114 replies to this topic

[PKDfan]

A question for you Sir, are you saying that "Prism" is better for a Mak than the "dielectric mirror" one?

That is such an excellent question MrsM75 that i'm at odds to answer it unequivocably; so the best i can say is, it depends upon the target and the scope used.

Yes for low contrast planetary many swear by a prism. For me its not so clear as my little scope is faster ratio than what a prism likes and my slower larger scope i've parked for awhile trying and testing out my little Evolux.

The limited field of the 1 1/4" prism isn't ideal for wide star fields and the very high quality mirrors i have i cannot detect any problems so there is little need for the prism, as of right now.

I do intend to test the prism again in my F/9 100ED with my many new eyepieces as your question has me contemplating using one again and finding an exact answer to your question which has always bothered me.

Like should i invest in a Zeiss spec prism over the Celestron ?

So the short answer is I Don't really know the answer to your excellent question. Best i can say is maybe. Sorry for such a lame answer.

It really depends upon how flat the mirror is in the dielectric and how good your telescope is and how steady and clear your skies are.


Lance

Clear Steady Skies

[Astrojensen]

From my own experiments, comparing many different diagonals, I can attest to TOMDEY's results: Diagonals vary WILDLY in optical quality, and FAR more than eyepieces do (as long as you don't buy the very cheapest eyepieces). The major problem is that I've tested some not so inexpensive diagonals that turned out to be very poor performers, compared to others, some of which were very inexpensive... In other words: You can't automatically just assume that "you get what you pay for". And that's a problem. 

 

The only real way to find out, whether your diagonal is good or not, is to test it directly on challenging objects. Planetary details, very tight double stars, etc. Look at very bright stars, and notice how much scattered light there is around them. Compare the view with and without diagonal, and, if possible, between diagonal and diagonal. Notice the differences. 

 

I did so with a number of diagonals, and the comparison was shocking. A $200 2" dielectric diagonal was much worse on planetary details than a $15 no-name Japanese prism diagonal. Now, I'm not saying this will always be the case, but it's something that shouldn't be ruled out without testing. Just because it's expensive doesn't mean it's good. 

 

Only my very most expensive Zeiss and Baader diagonals came close to the view without a diagonal. In all other cases, the difference between diagonal and no diagonal ranged from somewhat noticeable to blatantly obvious, meaning that some of them (the dielectric was by far the worst in my case) severely impacted the visible details. I had NOT anticipated the huge quality spread before I began testing.  

 

A downside to observing without a diagonal is of course that it can get very uncomfortable... So purchasing a high-end diagonal is a very good investment. And one I would DEFINITELY do BEFORE investing in high-end eyepieces. 

 

Just remember to actually test it, and not rely on price or brand name alone. 

 

 

Clear skies!

Thomas, Denmark

[quilty]

Along my experience a good (what is premium?) diagonal matters much more than a premium ep.
Contrast and crispness and colours.
A good diagonal might be a cheap semi plastic Celestron prism (my last one purchased new for 10€) increased my vision more than any perceived diff between any of my eyepieces.
At crispness, contrast and colour. At viewing comfort it's the ep what matters, such as AFOV, edge correction, ER, blackouts...

Note: It's simple to make a decent 1,25" prism but ther're 100 ways to mess up a dielectric mirror diagonal

Edited by quilty, 18 May 2025 - 02:57 AM.

[Astrojensen]

A question for you Sir, are you saying that "Prism" is better for a Mak than the "dielectric mirror" one? 

Prism diagonals, if well made, have lower scattered light than dielectric diagonals. This is very important for planetary observing. If my telescope is slower than f/6 (f/8, f/15... for example), I will always prefer a prism over a mirror diagonal. 

 

It is interesting to note, that small prisms, in the 1.25" size, are much easier to make than dielectric mirrors. This dramatically increase the chance of getting a really fine prism, compared to getting a really fine dielectric mirror. 

 

At least, I've personally not yet seen a mirror diagonal that was better than a prism diagonal in slow telescopes. 

 

 

Clear skies!

Thomas, Denmark 

Edited by Astrojensen, 18 May 2025 - 02:57 AM.

[Jon Isaacs]

I very very much agreed with you Sir. I doubt a Televue eyepiece would show the same quality in a $5 all plastic diagonal with plastic mirror. 

 

I actually find the diagonal to be more important than eyepiece, because on axis, tbh when "on xias" all eyepieces look same in my Maks. 

It the fast focal ratio scope that has to deal with the outer edges correction.

 

Hopefully, this is not about a $5 diagonal.  The 1.25 inch Celestron Prism diagonal is $45 and is a very good performer, especially at Mak focal ratios.  

 

As triplemon said, the diagonal of a slow scope only needs to be flat over a very small region.  1/10th wave flats are for Newtonians where nearly the entire mirror contributes to each point on the focal plane.  With a 1.25 inch diagonal and an F/12.5 scope, the light cone is about 50mm from the focal plane.  That means light cone is only about 50mm/12.5 = 4 mm in diameter at the diagonal.  That is the scale of the flatness necessary.

