64 replies to this topic

[Rob Moore]

The right diagonal for visual with a fine APO refractor

[Rob Moore]

I noticed a big difference between my AP mirror diagonal and my Baader prism diagonal when viewing planets and double stars. I get a slight very faint glow around double stars with the mirror but just black space right up to the airy disk with prism. Just wanted to share my experience so others can get the absolute best view.

[Rob Moore]

On the moon I prefer the mirror diagonal because prism causes slight chromatic aberration. But for planets and double stars the prism is the way to go.

[ken30809]

I noticed a big difference between my AP mirror diagonal and my Baader prism diagonal when viewing planets and double stars. I get a slight very faint glow around double stars with the mirror but just black space right up to the airy disk with prism. Just wanted to share my experience so others can get the absolute best view.

Which Baader prism were you using and what's the 'F' ratio of your scope? I've been thinking about trying a prism. Thanks.

[Rob Moore]

I am using Baader T-2 90° 32mm Prism Diagonal with 1.25" Nosepiece and Eyepiece Holder # PRISM-1 2456005K. I just ordered the 2” Baader Zeiss spec prism so am anxious to try it out. My scopes are F 6 to F8. It’s works good in all of them


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[Rob Moore]

Baader 2" Clicklock 90° Star Diagonal with Carl Zeiss Spec Prism # PRISM-2Z 2456117. This is the one I just ordered. $582.00 so I hope it’s good. I am sure it will be.


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[Spikey131]

I’m sure you will be satisfied with that excellent Baader diagonal.

 

But your comparisons noted above - were those observations made on the same night?

[Rob Moore]

I have compared them many times over weeks on the same object going back and forth. The prism is definitely better on stars and planets.


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[Rob Moore]

I have been a strictly visual observer for almost 40 years and this is my experience. Take it for what its worth.


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Edited by Rob Moore, 06 July 2025 - 03:47 PM.

[Rasfahan]

Interesting to read about your experience. For my TS Photoline 130 (f/7 FPL53) the Baader 2“ BBHS prism significantly increases visible CA on Jupiter and the moon. It’s best with a mirror diagonal. Interestingly, it decreases CA on my Takahashi FS-60CB. In the FS-60Q and in the FC-76DCU I can see no difference. I like to use a prism for solar h-alpha viewing because of the reduced scatter.

[Rob Moore]

I haven’t noticed any color on Jupiter but at higher magnification on the moon I see it. That’s why I said i prefer mirror diagonal on moon


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Edited by Rob Moore, 06 July 2025 - 03:51 PM.

[Eddgie]

Interesting to read about your experience. For my TS Photoline 130 (f/7 FPL53) the Baader 2“ BBHS prism significantly increases visible CA on Jupiter and the moon. It’s best with a mirror diagonal. Interestingly, it decreases CA on my Takahashi FS-60CB. In the FS-60Q and in the FC-76DCU I can see no difference. I like to use a prism for solar h-alpha viewing because of the reduced scatter.

I own the same Photoline model as you do, and my experience is identical to yours.  With a prism, the chromatic error is easy to see, and rather pronounced on Sirius and still visible on bright doubles.

 

I have a Baader 2" mirror, and with it, the instrument is virtually color free when used visually.

 

The sperhochromatism will be worse with a 2" diagonal because of the much longer light path. The longer the converging cone travels through the glass, the more spherochromatisim will occur.

 

This plot is done at 550 nanometers (green) and even in green light there is some error, but the error is much worse in blue. Since our eye is not very sensitive to red, we don't see the red fringe, but at f/7, even a 1.25" diagonal will produce meaningful spherical aberration in blue and red.

 

This means that energy that should be falling into the center of the Airy Disk is not falling into the diameter of the Airy Disk,  The goal of a good optical system is to put as much energy into the Airy Disk as possible, and anything you put in your light path that causes that energy not to be as well concentrated as possible will lower contrast.

 

It can also shift the color of color contrast doubles or color stars. To get the purest color rendition, as much energy as possible should be kept into the Airy Disk. 

The 2" diagonal would likely be much worse. 

 

 

I recommend mirror diagonals for scopes of f/7 and faster unless those scopes were designed and marketed as providing their best performance with a prism diagonal, in which case the use of a mirror will not cancel out the Sperhochroamtism the designer build into the system. 

Edited by Eddgie, 06 July 2025 - 04:27 PM.

[Rob Moore]

All I know is what I see at the eyepiece. I don’t get any glow around stars with the prism. I guess it will just come down to personal preference. Which image do you prefer. Everyone is going to be different of course.


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[Rob Moore]

That’s why I started this thread to get different opinions to see what other people see.


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[ken30809]

That’s why I started this thread to get different opinions to see what other people see.


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I think I will see a nice improvement with my TSA-120 and TOA-130 with the prism diagonal. I'll know soon. Thanks for your post.

[Wildetelescope]

Several long threads on this.  Boils down to less background scatter vs spherochromatism.  Because of this most folks that like prisms use them with f8 or greater focal ratios.  I like my Baader Zeiss on planets with my F9 refractors.  I use my AP Maxbrite with anything faster.  Talking about subtle differences for the most part.  Matter of personal preference.

 

JMD

[Aibrahim]

I have the baader Prism diagonal, AP and takahashi mirror diagonal. The Baader is the most superior out of all these. Specially on planets and double stars and the moon. No CA when used with my TOA 130 or 150 or any of my cassegrains

[Psion]

All I know is what I see at the eyepiece. I don’t get any glow around stars with the prism. I guess it will just come down to personal preference. Which image do you prefer. Everyone is going to be different of course.


