Quote:
Originally Posted by Bart
I am unable to detect any stars in the field after it is set up....Using 2x2 binning etc, I point it to an object I wish to image and I just cannot find any stars.
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It is pretty unlikely that you find a bright enough star in the oag "by luck" (i.e. if you just point the main camera at the object you want to image). Remember the FOV of your OAG is tiny compare to the small guide scope. The reason rotators are so popular is to fix this problem, the accessible area around the main chip with the rotator is 10-20x bigger.
You can manually rotate the image train in most setups. So plan ahead with a star chart (e.g. TheSky) and find a bright enough star near the object and put the OAG in that orientation (hence why put the OAG above the main imager and remember which way is north on the camera). Otherwise you will need pretty long guider exposures, which may negate the whole point of guiding if the mount needs fast corrections (i.e. gear noise).
To determine how bright a star you need is easy use 1 sec exposure on a 6th mag star to start, look at the Signal to Noise Ratio (SNR value in Maxim information window). Every magnitude fainter needs 2.5 times more exposure for the same SNR, so an 11th mag star needs a 100 sec exposure for the same SNR as the 6th mag star, ouch. Usually SNR > 10 is desired for guiding, SNR > 100 is fantastic, you can go as low as SNR~3, but things get dodgy below ~10 in my opinion.
The optional steps I listed in the previous post are there to reduce the noise term hence making the SNR bigger for the same exposure and allowing you to guide on fainter stars. Which means sometimes no rotating the camera!!!
Best,
EB