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Prism or mirror for FS-102?

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#1 Eric H

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Posted 01 January 2020 - 09:56 PM

Subject title says it all. Whats better?



#2 BillP

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Posted 01 January 2020 - 10:16 PM

Well...I think the answer here would be more a matter of principle than anything else!  I mean you are in a bit of a refractor pursit land having a fluorite Apo, and a nice classic doublet makes it purer still, and of course a Takahashi branding is the icing on the cake.  Now with all that purity in hand, why spoil it with mirrors in the optical chain shameonyou.gif.   I think you need to keep that little OTA an all-glass affair and go with a nice prism  hamsterdance.gif


Edited by BillP, 01 January 2020 - 11:14 PM.

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#3 Defenderslideguitar

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Posted 01 January 2020 - 10:51 PM

Agree in totality with Bill P..    There is just something about a prism diagonal for a refractor such as this........prism over mirror     yes

 

I sought and found at Neaf 2019  an APM 2 inch/1.25 prism diagonal  that is wonderful on the C-102fl   and  the Tak's fc-100, fs-128

Can't go with getting a good quality prism   several good ones fit the bill....enjoy


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#4 Steve Allison

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Posted 02 January 2020 - 01:30 AM

I bought a 1 1/4" Baader Zeiss prism diagonal for use with my own FS-102 because I wanted to feel I had the best optics available for my fun hobby of visual observing. For the same reason, I found and purchased a .965 Nikon 5 mm orthoscopic eyepiece, which I understand was one of Thomas Back's personal favorites.

 

The above combination gives me 164 power's worth of optical perfection, and I don't have to wonder if I am missing anything that 4 inches of aperture can possibly show.


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#5 aa6ww

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Posted 02 January 2020 - 01:58 AM

I always took the advice of AstroPhysics as in, always recommending  Mirror diagonals over Prisms for their Pristine AP line of excellent refractors. Always had an AP Maxibrite as my first 2" diagonal and still have it, with a  Televue 2" Everbrite as my second choice. 

 

The Test of time has been excellent to both of these Mirror diagonals as solid excellent choices.

 

...Ralph 


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#6 Erik Bakker

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Posted 02 January 2020 - 04:09 AM

This discussion comes up from time to time on CloudyNights, so a good search here will give you quite a few results.

 

Having had both, my FS-102 and myself preferred my 2" Tak mirror over my Zeiss ca. 31mm prism diagonal (1 1/4" and NLA) or regular 1 1/4" or 2" aluminum or dielectric coated mirrors. Better contrast, less scatter. Very apparent on Jupiter. For the finest throughput, no diagonal is even better, but I find those advantages loose out to ergonomic awkwardness often, depending on your size of mount and position of the object in the sky. Zenit observing is a nightmare without a diagonal, but Venus or Mercury low in the sky do well, as do the other planets when they are low in the sky. Your mind needs to be able to cope with the change in orientation though.

 

A wonderful comparison on many diagonals was made by William Paolini on CloudyNights some years ago. You can find it here.

 

The classic Zeiss prism diagonal or even the 1 1/4" Tak prism diagonal are fine choices, as are the best made 2" mirror diagonals. The latter are a better choice if you also want to observe with 2" eyepieces. For binoviewer work, or medium-high power only, 1 1/4" in a prism works fine, though I enjoy the added feel of stability a 2" diagonal gives when using a (top) heavy binoviewer with it.

 

Here's my personal favorite, the 2" Tak mirror diagonal, as shown on my FS-102NSV:

 

Tak 2%22 diagonal.jpeg

 

DSC_1895 Tak 2%22 diagonal CN.jpeg

 

Note it only has 1 set screw (shown in smaller picture) and no brass ring to clamp an eyepiece. Works fine in real life for me though.


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#7 Doug Culbertson

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Posted 02 January 2020 - 07:27 AM

I have both the Baader Zeiss spec T2 prism with a 2” nosepiece and 1.25” micro focuser eyepiece holder and a Baader 2” BBHS mirror diagonal. 
 

When I had first light with my FS-102 a couple of nights ago I found that when using the 24mm Panoptic and 18.2mm DeLite (parfocal) and other 1.25” eyepieces  with the prism I had to rack the focuser all the way out and then use the micro focuser to reach focus. With the mirror the focuser was racked out about an inch as shown below. This may or not matter to you, but I thought it worth mentioning. 