 

A decent diagonal is all you need. The biggest contributors to the high power views are the seeing and the size and aperture of the telescope.  A decent diagonal and a decent eyepiece will show whatever the scope and the atmosphere will allow.

 

Jon

[Procyon]

It's good to read up what other Mak users prefer, which I'm sure you do MrsM75, for instance a thread like this about Mak users and diagonals can be very informative: https://www.cloudyni...ak-cass-scopes/

 

Having said that, buying a very decent diagonal can never hurt, sure it can feel like having to give over hard earned money and that will come with some obvious pain, but I find the pain goes away about an hour after you click the buy button lol. The enjoyment and peace of mind it brings though, lasts for years, maybe decades.

 

I would advise you to get a 1.25" Baader Prism with a VERY practical 1.25" Baader clicklock or read up if this might be a better option as an eyepiece holder: Baader 1.25"/T-2 Eyepiece Holder with Helical Microfocuser. You also need the nosepiece. This diagonal setup has tested and used for over a decade and everyone's happy with it. I loved mine 10 years ago  It's very practical in many ways, such as if ever you decide to try binoviewing planets, it's short light path length will be a great advantage.

 

Sure this will cost about 250, but in the long run, say over 10 years, that extra 100-150$ or that extra 10$ per year over 10 years will be worth much more to you if you intend to stick with this nice hobby.

Edited by Procyon, 18 May 2025 - 03:00 PM.

[Jon Isaacs]

 

Sure this will cost about 250, but in the long run, say over 10 years, that extra 100-150$ or that extra 10$ per year over 10 years will be worth much more to you if you intend to stick with this nice hobby.

 

The difference in the views between the $250 diagonal and the $50 diagonal will be minimal. MrsM75 would be wiser to invest that $200 in a telescope fund.  

 

Jon

[Jon Isaacs]

I can attest to TOMDEY's results: Diagonals vary WILDLY in optical quality, and FAR more than eyepieces do (as long as you don't buy the very cheapest eyepieces).

 

 

Tom Dey tested the entire surface of the diagonal.  That is what you do with a diagonal meant for a Newtonian.  For a refractor or CAT, as explained above by myself and others, the global flatness is not critical.  This idea, it is something that Roland Christen of Astro-Physics has stated a number of times.  Of course Roland's MaxBright diagonal was up there at the top of the list in Tom's testing.. 

 

Roland is committed to excellence but is a straight shooter.

 

Jon

[vrodriguez2324]

Eyepiece

[Bill Barlow]

MrsM75, I would suggest the Takahashi 1.25” prism diagonal.  It is priced around $175.  I have used it extensively in my smaller SCT’s like a C5/C6 with excellent results.  I have compared ot to the TV 1.25” dielectric diagonal and the Tak prism produced better contrast and tighter focused Airy discs.

 

Bill

[Procyon]

The Tak looks great also. 

 

The difference in the views between the $250 diagonal and the $50 diagonal will be minimal. MrsM75 would be wiser to invest that $200 in a telescope fund.  

 

Jon

I dunno, she might not be ready right now to get another scope. She does have 4 Maks waiting for a better diagonal though. Thought it would make a nice upgrade until she gets another scope, plus she can just carry the diagonal to the newer scope if ever she decides to get another one.  I'm pretty sure she would enjoy using the clicklock system. It really is very user friendly for so many reasons, as well as being top notch. But that Takahashi might be another good option. I still don't know much about that $70 GSO she has, maybe it's sufficient, what do you guys think?

 

Probably would be a good idea to read Bill's big diagonal shootout also. https://www.cloudyni...omparison-r2877

 

In assessment of scatter around Jupiter and bright stars, straight-through observing without any diagonal was very clearly and distinctively better, perhaps showing only half as much scatter as the best diagonal.  After that, the diagonals came in showing scatter in the following four tiers, from least scatter to most:

 

a.       Baader T2 Zeiss Prism, followed by: Baader 2" Zeiss Prism, Takahashi 1.25" Prism

 

I don't think you need the premium Zeiss BBHS version though.

Edited by Procyon, 18 May 2025 - 04:45 PM.

[BlueMoon]

 

Premium diagonal vs premium eyepiece, which matter more to get the most performance out of your scope visually?

The degradation in the performance of one effects the other. A poor diagonal isn't going to allow that premium eyepiece to work at its potential and vice versa.  

[Mike W]

Why not buy a used TV off C/N like I just did? 

[25585]

I use the Premium Amici, Baader Astro.  https://www.baader-p...hs-coating.html
 

The APM is almost as good   https://www.teleskop...d-coating-11933
 

My others are unused. 

[Jon Isaacs]

I use the Premium Amici, Baader Astro.  https://www.baader-p...hs-coating.html
 

The APM is almost as good   https://www.teleskop...d-coating-11933
 

My others are unused. 

 

 

Even Baader admits their very best Amici prisms show the roof line. 

 

Star diagonals for the stars. Especially double stars.

