Diagonals - expensive vs mid range

[heliumstar]

For people with high-end optics - which diagonals are you all using? Did you go for high-end/expensive diagonal as well or went with run of the mill dielectrics. Reading reviews online, some people observe better colour saturation on planets and stars with Baader BBHS and I'm wondering if it's worth spending ~£400 on that one and risk silver deteriorating in a year or two I had 1.25" Baader BBHS awhile back but can't really remember if it was that better than generic ones.

Edited September 4 by heliumstar

[Mr Spock]

I use the standard mirror diagonal for most things. For planets I use the T2 Zeiss prism as it gives a bit more 'bite'. Both are very good at what they do.

Whether it's scope optics, diagonal or eyepiece, it's always worth getting the best performance. Then you can be sure nothing is left behind.
 

[Louis D]

I've been using 2" GSO dielectrics under various brand names for years with no complaints.  I mostly observe planets using large Dobs, so choice of diagonal makes no difference with them.

[Highburymark]

I just use T2 Baader diagonals, (Zeiss prism and BBHS mirror). They’re cheaper than the pricey 2” versions (though come without clicklock eyepiece holders, which are an extra cost), with a very short light path for binoviewing, and can be configured for 2” eyepieces - only eyepieces with the widest field stops vignette. Both T2s offer a small benefit (brightness in particular, and the Zeiss prism cuts scatter) over the dielectrics I used to own - they are also much better configured for my night vision eyepiece. Otherwise, the several dielectrics I used were all excellent - we are not talking dramatic differences here. 

[Highburymark]

And I don’t think you need to worry about silver deteriorating with the BBHS units - there are many users out there who’ve had them for a long time. I think it was earlier silver coatings that tended to cause problems.

[Rob]

I use William Optics CF Dielectrics 2" & 1.25" and Takahashi 1.25" Prism for Planets. This combo suits me fine

[Paz]

I mainly use baader t2 diagonals, but although the quality is good the other benefit is solidity and t2 connectivity.  But I would say the question of whether to go for a mirror or prism would be something to also take a view on as that can matter. If I'm using a fast scope or doing dark targets I'll tend to prefer a mirror, if I'm doing bright targets or using a slow scope I'll prefer to go with a prism. If I had to have one diagonal to do everything I would have to think about it a bit.

On cheaper diagonals I think the quality can still be very good, but something to look out for is if they have restricted apertures but you want full aperture.

[wookie1965]

I really don't know the difference between a mirror and a prism diagonal. 

 

 

 

Edited September 4 by wookie1965

[John]

I use a Baader T1 Zeiss prism diagonal with my Tak FC100-DL, Tele Vue Everbright 2 inch diagonals with my ED120 and ED102SS refractors and an Astro Physics Maxbright 2 inch with my Tele Vue 85 refractor. All these seem to work very well indeed as far as I can tell. I occasionally change the diagonal / scope combination but can't really tell much difference when I do. I tend to avoid using the prism with my fastest refractors which are F/6.5 and F/6.

In the past I've owned GSO's William Optics, Baader and a few other mirror diagonals and they seemed ok. I guess my move to the more expensive brands was a "belt and braces" approach to keeping all links in the optical chain as good as possible.

 

[JTEC]

I use the Baader (Zeiss) prisms and the BBHS mirror diagonals with my TEC140, mono and when binoviewing, with the Clicklock system.  As far as I can tell, these are about as good as it gets.  I tend to use the prisms for deep sky and the 1.25” BBHS for lunar and planetary.  The latter is perhaps a touch warmer and Bill Paolini found that they presented star colours better but, honestly, both are great in either application.  If I had to choose one, it would probably be the 2” prism because it allows most fitting options - with the 2” Powermate and bino, for example.  I did have a problem with a blotch that developed on one of the BBHS diagonals that was barely 2 years old - thanks to FLO, this was sorted out by Baader.  Es Reid’s advice was to ‘stick with a prism’.  TEC, on the other hand warned against the possibility of a prism introducing chromatic ‘errors’ with the refractor and, reportedly, use the Astro-Physics Dielectric.  I owned one and sold it and, fwiw, found the Baaders marginally superior.  Just to throw a cat in amongst the pigeons, I’ve experimented with viewing straight through, that’s to say, without any diagonal at all, and to my eye with the refractors I’ve had the opportunity to use, that yields a slight improvement over use with a diagonal of any kind. But it’s bloomin’ uncomfortable! 

[Second Time Around]

I wonder if the crossover point for prisms v mirrors has simply been passed down ad infinitum and as a result is now accepted as fact but isn't quite right.

Here's an extract from a comprehensive field test by author Bill Paolini on lots of diagonals. Here he's comparing the various prism and mirror diagonals at f/6.25, where the received wisdom is that mirrors will do better. I think many will be surprised!

"...After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminum, silver, and dielectric mirror diagonals were tested. First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!" I felt this was a rather obvious tell. I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences. Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB.

As more and more field observations were conducted with the 80mm APO on Jupiter, it became apparent that the prisms were providing another level of performance that the mirrors were not....."

Bill went on to state:

"While most manufacturer's seem to advertise f/7 or slower as a focal ratio for optimum prism performance, based on the observations of this comparison I feel that f/6 would also be perfectly adequate as very little color was generated even at this focal ratio."

I use prisms as they have a shorter light path than mirrors, and so give me more space for filter wheels, reducers or eyepieces such as the APM Superzoom that require extra infocus.

I bought both T2 and 2 inch sizes of the Baader models with BBHS coatings as these have increased sensitivity in the infrared - a big plus with a night vision device.

