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Diagonal prism or mirror diagonal for Mak.?

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18 replies to this topic

#1 paulsky

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Posted 19 June 2017 - 01:02 PM

Thank you for all your opinions.
Finally I got a Maksutov 127/1500.
One last question: for this telescope is better, a diagonal mirror or a diagonal prism?
Thank you
Paul



#2 infamousnation

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Posted 19 June 2017 - 01:13 PM

prism, as long as its a good one

 

An exceptional mirror is better than an average prism though.

 

Shorter focal length scopes like refractors sold by televue and astrophysics include exceptional mirror diagonals and deliver exceptional performance.  Longer focal length scopes can get away with using prisms, and if the prism is exceptional, may even gain some benefit due to a reduction in scatter.

 

Some observers will even go so far as to remove the diagonal all together, but that makes for an uncomfortable experience.

 

I think most important is that it is a good diagonal first, and prism vs mirror is less important.



#3 Richard O'Neill

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Posted 19 June 2017 - 01:14 PM

Theoretically, a mirror.



#4 Richard Whalen

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Posted 19 June 2017 - 01:16 PM

To answer that you  need to do a star test without any diagonal, determine if it is over or under corrected. If you are not able to do this your best bet is a high quality mirror diagonal.



#5 NiteGuy

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Posted 19 June 2017 - 01:18 PM

For a Mak, use a mirror with no more than 96% reflectivity (higher reflectivity dielectric coatings are subject to light scatter).



#6 junomike

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Posted 19 June 2017 - 02:35 PM

I'd go with a mirror also to maximize transmission (although very little difference).

 

Mike



#7 infamousnation

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Posted 19 June 2017 - 02:59 PM

I'd go with a mirror also to maximize transmission (although very little difference).

 

Mike

Are you sure a mirror would transmit more light than a prism?

 

https://www.edmundop...-mirrors-en.pdf

 

The equations in this writeup (snells law) suggests the opposite.

They also suggest that a prism performs better as its temperature changes.



#8 paulsky

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Posted 19 June 2017 - 03:00 PM

Thanks to all for your response,
I have asked because I had read that a diagonal prism of good quality was better for a refracting telescope, and I do not know why I had thought that perhaps for a Maksutov could be a good alternative ..



#9 infamousnation

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Posted 19 June 2017 - 03:13 PM

For a Mak, use a mirror with no more than 96% reflectivity (higher reflectivity dielectric coatings are subject to light scatter).

Dielectric mirrors with a high reflectiveness of say 99%+ will scatter light if the surface is not flat.  If the surface is very very flat, they will not scatter much light.  As the surface becomes less reflective, it will not make scatter go away.  In fact the light which is not reflected at the right angle, will either be absorbed or scattered, increasing the amount of scatter.  If the coating is not applied perfectly, it will make the surface less flat, this is why many dielectric mirrors have problems with scatter, even though they have a high reflectivity.  As more layers of coating are applied, the uneven waves in the surface become even more uneven, increasing scatter.

 

Then the scatter of the mirror is dependent on having a very flat surface to start with, and then applying the coatings very precisely, which is expensive process. 



#10 infamousnation

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Posted 19 June 2017 - 03:16 PM

Thanks to all for your response,
I have asked because I had read that a diagonal prism of good quality was better for a refracting telescope, and I do not know why I had thought that perhaps for a Maksutov could be a good alternative ..

The main downside to prisms, is that when the light enters in a diverging or converging cone, it causes aberrations.  This is most pronounced at shorter focal ratios.  This is why short refractors come with mirrors.  But your mak has a larger focal ratio, so would be even less affected by this process.

 

Either one will be good, it is more important that you get a good one, than a certain type.


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#11 junomike

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Posted 19 June 2017 - 03:31 PM

 

I'd go with a mirror also to maximize transmission (although very little difference).

 

Mike

Are you sure a mirror would transmit more light than a prism?

 

https://www.edmundop...-mirrors-en.pdf

 

The equations in this writeup (snells law) suggests the opposite.

They also suggest that a prism performs better as its temperature changes.