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If you look into the Baader T2 90° mirror, you will see how much light is reflected by the prism edges. If you blacken the surface, you will achieve better results.

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[Scott99]

I use the 2" Maxbright or the Tak 1.25" prism diagonal.  I've never done must comparing of these two diagonals, I have no comment on mirror vs. prism.

 

One thing I have noticed with these two is that the open mirror diagonal is a magnet for dust and crud, while the closed prism seems to never get any dirt on it.   Therefore, I open up the Maxbright and clean it on a regular basis.  I also blow it out w/ Ultrajet compressed air blasts after every session. 

 

If I was to compare these two, I would want to clean the Maxbright until the mirror is spotless under a magnifying glass before I started the comp

Edited by Scott99, 07 July 2025 - 12:41 PM.

[Bill Barlow]

I have done quite a few side by side comparisons of prism and dielectric diagonals (both 1.25" and 2") in f/10 6-11" SCT's and f/6.65-7.5 3-3.6" refractors.  What I have found is that the prism diagonals (Tak and Baader) show better focused Airy discs and better planetary contrast than the dielectric diagonals (TV, AP, Vernonscope).  But for some reason, when viewing deep sky objects like galaxies and PN, the dielectric diagonals were slightly brighter.  

 

The differences are mostly slight but visible.

[ris242]

If you look into the Baader T2 90° mirror, you will see how much light is reflected by the prism edges. If you blacken the surface, you will achieve better results.

Does the edge affect the view? You are only looking down the centre.

[GGK]

The right diagonal for visual with a fine APO refractor

There are several topics on a few forums discussing why some APOs perform best with a prism diagonal while others do not. Rather than restate the posts, here's one from Cloudy Nights: 

 

https://www.cloudyni...prism-diagonal/

 

I also find this paper from the CFF Telescopes website interesting:  https://cfftelescope...olor-correction

 

The body of the text is copied in below.  The paper was written by Pal Gyulai, Optical Designer of refractive optics, CFF Telescopes

 

Color corrections of our lenses

Photographic vs. Visual color correction

 

The color correction of Visual-tuned lenses is designed to give peak Strehl ratio where human eye is the most sensitive.

 

For this reason, these lenses deliver best possible image sharpness and contrast for visual observations. Technically speaking, they are tuned to reach peak Strehl ratio in a limited range of wavelengths (430 nm – 700 nm).

 

Our recommendation is for these Visual tuned lenses to be used with mirror diagonals. This type of lens gives excellent photographic results when used with IR/UV filters.

 

The color correction of Photographic-tuned lenses is designed for covering a sensibly wider range of wavelengths. These lenses will be better corrected for near IR and violet wavelengths. Depending on the type of the photographic sensor used and its sensitivity range, images can be taken without the need of an IR/UV filter.

 

The acceptable corrected wavelength range is usually 400 nm – 1000 nm. As photographic sensors are sensitive over this range, this type of color tuning nicely matches the needs of photographers.

 

It has to be noted that Strehl ratio becomes slightly lower in red and blue colors and experienced planetary observers might detect the difference under excellent seeing conditions. To get optimal Visual performance from these lenses, some glass needs to be added in the light path, and in our case, this can be represented by a prism diagonal (1.25″ or 2″ size). As prism diagonals typically have better optical quality, better light transmission and less light scatter than mirror diagonals, we encourage all amateur astronomers to use them.

 

Considering that APO telescopes tuned for “Photographic” color correction can be used both for photography and for visual “work” (with prism diagonal), we believe this tuning of color correction is optimal (Zeiss tuned the color correction of their APQ series of apochromats very similarly) and it shall be used as the ‘default option’ for our apochromat objectives.

 

Post 7 and 9 by Astrojensen in the CN linked topic above is interesting to me because improvement was seen in a 2" prism diagonal but not is a smaller T2 prism diagonal of the same Baader design.

 

I use both prism and mirror diagonals, depending on the scope and targets. 

 

Gary 

[Dean J.]

Interesting.  I'm pretty sure that my AP 110GTX would fall in the "Visual tuned lens" category and thus my AP Maxbright diagonal would be the recommendation for that scope.  I'm wondering about my TEC 140FL.  If I'm reading Eddgie's post above correctly then I should be using the mirrored diagonal with that one too.

[Spikey131]

Curious that this thread is going when Jupiter is unobservable….no chance to try out my various diagonals.

 

I have to say that (by memory only) I have notice little difference between my quality mirror diagonals and Baader prism.

[Wildetelescope]

I think that if you are happy with the view you are getting from your good quality mirror diagonal, then there is minimal reason to spend a lot of money on a good quality Prism diagonal, UNLESS you have previously looked through one and decide that you really like it.  In my case, my motivation for the Zeiss diagonal was that it slightly shortened the light path, allowing me to bring all my eyepieces to focus in my 152 mm F9 AP scope.  Now that I have it, I prefer to use it for planetary with my TV102 and my two AP refractors.  But I would be perfectly happy with the Max brite, or even my Orion Dielectric diagonal otherwise.   There are cheaper ways to cut down the back ground scatter than a Zeiss Prism:-)    That said, if you have long focal length scopes and are curious/can afford a good prism, they are very nice. 

 

Cheers!

 

JMD