Reflecting on 3 Diagonals and my wallet

[DaveGibbons]

Hi All ,

This will be a fairly brief comparison of 3 different diagonals I have had the opportunity to test in the same telescope over 2 nights and 5 hours observing . It is purely my opinion on what my eye's tell me whilst viewing a range of objects . It is not scientific or technical in any way and I was not intending to write this but thought someone may find it of interest!

I recently purchased an as new Skymax pro 150 Maksutov . Technically second hand but it had never been out of the box in anger and never mounted or viewed through . It is the latest version with standard SCT threads on back along with the natty green dovetail. I always find “first light” exciting as all is about to be revealed . Will I be disappointed ? Will this particular set of optics be up to scratch? Will it do what is says on the tin?

In particular I was keen to try it out with my Altair lightwave 2” sct fit 99% reflectivity diagonal. As we all all know the standard mirror diagonal is not the best , right? Many people have commented on this over a number of Astro forums and it is often stated as the first “upgrade” you must do.

So out goes the scope about 5pm..Out of centrally heated house into near zero temperature . Figure minimum hour's cool down with this type of scope (it took over 2 hr's).

Anyway first target I view is the double double using Altair diagonal. This is after an hour and I can still see the heat plume giving that distinctive wedge shape in out of focus intra and extra focal pattern although they are split well and I can already see despite disturbed view that the optics are excellent . I go back out at 8pm and all vestiges of temp difference gone , the scope is in perfect equilibrium now , so I can evaluate the optics. They are superb absolute textbook diffraction patterns on all four components and as always the Maksutov optics excel on double stars , time to chase down more and more difficult pairings. Delta Cygni next followed by the more challenging Lambda Cygni a sterner test. I am now at the limit of seeing condition and can split this sub arc second pairing but the fainter component is going in and out of that elongated smear you get when conditions are not perfect .

I decide to just check with a straight through configuration that my Lightwave diagonal is as good as it looks and although requiring a contortionist pose I screw back on the original visual back and put in 1.25” adapter and 6mm orthoscopic eyepiece . Yup that diagonal is converting every photon and delivers all to my eye with absolutely no difference in view to straight through configuration . As the original visual back is now on I decided to use my William optics push fit dielectric just to see if any difference. I use the same reference doubles and pleased to say absolutely no difference in optical performance .It does sit slightly further back from the scope than the screw fit SCT but no issue with either.

I now change targets and start looking at some of the tighter open clusters in Cygnus and Cassiopeia .The double cluster is just clearing tall trees and although all can't fit in the Mak's smaller field of view the stars are just pin prick jewels the deep reds of the old giant stars so beautiful , they are apo like. Anyway I am now starting to get cold so pop in for a cuppa and in my now cold conservatory sits the unloved original supplied diagonal. I decide to see just how much better my premium diagonals are so shove it outside for half hour whilst I get a warm.

Back out and straight on it Lambda Cygni . The view is exactly the same ...no it's slightly better the fainter component occasionally showing as a star not a smear a clean split. Assuming seeing has improved I put William optics diagonal back in and yes it presents me with exactly the same quality of image and I mean exactly the same . I now go on a tour of all those objects previously viewed with the premium diagonals , not one iota of difference ! Nothing in terms of contrast, colour or resolution . My mind and wallet tell be there must be a difference, perhaps the good but less than perfect seeing is a leveler ? By now cold has got to me and early work in morning, session over.

Next night is clear as a bell and I leave the scope and accessories in the unheated conservatory . All back out in garden at 4.30pm and start observing at 7.30 . Seeing is almost perfect . 3 hours later I can say without any shadow of doubt there is absolutely no discernible difference in optical performance not one iota visually  on double stars, open clusters and Messier objects . I myself have had some really crappy quality diagonals with new scopes(mostly Meade and Celestron) but the OEM one that came with the 150 pro is excellent . Either I got lucky or Skywatcher have improved quality or perhaps the premium dielectric examples are a bit smoke n mirrors who knows ? Expensive means better right? Certainly aesthetically they are beautiful but I also rather like the rugged utilitarian look of the Skywatcher.

Like I said at the start not very scientific and it may just be a case of my eyes not being able to see the minute differences between optical surfaces or maybe just maybe we convince ourselves expensive must be better . Blind testing anyone !

Cheers

Dave.

[alan potts]

I convinced myself expensive is best, though they do have much better build quality. I have 3 TeleVue diagonals and some time after bought a WO at about a 1/3 of the price, cant tell them apart if I am honest.

Alan

[John]

Very interesting review Dave

I too find it very difficult / impossible to see differences in performance between competant and premium diagonals while others report very clear (to them) differences.

