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SkyWatcher’s 20" goto truss dob!

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#276 IVM

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Posted 14 December 2018 - 01:34 PM

Ari, could you post a couple of single frames, with exposure time, of the in-focus pattern from your new mirror?



#277 Arctic eye

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Posted 14 December 2018 - 02:49 PM

Here you go. Single focus frames, enlarged 2x, green filter, exposure 1 millisecond. This looks otherwise identical to the original mirrors focus images, but now with added astigmatism. Blobs, blobs, visually stars look like globular clusters, stacked and sharpened moon images are grainy.

 

Edit: By the way, the size of the single in-focus pattern is 5" in diameter...

 

Singleframes-focus-1mS.jpg?img=full

 

 

Here is also temperature record from those evenings from an airport 5km noth from me. Temperature was more stable at my spot, but this gives an idea. I calculated with "telescope mirror cooling" program, that there should have been no trouble at all for the mirror to follow that temperature. 

 

Temps1.jpg?img=full


Edited by Arctic eye, 14 December 2018 - 03:13 PM.


#278 IVM

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Posted 14 December 2018 - 04:03 PM

Thanks! That turbulence is terrible. This is what I see on most nights, especially the first couple hours after set-up. Good seeing is rare here, and when it's good, it looks like your picture from April and, to my eye, in line with the manufacturer's stated 1/4-wave QC criterion.



#279 Arctic eye

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Posted 14 December 2018 - 05:28 PM

.. in line with the manufacturer's stated 1/4-wave QC criterion.

So the conclusion from you is, that it is like Synta intended it to be? And the conclusion from me is, that it just does not perform in high-res photography, even when in the same situation the 16" does perform well. Combining these two conclusions, after two and a half years of extensive and futile attempts with this scope, I step aside and start planning for the next scope. 

 

Those of you who use this scope for DS or medium power L&P, don't worry about me, continue enjoying. It is a great scope for that. After all, what I do is a special thing, and I do it in difficult conditions. Those of you who maybe try high-res imaging with it; If you succeed, let us hear how you did it.

 

If SkyWatcher answers me something that makes any difference for anyhing, I will let you know.

 

Clear skies!

 

400p_500p_comparison.gif?img=full


Edited by Arctic eye, 14 December 2018 - 05:33 PM.


#280 dgoldb

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Posted 15 December 2018 - 12:00 PM

I'd like to see the skywatcher rep respond to these issues.  Skywatcher is usually a pretty good company.  Would like to hear what they have to say about this.  



#281 Jason_J

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Posted 16 December 2018 - 12:59 AM

Ari, the "cell-induced deformation" of the primary mirror is demonstrated here:

 

http://www.dreamscop...hN-18-point.htm

 

This company recognizes the problem and aims to limit it to 1/82 of a wavelength peak-to-peak in their high-end products.

 

Also, a very interesting mirror cleaning method:

 

http://www.dreamscop...irstContact.htm



#282 hakann

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Posted 16 December 2018 - 12:02 PM

I has not read all inputs here, but maybe one get for one pay for.
I'm not defending manufactory.
It's still a 20" telescope and rather portable.
It do not cost much to let Rohr test the primary in Germany and then the manufacture can mean it's still ok.
In ATM industry there are never documents, and 'if' numbers they can be whatever.
20 nm RMS on surface ( diffraction ) is a killer tolerance that demand great test rooms in a 500 mm primary.
Bring in sag, cool-off/temperature, gravity etc and no heat in any form can be in room.
A serious pro test is very expensive, near twice the price of a CZ Quartz f/4 18".
So it become a trust situation and then the EP will talk in the end.
One can build a telescope in many ways.
I did rely on source whit good reputation as CZ, Antares, SDM and TV, Starlight, Glatter, Cat Eye etc.
It's still a 'classic' but in a 20" box and 40 x 2.6 mm mm CF trusses, but the total is near 25K on a project like that.
Worth it ? ( it's build to be stabile, not portable )
Well sky is still there and 25K in the pro world in optic's is nothing in this size.
But do we need that for visual use ?
Dream Cellulars has nice designs for decent price.( ex +20K for a 20" whit a diagonal included )
It's like a bino for astronomy.
One can get a cheap China 60 mm ( vs hand hold ) and one will see the stars to, but I got me a Zeiss 2060S.
Zeiss has great optic's but not perfect, but I would not pay +8K for it but I found one used in Germany at 1.000 Euros.
Ari, If I was you I sold this scope and checked out the used market for a more premium telescope for maybe the same price.

