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Old 06-06-2011, 03:19 PM
binofied
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binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Size vs resolution gains?

Does aperture increase also increase resolving power in Astrophotography if you use the same camera. Wondering because the longer focal length also changes the image scale, making the stars blobbier?

I have a question about aperture of the telescope vs the resolving power in astrophotography. If I was talking about visual work I would say that about the 10 to 12" range you start to be able to fully resolve stars in a globular cluster. Looking in say a 6" telescope you are aware of the round shape and peppery texture but individual stars don't pop out at you like they do in 12" or over. Then I am seeing these cheapish GSO RC's in four sizes. I currently have an oldish 10" Meade LX200 OTA (15years). I have done some tests against an 8" GSO RC and found VERY simalair results the image scale is almost the same as I use a 0.63 reducer on my 10" so it's about 1600mm the 8" GSO was also about 1600. The star FWHM's were identical and brightness on the Meade was about 20% brighter on peak star values (taking bias etc into account).

So this leaves the question what is there to resolution or image depth gain by using say the 10" or 12" GSO over my Meade 10"? Would the larger instruments give lower FWHM's on the stars (in arcsec terms, not pixels) would I get more detail in nebula or galaxies with the larger tubes. I was thinking I would need to use a 0.75 reducer to get the focal length to be a better image scale match for my ST2000. Our seeing is about 2 arcsec so image scale of 1 arcsec/pixel is OK.

The below FOV image & scales are for my camera, SBIG ST2000 7.4 uM pixels 1600x1200 11.8mm x 8.9mm chip size.

Aperture Image Scale FOV FL
6" 1.25' 33x25" 1216mm
8" 0.94' 25x19" 1624mm
10" 0.75' 20x15" 2032mm
12" 0.63' 17x13" 2432mm


With 0.75 focal reducer
Aperture Image Scale FOV FL
6" 1.67' 45x33" 912mm
8" 1.25' 33x25" 1220mm
10" 1.00' 27x20" 1524mm
12" 0.84' 23x17" 1824mm
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