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Re: Angular Distance Between Stars By Camera and Sextant
From: Marcel Tschudin
Date: 2012 Sep 17, 11:45 +0300
From: Marcel Tschudin
Date: 2012 Sep 17, 11:45 +0300
Greg, regarding your comment
I'm trying to keep to simple hand held methods. The best way would be a motorized tripod set to latitude that has the camera shutter held open. This would provide flexibility for ISO and f stop settings. No room or money for smart tripod gear though. I'm satisfied with the hand held lunar camera trial results so far and will be trying for a full lunar calibration series using the 50 mm lens.
I understand that for "normal" observations it makes sense to use a "simple hand held method". However, for calibration, where one aims at a higher accuracy a slightly more sophisticated method is justified. For the type of exposure times you are likely using a tripod gear should not be required. Let's make a simple estimation: The sky moves about 360 degrees in 24 hours or in about 86400 seconds, i.e. during one second exposure time about 15 seconds of arc, or during 1/8th second exposure time about 2 seconds of arc. I consider this to be completely sufficient for the type of calibration you want to do. The star blotches result from making these calibration photos handheld and possibly also from being slightly overexposed.
It looks like the calibration with star distances might be better than you feared. To my understanding the refraction should be added to each of the stars separately before calculating the apparent distance, as indicated by Brad. I do not have the tools to calculate local star positions. If Brad's value is correct then your result from the photo with the blotches differs only by rounding errors, i.e. by less than 0.05 moa. I consider this a "lucky" value since your present calibration is not better than +/-0.5 moa.
Marcel
It looks like the calibration with star distances might be better than you feared. To my understanding the refraction should be added to each of the stars separately before calculating the apparent distance, as indicated by Brad. I do not have the tools to calculate local star positions. If Brad's value is correct then your result from the photo with the blotches differs only by rounding errors, i.e. by less than 0.05 moa. I consider this a "lucky" value since your present calibration is not better than +/-0.5 moa.
Marcel