 

 I ended up sticking with the mirror as I was looking at the moon and really couldn’t tell much of a difference between the two. 
 

gallery_7167_7839_73564.jpeg


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#8 25585

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Posted 02 January 2020 - 08:07 AM

Liking correct image to match charts etc, prism is my choice, having the good APM roof prism and excellent Baader Amici Astronomical BBHS diagonals, each 2" fit. 

 

I also have several mirrors for shorter FL scopes - 2 types of Tele Vue, the Astro-Physics. They will all outlast me.


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#9 Cometeer

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Posted 02 January 2020 - 09:26 AM

I did a little comparison with a FS-60Q and five 1.25” diagonals. None of my 1.25” diagonals are anything like Zeiss/AP quality, but they also cost a fraction of the price.

The main shootout was between the Baader T2 prism (non-Zeiss), Lumicon Lumibright mirror, and Astrotech dielectric mirror (has many clones). I also added into the mix the Celestron prism and mirror that come default with many Celestron scopes.

The results I found were that the Baader prism and Lumicon were the best, with the Baader just slightly besting it. Trailing quite a bit behind was the Astrotech dielectric. The Celestrons performed the most poorly of the bunch - I recall the prism had a slight edge over the mirror.

The difference in the views between the Baader prism and the Astrotech dielectric was most greatly seen with planetary observation. The difference was not as profound with deep sky.
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#10 peleuba

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Posted 02 January 2020 - 09:31 AM

Subject title says it all. Whats better?

 

 

You cannot go wrong with either the TeleVue Everbrite or the A-P Maxbrite.  I prefer these over any prism and own samples of both.   These are dielectric mirrors.  Baader makes a very nice mirror diagonal, too.

 

But to answer your question specifically, you'll need to evaluate the optics in the telescope.  A prism will alter the spherical correction of the objective and it will also add some color.  If these two aberrations are of opposite sign of that what is in the objective, the view with a prism in the focuser will be slightly improved.  

 

The only downside that I can think of when using a high quality mirror diagonal is that it will add some slight scatter to the image.  

 

To me, the upside of a high performing mirror diagonal outweighs any benefit that you may receive from a prism.  But, you'll get some differing opinions on this.


Edited by peleuba, 02 January 2020 - 09:33 AM.

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#11 Eric H

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Posted 02 January 2020 - 11:00 AM

Thanks all!



#12 aa6ww

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Posted 02 January 2020 - 11:46 AM

I've always bought the big name diagonals for my Takahashi Flourites and Triplets, TMB and APM refractors to give my scopes every performance advantage I can coupled with high quality eyepieces from Televue, Explorer Scientific but in all honestly, I've never seen any visual difference between a AP Maxibrite, a TV Everbrite or a $100 Celestron  or Meade 10th wave Diagonal. 

 

This doesn't stop me from owning the big name diagonals however. I think this is more in our heads then anything else. There may be a detectable visual advantage if your eyes are 20 years old but after 50, I believe the differences mostly show up on a data sheet and not in real world visual observing.

 

The quality of your eyepiece has a much more significant advantage then the quality of anyone's 10th or 20th wave diagonal, but even the best 2" eyepieces from Televue still cant compete with a very inexpensive 1.25" binoviewer with a simple set of plossls in them. 

 

Just being realistic.

 

...Ralph 


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#13 peleuba

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Posted 02 January 2020 - 12:09 PM

I've always bought the big name diagonals for my Takahashi Flourites and Triplets, TMB and APM refractors to give my scopes every performance advantage I can coupled with high quality eyepieces from Televue, Explorer Scientific but in all honestly, I've never seen any visual difference between a AP Maxibrite, a TV Everbrite or a $100 Celestron  or Meade 10th wave Diagonal. 

 

 

Understood.  And to add to what you said,  the actual surface area in use on a mirror diagonal is quite small.  If a diagonal is rated to be .1λ across the entire surface, the accuracy of the portion in use is closer to .025λ or 4x better. 

 

The only aberration that mirror diagonals can contribute to the image is astigmatism.  Scatter is not an (Zernike) aberration but can certainly affect the view.

