Ideal aperture setting in Backyard EOS

[goldwater58]

I'm quite new to astrophotography and have a question regarding setting the appropriate aperture setting in Backyard EOS.  Do I simply match it to my telescope's focal ratio?  I'm using an 80mm f/6 refractor with a 480mm focal length and intend to use my camera (Canon T3i) in a prime focus fashion.  I'm looking to capture some DSO's if that helps.  Again, I know this is more than likely a very noobish question but all help/advice is appreciated.

[s3igell]

When you are doing Prime Focus DSO Imaging (DSLR connected to a Telescope), the Aperture and Focal Length (and therefore the Focal Ratio - f/stop) are permanently dictated by the Optics utilized (Scope and any Barlow or Reducer).  The only Variables that you control are Exposure Length and ISO.

Exposure Length should be a "Best Compromise" between the following factors: 

1. Tracking quality - your Mount Quality, Pixel Resolution, Seeing Conditions, Scope Balance and Auto-Guiding will combine to dictate How Long you can Expose before the Stars start to Drift across more Pixels than you are willing to accept
2. Light Pollution and - under anything less than Perfect Skies the Light Pollution will eventually build-up in your Image to the point of swamping the Data you are trying to Collect
3. Camera Noise - the Sensor on your DSLR will contribute Noise in addition to the Light Pollution

ISO should be a "Best Compromise: between the following factors:

1. Unity ISO - each Camera Model has a combo of Sensor and Electronics that produces a ISO - usually between 400-1600 - where Light Amplification vs Noise vs Dynamic Range is best
2. ISO Noise - Sensors produce / amplify the Electronic Noise at higher ISOs
3. Histogram - the Captured Image Data needs to be such that the Histogram Curve's Left-side is separated at least a bit from the Left Edge of the Histogram

A good number of variables to consider and explore...

[astroman133]

When using a DSLR at prime focus the aperture is not selectable. It is fixed because the aperture and focal length of the telescope are fixed. In fact, I do not believe that the f/stop is even displayed unless an EF lens is connected to the camera.

[goldwater58]

Thanks for that info guys.  When I was messing around with Backyard EOS and my camera in the living room, I didn't realize that attaching/detaching the camera lens would alter the program's interface.  With the lens detached, I no longer see aperture as an adjustable option which makes a lot more sense.  Thank you for all the other tidbits as well!

[Dirty Harris]

Hey there Goldwater 58, just a couple other things that may be of help. I too have a Canon T-3 Rebel similar to yours. I have found that when trying to shoot DSO that a solid polar alignment is critical, whether using a guide scope or just going with guts poker alignment its really pretty important. One of the great things about BYE is that under "Frame & Focus" you can lock onto a star for focus, but it will also let you know how well your polar alignment is. If the star drifts greatly while trying to focus then your alignment is in question and may need to take another shot at it. The other sweet thing with BYE, deals with focus, it allows one to take individual "Snap Shots" of your image, and allows you to be able to view them on FULL SCREEN there by determining  how well focused and how over or under exposed your photo will be. The versatility of BYE is very user friendly. With my T-3 I usually start with an ISO of 2400 and exposure time of 8-10 seconds just to get an image and then adjust up or down from there. Hope this may help some, welcome to the world of Astrophotography and BYE. Dirty Harris Colo.USA.