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SkyWatcher’s 20" goto truss dob!

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#251 Arctic eye

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Posted 30 October 2018 - 01:57 PM

Hello. A little update here to my sad saga with the SW 20" dob.

 

So as can be read from this thread, I found my primary mirror to have a bad  print-through problem and made a reclamation to the vender in March. It took then 4 months for the reclamation to proceed, first into the European distributor, and then further into the Synta factory. Some discussions followed and finally, in the beginning of August, I was told that I will get a new primary mirror! I was given an estimation of 6-7 weeks for the mirror to reach me. Now it has been 12 weeks, no mirror yet... Seems I have to wait some more, maybe it's the slowest camel in the silk road that carries my mirror. It's only 7 months passed on my reclamation, I am quite a patient man 4.gif

 

 

Meanwhile, I have had the bright Finnish summer nights and lots of time to prepare the scope for the new mirror (old mirror is already packed, ready for the return). So I decided to build motorized collimation for my scope. I purchased some geared DC motors and small gogged belts & pulleys, capable of delivering 0,6 rpm rotational speed to the collimation screws that I modified from micrometer adjusters. I added stronger springs to the screws, but also left the original screws and spings in Place next to the new ones, to give additional guidance and spring force to the system. I only made motor adjusters for 2 corners, the 3rd corner doesn't need adjustment. I also made a hand controller with 3 meter cord and 9 volt battery inside. So now I have precise and easy adjustment for my empty mirror holder lol.gif

 

Some photos:

Moottoriosat.jpg?img=smaller Ruuvi1.jpg?img=smaller  Moottoriosat3.jpg?img=smaller

 

Moottoriosat4.jpg?img=smaller  Coll1.jpg?img=smaller

 

500p4.jpg?img=smaller Coll2.jpg?img=smaller


Edited by Arctic eye, 30 October 2018 - 02:00 PM.

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#252 Arctic eye

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Posted 27 November 2018 - 04:44 PM

Today, after more than 8 months, I received the new replacement mirror for my 500P!   banjodance.gif elephant.gif hamsterdance.gif jump.gif snoopy2.gif whee.gif

 

The dealer I bought the scope from, disappeared two months ago, nobody seems to be able to reach him... So I managed to reach the European distributor directly, who hadn't been able to reach him either. Because of that the mirror had been stuck in the distributor in UK for several weeks. I sent the old mirror directly to UK and now I have the new mirror. 

 

I was happy to see that the new mirror is of the newest design with the main plate been done from two separate glass plates glued together. Let's see how the star test is going to look like. Of course it immediately turned into cloudy weather, so need to wait, but I'm already used to that. And maybe clouds is a good sign!

 

 

a1.jpg?img=full

 

a4.jpg?img=full

 

a3.jpg?img=full


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#253 Kunama

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Posted 27 November 2018 - 05:59 PM

Toivottavasti tällä onnistuu waytogo.gif


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#254 Arctic eye

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Posted 06 December 2018 - 04:49 PM

Finally I had some clear weather for star testing the new mirror. Seeing wasn't great, but it was good enough, especially when using stacking, to reveal the optical quality of the new mirror. Unfortunately the new mirror is even worse than the original mirror!!! There is at least equally strong print-through and now also astigmatism present. 

 

First night I couldn't believe my eyes! I used 2,5x powermate, ir-pass filter and ASI183MM camera. Stupidly I had the filter in front of the powermate and thought that it might cause some SA. It was -3C outside and I thought that maybe I had too short cooling time for the mirror and that would cause torsions to the mirror structure, thus having the print through visible. I checked visually with 8mm eyepiece and stars looked like hairballs…

 

Next night I ensured longer cooling time. It was -2C and the scope was carried out from an already semi-cold storage. This time I used green interference filter in front of the camera. Same results; print-through and astigmatism. I used about 10 waves defocus, the secondary mirror CO is 25%. Below are images from the second night.

 

Third night the scope had been out for 24 hours and ambient had been steady -2C fro the whole time; Mirror was cooled down evenly. To make sure it was nothing with my imaging train, I first star tested visually to Capella with 6,5mm orthoscopic ep. I know, not optimal, but the two bright print-through rings, with 12 brightenings each, were strikingly visible around 10 waves defocused Capella, making it look like a brass clock gear wheel! Have never seen anything like that. When defocused more, Capella looked like pizza sliced to 12 pieces! Then put the imaging train on, with similar results as nights before.