-Just my 2 cent's.

Edited by hakann, 17 December 2018 - 04:52 AM.


#283 IVM

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Posted 16 December 2018 - 04:12 PM

Precisely - hopefully one gets what he pays for. I understand that in this hobby, the aesthetic aspects of ownership play some role, perhaps even appreciation of artisanship, but I am speaking like someone who, among other things, procures research optics by day. I would not mind getting a 1/8-wave 20" as the luck of the draw, but I would not willingly pay for it, if the maker said their QC was to 1/8 wave, given a reasonable alternative (like 1/4 wave). I am not sure I would, even if I were watching planets from the Gulf Coast, or the Caribbean or near a professional-quality observatory location with attendant seeing. (Instead, I am doing visual deep-sky from locations resolutely average for seeing, although I tend to use high to what some might call very high magnifications - 300-400x routinely.) In all the years, before I got the SW, that I had my 16" with Meade optics, we had subarcsecond seeing only once or twice, and the scope then showed a distinctly subarcsecond resolution, on Jupiter. Note that it only had to be 1/4-wave (the whole system) to do that.

 

Now, it seems possible to me (merely an intuitive supposition of possibility) that the structure of this mirror (20" SW) may not permit 1/8. Not that a ribbed sandwich structure in general cannot (it can), but specifically this implementation. If so, and it only permits reliably, from the standpoint of the manufacturing process, achieving the stated SW criterion for these scopes (18-20", 1/4 wave, individually tested), I'd call it good engineering. They never said they wanted to compete with custom 1/8-wave mirrors (wherever an amateur might actually make use of such quality!). Actually, I suspect that the usually higher quality of small-batch mirrors is primarily for the practical reasons of the manufacturer. If you are making a few mirrors (I suspect), the variability is high and you want to shoot far away from your QC bound to satisfy it without too much costly back-and-forth, but if you are making many, on the line, you can get the variability down and then afford to aim on average just below your QC target.



#284 hakann

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Posted 17 December 2018 - 03:53 AM

It’s no way this glued mirror in a telescope at 5-6K include material, labour, and 3 or more parts make money plus freight plus taxes has 10 nm RMS.
Not 20 nm RMS and not 30 nm RMS either.
10 nm RMS is not even very easy to test.
I has a price of MRF on a 20” ( pocket milled Zerodur at 6;1 whit 6 mm ribs/surface at f/3.5 ) and that was between those tolerances ( 12 nm RMS @632, equal to around 1/6 PV wf ) and including test it was 40K.
No one can do do magic, not even the Chinese, but what I say it will be ok in most seeing vs price.

This ideas whit 8 or 10 or 20 waves is odd when no AMT mirror has been on a real pro test ( as far as I know or heard about ) whit modern state of the art testers and test rooms that investments is in the millions.
But a experienced eye can tell in the EP if it is ok or decent or great mirror or real bad one
Personally I think the ( Chinese market ) or the premium market do it good and it’s ok whit 1/4 wave ’number’ or ’trust me’ since their charge is for a market that is not willing to pay and this market can’t or is not ready for real numbers anyway ( or even need it )

 

-If you get a KIA, you get one, not a BMW.


Edited by hakann, 17 December 2018 - 11:00 AM.


#285 Cathal

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Posted 07 April 2019 - 09:43 AM

I recently purchased one of these scopes, first impressions were "This is great!" but as time passes and I'm becoming more familiar with the scope and getting more time under the skies with it, as well as getting the opportunity to perform more testing, I'm no longer impressed with the value for money paid. I've been keeping another CN thread (linked here) updated with my impressions and feedback. Thought I'd post here to ensure that the information was in this thread as well, as it appears pertinent to the discussions ongoing.

 

I have two definite issues with the build quality/assembly and the design/engineering of my particular example, and a third issue with the optical quality of the scope.

 

I'm having pretty severe issues with the goto performance, I'm generally unable to get targets into the FOV of any eyepiece, and having the requested target near the edge of the finderscope FOV. Not really acceptable in a scope costing multi-thousands of Euro. Currently working with the vendor and (hopefully) the European distributor on how to resolve this. May require a replacement base. The scope can't even return accurately to the first alignment star immediately after alignment. Cable is tight, clutches are tight, using a crosshair eyepiece for accuracy/repeatability, time and location are correct, base is leveled correctly, scope moved same direction for final alignment, scope appears to be properly balanced. Also appears to be a binding issue somewhere when slowly driving that altitude cable causing objects to jump in the FOV..

