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Guiding question

Posted: Tue Jul 01, 2014 9:58 pm
by eoliveira
Hi Allan,

I recently purchased your book "Getting started: Long exposure astrophotography". It is a great book with a sweet spot between depth and coverage. Congratulations! My question is related to the topics you have covered in the section on guiding (page 60 and 61). Doing the maths for my setup (Atik 314L+ mono on an 8 inch Ritchey Chretien, FL=1625mm and an 80mm guidescope, FL=400mm with an Orion starshoot autoguider), I got the following figures:
Resolution of main telescope/main camera=0,818 arcsec/pixel
Resolution of guidescope and guidecamera=2,68 arcsec/pixel
Does that mean (following the same reasoning of the book), that when the guide star is off by 1 pixel, the mount will be corrected by 2,68 arcsec, which translates into 2,68/0.818=3.2 pixels in the main camera? Is this correct? Is this still reasonable? How much correction movement in my main camera can I tolerate before I get a high deterioration of the resolution in my image?

ps. maybe my calculations are wrong, because I could not figure out how did you got the "2.09 pixels" figure mentioned on page 60 of the book

Thank you,

Eduardo.

Re: Guiding question

Posted: Sat Jul 05, 2014 9:23 am
by AllanHall
If your guide camera is off by 1 pixel, that will move BOTH your guide scope and imaging scope 2.68 arcsec. If your imaging scope moves 2.68 arc/sec and the resolution is .818 arcsec/pixel that means it will move (2.68/.818=3.27) 3.27 pixels, or since there is no fractions of a pixel on your camera sensor, 4 pixels. So yes, you calculated it correctly.

Is it reasonable? That is really up to you. In my opinion, 4 pixels is too much for a DSLR and WAY too much for a monochrome CCD. I would not want my mono CCD above 2.

Fortunately there is an easy fix, simply use a barlow on your guide camera. This will effectively increase it's focal length and therefor its resolution. I would recommend a 1.5x or 2x at the most. Any more than that and you may dim the light too much to find and use guidestars well.

Allan

Re: Guiding question

Posted: Sat Jul 05, 2014 9:38 am
by eoliveira
Hi Allan,

Thank you very much for your reply. That´s great advice and much appreciated. Clear skies!