 

The issue with cheap mirror diagonals is the manner in which they are constructed/mounted which can cause warping (astigmatism) across the surface.

 

The reason to use a top tier dielectric is for the durability and reflectivity that can stand up to years of use and (careful) cleaning.  The top two that come to mind, Everbrite and Maxbrite are manufactured to high tolerances and are substantial pieces.


Edited by peleuba, 02 January 2020 - 12:10 PM.

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#14 Wildetelescope

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Posted 02 January 2020 - 03:28 PM

Thanks all!

I have a maxbrite diagonal, a couple Orion dielectric diagonals, and the Baader Zeiss prism diagonal.  The Baader costs about 2X what the Maxbrite cost me 10 years ago.  All these diagonals give me nice views.  Like others, I do not see much difference between the mirrors.  The prism is my newest one, and I have not yet done a head to head comparison.  However, I DO seem to see a touch more contrast with it when used with my refractors(but it might be my memory playing tricks).  All my current refractors are F8 or greater.  I have heard that prisms do not perform as well with faster scopes.  I will test that out when my 80 mm F6 gets here.  All in all, I think if you get a good dielectric mirror or prism (Baader, Baader Zeiss) or Tak, you will be happy.  I will point out that the Clear aperture of the Baader Zeiss is  ~34 mm, which is smaller than the field stop for some of the longer focal length eyepieces, like my 35 mm Pan, and might give some Vignetting.  I do use my 35 mm Pan with the prism, and I might be seeing some vignetting, but it is not very evident or objectionable to me.  The Mirrors do not have this issue.  

Hope this helps.

 

JMD


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#15 Nippon

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Posted 02 January 2020 - 05:49 PM

I have a Vixen 1.25" Everbright and a Baader 1.25" non Zeiss prism that I have compared in my Vixen ED103s and prefer the prism. It is certainly not a huge difference but subtle showing a tiny bit more contrast.


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#16 Steve Allison

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Posted 02 January 2020 - 06:45 PM

Prisms are forever. Mirrors, not so much.


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#17 BillP

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Posted 03 January 2020 - 01:16 PM

The reason to use a top tier dielectric is for the durability and reflectivity that can stand up to years of use and (careful) cleaning.  The top two that come to mind, Everbrite and Maxbrite are manufactured to high tolerances and are substantial pieces.

Hmmm.  Not sure the durability assertion can be substantiated empirically since dielectrics have not been around all that long in comparison to prism.  No reason that the reflectivity of a prism would ever change so reflectivity never an issue.  But a dielectric's reflectivity should exceed an aluminized/silvered mirror over decades.  As far as durability, no reason either as have old prisms and 100+ year old eyepieces where the glass has stood up to a century of cleaning and still as "smooth as glass".  IMO most purported benefits of dielectrics, are more contrived than real as prism diagonals have no history of being poor in durability or reflectivity.  So just the marketers at work trying to justify what a lower cost to manufacture product should be embraced.

 

Basically, the dielectric is the new technology on the block.  It is purported to be more durable from the perspective of harder surface so less scratch resistant.  However, no one is offering durability stats relative to longevity of the sometimes 40+ dielectric layers not separating due to uneven expansion/contraction stresses during thermal acclimation.  Of course, prisms do not have this issue.  Scatter is also an issue any time one applies so many layers and while there are one or three top brands that I have tested that come close to a low scatter prism, none reach the prism on excel over it in this area IME.  Not a biggie but all dielectrics I tested, including the premiums, show astigmatism on-axis for the first 15 min or so of thermal acclimation.  Not a biggie but not something I have seen in prisms. 

 

Personally I prefer prisms overall.  But there are some high quality non-prisms that pragmatically are just as good, and for me these would be the Baader BBHS Silver Mirror, then either of: Baader Maxbright Dielectric, AP Maxbright Dielectric, TV Dielectric.

 

PS - I believe that dielectric coating failures or improper application will cause wavefront issues.  Proper application of so many layers over a small wavefront error on the substrate will also make that wavefront error increase.


Edited by BillP, 03 January 2020 - 01:36 PM.