 

The mirror design prevents collimation from being held in place. The glue holding the aluminium shaft to the mirror glass allows an unacceptable amount of flexure. See these video clips here and here for the amount of movement possible from finger pressure - and remember that the weight of the mirror puts a bigger torque on the glue. I get a ~7mm movement of laser spot movement between horizon and overhead. I can get this down to under 1mm by restricting the movement of the mirror with additional clamps between the wavy mirror plate and the collimation plate that the mirror is mounted on, but my star testing suggests that this creates issues with the figure - similar to pinched optics. Pretty much, the mirror support system is less stiff than noodles of the same size would be. Hopefully my vendor will get feedback on how to ensure adequate stiffness of the optical system. May have to get a replacement mirror that does not flop about under its own weight. (may also have to get a replacement mirror based on the below issue as well..)

 

The third issue is pretty poor optical performance for a 20". After a very lacklustre view of Jupiter, with less contrast than my 8" SCT, I figured I should delve into the optical quality. I appear to have fairly severe undercorrection of the optics (secondary breakout is *very* asymmetric as well), zones present, with print-through of the support ribs visible in stacked intra- and extra-focal images at ~25 waves defocus. Astigmatism is present as well. Definitely does *not* come close to the 1/4 wave performance minimum as stated by the US Skywatcher rep earlier in this thread. I doubt that there is much issue with the secondary, given the radial appearance of the aberrations.

Winroddier analysis of a defocused Regulus in green channel at an altitude of ~40 degrees, after a number of hours of thermal equilibrium and collimated at the same altitude, the fits images were generated from thousands of captured frames over multi-minute captures here: (Note - the fits images are not circular due to a portion of the shroud intruding into the light path - I had left a gap at the bottom of the shroud to help speed mirror thermal equilibrium and I didn't ensure that the shroud was clear of the light path. Fits images are within a pixel of each other in average radius. Next set of defocused images I take I'll have a vernier calipers to get equal defocus values in mm)

Winroddier_500p.PNG

 

If anyone is thinking of purchasing one of these, make sure you have a good relationship with your vendor and a good warranty in writing. My example would be excellent at very lo powers only, if the goto was adequate. As it currently stands with my particular example, it's a bit of a bear to use at low powers with the goto and drive issues, and not pleasant at high powers because of the mirror flop and undercorrection of the mirror. It appears that I and my vendor have a bit of work ahead to ensure that I get the value of my multi-thousand euro investment in this scope, to get optics that maintain 1/4 wave from horizon to overhead, and a Goto system that puts objects somewhere in a low power eyepiece FOV. Shouldn't be too much to ask for really..


Edited by Cathal, 07 April 2019 - 01:14 PM.

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#286 Arctic eye

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Posted 07 April 2019 - 01:21 PM

I can also give a short update on my situation.

 

When I received the replacement mirror from the factory and found out it was even worse than the original mirror, I gave feedback to European distributor who forwarded it to the factory. I had been told that the replacement mirror had been tested at the factory prior sending it to me, so I asked how they had tested it, since it was obviously bad. I also asked if there is something that can be done with the situation (My vendor had disappeared and still is). Also I wanted to verify that the mirror was supposed to work in sub-zero temperatures.

 

Well, the answer from the factory was that yes, the mirror is supposed to work in sub-zero temperatures. And that yes, they had tested it - by looking with a 10mm eyepiece! And to my question if there is something that can be done; I should try watching with a 10mm eyepiece and see if I still see something abnormal...

 

10mm eyepiece... So this thing is for 200x power max? At that stage I had had enough! Clearly Synta was not able to produce this mirror type even close to diffraction limited, so I had to do something else. I ordered a 600mm F4 optical set, salvaged the goto-base, focuser and secondary holder, and started to build a new OTA for the new optics and enlargement pieces for the goto-base. I am planning to donate the dismantled faulty optics and left-over parts for some local astronomy club for a low power light bucket project. I will not sell it, because It is against my ethics to sell anything like this...

 

I am planning to have the rebuild ready during the summer. This was my last AberrationWatcher telescope I will ever likely buy tongue2.gif   Atleast for myself.


