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DSLR Attachment Method - Eyepiece Mount vs Star Diagonal

Astrophotography DSLR SCT
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#1 9Joeker9


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Posted 19 September 2023 - 02:16 PM

Hello Fellow Stargazers,


Quick question as I am still new to this...For Aperature size and light gathering, do I lose aperature if attaching my DSLR using a T-Ring with an eyepiece attachment vs directly attaching the T-Ring directly to the Star Diagonal? Seems the 1.25 eyepiece attachment narrows the field? I have a Baader T2 Zeiss Prism Diagonal, and my T-Ring has a T2 threat...so I can attach it directly, but it is tough to screw on.


Any thoughts are definately welcome!


Thanks a bunch.

#2 bobzeq25



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Posted 19 September 2023 - 02:27 PM



Essentially nobody uses a star diagonal for this.  There's little value, and some degradation in image quality, noticeable after the intensive processing in DSO AP.


One goes straight through.  Tries to make the tubing size not a factor in vignetting (using M48 can help there, there are M48 T-rings), also more noticeable after intensive processing.  Tries to use threaded connections as much as possible to minimize small movements.  Your eye hooked to a smart brain can ignore a bit of motion.  Dumb tiny pixels - not so much.


Underestimating the importance of those last two sentences is a major reason why beginners frequently mess up by not getting a proper mount and/or not autoguiding.



Edited by bobzeq25, 19 September 2023 - 02:29 PM.

#3 kathyastro


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Posted 19 September 2023 - 03:11 PM

The big problem with using a diagonal with a DSLR is focusing.  The light path can easily be too long to place the camera sensor at the focal plane, because the camera body + T-ring adds 55 mm to the light path.  Plus the diagonal flip the image, although this is easy to correct in processing.


The best option for attaching a DSLR is to replace the diagonal with a suitable T-adapter.  Which adapter to use will depend on what scope you have.

#4 BlueMoon



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Posted 19 September 2023 - 03:35 PM


Seems the 1.25 eyepiece attachment narrows the field?

Yes, it does. However, eye piece projection imaging is usually done for planetary where the reduction in field isn't normally a problem. As Kathy and Bob point out there are issues to be aware of.


My DSLR rig. The image on the left uses a 2" diagonal and eye piece projection attachment (green lettering is a 1.25" 18.2mm TeleVue DeLite between the camera and the diagonal using a 2" to 1.25" adapter), no corrector used, for when I do planetary. The one on the right uses a 2" corrector (just in front of the camera) with a DSLR adapter for DSO and wide-field. The light path distance remains close between the two configurations because the diagonal takes the place of the corrector so focusing isn't a hassle. Hope this is helpful.


Clear skies.

Attached Thumbnails

  • AZMP_Meg90FD_afocal_CN.jpg
  • AZMP_Meg90FD_prime_CN.jpg

Edited by BlueMoon, 19 September 2023 - 04:10 PM.

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