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Skywatcher Stargate 500p - opinions/advice?

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#51 Cathal

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Posted 24 May 2019 - 04:52 PM

As I understand it, the Smax number is the theoretical max Strehl possible compared to an unobstructed aperture, most likely calculated from the mask generated from the two fits images. Refractor runs in WinRoddier show Smax at 1.00 so it's understandable that an obstructed optic would have a lower Strehl by definition - and the advertised Strehl would be the comparison ratio of the calculated Strehl to Smax. So for my runs, the Smax of 0.8 is the theoretical max, and the actual returned number of 0.25 and 0.36 as returned from the runs with defocused disks in the ~30 waves defocus range are definitely more indicative of my optics. The 3rd run above does have defocus disks that are too small to get useful/trustable numbers from - about 8 waves of defocus I think, but it does have a nice set of images to show the difference in breakout. I ran the third set through Winroddier just for curiosity purposes to see if the aberrations continued to show in that run. I don't think I could read too much into the numbers or analysis as the deficus disks are too small as per Winroddier forum postings.

 

I think the severe undercorrection on my mirror is most of the problem, as I really do not ever have a point of best focus, I have more of about a mm where the zone of least confusion is mushy and not dropping near to a point.

 

Yep, the seeing was not good that evening, but things are typical of this in Ireland with a fairly consistent jet stream overhead for most of the year. These are other videos from that attempt to record a startest. My priority was to try to get the difference in shadow sizes, as that alone is enough to show a Skywatcher rep that the mirror is flawed optically. All of the other optical defects present, the zonal artifact and the 12-spoke pattern seen, definitely contribute to the problem, but not as bad as the undercorrection. That's based on unticking the "higher orders" polynomial selection in the floating window, and seeing the effect the run had on the numbers.

 

https://youtu.be/vHj10gfUvKY - A slow run through from outside focus to inside, might be a little more clear

https://youtu.be/5HjQGQzKA8Q - I could see cloud coming in for this one, so I just continued racking through focus a number of times.

All of the star test videos start from outside of focus and head inwards, so the large-shadowed video portions are all intra-focal.

 

One item of interest in the as-captured avi files, I can see a clear Poisson spot in the centre of the disk, with quick rays from the bright portion of the disk as seeing wobbles the lightpath around.

 

But yes, I'm hopeful that with the star test videos, and the other files I've provided, that the vendor will work on my behalf to get a replacement mirror from the distributor. As it stands I am of the opinion that I have two issues with the mirror that individually would warrant a replacement. One being the optical issues showing up in my recent posts, and the second being the mechanical issues causing the collimation problems. There's no way that a commercial scope from a well-known manufacturer should have had a mirror of that quality pass QA before shipping, and there's no way that a Newt should have that much movement of the central collimation zone when moved in altitude.

 

I'm not looking for perfection, just good enough to work as-described, which shouldn't be too much to ask.

 

If the distributor does provide a replacement, I would like to get the mirror tested in the UK before having it shipped to me in Ireland. Any suggestions for this? Would anyone advise against Orion Optics to test?


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#52 Asbytec

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Posted 24 May 2019 - 06:37 PM

"Refractor runs in WinRoddier show Smax at 1.00 so it's understandable that an obstructed optic would have a lower Strehl by definition - and the advertised Strehl would be the comparison ratio of the calculated Strehl to Smax. So for my runs, the Smax of 0.8 is the theoretical max..."

Great. Thanks for the clarification. Some will define Smax of an obstructed aperture as 1 given the added diffraction present. It's the best it can be after diffraction. I still cling to the idea Strehl is a measure of aberration alone...after diffraction is taken into account. But measured at the unavoidable diffraction at the clear aperture and treat the added obstruction diffraction separately as a Strehl-like number. However, that Smax is 0.8 also makes sense as it measures final system peak intensity from obstruction and aberration in a single number. And probably more true to the definition. In this case, any aberration will push an obstructed scope below the diffraction limit. Whatever that means.

I agree, though, something is not right with a 33% obstruction given the shadow sizes and large breakout distance with a parabola (sic). The result is not worthy of a premium mirror. I still think performance centers on your best defocus location, if you can find it. I imagine Win Roddier assumes deviation from a reference sphere centered on paraxial focus. I see over correction, you see under correction. Probably a little of both across the pupil (which kept me awake, btw :) ). Be that as it may, that you are mushing through focus suggests a large caustic focus. That, and customer dissatisfaction, is enough to warrant a replacement, IMO. Godspeed.