Maybe the giant planets will provide more obvious differences when they are better placed ?

[Ruud]

Thank you Dave, for sharing your impressions. You are no doubt right! Most diagonals perform equally well.

There's good reason to expect why this is the case. Suppose your old diagonal has 95% reflection and is otherwise good. You will see no perceptible gain from moving to a 99% diagonal. The gain will be 2.5 x log(99/95) = 0.045 magnitudes. You'll not notice that.

Mirror flatness isn't all that important either. The light of each part of the view reflects off only a fraction of the mirror. Here is the light of one star reflecting of a diagonal that is far from flat:

                                                     undulated diagonal surface

In practice, if a mirror is 1/10th wave overall, it is 1/40th wave effectively when it comes to reflecting the light cone of a star. Even reflecting the light cone of a planet involves only a minor portion of the mirror, though it will be larger than that of a star and some minor image degradation may result from this. Nothing much to notice here either.

What matters more than extreme reflectivity or extreme flatness is the roughness of the diagonal. The smoother, the less scatter. You will notice scattered light as halos around bright stars and planets. Microscopic scratches from cleaning are a source of roughness on mirrors.

I bought my first replacement diagonal to take the place of an old aluminium enhanced (overcoated) Televue diagonal. Twenty years of cleaning had taken its toll on its surface. There was a small dull spot to the side. I got a dielectric one because that can withstand casual cleaning without getting scratched. It's wonderful for that reason! It will not turn rough over time an although theoretically a dielectric mirror starts off with a tiny bit more scatter than an especially smooth overcoated aluminium one, in the long run a dielectric mirror is a better choice. (That's a personal opinion, I know, but  star diagonals do require regular cleaning and I want cleaning to be fast and easy without worry.)

When I got a Nexstar 6SE, I replaced it's diagonal as soon as I could. It suffered from an enormous amount of scatter. Very poor polishing  on that one! Flatness was okay. The dielectric replacement was an enormous improvement!

[johninderby]

I’ve owned an Astrophysics mirror diagonal and other than the build quality couldn’t tell the difference between it and a WO or a Revelation quartz. I have however seen a difference between the previously mentioned diagonals and one of the lower priced dielectric diagonals such as the Skywatcher which have a bit less contrast.

Have been trying out a Baader T2 prism in my little 127 Bresser mak and while it is excellent not much if any real difference to a mirror diagonal. Maybe I’ll change my mind after further use. 🤔

Edited December 16, 2019 by johninderby

[Highburymark]

The only time I saw a really significant difference was replacing a stock Celestron C6 diagonal with one of the dielectrics - WO i think. Huge improvement. Now I tend to use Baaders for T2 connectivity but like others don’t see any major difference.

But as mentioned in your report Dave, perhaps Skywatcher is including better diagonals with its larger Maks these days? 

[Mr Spock]

I use a Revelation Quartz Dielectric 2 inch Diagonal and can't see any difference between that and more expensive ones. 

[Carbon Brush]

Thank you for posting. It is a very good report.

Although I have not made thorough comparisons in the way you have, your observations are broadly in line with mine.
As a general rule, I have found the weight (or hopefully lack of) and quality of fitting (compression band etc) tend to be more obvious than the optical differences.

Maybe you did get lucky with the standard diagonal. Scope manufacturers have historically tended to tell us very little about about performance specification.
This allows them to turn out packages with good/mediocre/poor components and it can be the luck of the draw.

David.

 

 

[F15Rules]

Great write up, Dave.

I too came to very similar conclusions several years ago..any differences I could/can perceive are pretty small.

My own personal preference these days is for prism diagonals rather than mirrors, as they do seem to me to offer a little less light scatter and improved contrast - I've always felt that good prisms have shown me the blackest backgrounds - but I have no way of proving it scientifically.

I currently have a Baader T2 prism and an older unbranded (I believe Japanese) prism. I find it pretty much impossible to perceive any real difference in the views. But the build and feel of the Baader is what really separates the two.

I also have a very nice 2" WO Enhanced mirror diagonal (as it happens I got this from Dave, the author of the above report), and having previously owned a top of the range WO 2" Durabright Dielectric, I honestly prefer the so called "lower end" Enhanced version..go figure, as our US cousins might say!

Thanks again for the write up Dave, I suspect you have helped to reassure a good few fellow Stargazers with your observations!

Dave

[Alan White]

Very interesting to read your findings, these mirror my own observations over the years and varying diagonals.

My worse ones, were much older cheapies that came with poorer quality kit than I now purchase.

I at present have a WO and a TeleVue diagonal and performance wise can see no difference, build wise the TeleVue has the edge as it should do based on cost.

A while back my SW ED Pro scopes came with nice and well performing SW diagonals.