 

I wanted to exclude all other possible causes, so I examined for possible collisions or tensions within optical train - none. The secondary is also of cellular structure (but with 10 ribs), so I took it off and replaced with the traditional secondary from my SW400P scope. It is smaller, and it cutted off some light, but still the same print-through and astigmatism visible there.

 

Seriously Synta? After 8 months you sent me this? Have anybody else star-tested their 500P:s? At least I haven't seen any images. Or am I the only one using these below 10C temperatures and these only work in room temperatures? I don't think so, unless it's made from two different glass types, which would be stupid. Anyway, seems that I have to order different kind of mirror (conical back) from different supplier now, because Synta seems dead-end. Still would like to see a star-test image from someone else who owns 500P.

 

 

 

 

 

SW500P-Capella-10w.jpg?img=full

 

 

SW500P-Capella-18w.jpg?img=full

 

 

SW500P-capella-10w-singleframes.jpg?img=

 

 

Capella_10w.gif?img=full


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#255 macfly51

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Posted 06 December 2018 - 06:04 PM

Hi,

 

Are you sure a tube does not touch the mirror?

 

The night it happened to me, the pictures were catastrophic!



#256 Arctic eye

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Posted 07 December 2018 - 11:39 AM

Hi,

 

Are you sure a tube does not touch the mirror?

 

The night it happened to me, the pictures were catastrophic!

Yes, I checked, no contacs anywhere. And the center shaft nuts are not contacting the flange of the shaft that is against the glass.  Nothing whatsoever.

 

Macfly51, you have the same scope? Have you done a star test?  Used the scope near 0C temperatures?



#257 IVM

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Posted 07 December 2018 - 01:32 PM

Ari, I too would be interested to hear more from Macfly51. In the meantime, let me offer this info and opinion. Earlier this week I was at our remote site with it, and it was -8C. The lowest so far was about -18C, and on that night I was going after very faint, almost stellar objects in M81, no problem (the seeing was excellent). I don't see any astigmatism in your last images - maybe a bit of tube current (hypothetically). Also it's nonobvious to me that the very moderate out-of-focus 12-fold spikiness impacts the in-focus pattern appreciably. In my experience it doesn't, but I don't do planetary imaging. It's easy to believe that a given 20-inch can be worse in that application than the given 16-inch. However, F/4 ("F/3.95" as SW calls it) can strain the performance of the Barlow - this is also something to consider. My typical in-focus pattern is like in your post from April 11 (where, I assume, the middle is a bit overexposed for presentation and so the broken rings are brighter than they really are relative to the Airy disk). Considering how tiny the whole pattern is (we are talking 20" of aperture after all), I think it is quite decent in practical terms. Again, for 20 inches. I should note that a degree of steady, unequal triangularity noticeable in this the pattern is indicative - probably - of the pressure not being equally supported during the figuring by the 12 ribs of the sandwich, but a) this is to be expected in practice and b) I think we are really splitting hairs here. I am delighted when the seeing is so good that this may be visible, and then look at the objects, not the pattern. I like the idea of this mirror and deem its execution quite acceptable in my sample, but the structure in this scope is even better. If you get a center-supported super-premium mirror for it, it might be an absolute killer. Also it seems easy enough to put in a more traditional cell and a solid thin mirror in there. I wouldn't do it for visual deep-sky, which is what I do and what I think the scope was designed for, but maybe this is what will meet the requirements of your more ambitious application. Clear skies to you!


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#258 Axunator

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Posted 07 December 2018 - 03:21 PM

I don't see any astigmatism in your last images - maybe a bit of tube current (hypothetically).


Ari’s star test images are textbook examples of astigmatism - eccentricity of the diffraction pattern that flips 90 degrees on the other side of focus.

Considering the hardship he has had with his quite expensive scope and now the second mirror, I can only admire the calm analytical tone he manages to maintain in his posts...

#259 macfly51

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Posted 07 December 2018 - 06:37 PM

Hi, 

 

Indeed, I am a happy owner :)

 

Certainly next to my friends who have 600 carved by artisans, the image is a little less stitched, but for a price 3x higher here in France ....