Edited by Asbytec, 24 May 2019 - 06:39 PM.


#53 Pinbout

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Posted 30 May 2019 - 10:13 PM

Most of the defocuseses are too far out to read correction, good for print thru and zones like in the 70-80%

 

the one very close to focus needs to be a little more defocused but you can see the correction a lot better a lot.


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#54 Asbytec

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Posted 30 May 2019 - 10:34 PM

Most of the defocuseses are too far out to read correction, good for print thru and zones like in the 70-80%

 

the one very close to focus needs to be a little more defocused but you can see the correction a lot better a lot.

What kept me awake nights wondering was the very tiny shadow inside focus in run 3 left image (not outside as labeled, IMO) in the middle of the previous page. It seems to suggest the inner zone is focusing well short of best focus, which the OP cannot clearly find along the caustic. But, the outer ring inside focus seems close to equal brightness as the inner ring at the shadow, but the outer ring is definitely brighter outside focus suggesting over correction. I think the 70% zone (a valley) is focusing well short of the over corrected marginal zone, too, and close(?) to paraxial focus if over corrected.

 

The medial zone is clearly seen as a valley (blue) in the wavefront profile while both the marginal and paraxial zones are mostly green which should mean they are close to the desired profile and coming to focus very close to each other. This might suggest, then, both marginal and paraxial zones are coming to focus outside the medial zone so there is some hybrid of both over correction (edge) and under correction (center) present with respect to best focus (not paraxial focus). But, I cannot explain why the 33% obstruction shadow inside focus is so tiny with a bright ring around it that far from best focus unless it is focusing very short, too. If the center is under corrected, the shadow should be larger. But, it's not. Weird. smile.gif


Edited by Asbytec, 30 May 2019 - 10:36 PM.


#55 Cathal

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Posted 30 May 2019 - 10:49 PM

Run 3 left image is very certainly outside of focus, run 3 very certainly inside of focus.

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#56 Arctic eye

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Posted 31 May 2019 - 01:44 AM

It might have something to do with the print-through. In my mirror(s) the print-through was visible in star tests in two ways. One is the 12 radial lines, but second is that the correction error bright "circles" were actually not circles at all. They were of 12 circle sectors or oblong blobs that in the focus turned into 12 superimposed star images on top of the real one. That's why my lunar images were so hairy. So it's not just the print-through of the radial supports, the whole mirror surface between the radial supports is warped. So it is a hybrid. It's like a bad optical interferometer of 14 sub-apertures (center, edge and 12 sectors)…

 

Hope you get it sorted out. Do not trust their replacement mirror, do ask for or arrange a test. 


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#57 Pinbout

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Posted 31 May 2019 - 01:10 PM

Run 3 left image is very certainly outside of focus, run 3 very certainly inside of focus.

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showing a way under corrected mirror.

 

btw, the image on the left should ALWAYS be inside focus, by convention... just so it won't confuse people.

 

I know you named the left image "out" but who reads when looking at pics lol.gif



#58 Pinbout

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Posted 31 May 2019 - 01:12 PM

 

something is not right with a 33% obstruction given the shadow sizes and large breakout distance with a parabola (sic).

cause he has them in reverse order when displaying them. bangbang.gif



#59 Cathal

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Posted 31 May 2019 - 03:27 PM

Understood about the convention of relative positions of in / out, I'll make it clearer for the next set...

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#60 Asbytec

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Posted 31 May 2019 - 09:06 PM

showing a way under corrected mirror.

 

btw, the image on the left should ALWAYS be inside focus, by convention... just so it won't confuse people.

 

I know you named the left image "out" but who reads when looking at pics lol.gif

Just general interest at this point, mostly because I think it's an interesting star test to study over morning coffee. Has Skywatcher replied about a replacement?? 

 

But, I have to insist run 3 is not consistent with runs 1 and 2 where, in the latter, the radial grooves and the medial zone are brighter inside focus (and the wavefront profile) suggesting they are valleys. Run 3 left image labeled outside shows them being brighter suggesting they are hills. If that really isn't the case, then it's severely over corrected, at least by appearance, and the convention would be correct.

 

What intrigues me is I suspect a more complex star test is at hand, not simply pure correction error. I believe the OP is fully competent with WinRoddier and set the defocus distance the same. Yet, the shadow differences are extreme while the diameter of the diffraction artifacts are roughly similar. If it is over corrected, then inside focus should show a significantly larger slice through the marginal zones and be consistent with the brighter marginal ring outside focus (in the right hand image).