 

IME yes.  The difference isn't staggering but noticeable on DSO's.

 

Mike



#12 infamousnation

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Posted 19 June 2017 - 08:37 PM

Here is an article that explains total internal reflection.

 

http://www.physicscl...rnal-Reflection

 

It shows how all the light gets reflected.  Its how prism diagonals work and fiber optics.  Its really cool actually.



#13 NiteGuy

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Posted 19 June 2017 - 08:40 PM

 

For a Mak, use a mirror with no more than 96% reflectivity (higher reflectivity dielectric coatings are subject to light scatter).

Dielectric mirrors with a high reflectiveness of say 99%+ will scatter light if the surface is not flat.  If the surface is very very flat, they will not scatter much light.  As the surface becomes less reflective, it will not make scatter go away.  In fact the light which is not reflected at the right angle, will either be absorbed or scattered, increasing the amount of scatter.  If the coating is not applied perfectly, it will make the surface less flat, this is why many dielectric mirrors have problems with scatter, even though they have a high reflectivity.  As more layers of coating are applied, the uneven waves in the surface become even more uneven, increasing scatter.

 

Then the scatter of the mirror is dependent on having a very flat surface to start with, and then applying the coatings very precisely, which is expensive process. 

 

Reflectivity has nothing to do with scatter. It's the process...enhanced aluminum vs. dielectric. As far as reflectivity goes, it just happens that enhanced aluminum is usually 94 to 96% and dielectric is usually 98 to 99%. Sure, a nice flat surface accuracy and smooth polish will help any secondary optical surface perform better but scatter is much more prevalent with and largely a (dis)function of the dielectric process itself.



#14 dotnet

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Posted 19 June 2017 - 11:54 PM

BillP posted a shoot-out test of prism and mirror diagonals here on CN, search for it.

 

I'm now using a Baader T2 Zeiss prism with my F/12 Mak. Haven't looked back.



#15 Peter Besenbruch

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Posted 20 June 2017 - 06:07 PM

If I was concerned with planetary viewing on a Maksutov, it would be prism all the way. Prisms really do have less scatter than dielectrics. For deepsky, I don't much care one way or the other. The one advantage of dielectrics over other mirror types is durability. I describe the dielectrics as "dishwasher safe." Prisms are also quite durable.

 

In practice this means I have a 2" dielectric for lower power viewing, and an 1.25" prism for lunar and planetary views.


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#16 Jeff Morgan

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Posted 20 June 2017 - 06:48 PM

I would try both myself.

 

Here is a nice article on the various mirror and prism diagonals:

 

https://www.cloudyni...omparison-r2877



#17 T1R2

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Posted 20 June 2017 - 06:50 PM

I go with prism's 100% of the time in long focal ratio scopes, to me stars look better, they have less scatter, and better contrast..... they bend light, thats something that black holes do and to me that is really neat.

 

and I also prefer the prism in DSO's observation not just planetary


Edited by T1R2, 20 June 2017 - 07:02 PM.

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#18 Jeff Morgan

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Posted 20 June 2017 - 10:27 PM

I go with prism's 100% of the time in long focal ratio scopes, to me stars look better, they have less scatter, and better contrast..... they bend light, thats something that black holes do and to me that is really neat.

 

lol.gif

 

waytogo.gif

 

(For the record, I currently use an AP Maxbright - haven't tried a quality prism yet.)


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#19 SeattleScott

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Posted 20 June 2017 - 10:39 PM

Responses above sum it up pretty well. Prisms are popular with apos above F7 or so to eliminate light scatter from the use of mirrors. With a Mak, you don't have to worry about color aberrations from a prism and it should help reduce light scatter a smidge. Of course you already are bouncing light off two mirrors so using a prism won't eliminate light scatter from that. But at least those mirrors are not dielectric so they may not have much light scatter. 

 

So yeah in theory a prism would be better, at least for planetary. But would you be able to discern the difference between a premium mirror diagonal and a premium prism? Probably not until you have logged a good deal of observing experience. And maybe not even then, given the light scatter from the Mak.

 

Scott


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