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Prism or mirror diagonal for my Celestron C8?

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#1 DennyD

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Posted 15 September 2021 - 12:26 PM

I just got a Celestron C8 Powerstar SCT with a focal ratio of f/10. Would the Celestron prism diagonal or a mirror one word better with my scope? I read several discussions on using them with refractors, but nothing really regarding SCTs.
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#2 Notdarkenough

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Posted 15 September 2021 - 01:00 PM

Did you buy this as part of a kit, or an optical tube only? If kit, what diagonal came with the kit?


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#3 Nippon

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Posted 15 September 2021 - 04:49 PM

Prism diagonals work fine in any F/10 scope. I find they have less scatter and better contrast. If you prefer to use 2" eyepieces then a 2" mirror SCT diagonal would be the way to go. A 2" prism would be heavy and expensive and may take you even further from the the optimum distance between the primary and secondary mirrors than a 2" mirror diagonal does. C8s are designed to provide optimum performance with a 1.25" diagonal.. 


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#4 DennyD

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Posted 15 September 2021 - 04:54 PM

Did you buy this as part of a kit, or an optical tube only? If kit, what diagonal came with the kit?



#5 DennyD

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Posted 15 September 2021 - 06:48 PM

Bought the C8 from a private party who included the prism diagonal.

#6 C0rs4ir_

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Posted 20 September 2021 - 07:08 AM

i use a 1,25" baader prism (less scatter) for planetary observing and a good 2" Mirror diagonal (brighter) for deepsky stuff. (f/10 telescope)


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#7 Visit-the-Moon

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Posted 20 September 2021 - 07:24 AM

A high quality prism diagonal will beat most mirror diagonals, there is generally less light scattering and sharper images at high magnification. It depends upon what you are happy with, but to me field curvature becomes quite noticeable in 2" eye pieces in the standard SCT design, so a 1.25" diagonal is fine. If you want something really good, a Takahashi prism diagonal is excellent and also fairly priced. They are very light as the body is made from engineering plastic and they adjust to temperature change pretty quickly. I do use a high quality mirror diagonal in another telescope for low power wide field observing with 2"eyepieces. You can only tell the quality of a diagonal by comparison at the eyepiece. 

 

Here is a nice review that might help you to make a decision: https://www.cloudyni...omparison-r2877


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#8 Cpk133

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Posted 20 September 2021 - 10:39 AM

a Takahashi prism diagonal is excellent and also fairly priced. They are very light as the body is made from engineering plastic and they adjust to temperature change pretty quickly. I do use a high quality mirror diagonal in another telescope for low power wide field observing with 2"eyepieces. You can only tell the quality of a diagonal by comparison at the eyepiece. 

 

If it was anyone else, it would be cheap plastic, but since its Takahashi, it's "engineered" plastic.  lol.gif


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#9 dpippel

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Posted 20 September 2021 - 11:57 AM

Prism diagonals work fine in any F/10 scope. I find they have less scatter and better contrast. If you prefer to use 2" eyepieces then a 2" mirror SCT diagonal would be the way to go. A 2" prism would be heavy and expensive and may take you even further from the the optimum distance between the primary and secondary mirrors than a 2" mirror diagonal does. C8s are designed to provide optimum performance with a 1.25" diagonal.. 

Doesn't a prism of the same size always have a shorter light path than a mirror?


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#10 C0rs4ir_

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Posted 20 September 2021 - 12:26 PM

Doesn't a prism of the same size always have a shorter light path than a mirror?

that also is my experience. but dont ask me why.. somehow light doesnt travel in the prism.. but beam instantly : D

 

well i guess a prism is behaving like a big lens.. so the way of the prism is not counting as lightpath.


Edited by C0rs4ir_, 20 September 2021 - 12:35 PM.


#11 Nippon

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Posted 20 September 2021 - 12:50 PM

The convergence of the light cone is affected by passing through the glass of a prism and is not affected by reflection in a mirror.


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#12 Nippon

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Posted 20 September 2021 - 12:53 PM

i use a 1,25" baader prism (less scatter) for planetary observing and a good 2" Mirror diagonal (brighter) for deepsky stuff. (f/10 telescope)

I have heard this claimed that a mirror is brighter than a prism or in other words has higher transmission. But I have never observed an increased brightness of my 2" dielectric versus my prism 1.25" diagonals. 


Edited by Nippon, 20 September 2021 - 12:59 PM.


#13 C0rs4ir_

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Posted 20 September 2021 - 01:01 PM

I have heard this claimed that a mirror is brighter than a mirror or in other words has higher transmission. But I have never observed an increased brightness of my 2" dielectric versus my prism 1.25" diagonals. 

well in my case its just 3% difference on the paper, its true. now im thinking of a baader T-2 Zeiss prism with BBHS coating.. a prism to do it all..



#14 dpippel

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Posted 20 September 2021 - 01:12 PM

The convergence of the light cone is affected by passing through the glass of a prism and is not affected by reflection in a mirror.

Yes:

 

https://www.cloudyni...th than mirrors.


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#15 Nippon

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Posted 20 September 2021 - 02:06 PM

Always nice to see real data. I think I figured it out a long time ago when I noticed just a flat glass filter slightly changed focus.


