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Three diagonals - which will triumph?


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Today I became the owner of three diagonals after a cheap Celestron 1.25” prism diagonal joined my TS dielectric diagonal and a SkyWatcher starter diagonal that came with a Skymax 102 (both 1.25” diagonals). 

I researched the Celestron online before buying it, and learnt that it is at least as good as a Zeiss, but it also covers the view in blue chromatic aberration and gives a soft image. It has very little scatter, except when it has ridiculously large amounts of scatter!

I got the Celestron unit to see if scatter is possibly lower with a prism, and because of the potentially beneficial refractive properties of prisms. Prisms increase spherical aberration, so if a scope is undercorrected it might benefit from a prism. Also prisms push color error to the blue end, so if a scope is poorly corrected in red, the prism can again offer some improvement. Of course, a perfect telescope can only suffer from these prism aberrations (corrections?), but who has a perfect telescope? Tak owners, put your hands down.

I don’t have a clue if my Zenithstar 66 is undercorrected, and I have never seen any color error, but it’s not a very expensive experiment to conduct. 

I do not like the SkyWatcher diagonal - it disassembles far too easily and the thumbscrews collide with wider eyepieces as they are too near the top of the barrel and the screw heads are the wrong shape. The Celestron is quite similar with the same troublesome thumb screws, but feels a little more solid… maybe it is just heavier. At least it doesn’t disassemble. The coatings on the Celestron’s prism do the job - I needed to use a flashlight to prove to myself it really was a prism and not a mirror diagonal. 

The TS is chunky and looks and feels like a solid, quality unit. But some of the money spent on the diagonal has gone into styling it like a solid, quality unit… Hopefully most of the cost went towards a spectacularly good mirror?

My testing strategy is Lyra, Lra, Lyra. The ZS66 only splits one of the pairs of the Double Double with a 3mm eyepiece and the TS diagonal. Will the Celestron do any better? And will the SkyWatcher do worse.? My money is on no observable differences between all three, and I will probably get the diagonals muddled in the dark anyway. By the way, if my back holds up, and I can reach focus, I will also try the straight-through view too, so there is a baseline of a “100% Strehl diagonal”.

Staying with Lyra, I’ll look at the scatter around Vega with my most forensic and least scattery eyepiece, a Vixen SLV 6mm.

The Ring Nebula provides a chance to see if faint fuzzy objects appear any different in the SLV 6mm with each diagonal.

Finally, can any of the diagonals reveal Polaris B - another double that escapes my ZS66 under my city skies? The B component should be in range of the ZS66 - it’s not that faint - but I have never had more than hints. 

Clear (but not dark) skies are predicted tonight, so hopefully the answers will soon be in!

LATER, MUCH LATER…

Well, I have had a surprising and eye-opening night.

The first target was the Double Double.

Celestron diagonal - both pairs split, the components appeared white or slightly bluish stars and were pinpoints - I immediately feel I haven’t had a view this good in a long time.

SkyWatcher diagonal - both pairs split, they however appear blue-green; they are pinpoints with a little bit of scatter. Not bad. So why has this double been double trouble this year?

Finally my “fancy” TS diagonal I always use - no sharp focus point, blue-green stars , only one pair splits and a bad split at that, stars are messy, not pinpoint. Who would have thought - my special dielectric bought as a welcome gift for my Zenithstar has been sabotaging views for months?

Celestron clear winner. TS fails hard.

Vega next…

Celestron - clear diffraction rings in 3 mm eyepiece, very diffuse blue haze around the star. Numerous, pinpoint stars in 13.4 mm eyepiece.

SkyWatcher diagonal - subdued diffraction rings at 3 mm (129x), red edge to Vega. Strong blue outside focus, strong red inside focus. When seeing is poor, kaleidoscope around star. Spiky stars at 13.4 mm, less numerous than in Celestron.

