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SkyWatcher’s 20" goto truss dob!

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#226 Redbetter

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Posted 12 April 2018 - 09:13 PM

If you use a large thin blank then wouldn't it require a rather soft touch to prevent print through?  And wouldn't that mean much longer polishing/figuring times?  Time is money.



#227 Pinbout

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Posted 12 April 2018 - 10:31 PM

I agree, it is not the fact of having the ribs per se.... rather the pressure..... but don't you agree that it would make a fine clock... cool.gif

2p” is to big for a clock but it’s just right for an end table 


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#228 Pinbout

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Posted 12 April 2018 - 10:32 PM

If you use a large thin blank then wouldn't it require a rather soft touch to prevent print through?  And wouldn't that mean much longer polishing/figuring times?  Time is money.

So is redoing it.



#229 Starman1

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Posted 13 April 2018 - 12:27 AM

As the glass heats up in polishing, the ribs retain the heat more easily.  As a result, those sections of the mirror shrink more when the mirror cools,

and get polished off more when the mirror is warm.

To avoid that, shorter polishing periods should be used and lots more testing after the mirror cools to room temp.

That increases the length of time needed to make the mirror and increases cost significantly.


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#230 Redbetter

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Posted 13 April 2018 - 12:42 AM

So is redoing it.

 

My point is that in a production environment the natural tendency is to try to reduce the time to the shortest possible.  In this case it would seem that they went too short by being too aggressive.  So much for the weekly production record...  bawling.gif whistling.gif



#231 starman876

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Posted 13 April 2018 - 06:24 AM

so the reviews from people who really like the scope. I take it these were people who were awed by the size of the scope and had no clue what to look for as what a good mirror should look like?  This can be expected results if one cherry picks the comments from reviewers.  would also be useful if the vendor who claims the optics are really good and who claims they all get tested to provide some tests results.  I am sure they are capable of doing a DPAC.  Let's see some DPAC results.  


Edited by starman876, 13 April 2018 - 06:25 AM.


#232 vhinze

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Posted 13 April 2018 - 07:41 AM

I have seen the light, print through it is.

 

Googling "mirror print through" produced this link:   http://www.dreamcell...rintthrough.htm

 

These guys get around to using the term primary ripple as a synonym (which it is) for print through and go on to talk about its role in producing the symptoms of surface roughness. The term "Mid-Spatial Frequency (MSF) errors" pops up again in the discussion as well. There's the reason for the striking similarity to the figures I keep mentioning.

 

They also show how easily print through can be quantified using a null test, with illustration of an uncorrected and corrected blank. After this fiasco, Skywatcher would be wise to include documentation of a null test on every mirror,

I'm left wondering what they were thinking, if they were indeed testing every mirror, this should have stood out like a sore thumb. "We don't provide such tests, trust us."   Talk about shooting yourself in the foot, Skywatcher may end up ranking right up there with Tasco!

As for polishing pressure, these Dreamcell guys mince no words, prescribing a pressure of 0.25 psi or the weight of a 50 pound tool covering the surface of a 16 inch blank. Quoting them, "This should put the high pressures used by the outside vendor into perspective."

I was also struck by the construction of this company's blanks, with a far more intricate webwork spreading the load bearing over much more of the mirror back-plane. It appears they've paid their dues and done their homework.

 

Suiter, as always,was right, just that ribbed mirror blanks weren't singled out in his treatise.


Edited by vhinze, 13 April 2018 - 09:27 AM.


#233 dgoldb

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Posted 13 April 2018 - 10:36 AM

While unfortunate for the OP, a single sample is too little to base any conclusions about the product / design.  Manufacturing lines optimize over time.  Sounds like OP may have gotten a very early sample.  Quite possible things have improved by now.  I'd say OP should talk about warranty replacement and see if a new mirror is the same.  



#234 vhinze

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Posted 13 April 2018 - 10:50 AM

Hard to see how figuring a mirror wouldn't require a null test as part of the process. Is it really asking for the sun, moon and stars to expect documentation on an $8000 outlay?  I think not.  If the process has indeed been optimized, there's no need to take it on faith.

 

If you'd like more samples, scare up this thread:

 

20" f4 SkyWatcher Stargate GoTo Dob mirror flop                     on this forum.

.


Edited by vhinze, 13 April 2018 - 11:06 AM.


#235 dgoldb

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Posted 13 April 2018 - 11:06 AM

Hard to see how figuring a mirror wouldn't require a null test as part of the process. Is it really asking for the sun, moon and stars to expect documentation on an $8000 outlay?  I think not.  If the process has indeed been optimized, there's nothing to hide.

 

"Quite possible things have improved by now."  Trust us.

 

I know, "if you don't like it, go elsewhere" I did.

I never said "trust us."  I'm not affiliated with skywatcher...  I just said it is too early to draw conclusions about the product in general based on such limited reports.  