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#287 IVM

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Posted 07 April 2019 - 05:30 PM

Sorry to hear that, fellows. Cathal, I also had the binding issue in altitude out of the box, which completely screwed up the goto. You mentioned in the other thread that the front altitude bearing supports are 5 mm closer than the ones in the back. That's exactly what I had, and that was the issue. You need to re-seat the front supports (the right one was the obvious offender in my case) so that there is no binding. The choice of the alignment stars seems to be trickier than the manual makes it sound, but you need to get past the binding issue first. I now get excellent go-to's (10 arcmin?) and tracking (1 h undetectable drift).

 

Further down your list to troubleshoot, let me ask if the laser spot moves on the primary or only in reflection, when you move it in altitude. Also, what's Regulus like in focus, not stacked?

 

I have no serious criticism whatsoever of the scope. My review of it, therefore, is simply the observing notes and sketches in my blog (link below) starting from January 2018. In the notes, 20" F/5 is our Obsession; 20" F/4 is the Skywatcher. I've had more trips to remote sites with the Skywatcher than I transcribed for the blog. (The Obsession, for comparison, is observatory-bound under brighter rural skies.) As a rule these days, I go after structural detail (some of it starlike) in faint, small Arp peculiar galaxies, beyond the NGC for the most time. So the magnifications may be 360 and up, and this is done (with the SW) under very dark skies at elevation. I think that while the Skywatcher is not a universal Newtonian, it's a definitive and long-overdue step away from the aged "Dobsonian" paradigm and a little marvel for those who travel to very dark skies for visual deep-sky work and want to avoid the wheelbarrow-and-ramp arrangement, since the scope is hand-portable short distances while not being a compromise ultralight design.



#288 Cathal

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Posted 08 April 2019 - 04:27 AM

For me, the laser dot does not move appreciably on the primary with altitude - the truss system and secondary support appear to be more than sufficiently stiff for the required purpose. That portion of the scope is really well engineered and put together, unlike the other portions!. The only movement in the optics at this point is in the silicone between the mirror and the aluminium shaft as the mirror settles back to neutral when pointing straight up. In focus, stars are "mushy", and appear to be mushy in a consistent shape and pattern. I'm familiar with poor seeing and how that would affect the view, and this isn't suggesting to me that it's poor seeing alone. I'll make sure to get a stack of in-focus images on my next run.

 

Interesting that mine is not the only one with a squareness issue with the base. I couldn't feel any binding or rubbing when moving the scope in altitude when the cable was not attached, and I have about 1mm of clearance at the front guides all of the time - as far as I've been able to see. I'll investigate later today or tomorrow if there is any rubbing at any stage in the scope movement, and I can lubricate and adjust if needed if there is any. The screws holding the guides and rollers are overtightened at the factory, which makes it *really* difficult to loosen and adjust. I've not noted any change in the sound of the altitude drive that would suggest that it's needing to provide more torque than was needed. Thank you for the pointer and suggestion.

 

It is a relief to hear that someone has got good goto results with this type of scope, which gives me some hope for that part of the scope at least. I've always taken the suggested stars that the handset requests. It would be interesting if the suggested stars were not the ideal ones for getting good alignments. Does anyone have useful advice on that? The weather forecast is not conducive to stargazing for the next few days to test, one of the disadvantages of this country for this hobby!

 

I'll continue my updates in the other thread, and I'll report back to this thread if there's any major update.



#289 IVM

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Posted 08 April 2019 - 01:04 PM

When I was researching this scope (and that was years, because SW USA brought them out - go-to or not - much later than the rest of the world), I came across a review on the Australian forum where a member had (the only user review) severe binding of this exact type on a manual version. I even think it was the 18-inch model. And the early French review here sounded to me like they had the same issue, although the subsequent "resolved" post seemed to imply that it was something in the drive box - not sure.

 

The deal is that there is only one structural part of this scope that is neither cast aluminum nor CNC'ed out of solid aluminum, and that's the square frame that holds the altitude supports on the base. It was evident already in the open box that one of the four supports on mine was not parallel to the others. Remember we are talking millimeters, and it would be unnoticeable and inconsequential on a traditional Dobsonian. Nonetheless, in both the Australian fellow's and in mine, out of the box the altitude trunnions only seated themselves in the front after pausing on top of the supports then snapping between them under the weight. That won't do, however, for the cable drive as it would then repeatedly get stuck and throw off the alignment, even if it's not so dramatic as to necessarily notice as arising from the grazing on the side limiters. The plastic limiters cannot be adjusted to accommodate for it. The support needs to be re-seated on the frame to make it parallel to the rest. The frame is evidently put together from L steel one corner at a time - an awful manufacturing practice incongruous with how the rest of this scope is made. So the last, unsquare corner is where the offending support is mounted.