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#18 doctordub

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Posted 03 January 2020 - 01:54 PM

At F8 and slower, I prefer prism diagonals. In January of 2015, I had the best view of Jupiter, the Orion Nebula, and Comet Lovejoy passing the Pleiades. I was using a Baader Zeiss Prism T2 diagonal with my Takahashi FS-152 NSV.

CS

Jonathan


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#19 Ihtegla Sar

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Posted 03 January 2020 - 03:12 PM

Since primary mirrors need to be recoated every six years or so I have a hard time believing any mirror, dialectic or not, will keep its coatings pristine over the long term.

#20 peleuba

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Posted 03 January 2020 - 11:39 PM

 

PS - I believe that dielectric coating failures or improper application will cause wavefront issues.  Proper application of so many layers over a small wavefront error on the substrate will also make that wavefront error increase.

The  U.S.  military uses dielectric coated optics - in the desert - specifically for their durability.  Dielectrics are not new, they were introduced to the amateurs more then 20 years ago but have been around for military and aerospace for longer.  Are you saying that a dielectric optic needs be 100 years old or more to satisfy your criteria for durability?

 

And, a dielectric coating does not degrade or “fail” over time leading to wavefront errors in the the manner which you imply unless the underlying substrate has some inherent strain.  All of the error occurs during the application of the coatings and is almost exclusively seen at the very edge - out of the FOV.  It’s pretty easy to detect an error in the FOV as the star image will be astigmatic.   Presumably this is caught in QA prior to getting in the hands of amateurs.

 

What I said about prisms is true.  They contribute SA and color error to the image.  Whether or not this improves the view is totally dependent upon the type of error in the objective.  The ONLY error that a mirror will contribute to the image is astigmatism and only if the underlying substrate is warped or pinched.  

 

Agree that mirrors scatter more light then a prism.
 

Happy New Year


Edited by peleuba, 03 January 2020 - 11:47 PM.

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#21 peleuba

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Posted 03 January 2020 - 11:48 PM

Prisms are forever. Mirrors, not so much.

yes and so are the wavefront errors they contribute.  



#22 Steve Allison

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Posted 04 January 2020 - 12:32 AM

yes and so are the wavefront errors they contribute.  

Touche' !


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#23 Uwe Pilz

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Posted 04 January 2020 - 02:16 AM

I am (along others) a double star observer. Double stars are most demanding against (additional) aberrations, because you don't use the diffraction limit itself, but try to detect irregularities in the Airy disc. I used many different diagonals and a Baader prisma. Most of them introduced some kind of aberration, mostly coma (the first ring is brighter at one side). The prism was particularly bad with this very close look at the Airy disc.

I ended with two WO prisms (1,25" and 2") which are flawless. I had a TS quartz diagonal which was only average and introduced coma.

 

If you use a extension tube to get the focus it is wise to use the diagonal first and then the tube. This way you use a much larger area of the mirror, which reduces additional aberrations.


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#24 cupton

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Posted 04 January 2020 - 03:19 AM

I have both the Baader Zeiss spec T2 prism with a 2” nosepiece and 1.25” micro focuser eyepiece holder and a Baader 2” BBHS mirror diagonal.

When I had first light with my FS-102 a couple of nights ago I found that when using the 24mm Panoptic and 18.2mm DeLite (parfocal) and other 1.25” eyepieces with the prism I had to rack the focuser all the way out and then use the micro focuser to reach focus. With the mirror the focuser was racked out about an inch as shown below. This may or not matter to you, but I thought it worth mentioning.

I ended up sticking with the mirror as I was looking at the moon and really couldn’t tell much of a difference between the two.

gallery_7167_7839_73564.jpeg

Noticed the same thing with my Vixen ED80 and my 1.25 Tak prism diagonal. Focuser is out quite a bit, almost to the end in order to get to focus. When using my 2” dielectric it’s barely out. Maybe an inch or so.

#25 Piero DP

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Posted 06 March 2020 - 08:34 AM

I would be interested in reading more comparison between mirror and prism at high power (e.g. planetary and double stars observing). 

 

When it is said that a mirror introduces more light scattering than a prism, how much detail is lost? Could you describe features that were noticeably enhanced with a prism and lost or washed with a mirror? 

 

Let's assume a decent mirror diagonal, like the Baader BBHS Silver mirror, and a decent prism like Baader T2 zeiss prism or 1.25" Takahashi prism. 




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