 

Also, thanks to a laser thermometer, I had fun watching the temperature drop of our mirrors here when it is less than 0° ... the Stargate descent significantly faster than slabs 50mm thick!


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#260 dgoldb

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Posted 07 December 2018 - 07:25 PM

This is truly sad and pathetic.  That mirror appears very poor.  I am very sorry for you, OP.  Rest assured you have saved many CN'ers from buying this scope in its current iteration.  


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#261 IVM

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Posted 07 December 2018 - 07:43 PM

OP was about the manufacturer's Australian press-release in 2015 ;) This is one of the longest-running telescope-specific threads.



#262 a__l

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Posted 08 December 2018 - 01:16 AM

https://www.youtube....h?v=xu_ce46wUHw

 

With a similar mirror. There is a test in autocollimation.



#263 Arctic eye

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Posted 08 December 2018 - 07:29 AM

My typical in-focus pattern is like in your post from April 11 (where, I assume, the middle is a bit overexposed for presentation and so the broken rings are brighter than they really are relative to the Airy disk).

Actually not overexposed, in both of the mirrors I have had, the pattern is really bright relative to the airy disc. But propably there is a variety of quality of these mirrors out there, and hopefully I have already hoovered the worst units out of the market now. And also my application is extreme, up to lower-high powers at least the first mirror was good enough. this latter mirror I suspect. 

 

Without any barlows, only the eyepice, a little defocused Capella looked like this: Capella-visual-defocus.jpg?img=full

 

 

It would still be valuable to understand, whether my mirrors represent the average quality of these mirrors, and they are just not ment for planetary applications, or if I actually just have two extreme examples (+ 3rd one in French review)? Especially because my latter mirror represents the latest design of the mirror. I have noticed that in both of my mirrors, the smaller back glass disc of the structure is waved, like it had been pressed against the ribs in the bracing stage. If there were residual stresses, that could distort the whole structure and pass that to the optical Surface through the thin glue? So, to you other owners, is your back plate waved also?

 

Like this:

backplate1.jpg?img=full

 

 

Right now I am thinking that if the optical thin disc was actually made correctly, and the problems come from glueing it hardly against the other plate, that is distorted by the structure, could I somehow dissolve the glue, separate the plates, and then glue them back together with 100 drops of silicone or something? Quite radical, but I have little to loose with this mirror .

 

In the YouTube video I dont see the 12 ribs pattern in the test. I will buy a lottery ticket tonight.


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#264 Darren Drake

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Posted 08 December 2018 - 09:03 AM

If I remember correctly there was a review of one of these in S&T I believe? a while back and there was an actual foucault image that showed no print through.   So these can and have been made it appears to a reasonably high level of quality....



#265 IVM

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Posted 08 December 2018 - 11:57 AM

Indeed the back plate is rather strongly waved. This is an unambiguous indication that SW meant it when they said it was "fused", not glued. The plate evidently was very hot when this was done.

 

Ari, in your place (just chatting as between two owners of a still pretty rare scope, not actually dispensing advice), I would ask specifically what in the in-focus pattern is unacceptable. I'd call it unacceptable if the resolution (i.e. the overall size of the bright part of the pattern) would be decidedly worse than the theoretical ideal for half the aperture.

 

If I were a planetary imager trying to upgrade from 16", I'd naturally want a higher resolution than the last scope, but this is luck of the draw if the optics you are buying are not advertised with individual test results. Since the difference between 16 and 20 in resolution is even ideally only 5/4, the informative test would be "other things being equal", and one worry here is what happens to the performance of the Barlow/teleextender when you compress your objective F-ratio. Not being an expert, I would nonetheless expect the normal good Barlow to start falling apart optically, for critical applications, at around this focal ratio. Your April in-focus pattern looked fine to me for 20" out under the stars, assuming it's size was what it seemed. Your new in-focus pattern is stacked, so it's hard for me to judge, but the only notable feature in it seems to be a bit of a thermal "lip", not necessarily from the mirror or the shroud tube.

