mm mini (approx. 2 arcsec/pixel) that better matches the pixel size of the 1600 / 1000mm scope (0.77 arcsec/pixel) combination.

 

Hoping this will settle down my guiding. 

 

Wonder whether my logic is correct and if others have had similar experiences when switching imagers.

 

Incidentally, I have an OAG coming as well.



#2 kathyastro

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Posted 16 July 2020 - 09:16 AM

I wonder if the large pixels of the old guide camera were making it blind to seeing conditions.  That is a good thing.  It is possible that the smaller pixels of the new camera have cured this blindness and you are now chasing the seeing.  You could try longer guiding exposures to average out the seeing, and see if that improves the images.



#3 happylimpet

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Posted 16 July 2020 - 09:25 AM

dont forget that many folks guide using guidescopes delivering fractions of the full resolution of the imaging cam. Centroids of guidestars are determinable (is that a word?) to tiny fractions of a pixel size.

 

Nonetheless, Im sure the 120mm will improve things.

 

Note I have a 1600mm, and guided with a 120mm before i upgraded to a 290mm with an OAG, so same image scale. I binned the pixels to 7.5microns (now 5.8 microns) and that was fine.


Edited by happylimpet, 16 July 2020 - 09:26 AM.



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