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Time for yet another cry for help when it comes to choosing diagonal. I have read the many similar threads and gathered some knowledge (too many to start linking). I have come some way in my process and now that it is coming to final decisions I would like to hear from the experts. Not many of threads I have read end with the OP returning to deliver some review/verdict of his/her final decision. While I wait for response on some thread where I asked about the result, the eagerness in me forces me to write my own thread. Perhaps some of the people asking these questions before can now answer in my thread as experts!
I have the Nexstar Evolution 9.25 and am currently using the stock diagonal. My eyepieces are the Baader 8-24 mm zoom and the stock 40 mm Plössl. I would also like to upgrade EPs and there I'm looking at something better in 24 mm range, as well some nice low power for more FOV. I'm following threads about EPs and SCTs with great interest for this (on CN). Can say that I'm currently leaning towards the 1.25" 24 mm ES 68° and 2" 36 mm Hyperion aspheric (if going 2" route).
I'm thinking 2 alternatives (including a budget alternative for one of them). I'm looking at Baader mainly for ClickLock (and expect good optics):
Baader T2 Zeiss prism with a 1.25" ClickLock EP (T2 part #08) -OR- the 2" prism with 2" ClickLock (splurging that is). The budget alternative would be to get the non-Zeiss T2 prism instead for the 1.25". Worth noting that I would like to get the Celestron f/6.3 Reducer/Corrector. This would be for future purposes of delving into EAA but of course I would use it visually as well (especially if choosing the T2 route). The reasoning for my alternatives:
Go for the 2" Zeiss prism to theoretically get the best of the best in visual terms. I would make better use of the 46 mm baffle tube opening. Theoretically possible to combine with the R/C thanks to relative short light path (although not necessarily needed with 2" EPs). Downside of going to 2" accessories would be the cost, EPs, filters etc., on top of diagonal. Would not be able to spend all these costs at once. Cheaper route with T2 prism (especially the non-Zeiss), not only diagonal but also the other accessories. Cost of the R/C would be comparable to e.g. the 36 mm aspheric and give similar power and FOV with the 24 mm ES, i.e. the 24 mm would act as both. Extra plus is the ClickLock clamp for 1.25" with built in fine focusing not involving the mirror. Downside of knowing that not all light coming out of baffle tube is used. To get the wide FOV (24 mm + R/C) I'm adding glass to the optical train (theoretically not a good thing). I'm leaning towards the T2 as it would be a cheaper diagonal and for EPs I would only need the 24 mm and then the reducer instead of a 30-40 mm, so saving the expense of one EP. Then I would already have the reducer for continuing into EAA. The questions I hope the experts here can help with:
The old reducer vs 2" diagonal question. With R/C and the 24 mm I can get roughly the same mag and FOV as e.g. the 36 mm Hyperion (technically 38 mm vs 36 mm and 68° vs 72°). Also reading good things about the ES 68° and with R/C the EP should behave the same. Am I missing something here? The logics say that the I would lose some contrast with the R/C (not using full opening + adding elements), correct? Possibly flatter fields though (not important now, hopefully the EP threads might tell soon enough). The Zeiss vs non-Zeiss T2? Big differences? I have read a few posts on this so most to get some updated views here (have read that Baader has changed some things over the years). Using the R/C (f/6.3) with these prisms. I know f/7 is mentioned as "the limit" but also remember BillP's test where he was happy down to f/6 with the prisms (in 2014 at least). Perhaps most important: have I missed some other obvious alternative here? Maybe I have forgotten some question here but perhaps for the best as I assume those who have gotten this far are tired of reading now. Thanks for getting here though!
How to know this? I have read that with 65/420 scopes you cannot use 2" diagonals because there isn't enough backfocus.
By Bino Pete
Celestron XLT 2" diagonal
Includes 2" - 1.25" adapter
The diagonal is in excellent condition
May swap for Baader Hyperion 68 degree 10mm eyepiece or Baader Aspheric eyepiece 31mm or 36mm - these must be in excellent condition with caps etc
Price includes postage (1st class signed for)
Cash payment or fees paid via Paypal.
We've just discovered that the Bresser Skylux 70/700 that Santa brought to my daughter is missing one piece: the prism/diagonal. My husband went to the shop (supermarket) where the telescope was bought at a very convenient price (like 50% discount), but at this time we fear there will be no other telescopes left, so we'll probably have to options:
- Get the refund for the telescope and save some money for another telescope, at a higher price
- Stick with the telescope and buy the missing piece on eBay or Amazon... maybe we can find a better one which will improve the telescope performance?