 

I don't think its unreasonable to want documentation, but I also understand why they don't.  People obsess over numbers and it would likely cause more trouble than its worth.  Also, tests are fairly easy to read incorrectly or run incorrectly.  It is entirely possible this scope was tested and passed.  Better to rely on reports from users than manufacturer-provided testing.   



#236 vhinze

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Posted 13 April 2018 - 11:30 AM

Yes sir,

 

I understood you weren't shilling for Skywatcher. Sorry I left that impression.  Do me a favor though, and follow the link I posted, look at the null test images illustrating this particular issue and see how an optical degree isn't required to recognize good from bad, no numbers required...

 

Skywatcher could do that if they wanted to quickly put this to bed.


Edited by vhinze, 13 April 2018 - 11:34 AM.


#237 Pinbout

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Posted 13 April 2018 - 12:19 PM

 

Hard to see how figuring a mirror wouldn't require a null test as part of the process

cause we have IF, we have KE with zones or we do KE flip diff, with a lot more zones [like 21] and the overall reading is a lot more sensitive then using a standard 7zones.

 

https://www.youtube....hyN0oJICs2L6aP7

 

 

but I do like null test, either against a flat or using a ross null - I like the ross null cause its close to radius of curvature so it doesn't take much to change the set up from RoC

 

https://www.youtube....2L6aP7&index=24



#238 starman876

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Posted 13 April 2018 - 12:57 PM

Takes one hell of a flat for a 20" mirror.  I have a 16" flat so I would have to test one in zones and then piece it together.  I would think a 22" flat be one heck of an investment for anyone. 



#239 Arctic eye

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Posted 13 April 2018 - 01:13 PM

There is something positive here, maybe? Because of this we now have something you dont see very often; real life comparison images on what this kind of an error causes on final images.

 

I did a comparison by setting up my SW16" goto Dob next to my waste of space stargate 20". They have only 10% difference in focal length. Had them cooled down, collimated with the same procedure and tools, Goto aligned both and then I started imaging in turns. First I used ASI183MM, ir pass filter and APM 1,5x barlow. I took video with the first scope, then changed the imaging train to the other scope, keeping FC connetcted to the camera all the time and took same amount of frames and then back to the first scope again. I imaged Sinus Iridum in the moon, because I had earlier comparison material on that. With the 1,5x barlow I get a medium image scale, something that I used to have with my earlier 12" scope. With that scale the 12" produces crisp images. I stacked the videos with similar settings and scaled and rotated to match each others. Below is a GIF animation of the final images:

 

_full.jpg

 

 

In live view 16":er snapped nicely into focus. 20":er didn't have a sharp focus at all. Other observation from my 10 Terabyte Via Dolorosa is, that optical aberrations enforce the effects of seeing and thus one easily blaimes seeing, when it is actually the optics...

 

 

On another day I decided to do it again. This time I was using my "high scale" imaging train; APM 2,7x barlow, ir-pass filter, ADC and ASI290MM. Otherwise the same procedure as above. This time the target was Platon. I let you to guess which image is imaged with which telescope...

 

_full.jpg

 

 

 

I have 3 SW reflectors and one refractor. 12" SW Explorer on NEQ6 mount, 16" Flextube goto Dob and this 20" Stargate. 12" is excellent with its traditional style mirror, but it takes way too long to cool. 16" with its Celestron SCT style conical mirror is excellent and cools down quickly. This 20" rib-structure bathroom mirror cooles down quickly but... 

 

By the way, usually people talk about "first-light". I have decided that this Platon-image was the "last-light" of this mirror. Now I am waiting an answer for my reclamation, waited for 4 weeks already. If they dont handle this correctly, I will order an 20" conical mirror like I have in my 16". Telescope Service in Germany sells those with a measurement report and it fits rigth into my mirror holder. But lets see first how this goes. Mistakes happen and I still have faith in Skywatcher. At the end, my other SW telescopes are superb.


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#240 vhinze

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Posted 13 April 2018 - 02:00 PM

Commendable philosophical attitude. Bathroom mirror? More like fun-house mirror.

 

clear skies. 



#241 starman876

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Posted 13 April 2018 - 04:49 PM

wow. you guys are thoughlol.gif



#242 Bram Weisman

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Posted 13 April 2018 - 11:12 PM

I purchased a 20" goto in June or July of last year.  This was after seeing the demo at TSP.  The first big surprise I got was the difference in mirror construction vs. the demo.  My top plate is not a single plate, but a sandwich.  BTW, people are talking about 3/4" plates, but they are more like 5/8".  Here's a shot of the edge so you can see: https://www.cloudyni...60835-img-5702/

 

Skywatcher hasn't told me why they did this, but here's my theory:  The plates warp in the furnace and what's left is difficult to support for figuring.  But it's easy to cut the top plate after the fusing process into something that has a perfectly flat surface.  Meanwhile, they figure the top half of the sandwich separately and later glue them together.  So I think that the new process is probably faster and yields a better quality mirror. I just pray the sandwich never separates.  I can tell you that the views through the demo and my production model are stunning.  I'm not perfectly happy with the overall experience.  There have been mechanical issues, and mirror flop.  But I'm not throwing the baby out with the bathwater, yet.  That said, the OP may have gotten a real lemon and SW should make him whole.

