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Which T-Ring is right for me?

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#1 odie714


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Posted 19 January 2021 - 09:18 PM

Hi all, 


I am about to pick up my first AP full setup (Orion 8" f/3.9 Newtonian, Atlas EQ-G, Skywatcher Quattro F4 Coma Corrector, Orion StarShoot AutoGuider Camera and 80mm guide scope) and I am a little confused on what to grab next. I know I need a T-Ring for the A7iii I want to use in this setup, but I'm not sure which one to get. I see a few options online like this or this, but I am not sure if either are 2" T-rings. I found this one that I'm sure is 2", but it is crazy expensive for a little piece of plastic. Am I missing something? Also, what is the major difference between a 2" and 1.25" T ring? Will it effect my photos much? Is there anything else i need in my optical train before I start shooting? A 1.25" T ring would really help with using a barlow (2" ones are harder to find) and possible eyepiece projection, but I also don't want to use it if it has major drawbacks, especially for a full frame. Lastly, if i were to do eyepiece projection, would something like this work, and what are the downsides of eyepiece projection? Any help would be appreciated!

#2 rk2k2



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Posted 19 January 2021 - 10:04 PM

My recommendation is go for the minus t-ring (the third you listed).  What I have come to understand is T rings are 'regular' (if they don't specify other words) and have 42mm threading (1.25").  WIDE is 48mm or also referred to as 2".  The 'minus' designates a shorter adapter where as regular ones have the appropriate backspacing for the camera it is intended for.  I recommend wide minus rings mostly because you can 'add' length with spacers, where as you can't reduce the length (shorten the light path).   Ideally (for me), the minus T-ring will have no more light path distance than what is required for the bayonet mount portion.  Some T-rings come with a section (nosepiece) that is removable from the bayonet section which results in as short a light path as possible. 

#3 spereira


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Posted 20 January 2021 - 08:10 AM

Moving to Beginning Deep Sky Imaging.



#4 BobE102330


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Posted 20 January 2021 - 09:15 AM

Given your sensor size, go for the wide ring to minimize vignetting. I get some minor vignetting with a crop sensor on that scope. A standard length ring should get you 55 mm back spacing. What does the CC need? 

I started with the Orion f3.9 and a DSLR. It’s possible to learn AP with it, but you need a high tolerance for frustration.  With a DSLR you won’t be able to use an OAG due to back spacing required. You’ll want to guide, or you’ll be limited to very short exposures. Mounting a guide scope can be a challenge on the Orion. You shouldn’t use the finder mount, too much flex. Your learning curve will be faster and less frustrating with a shorter focal length  scope.  Of course with Galaxy season nearly upon us you’ll want all the focal length you can get. Or shoot Galaxy clusters. 

If you are buying a t-ring why consider eyepiece projection at all?  You’re better off cropping than adding more glass in the optical train. 

Orion has a 2” Barlow that has worked pretty well for me. It wants 55 mm backspacing, so a t-ring with a nosepiece makes it an easy drop in. Guiding and focusing, that’s another story. 

For focusing, get a Bahtinov mask unless you’re going to add an electronic focuser.  Focusing without a Bahtinov generally won’t get you close enough for AP. 

Enjoy the journey.  

#5 michael8554


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Posted 20 January 2021 - 10:19 AM

Here's a 48mm T-Ring, but not sure if your Sony has the E mount:



Edited by michael8554, 20 January 2021 - 10:21 AM.

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