[JeremyS]

For 1.25-inch fitting, the Tak prism diagonal can’t be beaten. I have several.

For 2-inch, I use Baader BBHS mirrors (T2 or full spec) or Takahashi mirror.

[Highburymark]

28 minutes ago, JeremyS said:

For 1.25-inch fitting, the Tak prism diagonal can’t be beaten. I have several.

For 2-inch, I use Baader BBHS mirrors (T2 or full spec) or Takahashi mirror.

I think it’s the same Takahashi prism that’s in the Tak turret Jeremy - and I agree that’s every bit as good as the Baader Zeiss. I use the turret with my best eyepieces on Saturn, Mars and Jupiter for close detail, and it’s excellent. For £110 or whatever the 1.25” Tak prism costs, it’s a bargain.

[JeremyS]

3 minutes ago, Highburymark said:

I think it’s the same Takahashi prism that’s in the Tak turret Jeremy - and I agree that’s every bit as good as the Baader Zeiss. I use the turret with my best eyepieces on Saturn, Mars and Jupiter for close detail, and it’s excellent. For £110 or whatever the 1.25” Tak prism costs, it’s a bargain.

Ah yes, if we include turret diagonals, I have to agree the Tak turret is superb. It is indeed the same prism as in the standard diagonal. Lovely.

[JTEC]

19 minutes ago, Second Time Around said:

I wonder if the crossover point for prisms v mirrors has simply been passed down ad infinitum and as a result is now accepted as fact but isn't quite right

Well, the TEC140 is f7.  I asked the TEC opticians which to use and they recommended using a mirror not a prism for exacting work like planetary observation. That said, I’m a tourist these days and, while I do try to optimise my viewing and strive for quality, I couldn’t claim that what I do qualifies as either ‘work’ or ‘exacting’. 🤣 Nor have I ever been troubled by ‘false’ colour when using the prisms.  I do agree with the point regarding mirrors and scatter though - and the presence of dust on surfaces will compromise any meaningful comparison. 

[JeremyS]

Another consideration not often discussed is light path length.

I think a longer light path prism diagonal might be detrimental compared to a shorter one. Why I prefer mirror for 2-inch, but the Tak prism for 1.25-inch fitting.

[Mr Spock]

I used to have a Tak prism. I sold it when I compared it to the T2 Zeiss on Jupiter. The Zeiss was able to tease out finer detail. 

[omo]

There is an article on Baader's site that explains a bit about longevity and more:  baader bbhs properties

I have both T2 Zeiss prism and mirror, i cant see much of a difference between either but i tend to keep the prism for planetary and use the mirror for the bino.

[Don Pensack]

In theory, a mirror diagonal has the advantage over the prism:

--no added CA, especially off axis

--1 reflective surface as opposed to 2 refractive surfaces plus a reflective surface.

--higher reflectivity at off axis angles (refers to uncoated hypotenuse, because silver coating creates equal reflection to a mirror diagonal on that surface)

A multi coated prism should pass 100% at exactly 45°, but the light loss increases rapidly with off axis angles.

--only one surface to figure versus 3

--a prism can interfere with the insertion of some accessories, like a Barlow or a deep-insertion eyepiece (like some 2"/1.25" eyepieces

--light passing through glass encounters inclusions and tints that a mirror will not have a problem with.

 

But, in practice:

--a prism is more likely to be collimated (there are exceptions)

--a prism is more likely to be clean on the reflective surface

--a good prism's refractive surfaces will be multi-coated, yielding a higher transmission than a reflective diagonal

--a prism's surfaces, if well maintained, will last longer than a reflective surface (though dielectric coated mirror diagonals may dispute that, but we don't have a hundred years of data, yet).

--due to refraction, a prism might adjust or correct certain optical errors in a refractor that a mirror cannot.

 

One final note: AstroPhysics, Tele Vue, Stellarvue, TEC, and Vernonscope all sell or recommend mirror-style diagonals.

Think about that.

[globular]

So we’re all agreed. A reflector is better than a refractor.

 

🧨

[StevieDvd]

9 hours ago, globular said:

So we’re all agreed. A reflector is better than a refractor.

 

🧨

On reflection, yes.

[Mr Spock]

So... has anyone used a prism as a secondary on a Newtonian?

[JTEC]

Has anyone experimented with viewing straight through?  I believe the optical design of some scopes - by AstroPhysics?- took account of the fact that a diagonal would be used, but presumably that doesn’t apply to most.  When I’ve tried straight through viewing with the TEC and, most recently with one of the Stellalyra triplets, it’s been my impression that it gave a slight edge to image quality.  After all, the diagonal is only there to save our necks and, except for the situation that Don mentions where a prism can correct optical weaknesses in the scope, a diagonal’s contribution, especially if dirty, can only be in the direction of image degradation.  So, getting the best possible  image quality from your refractor might mean taking up yoga. 

[heliumstar]

Thank you all for helping me spend money Unfortunately BBHS's are out of stock at FLO so I'll order Tak prism (my 4th or 5th lol) and then order BBHS mirror and compare once they are back in stock.

Looking at the report, the lens looks like it's slightly undercorrected so in theory the prism should improve the performance

Edited September 5 by heliumstar

[John]

48 minutes ago, heliumstar said:

Thank you all for helping me spend money Unfortunately BBHS's are out of stock at FLO so I'll order Tak prism (my 4th or 5th lol) and then order BBHS mirror and compare once they are back in stock.

Looking at the report, the lens looks like it's slightly undercorrected so in theory the prism should improve the performance

Very similar test to that which my 130mm F/9.2 LZOS had. I used the AP Maxbright 2 inch mirror diagonal with that one and it worked superbly 🙂