6 replies to this topic

[johnrainey27]

I hope this is the correct Forum

I have a NexStar  8se. with a 1.25 inch rear opening and use 1.25 in eyepieces.  Is there any benefits to purchasing a 2 in SCT mirror diagonal that mounts to the 1.25 rear cell of the 8se. If I do this then the image that any new 2” eyepiece receives is still through the 1.25 in cone of the telescope. So is there more light and contrast to objects viewed? Any benefits?

Thanks for any advice

John

[Notdarkenough]

Some eyepieces can only be had in the 2" size. But, most of those are all about wider fovs, which are limited by the focal length of the C8 and the 1¼ size.

 

Here is another, similar thread you might find helpful. https://www.cloudyni...iagonal-on-8se/

 

Welcome to Cloudy Nights!

[12BH7]

https://www.amazon.c...,aps,187&sr=8-4

 

Here's a cheap way to get started on using 2" EP. I use this quite often on my 8" SCT with as large as a 35mm Panoptic. To be honest I don't see any reduction of FOV between using this and a 2" diagonal with 2" nose piece adapter. 

[Paul Skee]

I have found this information valuable when considering decisions such as yours:

https://astro.ecuado...t-and-maksutov/

[sevenofnine]

Welcome to C/N! 

 

I changed my 5" Mak to accept a 2" diagonal. I tested the result for visual vignetting both day and night. I could find none. The eyepieces used were 2" ES 28/68 and ES 40/52. The biggest advantage in my view is a much larger eye lens to look through. Good luck! 

 

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Edited by sevenofnine, 06 July 2025 - 06:15 PM.

[Lee D]

I hope this is the correct Forum

I have a NexStar  8se. with a 1.25 inch rear opening and use 1.25 in eyepieces.  Is there any benefits to purchasing a 2 in SCT mirror diagonal that mounts to the 1.25 rear cell of the 8se. If I do this then the image that any new 2” eyepiece receives is still through the 1.25 in cone of the telescope. So is there more light and contrast to objects viewed? Any benefits?

Thanks for any advice

John

The "1.25 rear cell of the 8se" of your question isn't correct (or maybe incomplete), at least as stated. Celestron C8 SCTs have a rear aperture of about 2 inches. Screw off the retaining ring attaching the stock 1.25" visual back to the rear of the telescope and measure for yourself.

 

You can buy a 2" diagonal that screws directly to the rear of the SCT, or a 2" replacement for the "Celestron Visual Back - 1 1/4" to attach a 2" diagonal to. 

 

I'm not saying you should, just that you should be aware of your options.

[Oldfracguy]

I hope this is the correct Forum

I have a NexStar  8se. with a 1.25 inch rear opening and use 1.25 in eyepieces.  Is there any benefits to purchasing a 2 in SCT mirror diagonal that mounts to the 1.25 rear cell of the 8se. If I do this then the image that any new 2” eyepiece receives is still through the 1.25 in cone of the telescope. So is there more light and contrast to objects viewed? Any benefits?

Thanks for any advice

John

It looks to me from the pictures of the NexStar 8se on the Celestron website that the scope is equipped with the standard male 2"-24 "SCT" threads on the back.  A visual back step-down adaptor is used to reduce down to a 1.25" visual back for use with the Celstron 1.25" Prism diagonal.

 

To answer your question, there are benefits to going to a larger 2" SCT diagonal like this one that screws directly onto those male 2"-24 "SCT" threads on the back of the scope:

 

 

 

The primary benefit for doing this is to use 2" eyepieces for a wider field of view.  The closer you can get the 2" diagonal to the back of the scope the better, for two reasons.  One, you reduce the optical path the light cone coming out of the baffle tube has to traverse; and two, you are able to focus with the primary mirror farther backward in the tube, which lowers the effective focal length, and hence focal ratio, of the scope.  Some 2" diagonals actually have female 2"-24 SCT threads and screw directly onto the male SCT threads, after first screwing on what is called an SCT Locking Ring.  That is a very thin, flat ring that screws back against the diagonal body to lock it in the correct orientation.  Here are some examples:

 

The SCT-threaded 2" diagonal:

 

https://agenaastro.c...al-2956100.html

 

and the SCT Locking Ring:

 

https://agenaastro.c...t-diagonal.html

Edited by Oldfracguy, 11 July 2025 - 08:37 AM.