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#16 Nippon

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Posted 20 September 2021 - 02:17 PM

well in my case its just 3% difference on the paper, its true. now im thinking of a baader T-2 Zeiss prism with BBHS coating.. a prism to do it all..

I think I read somewhere that a difference in brightness has to be nearly 10 percent before the human eye can perceive any difference. I have a Baader T2 non Zeiss prism and a Takahashi prism. I also have a Celestron 1.25" prism that came with a Celestron Edge 8. Both the Baader and the Takahashi have better build quality but I'm hard pressed to say the Celestron is inferior optically. I think a lot of the bad rap of the Celestron is a lot of them get shipped out of collimation. But they are quite easy to fix. 



#17 C0rs4ir_

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Posted 20 September 2021 - 02:28 PM

I think I read somewhere that a difference in brightness has to be nearly 10 percent before the human eye can perceive any difference. I have a Baader T2 non Zeiss prism and a Takahashi prism. I also have a Celestron 1.25" prism that came with a Celestron Edge 8. Both the Baader and the Takahashi have better build quality but I'm hard pressed to say the Celestron is inferior optically. I think a lot of the bad rap of the Celestron is a lot of them get shipped out of collimation. But they are quite easy to fix. 

the celestron 1,25" prism is mechanically very bad.. in contrast to the optical characteristics of the prism.. which is pretty good.. still i rather had none of those cheap accessories with a new scope and rather spend the saved money on good quality stuff.. kapitalism says no..


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#18 Nippon

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Posted 20 September 2021 - 02:38 PM

the celestron 1,25" prism is mechanically very bad.. in contrast to the optical characteristics of the prism.. which is pretty good.. still i rather had none of those cheap accessories with a new scope and rather spend the saved money on good quality stuff.. kapitalism says no..

I found the the mechanics of the Baader T2 1.25" prism very good. The Takahashi is a little less robust and optically they seem on par with each other. The Takahashi's collet type of eyepiece clamp is wonderful with eyepieces that are not undercut but problematic with those that are or are tapered. So between the two the Baader is compatible with any eyepiece on the market and the Tak not so much. 

 

Oh I think I have yet another opportunity to state that I hate eyepiece undercuts!


Edited by Nippon, 20 September 2021 - 02:43 PM.

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#19 C0rs4ir_

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Posted 20 September 2021 - 02:56 PM

I found the the mechanics of the Baader T2 1.25" prism very good. The Takahashi is a little less robust and optically they seem on par with each other. The Takahashi's collet type of eyepiece clamp is wonderful with eyepieces that are not undercut but problematic with those that are or are tapered. So between the two the Baader is compatible with any eyepiece on the market and the Tak not so much. 

 

Oh I think I have yet another opportunity to state that I hate eyepiece undercuts!

yes same.. the baader T2 1.25" prism is a solid performer. sadly never had anything Takahashi, but would be interested if it was better. tbh those undercuts safed a lot of my equipment / eyepieces when i was still new to the hobby (mount was swinging through, altitude lock lose)..

but now with the c9,25 + reducer/corrector i dont need a prism that has larger free diameter than 30mm for 2" eyepieces. so a top tier T-2 prism sounds great..

"Baader T2 BBHS-Sitall diagonal mirror" with 99% transmission in a wide wavelength spectrum.. i think i need it.. hope there will be a sct adapter..



#20 dpippel

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Posted 20 September 2021 - 03:00 PM

I've been using this APM 2" prism diagonal with both my f/7 refractor and f/10 C9.25" Edge for the past several months. I find it to be excellent both optically and mechanically:

 

https://luntsolarsys...adband-coating/

 

It's hard to find in-stock now, but I think it's a very viable alternative to the more expensive Baader offerings. There's a thread or two about it here.


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#21 Nippon

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Posted 20 September 2021 - 03:56 PM

I've been using this APM 2" prism diagonal with both my f/7 refractor and f/10 C9.25" Edge for the past several months. I find it to be excellent both optically and mechanically:

 

https://luntsolarsys...adband-coating/

 

It's hard to find in-stock now, but I think it's a very viable alternative to the more expensive Baader offerings. There's a thread or two about it here.

How heavy is it compared to a mirror


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#22 dpippel

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Posted 20 September 2021 - 05:04 PM

How heavy is it compared to a mirror

The APM with 1.25" adapter and caps in place weighs 653 grams (23 ounces) compared to my 2" Televue Everbrite with the 1.25" adapter and caps in place at 543 grams (19.2 ounces). Not a huge difference, but certainly noticeable.


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#23 Nippon

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Posted 20 September 2021 - 06:04 PM

Okay yeah less difference then I thought.



#24 PJBilotta

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Posted Yesterday, 01:27 AM

This thread prompted me to pull out my very nice dielectric to compare to my T2 prism. On every object I compared it on, the mirror just could not match the prism - particularly on contrast and scatter. This was especially apparent on the Veil Nebula, with the mirror showing a lovely ghostly ribbon, but the prism revealing fine detail against a significantly darker background. I'm now wondering why I'm keeping the mirror, except it is my 2-incher.
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#25 Bill Barlow

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Posted Yesterday, 10:25 AM

I’ve also found the Tak 1.25” prism diagonal a better performer than the TV 1.25” Everbright mirror diagonal in my smaller refractors and SCT’s.  
 

Bill




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