 

Side note: my mirrror assembly has a lot of cosmetic chips on the ribs but they don't affect the view in the EP.  My friend who works with glass is concerned about the chips at the joints casting doubt on the integrity of the joints.  I pray this mirror never comes apart.  SW should warranty that forever, IMO. 

 

I like the mirror concept for it's fast cooling potential.  If there's no warpage from the manufacturing process, I think it should be very effective.  Unfortunately I haven't had much observing time with it, because Okie-Tex and subsequent SE Texas weather have totally sucked.  But I did have one night in Columbus TX where the detail I saw in the M45 were breathtaking.  Total vindication of the optics.  And the moon is very crisp for me.  Not like the GIF above.  I feel for the OP.

 

There will likely be three Stargates at TSP this year: Robert Reeves', Skywatcher Demo and mine.  We'll all likely be near each other too.  I intend to demo the mirror flop to Skywatcher to make sure they understand it, and to test theirs and Robert's as well.  If anyone wants to come by and star test mine, you are welcome to.  Look for us on the upper field near the vendor area.


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#243 Kunama

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Posted 13 April 2018 - 11:32 PM

Toivottavasti SW antaa sinulle uuden peilin cool.gif  Tästähän voisi seinäkellon tehdä waytogo.gif

 

Great attitude to the issue Ari, hopefully SW do the right thing by you waytogo.gif



#244 Arctic eye

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Posted 14 April 2018 - 05:09 AM

I purchased a 20" goto in June or July of last year.  This was after seeing the demo at TSP.  The first big surprise I got was the difference in mirror construction vs. the demo.  My top plate is not a single plate, but a sandwich.  BTW, people are talking about 3/4" plates, but they are more like 5/8".  Here's a shot of the edge so you can see: https://www.cloudyni...60835-img-5702/

 

So this is what they have done to fix it, wow. When looking closely, I can see the same structure in S&T review image I think. Hopefully I can get one of those, I would love to give it a ride next winter when it gets -25C. If it can stand cooling from room temperature to that, then it's good.

 

There is more humour and irony around this, you know: When I ordered this scope, I also decided to order a new eyepice for that. Guess what I ordered and paid? Baader Morpheus 17,5mm... I received it finally this January, after they had tried several designs at Baader, just to find out that the scope itself is an early unsuccesfull design. Difference is that Baader did not deliver their unsuccesfull early designs, Skywatcher did. If I dont get this sorted out, maybe I can sell my combo into the Museum of Engineering lol.gif 



#245 Redbetter

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Posted 14 April 2018 - 09:23 AM

The chips at the joints could pose problems for freeze/thaw cycles.  Freezing water has a way of expanding cracks and separating things.  That might not be an issue on the back side of a mirror...hopefully. 



#246 Starman1

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Posted 14 April 2018 - 01:38 PM

wow. you guys are thoughlol.gif

Tough?  Or thorough?  Or both?grin.gif


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#247 starman876

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Posted 14 April 2018 - 10:24 PM

Tough?  Or thorough?  Or both?grin.gif

without spell check I am lostlol.gif


Edited by starman876, 14 April 2018 - 10:25 PM.


#248 IVM

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Posted 15 April 2018 - 12:17 PM

Back to the topic and the buyers' updates - no, personally I would not get in line for the 7 month lead time if the bulk of the Obsession were not an issue. Actually I probably wouldn't get in line regardless.

 

It is interesting to see the in-focus pattern from the planetary photographer. It's a bit overexposed in the middle as presented, emphasizing the pattern around the central spot, but actually not bad from 20" under real-life conditions (although the triangularity, assuming it wasn't ephemeral, is quite strong and in this case cannot be explained by a pinched mirror due to how they are mounted). F/4 might be challenging for a Barlow, and overall, a 20" that doesn't hit a wall in planetary imaging would be a great scope indeed.

 

Here are two of my recent sketches with this telescope, both detailed galaxies under dark skies, one at 250-360x, the other at 360-500x:

 

https://www.cloudyni...11662-ngc-3147/

 

https://www.cloudyni...12167-ngc-4054/


Edited by IVM, 15 April 2018 - 12:29 PM.


#249 Arctic eye

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Posted 08 June 2018 - 11:25 AM

Almost 3 months passed since I did the reclamation. Still waiting for a conclusionfingertap.gif ...  



#250 IVM

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Posted 12 June 2018 - 03:39 PM

Took this thing for its 4th run to our remote site. Observing notes here:

 

https://www.cloudyni...in-boo-and-ser/




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