TS diagonal - no diffraction rings around Vega which has linear rainbow effect with a line through the star at 3 mm, good bright stars at 13.4 mm however. Is the dielectric mirror cracked????

In the mirror diagonals the red or blue false color was extremely strong just inside and outside focus, but with the prism, this was not observed.

Celestron clear winner. TS fails hard again.

Polaris next. However I only used the Celestron prism. With 3mm eyepiece: vividly yellow primary with diffraction rings, very faint but clearly seen secondary. Confirmed in Stelle Doppie and Stellarium.

I am smiling ear to ear. Finally splitting the Double Double and resolving Polaris B (mag. 9.1) has seemed impossible at times with the Zenithstar. I had always expected more from it, and it looks like I was being held back by my fancy dielectric diagonal. 

Moral of the story: don’t always judge equipment by the price tag or the external stylings!

Edited by Ags
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very interesting read, thanks Ags, might consider getting one of these to try out too.

Wonders if we'll see a sudden spike in demand for the Celestron prisms and a big price hike 😉 

Edited by DaveL59
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Interesting comparison @Ags. I’ve heard of the occasional duff diagonal affecting views adversely, perhaps as you suggest, you have a poor example.

The Zeiss prisms do perform very well, I saw an improvement over a good mirror diagonal with mine, particularly at high power. Sure that would be an upgrade on the Celestron but they aren’t cheap! The standard Baader T2 prisms are likely still very good though, and the T2 connectivity is useful.

I had a WO diagonal which came with my ZS66 and was always able to split the double double with it in reasonable conditions. I occasionally used a 2.5mm Nagler in this scope on Mars (X155) and it coped pretty well, showing some detail at opposition. Nice little scopes 👍

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Can't beat a prism diagonal in a small refractor. Celestron and Vixen units are very good but folk are put off them due to the lightweight build and no compression ring. I use my Vixen 1.25" prism next to a Baader T2 prism diagonal and the views they both give are essentially the same. The Baader is much better built and the T2 system is far more adaptable, 2" or 1.25" nosepiece, clicklock or fine focus eyepiece holder, direct connection of binoviewer etc. But, as far as the views go with lightweight 1.25" eyepieces there really is nothing in it. The Takahashi 1.25" prism is another excellent, lightweight diagonal and of course it has the Tak collet which is great. The only time I switch out to one of my 2" dielectrics is when I want to use a low power, widefield 2" eyepiece, which isn't very often being under bortle7 skies.

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52 minutes ago, Stu said:

I occasionally used a 2.5mm Nagler in this scope on Mars (X155) and it coped pretty well, showing some detail at opposition. Nice little scopes 👍

Last night was the first time I felt I could push the scope down to 3mm (129x) and now I am thinking about an SLV 2.5mm as a starsplitter and Mars eyepiece.

I can't understand how dielectric is producing the multicolored distortions I see. I went back and forth between the diagonals many times to make sure. The difference was however stark and undeniable.

The only issue I had with the Celestron prism was a faint blue halo around Vega; this might be an issue viewing the Moon...

I will do more testing on other targets and different conditions.

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That's a great review and very useful.

The selling point for dielectrics is durability and reflectivity but surface smoothness and low scatter are important also, and even more so for things like doubles.

I've read plenty of reviews of diagonals and often they conclude there's little difference between them but I recently did some back to back tests on some diagonals and the same as yourself I also saw noticeable differences. 

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I’ve read reports from people who love the Celestron diagonal, so I’m not surprised to hear your experience Ags. For what it’s worth, I’ve had a couple that were below the quality of the WO and SW dielectrics I had at the time.   Particularly in transmission. I suspect Celestron may have upgraded the prism in recent years. 

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Thanks Ags, this was very interesting! Clearly it's true what they say that the diagonal is often the weakest link  in the optical path.

I have two Chinese knockoff 'dielectric' diagonals which I suspect are not actually dielectric (there is no colour when shining a torch on them off axis as is expected with most budget dielectrics). While they perform satisfactory in my Maks I often wondered if using  a prism will be an improvement.

