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

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Discussion Overview

The discussion revolves around the use of different coupling sizes (1.25" vs 2") for mounting a DSLR on a telescope, particularly focusing on the implications of stepping down the diameter of the light path close to the sensor. Participants explore potential drawbacks such as light clipping and vignetting, as well as practical considerations for astrophotography.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants express concern about the potential for light clipping when using a 1.25" coupling with a DSLR, questioning whether this could lead to a loss of light.
  • It is noted that modern telescopes are often designed for 1.25" couplings, which may mitigate concerns about light loss.
  • One participant mentions that the imaging chip of a DSLR is larger than those in cameras designed for astrophotography, suggesting that vignetting may occur near the edges of the camera's field of view.
  • A suggestion is made to use the camera 'straight through' with a 45° mirror to facilitate easier viewing of the live image without neck strain.
  • Participants share personal experiences with older DSLR models, discussing limitations such as the lack of live view and the use of external displays for better visibility.
  • There is mention of alternative methods for achieving live view functionality, such as interfacing with smartphones, which some participants find useful.

Areas of Agreement / Disagreement

Participants express varying levels of concern regarding the effects of using a smaller coupling size, with some emphasizing the potential for vignetting while others suggest that it may not be a significant issue. The discussion remains unresolved regarding the extent of light loss or vignetting that may occur.

Contextual Notes

Participants reference specific camera models and their features, highlighting limitations such as the absence of live view in older DSLRs. There is also mention of the practical challenges of using optical viewfinders in low-light conditions.

Who May Find This Useful

This discussion may be useful for amateur astrophotographers considering different coupling sizes for their DSLRs, as well as those interested in practical solutions for viewing images while using telescopes.

mishima
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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?
 
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mishima said:
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.
 
Thanks, I'll just try it then and look for the vignetting.
 
mishima said:
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.
 
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:
Hoodman_RAV.jpg
 
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.
 

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