 

I don't specifically test my scopes but I work with (non-astronomical) research optics and so I judge on the fly if the optical performance is reasonable for what the optics is supposed to be and for the task at hand. With this in mind, I think a) in the year that I've had this scope we haven't had enough of good seeing for a verdict on the following but b) it may not be better than my old 16-inch (which has Meade optics), but it is not notably worse either, speaking strictly of the resolution. There is, however, an obvious added "strain" in the eyepiece-end optics (oculars with or without a Paracorr) due to the F/4 versus F/4.5 on the old scope. For what I do, the new scope is a vast improvement over the old because of the light gathered.

 

Do let us know how it develops with your 20. Frankly if I did planetary imaging and tried the 20 and it worked, for whichever reason, worse than my existing 16, I'd try to send it back. If it were merely a tool, that is. The Stargate is surely fun to play with, seems a very modifiable structure too, better designed and executed than a 20 really needs in most conditions. I've gone so far as to contemplate putting a significantly larger mirror on it.



#266 Arctic eye

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Posted 08 December 2018 - 02:09 PM

There is also this test that had a very good sample in it: http://r2.astro-fore...508-1988-r-3976

 

So it surely can be good. And I hoped they had been selecting a good one for me as a warranty replacement, but unfortunately it turned out differently.

 

 
I have a 12" SW, 16" SW and this 20". The original 20" mirror produced worse images than the 16" when compared side to side. I estimate the original mirror to have been performing somewhere near what the 12" does, or little less. This new one, with added astigmatism, propably goes somewhere between 8" and 10" class. No use for me.

 

I've gone so far as to contemplate putting a significantly larger mirror on it.

 

My vendor has disappeared and I have contacted the U.K. distributor directly again to see if there is something that can be done about this. Based on the fact that it took 8 months with the previous round, I'm not holding my breath. If I'm left on my own, this is exactly what I am now turning into considering of; A 24" conical back mirror would fit easily with some modifications, but propably can't find one. A 24" traditional quality mirror would need some more modifications, but I think it could be done also.

 

However this turns out, it will turn out good! Either I get a good replacement mirror eventually, or I can return the scope and purchase a bigger and better scope instead, or I purchase a larger Premium mirror and modify the scope into a bigger one (and have the current mirror as a very expensive sandwich tray with a story to tell). This is hobby, you know.

 

 

 

 

 



#267 a__l

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Posted 08 December 2018 - 09:24 PM

There is also this test that had a very good sample in it: http://r2.astro-fore...508-1988-r-3976

 

So it surely can be good. And I hoped they had been selecting a good one for me as a warranty replacement, but unfortunately it turned out differently.

 

 

Strange report. 

One side Strehl 0.93, the other side quote: 

 

"Von einem 20-inch "Licht-Eimer" darf man nicht zuviel verlangen: Neben einer ausreichend langen Temperatur-Anpassungvon von ca. 3 Stunden"

 

0.93 it's a good mirror, it's not "Licht-Eimer"

This mirror has a curious fixation, glued glass parts and astigmatism is turned off in the test?

Also spherical 0,35 (1/3 ~) but PV 1/7~ and Strehl 0.93

 

There is a question for what purpose this report was made.


Edited by a__l, 08 December 2018 - 09:27 PM.


#268 IVM

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Posted 08 December 2018 - 10:27 PM

Not glued - fused, as has been discussed many times in this long thread, and again just now ;) I too found these results to be all over the place. I think this has to do with the extreme sag. This mirror is actually pretty heavy overall, but it is designed to be suspended on the central support only and has a thin outer edge. They seem to have set it down on the edge and the radial symmetry was completely lost. To properly "turn off" astigmatism, I believe, some semblance of radial symmetry must be physically preserved by the mirror on the bench. Otherwise you can get all sorts of meaningless numbers after running this software. Those who actually test optics: don't judge me too harshly for this supposition.



#269 IVM

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Posted 09 December 2018 - 05:45 PM

In comparison, the Ukrainian magazine (Youtube link in a post just above - the same one was near the beginning of this thread) tested it correctly from the standpoint of the physical setup: the entire "mirror box" was mounted onto the test bench (9:35 into the video). And generally, those folks were very thorough; they and their audience might be amateurs in astronomy, but it sounds as if it were a review by those with engineering degrees for those with engineering degrees.