 

Jon

[25585]

There is this review of 1.25" diagonals   https://www.baader-p...n_w_paolini.pdf

[Astrojensen]

Tom Dey tested the entire surface of the diagonal. 

Perhaps, but I tested mine visually at the telescope, in a long focal length refractor. And the difference was still very pronounced. 

 

My experience is that the optical quality of diagonals vary wildly, and that expensive is not necessarily equal to high optical quality. 

 

 

Clear skies!

Thomas, Denmark

[hfjacinto]

I’ve had a few 2” diagonals (Celestron, OPT, Orion and Baader) and all were pretty good, the Baader was the best built and the only one I kept. But all were good, a better eyepiece is much more important! But a good eyepiece can cost as little as $50 (zhummel planteries or paradigm) both are not expensive and pretty nice.

[Procyon]

There is this review of 1.25" diagonals   https://www.baader-p...n_w_paolini.pdf

Added it in my previous post = ) no biggie though, maybe worth repeating as it's a great read.

[vtornado]

For high power planetary and lunar, my Celestron 94115-A prism diagonal is my best diagonal.  You have several Maks which I assume are around f12, so the prism will work very well in them.    At faster focal ratios the prism can introduce some CA, so I assume this will degrade the image.   (Maybe the CA is worse than scatter of the mirror, but I have not experimented with it.  When I view planets I use my Mak or my f/9 refractor).  My best (most expensive) diagonal is a williams optics diaelctric maybe around $75.00.  So I have not compared my little celestron to anything top shelf.  I'm always tempted to try an higher class prism diagonal like a Baader or the Tak.  But so far I have not.

 

It is important to test your diagonal to make sure it is in collimation.  The prism's front face must be perpendicular to the optical axis.  The prism can shift in the housing.  

 

At low power I don't see a big difference between the prism and mirror.

 

My sharpest eyepiece combination is a 12.5mm Meade Superplossl with a 2x Orion vintage long barlow.  I bring this up because I believe it hints that the diagonal is more important than the eyepiece.

Edited by vtornado, 18 May 2025 - 02:52 PM.

[RichD]

Perhaps, but I tested mine visually at the telescope, in a long focal length refractor. And the difference was still very pronounced.

My experience is that the optical quality of diagonals vary wildly, and that expensive is not necessarily equal to high optical quality.


Clear skies!
Thomas, Denmark

I accept your experience, but as Tom says you do generally get what you pay for. The TV diagonal he tested, along with the AP and the lumicon, all expensive units, tested very well. Not a coincidence. You put the odds in your favour with an expensive unit.

Its certainly been my experience too, but I accept the differences between diagonals are not huge unless something is wrong.

In general I try to upgrade each element of the optical chain to a high standard over time, with the additional benefit that a high quality diagonal is such a nice thing to use and interact with each time you observe. I love the solidity of my 2" TV everbrite.

Having said that, i wouldn't blame someone at all for staying with a basic model and putting dollars towards a scope or eyepiece.

[Mike W]

Also the TV is a one piece design that will outlast the owner most likely= "Everbrite"

Edited by Mike W, 19 May 2025 - 10:15 AM.

[Astrojensen]

I accept your experience, but as Tom says you do generally get what you pay for. The TV diagonal he tested, along with the AP and the lumicon, all expensive units, tested very well. Not a coincidence. You put the odds in your favour with an expensive unit.

Odds being in your favour doesn't mean you can't lose. My worst diagonal cost me over €200... My best ones ARE very expensive, yes, but the €200 one has been thoroughly bested by far cheaper ones. 

 

What I am saying is that you need to VERIFY, not trust blindly. 

 

 

Clear skies!

Thomas, Denmark 

[Mike W]

Odds being in your favour doesn't mean you can't lose. My worst diagonal cost me over €200... My best ones ARE very expensive, yes, but the €200 one has been thoroughly bested by far cheaper ones. 

 

What I am saying is that you need to VERIFY, not trust blindly. 

 

 

Clear skies!

Thomas, Denmark 

Not so, I trust Televue to give me the best diagonal (or among the best) for the money. How many times have you heard of a bad TV or AP diagonal?

[Procyon]

Also the TV is a one piece design that will outlast the owner most likely= "Everbright"

The only disadvantage to the one piece design came when it was time to add a TSFLAT2 by removing the diagonal's nose lol. But that was with 2" diagonals. I still think it's worth noting for someone out there who has a refractor and wants to add that specific field flattener. I would love to have a TV, BBHS and AP diagonals, but I'm managing quite well with very practical and versatile 2" Baader Clicklock Diagonals, and 2" WO Durabrights. People underestimate the mechanical versatility some diagonals bring to the table.

 

I do feel a bit dumb having bought the 2" WO though, all to save 7mm of lightpath. It did help and do it's job at the time I needed that extra 7mm to get some eyepieces to come into focus. Will probably just keep it now, overall I like the Clicklock design more than the screw types, feels much more safer when things start flipping around lol.

Edited by Procyon, 18 May 2025 - 03:15 PM.