We have zero knowledge about lenses/telescopes, but my daughter is really interested in Astronomy (watching videos and reading about it all day long) and she was really excited when she got her first telescope...
Could you please give some advice?
By Ben the Ignorant
I checked 365astronomy's lowest-cost, made-in-China 2-inch 99% diagonal mirror, just 79€ when most 99-percenters cost at least 120€. I didn't fear any lack of resolution because of the price, when whole chinese telescopes routinely have 1/20 or 1/30 wavefront, a single small excellent mirror is nothing difficult to manufacture. Makers claim either 1/10 or 1/12 wave to remain conservative but the real wavefront in the latest diagonal production is way superior. I needed one for my newly-restored Astro-Professional 80mm f/7 semi-apo.
The nosepiece wobbled a bit, however. I tried to screw it, but it wouldn't move. How can it be both loose, and stuck? I tried harder to screw or unscrew it, but it didn't turn either way. Better open it and see what's wrong from the inside. The side panels were removed without a glitch. This revealed a matte-blackened frame, but the job was not complete, some shiny black spots remained:
Unlike all my other mirror and prism diagonals, the nosepiece and the other barrel are not threaded, they are held by lateral screws hidden in dark tunnels on the side of the frame, which is full-metal, single-block and very sturdy, by the way. The whole thing weighs 396 grams, an average mass for an all-metal elbow. One of the nosepiece screws was worn out, and turned without engaging anything. It's the little one on the Allen key; close to it is a functional screw with intact threads.
By sheer luck I'm a guitar tinkerer as well as a telescope tinkerer, so I had 3mm Allen screws in old guitar saddles, since I substitute non-resonant zinc alloy saddles with resonant steel ones. I installed the flat-tip chrome-plated screw, and solved the issue. However most people would have returned the diagonal, causing hassle to themselves and the seller. Better do a quick check on the gear before shipping it. Aside from that , the service from 365astronomy was courteous and efficient, as always.
I painted everything that's not the optical surface with blackboard paint. Went out to buy a little thing, when I came back the water-based paint was dry. Frame, anti-glare threads in both barrels, black paint for everyone.
The anti-glare threads are no longer shiny, but the foam is still more gray than black. After treating it, absorbtion of light is at the max:
You can see the difference between the painted section, and the part that touches the mirror, which I left as it was because the paint might stick to the mirror in time, and make maintainance annoying. Then I remembered some guitar saddles have V-tip screws that would match the V-notches in the barrels, and replaced all eight of them:
This time tightnening the screws didn't make the nasty noise of rough steel tips biting into aluminum, and giving birth to tiny shards that had to be cleaned away carefully, without pressure at all. Just holding the mirror vertically and touching it with a thick stitching thread. Metal dust would either fall or stick to the thread. Barrels that don't screw into the frame are maybe easier and cheaper to manufacture, but they also allow the top barrel to be oriented freely, so you can place the eyepiece locking screw where it's most ergonomic for you, nice.
During the changes I noticed the mirror is translucent at certain angles, so much so the silicone blobs that glue it to the frame are visible through it. That's not easy to photograph, though, here's the least failed attempt:
There is no metal film on the mirror, just the dielectric layers. Surprised me first, but then it came to my mind that all our interference filters for light pollution or imaging are also mirrors, but selective mirrors, so no wonder. Time to reassemble the diagonal, and test it.
But before that, I realized I had never shown the old-new Astro-Pro, so here it is. The setup, completed by a 34mm/68°, a bargain from First Light Optics, seems to have all its components matched together pretty well:
The contraption at the front is a sliding, tilt-lock (I'll describe it in detail in a future thread) 500 gram counterweight. But it proved too light for the 34mm anvil, I'll make another 700 gram tilt-lock, same as for my triplet. When the finder is in place the need for a heavier weight at the front will be an emergency. The diagonal I expected had the 365astronomy logo, this one bears the Starguider brand, proof we live in the time of clones.
My usual artificial star is flashlight-mounted but fainting batteries have interrupted experiments, so I relied on my bass amp's on/off diode this time (best compact amp I heard among 13 or 14 others, thanks Ampeg!). It has a very intense purplish-white shine that would prove very useful for testing a particular feature related to this thread's title.