 

However, added to the mix is the medial ~50 to 70% zone, which should be a valley and focusing short. But short of what, the paraxial or marginal zone or both? Or said another way and since we are dealing with spherical aberration, maybe the paraxial and marginal zones are focusing short or long relative to best focus at the 70% zone. So, I use best focus (which the OP cannot see well) then determine where the other zones appear to be focusing relative to it. The marginal zone appears to focus long of best focus which is normal for over correction and consistent with the right image. 

 

But, the left image (if inside focus) bugs me because the shadow is very small suggesting it is very close to focus inside best focus. But, at this defocus distance, the tiny appears to be cleanly "broken out" with a well formed and possibly slightly brighter ring around it, but it is less defined in the right image (if outside focus). That is consistent with over correction, too, in the star test, but not in the wavefront profile where both marginal and paraxial are "green" suggesting they are on the same profile and focusing near the same point probably further out from best focus and probably near paraxial focus. If so, then the center is a bit over corrected like it should be to focus near the marginal zone.  

 

This seems to be a more complex star test than simple correction error, but it does not explain (to my mind) the small broken out shadow and bright paraxial ring in the left image of run 3. This leads me to ask whether WinRoddier measures peaks and valleys from paraxial focus as I doubt it adjusts to best focus like we do visually. If so, then it might explain some of the results we are seeing. So, potentially there is a mix of over correction and under correction relative to the best focus location, and I have no idea where best focus would be located along the caustic of it exists at all because the OP cannot find it. So, I am wondering is the measurements are done from paraxial focus which explains the medial zone as a valley, but not the inner zone as over corrected.  bangbang.gif

 

Disclaimer: Unless I am missing something. smile.gif

 

No biggie, just an interesting puzzle. In any case, I hope Skywatcher makes good on a more premium S/Smax > 0.8. 


Edited by Asbytec, 31 May 2019 - 09:46 PM.


#61 Asbytec

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Posted 01 June 2019 - 05:49 AM

From my comment above, "But, the left image (if inside focus) bugs me because the shadow is very small suggesting it is very close to focus inside best focus. But, at this defocus distance, the tiny appears to be cleanly "broken out" with a well formed and possibly slightly brighter ring around it, but it is less defined in the right image (if outside focus)."

 

After (re)reading Mel Bartel's explanation of how to star test for a zone, it dawned on me the bright ring around the tiny shadow in the left image of Run 3 is the medial zone as seen close to focus and not the paraxial zone as I thought. Here is what Mel says along with some illustrations. It makes sense to me now. 

 

"Contrary to common belief, it is possible to determine a zone's location on the mirror, as long as there are not other errors that confuse the star test. Let's take the case of an undercorrected 60-80% zone. As the star is defocused outward, a bright ring will appear at the edge of the diagonal's shadow. As defocusing continues, the ring will propagate outward and dim slightly. As defocusing outward becomes extreme, taking up a good portion of the eyepiece's field, the ring will widen and dim, settling into position, revealing the location of the zone. Defocusing inward will show the opposite - a dark zone that expands and settles into position. As the zone becomes more modest, it becomes more challenging to discern its position on the mirror's face, hence the ZUP - Zonal Uncertainty Principle."

 

Scroll toward bottom:  https://www.bbastrod...tarTesting.html

 

The medial zone begins to show fairly early, and it implicates a zone closer to the center because it has not expanded to it's actual location until we are much further from focus (about 15 waves or more) as seen in runs 1 and 2. So, that explains a few things. 


Edited by Asbytec, 01 June 2019 - 08:47 AM.


#62 25585

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Posted 03 June 2019 - 05:49 AM

  • Buy 20" Skywatcher Dobsonian


#63 RaulTheRat

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Posted 08 July 2019 - 12:09 AM

I just bought a 450P from Teleskop Service. I got the chance to try it for the first time tonight, and am concerned I have a similar (and to my untrained eye, considerably worse) case of print-through of the mirror construction.

 

I'm attaching a picture of the intrafocal and extrafocal donuts my scope is producing, they are stacks of around 5000 frames each if I remember correctly, taken at around 60fps with 100-200 pixels of defocus (pixel size 3.8u). The star used was at a high altitude (70 degrees or so), the mirror had around an hour (I estimate, since at the time I wasn't expecting to find problems) to cool down, and collimation had been fairly carefully done with a laser and tublug before starting, although since I wasn't expecting to find problems, I hadn't collimated at the altitude of the test star as Cathal describes.

