T-Mount Try Out

The T-Mount system was devised by Tamron in the 1950s to allow their lenses to be used on many different cameras. It’s also the system used to attach a camera to the prime focus of a telescope. I’d never tried it, so I ordered a T-Ring and Sony adapter and gave it a go.

All of these images were made with a Sony A6600 APSC camera mounted on a NexStar 6SE telescope at prime focus using the T-Mount adapter. I captured multiple frames for each and processed them with astro photo capabilities added recently to Affinity Photo.

In prime focus astrophotography, you’re using the telescope as a lens, mounted directly to your camera. There’s no eyepiece involved, although you can insert additional optics into the light path. I used three different configurations.

1. Prime focus, with a Celestron f/6.3 Reducer Corrector. Equivalent focal length = 1.5 x 1500 x 0.63 = 1418mm

In the first image I used a Reducer / Corrector. This both widens the field of view / lowers the focal length, and flattens the field to enhance sharpness at the edges. This worked OK, but did have some obvious vignetting that was hard to deal with in post processing.

Luna 2.Prime focus. Equivalent focal length = 1.5 x 1500 = 2259mm.

Image 2 is the same setup, but straight from the telescope to the camera. There was no vignetting and I think the image quality is very high. The featured image at the top of this post is a crop from this photo. (Note: This one is posted on Flickr and is worth a click to see in greater detail. Click it twice when you get there to enlarge it.)

3. Prime focus with a Meade 2x Barlow lens. Equivalent focal length = 1.5 x 1500 x 2 = 4500mm

And finally, image 3 uses a Barlow lens, which is like a 2x extender. The image quality in this one is not as good. That could be due to degradation from the Barlow, a slight mis-focus, or vibrations / motion (or all three!).


Conclusions:

  • Check and double check all settings and adjustments.
  • These are longer focal lengths than anything I’ve ever tried before. Technique is super important and it’s hard to know if you’ve messed up until you get things on the computer later.
  • The straight prime focus method works very well. The image quality is the best I’ve gotten through the telescope, It’s better than the afocal approach (camera lens through an eyepiece) I used for this post.
  • The reducer / corrector works OK, but I’ll probably shy away from it unless I need a wider field of view. And if that’s the case I think I’d try using piggy-back photography first.
  • The 2x Barlow approach is challenging. The magnification makes any focus or motion issues much worse. This should probably be reserved for planets, and used as a second option to straight prime focus or piggy-back with a long telephoto lens.
  • There is a lot to learn about astro photography!

Thanks for stopping by and reading my blog. Take care of yourselves and each other. And if you can – make some (astro) photos!

©2021, Ed Rosack. All rights reserved

4 thoughts on “T-Mount Try Out

  1. Thank you for the information but a little over my head. I thank you for letting us know. Hopefully, one day I will completely understand everything involved but regardless, your images say it all! Well done!

    1. You’re welcome Dorothy.

      In retrospect, I probably should’ve added more explanations / info to this blog entry. It is a challenging subject!

      Ed

Leave a Reply to Ed Rosack Cancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.