#270 Jason_J

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Posted 13 December 2018 - 05:54 AM

Different types of glass expand/contract at different rates. The glass used in the mirror may be a different type to the glass in the 12 point support frame. When operating at extreme temperatures, as you are Ari, of below 0 degrees, the different contraction rates in the fame may distort the shape of your mirror. My guess is the lab test were all done at comfortable room temperatures, hence the design fault does not appear.



#271 Arctic eye

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Posted 13 December 2018 - 12:06 PM

This is something that also many of my local astronomy collegues has been proposing. That they have tried to cut costs at Synta, and used cheaper glass for the support structure.

 

I can’t see any tint difference between the parts, actually I dont think I can see any kind of a tint. If there wasn’t this aluminium shaft of unknown size within the mirror, glued by unknown amount of silicone-styrofoam mixture, I could have measured the volume of the mirror by submerging it into water, and then weighted it and calculated the density... Could I maybe check the refraction index by submerging the mirror partially into mineral oil or anhydrous glycerol? Third option would be trying to measure the thermal expansion between -20C and +20C, but need some tools for that.

 

I asked the factory if this mirror should work at low temperatures. Let’s see if they answer. Personally I can’t believe they would be so stupid or shameless in Synta to use two different glass substrates in the same mirror, or would they? At least my first mirror did not show any further drop of the image quality against dropping end temperatures towards the winter (similar image quality in +6C and in -20C).

 

I think the first design of the mirror (my first mirror) was bad, because figuring the surface against the rib structure caused flexing and thus print-through. In the second design (my second mirror) they tried to compensate for that by figuring the paraboloid into a separate thin disk first, and then glueing this in top of the original structure. But somehow they managed to spoil that too; Maybe they press the mirror too hard when glueing, and the glue seems to be extremely thin also, so again the underlaying print-through conducts to the paraboloid when the glue is hardened? But then I would expect the print-through still to be at least partly compensated for, and now it is not. So it is a mystery to me how they succeeded to unsucceed with this second mirror.

 

 

Not glued - fused, as has been discussed many times in this long thread, and again just now wink.gif I

It is both glued and fused.


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#272 Starman1

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Posted 13 December 2018 - 02:19 PM

The Ribs cannot be glued, but the interior support structure could be.

However, though this could cause a general ripple or astigmatism, the gluing of the center piece will not cause a 12 spoke print through.

I could see the spokes appearing in the star image as the mirror cools, only to disappear when the mirror is at the ambient temperature.

Have the tests been done on the mirror when cooled to the ambient temperature?

And, are fans being used?  Even a ribbed design might not track air temperature drop without fans.



#273 Arctic eye

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Posted 13 December 2018 - 03:58 PM

The ribs are fused to the bottom disc and to the top disc. But the top disc is actually two discs glued together (see below). You can see the devider seam at the first image. The glue is the whitish grid at the second and third image. The glued upper part of the top disc is the actual mirror, the fused rib & disc structure acts like a mirror holder only. 

 

a4.jpg?img=smallera5.jpg?img=smallera1.jpg?img=smaller

 

 

Have the tests been done on the mirror when cooled to the ambient temperature?

And, are fans being used?  Even a ribbed design might not track air temperature drop without fans.

Yes, it was fully cooled. At that point, the scope had been out for more than 24 hours at a constant ambient of -2C, which was nice for the test. I had the scope out continuously for three consequent days and nights, temperature was all the time -1C...-4C. And at this time of the year in Finland, day is almost as dark as night… When taken out from +14C to -2C and let cool (no fans), the first 1-2 hours the star test image looks even worse, of course, but then after one more hour it stabilises and then stays like seen in the images. 

 



#274 a__l

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Posted 13 December 2018 - 10:56 PM

Earlier models. No glue. No ribs

 

http://fidgor.ru/Obs...2/test_411.html

 

http://fidgor.ru/Obs...t/test_161.html



#275 Arctic eye

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Posted 14 December 2018 - 04:41 AM

That was a good find, thank you!

That is the exact 16” model I also own and been using for cross-testing against the 20”. Comparison images can be seen earlier in this thread, the 16” wins notably every time. I would imagine my 16” resembles the first test you linked.




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