'losing light' 1.25" DSLR coupling vs 2"?

[mishima]

I use a 2" coupling to mount my DSLR on the scope. However, I have an 90 degree mirror angle in the 1.25" size, and would like to use it sometimes for convenience getting my eye on the viewfinder/lcd screen.

I was wondering what drawbacks, if any, there were in stepping down the diameter of the light path so close to the sensor might be. Am I going to 'clip' the light path with the smaller diameter and lose light? I've heard most modern scopes are designed for 1.25" so maybe it doesn't matter?

 

[NFuller]

I was wondering what drawbacks, if any, there were in stepping down the diameter of the light path so close to the sensor might be. Am I going to 'clip' the light path with the smaller diameter and lose light? I've heard most modern scopes are designed for 1.25" so maybe it doesn't matter?

The imaging chip of a DSLR is usually quite large, at least compared with cameras specially made for astrophotography. It is quite possible that the light will become attenuated near the edge of the cameras FOV. You will see this as vignetting in the image.

 

[mishima]

Thanks, I'll just try it then and look for the vignetting.

 

[sophiecentaur]

I use a 2" coupling to mount my DSLR on the scope. However, I have an 90 degree mirror angle in the 1.25" size, and would like to use it sometimes for convenience getting my eye on the viewfinder/lcd screen.

I was wondering what drawbacks, if any, there were in stepping down the diameter of the light path so close to the sensor might be. Am I going to 'clip' the light path with the smaller diameter and lose light? I've heard most modern scopes are designed for 1.25" so maybe it doesn't matter?

Would it be a daft suggestion for you to use the camera 'straight through' and tape a mirror at 45° to the back so you can see the live view picture without cricking your neck?
I only recently came to astrophotography and needed a decent DSLR for it (the old one had more hot pixels than cool ones!). The Pentax K2s comes with a screen on a universal support so you can look at the live view from any angle. It was a nice surprise as I just hadn't thought of that when I bought the camera.

 

[mishima]

Hmm, I suppose I could rig up something like that. I guess I was really just curious if there was indeed some effect. It was too cloudy to mess with it last night.

My camera is an old Canon EOS 20d, it doesn't even have live view (20dA was first for that I think). I use the viewfinder for centering and then trial and error with the LCD for focus.

One thing I learned during the eclipse though was that if you connect it to a TV through composite, it uses the TV to display the image you just took. I bought a little 5V 7" LCD screen for 30 dollars (the original screen on the 20d is something like 1.5"). I'm hoping when that arrives it will make the experience less bone crunching.

I've also seen that Canon sells a 90 attachment for the viewfinder itself:

 

[sophiecentaur]

It's really not easy to use an optical viewfinder when you really need it in the dark. I'd bet you could afford another camera for the cost of a fancy Canon bolt - on (well, almost)
Many cameras will interface with a smart phone and give you a live view. I have one on mine which works pretty well. It doesn't do all the controls but it can be useful at times.