The secret hidden filter

[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. 

[John]

Interesting article but doing that work on the diagonal has invalidated the warranty and I wonder if 365 Astronomy would have liked to know that one of their products has some faults so that they could take it up with their supplier ?

What you are saying in effect is that this diagonal seems to have a decent mirror but contained in a housing which was not adequately internally blackened and where the 2" nosepiece was not securely fastened into the body of the diagonal. If I was the supplier I think I'd want to know about these issues 

Which one is it on the 365 Astronomy website ? - the lowest cost "99%" one that I can see is this one but it does not look like yours:

https://www.365astronomy.com/365Astronomy-2-99-Dielectric-Deluxe-Mirror-Diagonal.html

I've not come across a diagonal mirror that is partially translucent before but maybe thats the way they are these days ?

 

 

 

[Ben the Ignorant]

6 hours ago, John said:

Interesting article but doing that work on the diagonal has invalidated the warranty and I wonder if 365 Astronomy would have liked to know that one of their products has some faults so that they could take it up with their supplier ?

What you are saying in effect is that this diagonal seems to have a decent mirror but contained in a housing which was not adequately internally blackened and where the 2" nosepiece was not securely fastened into the body of the diagonal. If I was the supplier I think I'd want to know about these issues 

Which one is it on the 365 Astronomy website ? - the lowest cost "99%" one that I can see is this one but it does not look like yours:

https://www.365astronomy.com/365Astronomy-2-99-Dielectric-Deluxe-Mirror-Diagonal.html

I've not come across a diagonal mirror that is partially translucent before but maybe thats the way they are these days ?

I'm not sure this ruins the warranty, because I didn't change anything essential. Besides, I know I will never need the warranty now, nothing can go wrong in this diagonal, it's built like an atomic shelter; I checked it's very good optically and mechanically, inside and out. That's better than someone else's warranty on paper. 

I emailed 365astro to tell them I reviewed their Myriad eyepieces, I can do the same for this. However I'm not sure they will worry about a single screw; it's a non-event to me. Every optic or guitar I buy is completely dismantled, adjusted, and rebuilt, that's standard procedure for me, so I don't make a big deal of little defects I can deal with myself.

The mirror is not decent, it is excellent. I did many side-by-side's with my favorite 2" GSO mirror, and there was no difference except the violet color rendition, as I said in the post. The reference GSO has been compared to three other GSO's, a Sky-Watcher and a William diagonal, and proved as sharp as all the others, except the William unit that proved a little softer. I and two long-term observers could see no change in image quality save for that William (old, from the very first runs, when William quality was not consistent) mirror.

The inside had only a few shiny spots, seen on the photos, they were really no big deal either. The nosepiece was secured, three out of four screws held it strongly, but it jiggled a little bit, I didn't even notice at first. I'm not one for complaining or blaming. When my achromat was misaligned I didn't return it or complained, I added centering screw to the cell. 

You see, I am true caveman, I pick things up and work on them with whatever I have at my disposal. I don't complain if a rock is round, I bang it against another rock to sharpen it, and I make a knife. This saves me a lot of money.

Yes, the diagonal you show is the one I selected, but 365astro sent a clone. Because of the low price I knew I would get the mirror with the "imperfect" light curve, no problem. I think I prefer the look of the blue plates, anyway.

And come to think of it, if this diagonal was plated with aluminum or silver, it could not reject the violet, so a fully-dielectric, non-metallic surface was the right choice. I hope more achro and semi-apo owners will notice this, I just observed some tough targets with the 34/68 Maxvision, dark electric poles, dark branches against a white overcast sky, and I did see slightly less violet, in and out of focus. The effect is subtle, but coupled with some eyepieces like the 9mm Myriad, which reduce or don't worsen chromatism, it should be worth it, especially when you pay less for the benefit.

I'm gonna test that Myriad 9 and new mirror combination now; I had planned to do it, but forgot.