So following your example I just ordered the Celestron prism diagonal from Harrison's (sadly FLO does not appear to stock it). It seems a bargain indeed if performs as well as people say. My Maks are F12 and F15 so a prism should work even better than at F6. We shall see...  

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Posted (edited)

I have always wanted to get the Tak, and i think I will at some point. With the TS dielectric likely headed to the bin (can't in good conscience sell it) and not really liking the SkyWatcher freebie, I need at least one more diagonal.

I went for the Celestron over the Tak as I hear it works better (provides stronger correction) in fast semi apos like the Long Perng 90 f5.6 which I have my eye on as a travel / grab-and-go scope. It certainly seems combine well with the ZS66. If it's clear tonight, I want to continue the testing on some planetary nebulae and colored doubles. Color rendition seemed best with the Celestron prism while looking at white and yellow stars, it will be interesting to see how an orange-blue double like Albireo fares.

With the ability to go over 100x with the Celestron prism, some testing on the Moon seems worthwhile too.

Another thing I would like to try is the poor unloved C6 with the prism. I have been blaming the C6 for mediocre views, but it looks like the dielectric is the true culprit.

Edited by Ags
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34 minutes ago, Ags said:

Another thing I would like to try is the poor unloved C6 with the prism. I have been blaming the C6 for mediocre views, but it looks like the dielectric is the true culprit.

I rotated the corrector 180’ on my C6 and the views improved significantly.

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40 minutes ago, Gabby76 said:

have you verified the collimation of the TS

That's a very good point, it doesn't take much movement to put a mirror out of collimation, especially a 2". Whereas with the prism diagonal and this is another plus, the housing is cast at 90deg and the prism sits right in. If it does take a knock then the design on the Celestron allows an easy adjustment. There are two small grubscrews that hold the prism from the sides, loosen these slightly, turn over the diagonal so the prism sits flush against the 90deg housing and re-set the grub screws. Under the rear plate is a metal spring that keeps the prism tensioned against the housing, it can benefit from an occasional tweak to maintain the tension. Once done they keep collimation well, unless you drop them in the dark 😪.

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Posted (edited)

I don't have a laser to test the diagonals, but I can test with my SCT on the next clear night - using a camera, collimate the SCT without diagonal, then check collimation with diagonals. 

Unless I am mistaken, the TS dielectric is uncollimatable.

When switching diagonals I did notice some image shift, so I would guess collimation can be a factor.

Edited by Ags
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13 hours ago, Ags said:

collimate the SCT without diagonal, then check collimation with diagonals. 

One thing to watch out for here is the amount of play between the diagonal nosepiece and the focuser drawtube. However slight, it can have an effect on the apparent collimation of the diagonal, especially with a long focal length system. Self-centering adaptors do help. We'd all like a perfect bulls-eye when collimating our scopes but in Harold Suiters book "Star-testing astronomical telescopes" he sets up an experiment where he compares the views through a) a perfectly collimated system, where the cheshire eyepiece reflections are spot on and b) the same system that he had purposely put out of collimation, where the cheshire eyepiece reflections resembled almost a figure of 8. His findings were that there was no appreciable difference in the views given. The scope used was a 6" ED refractor.

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1 hour ago, Franklin said:

One thing to watch out for here is the amount of play between the diagonal nosepiece and the focuser drawtube

I haven't fitted a focusser to my SCT; I just have a fixed visual back and focus with the primary mirror.

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Posted (edited)

I don't think I am seeing significant collimation issues. All the diagonals put the object within a 4 mm circle, and collimation would show as coma/astigmatism and (in the case of a dielectric) a color change and dimming. But with the TS dielectric I am seeing a softness to the image combined with a rainbow line visible at high powers such as might be produced with a diffraction grating. I wonder if the dielectric layers were not deposited evenly, that might explain it.

Edited by Ags
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