 

Be careful with the lube. The cable drive depends on the surfaces being dry.

 

The manual talks of the sorting method for the suggested alignment stars that comports with their recommendations for selecting them. When engaged (also by default), the sorting does not actually follow the recommendations at all. It's a bug, and the alignment produced can be awful, depending on the sidereal time. But I found that following the written rules also does not produce especially good results, although it's a step forward from following the ranked star suggestions. In the end, I align on stars about 90 degrees apart that roughly anchor the area I want to observe in. I did it more than once and the results are good (on the low end of the error size for a large Dobsonian, which is 10-20 arcmin) to outrageously excellent (<10 arcmin), depending on the day. I must say that I was slow in troubleshooting the alignment (close to one year of dark-site runs, and I don't set up at home) since most of my targets need to be confirmed on a photographic chart anyway, and the tracking for visual is far less sensitive to alignment than the precision pointing is.

 

Cathal, I don't presume that you don't know this by mentioning it, but let it be said that a mushy in-focus star image in a 20-inch can (should) still be so small that all of it is inside the Airy disc of a smaller scope and similar to the atmospheric resolution limit on most nights. That said, the year+ since I got the scope has been unusual in the number of steady nights here, and nice enough Airy disks have been seen on many nights. The somewhat beat-up-looking in-focus pattern would not impress small refractor users, but it's a correct and correctly sized diffraction pattern on a steady night, after the scope itself stabilizes (3 h?).

 

I will freely admit to being not as well-versed in the practice of out-of-focus star testing as some members here, but the reason for that is that both at work and in this hobby it's hard these days to come across factory-made optics that's so bad that it significantly impacts the intended use. Out-of-focus testing is meant for fine-tuning the diagnosis of the cause, when the in-focus pattern is visibly degraded and limits the use. Lots of things can be seen in out-of-focus testing (even if done with the necessary precision, which is some work) that don't impact the in-focus pattern and practical usability any significantly. With this, of course, I am not saying that you don't have grounds for concern - I haven't looked through your scope and cannot judge, at least not yet.



#290 TonyStar

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Posted 14 April 2019 - 09:52 AM

Cathal,

 

sorry to hear about your issues with the SW 20". Good to see a Roddier test! I can't see the residual aberration values but I suspect the print-through is the least of the issues here, really astig and SA are killing the performances. One thing you could check if whether the astig rotates with the primary mirror. If it doesn't  replacing the secondary may get rid of the astig  (and perhaps some of the SA...)

Also if you do another Roddier test pick a star CLOSE TO ZENITH. Atmospherical aberrations are a lot more severe at 40 deg altitude.



#291 Cathal

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Posted 15 April 2019 - 09:01 AM

I'll try a star closer to zenith the next time I get a clear enough night, and I'll rotate the primary mirror 120 degrees to the next set of the collimation bolts. That should be rather enlightening.

 

As per Sutter on obstructed optics and spherical aberration, the distance from focus between the breakout of the shadow of the central obstruction in and out should be less than about a ratio of 3:1 for 1/4 wave. My example feels about 8:1 or worse - but I've to do a better job of actually measuring that. That ratio would suggest somewhere well outside the acceptable 1/4 wave of spherical aberration alone in my example.



#292 radicell2

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Posted 17 April 2019 - 09:12 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....

The print through wont show in a single pass test like Foucault so the results can be misleading to the newbie....It will show  (if any) in a double pass test using a flat.

 

Ric

 

(maker/user of fused cellulars since "83)



#293 RaulTheRat

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Posted 09 July 2019 - 07:24 PM

The blanks have gone through several generations of development to get to this point. The annealing process is done very slowly to ensure its done correctly.  

 

Hi I'm cross posting this here to make you aware of my posts in the following thread about the issues I've got with my brand new Stargate 450p

 

https://www.cloudyni...nsadvice/page-3

 

As you can see in that thread, and the attached star test images, my mirror shows massive print through of the 12 support ribs as well as serious astigmatism and zones.

 

 

0055+0059 WinRoddier.jpg
 
Extra and Intra focal.png
 
I go into more detail in the other thread about problems with poor goto performance also.

 

Maybe the vendor already has a replacement in stock, but if they don't, I'd appreciate some help from Skywatcher to make sure that the replacement is thoroughly tested before being sent to me to ensure I don't have any further issues.


Edited by RaulTheRat, 09 July 2019 - 07:24 PM.



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