But a quick quality test with the Ronchi grating, first. The finding was both reassuring and expected:
Straight sharp lines with a trace of barrel curvature (intrafocal), and no strange color effects, exactly identical to what I saw with the proven GSO diagonal. Excellent. Note the overwhelming blue-indigo-violet light given off by the diode, that makes all refractor testing harsher.
Hey, what was that talk about a hidden secret filter, you ask, that's been bogging you since you read the title of this thread? You see, price was not the only thing that interested me in this diagonal. APM of Germany also sells a clone of it, however they display something others don't, a reflectivity chart:
Reflectivity of the Zenitspiegel, zenith mirror with blaue Seitenteile, blue sideplates. Another rebrand. Unlike other, more costly dielectrics with 99% efficiency over the whole spectrum and a flat reflectivity diagram, this one absorbs some violet, it has dips in the beginning of the curve. And my semi-apo has some unfocused violet light that might as well not reach my eye. That 400 nanometer to 435-440 nanometer range semi-apos have a tough time with.
We'll make a cheaper dielectric layering, but since it can't be efficient over all colors, we'll taylor it so the inefficiency sits in the violet part, the engineers surely said. I knew the effect would be present, but to what extent? Would it be beneficial or just detectable? Enter the super-violet amp diode. Aimed at with a TS/Nirvana/William 4mm/82° clone.
Intrafocal defocus, 2 turns of the small wheel in the two-speed focuser:
Left: filtering dielectric, right: full-spectrum dielectric.
Left: Outer halo is thinner and bluer. Inner disk is extremely pale pink. Outer ring of the disk shows. Right: Outer halo is broader and more violet. Inner disk is pale purple. Outer ring is washed out. The extra defocused light reduces contrast.
The first pattern is preferable because it resembles a "white" full-apo a tad more; the whiter the pattern (paler colors), the better. The less difference between disk color and halo color, the better. In achromats the disk is hard lime green, and the halo is hard violet, strongly differing colors.
Intrafocal defocus, 3 turns of the small wheel in the the two-speed focuser:
These patterns are more readable thanks to the larger defocusing.
Left: violet-absorbing dielectric, right: full-spectrum unit.
Left: overall disk is pale indigo, two rings are distinct inside the halo. Right: overall color is pale violet, only one ring can be made out inside the halo. Some contrast is lost to the blurred violet light spread over the pattern.
All photos done at 400 ISO, no treatment, not even a crop. Visually, either in focus or outside of it, the effect is the same as in these shots. Violet is fainter. I looked at targets as harsh as the superviolet amp diode: mercury vapor lamps over a parking lot. The violet halos appeared a bit larger and in a bolder tone with the full-spectrum (and 50% more expensive) GSO mirror. Switching to the Starguider, mercury lamps were surrounded by a tighter region of blurred light, and it was more pale indigo than true violet.
The purple in defocused images was fainter, too, either outside or inside of focus. The patterns resembled less those of achromats. Try viewing the original images by clicking on these pics, constrast will be neater, and colors will be truer. The filtering quality of the diagonal might not seem much, but bear in mind the eye sees variances better than my phone camera does, and the targets were overloaded with purple light. Sorry, no Moon, no white Sun and no stars available for a real-world test. Only skies so overcast, a ceiling so low, the colors of lamps underneath paint the clouds. Pale green over a stadium, orange over the industrial.
Turbulence was supernaturally absent at the onset of the nightscape testing, at 140x I didn't comprehend what I was seeing cause the image didn't have the slightest tremor, but things were brought so much closer. Normally such enlarged things shimmer a lot but this time around they were still, something I always associate with small magnification. I was confused for seeing opposite things in the same time. Some time later a more "normal", but still very slight tremor presented itself.
The only problem I found in this secretly filtering diagonal is they don't advertise it as such. It might not be as efficient as a Baader Semi-Apo filter, but its reduced reflectivity makes it cost less, and makes it double-duty as a chromatism filter. Achromat and semi-apo owners should be told that.
While comparing chromatism between the new diagonal and the proven GSO, I also checked sharpness, brightness and contrast (outside of these chromatic effects). At times one seemed to have the minutest edge, other times the contender seemed a shade superior. When this occurs it means both are equal. They charge 1/3 less for the Starguider-365astro, though. Catadioptrics also have a little chromatism arising from the single "lens" acting as the corrector plate. I wonder if this discreetly filtering diagonal could help there, too; no one would put a Fringe-Killer or Semi-apo filter in their catadioptric, they would filter out too much good light.
Who knows if this affordable mirror could improve contrast there? How about you people experiment and research a bit? I have done my part.