 

In visual use, on a bright star a set of 12 bright diffraction spikes is visible with an eyepiece when the star is in focus (in addition to those caused by the spider).

 

As well as those 12 artifacts clearly visible in the attached image (as bright spikes in the intrafocal image, and as dark indentations in the edge of what should be the outer circle of the extrafocal image), I also see two distinct zones. Perhaps there's a lot more to see - I don't know, my star testing knowledge is quite basic.

 

I'm going to do some properly controlled videos tomorrow, following the WinRoddier documentation, for now these were just my naive attempt to get something in the limited time available so I could post this and advise the vendor about the issues.

 

I'm not particularly knowledgable about star testing, so is it even possible that what I'm seeing here is acceptable, or would resolve with a longer cool-down, or have I definitely got an unacceptable mirror? At the moment, I'm planning to do some more testing as described above, but perhaps I shouldn't waste my time if this is clearly beyond rescue? Should I test further or just get this replaced immediately?

 

 

Attached Thumbnails

  • Extra and Intra focal.png


#64 Asbytec

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Posted 08 July 2019 - 04:25 AM

I also see two distinct zones. Perhaps there's a lot more to see...

I see three distinct zones. The two under corrected bright ones in the intra focal image corresponding to the darker zones in the extra focal image. And one slightly bright, over corrected one near center just outside the shadow in the extra focal image, and its corresponding slightly darker zone inside focus near the obstruction shadow (with a bright ring in the middle, but gonna call all that a single zone).

 

Curious if there is some astigmatism, too. Hard to tell with such large images, but the secondary shadow seen to have a slight classic 90 degree flip to it. Might be better seen closer to focus. All mirrors have something going on, so I would not hazard a guess (at this point) as to how good or bad the mirror is based on the above. Do you have any Win Roddier results? 

 

Except for those radial spikes. Not sure what's going on there, you called it "print-through". Is that a Win Roddier thing or a manufacturing defect? 



#65 RaulTheRat

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Posted 08 July 2019 - 04:56 AM

I would not hazard a guess (at this point) as to how good or bad the mirror is based on the above. Do you have any Win Roddier results?

Except for those radial spikes. Not sure what's going on there, you called it "print-through". Is that a Win Roddier thing or a manufacturing defect?


I don't have any Roddier results yet, but tonight is supposed to be clear and assuming that it is, the plan is to put it outside for at least 3 hours to be absolutely sure of thermal equilibrium, then take some proper videos to stack for WinRoddier in accordance with the recommendations in the documentation about the amount of defocus and size of the images in pixels. These initial attempts were taken without having read the documentation in detail so aren't suitable for proper analysis in WR, but hopefully I'll have something that is tonight.

By print through I mean that the 12-rib structure of the mirror support seems clearly visible in the out of focus donut, and in focus there is a very obvious and unpleasant 12-spike pattern extending out from a bright star when viewed through the eyepiece. I don't have an image of this, I suspect it'll only be possible to capture with a camera by totally overexposing the central core of the focused star, as the spikes are surely much fainter than the core, but by eye they are very obvious.

Now it's just about possible of course that the mirror hadn't cooled properly last night - it's too early to conclusively rule that out, and so the rib structure could I guess show as stuff contacts at different rates whilst cooling, I am maintaining hope that this is the case and that it'll look better tonight, but I'm not overly optimistic given that by the end of testing last night it had been out for several hours and if there was any improvement from the beginning it wasn't dramatic.

Since I'll be capturing some files tonight to stack and test, is there anything else you'd recommend I also get such as in-focus patterns, videos racking through focus from one side to the other etc?

#66 Asbytec

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Posted 08 July 2019 - 06:18 AM

By print through I mean that the 12-rib structure of the mirror support seems clearly visible in the out of focus donut, and in focus there is a very obvious and unpleasant 12-spike pattern extending out from a bright star when viewed through the eyepiece.

 

I am maintaining hope that this is the case and that it'll look better tonight, but I'm not overly optimistic given that by the end of testing last night it had been out for several hours and if there was any improvement from the beginning it wasn't dramatic.

Since I'll be capturing some files tonight to stack and test, is there anything else you'd recommend I also get such as in-focus patterns, videos racking through focus from one side to the other etc?

Okay, thank you. Interesting. 

 

Yea, keep some reasonable expectations. It might turn out to be pretty good, just do not know. Shadow differences are not far off as shown in the images, provided they are at least close to the same defocus either side. So, there is a good sign. I am not a Win Roddier guy, but I learn by looking at results in passing. See how it turns out. You may have a descent mirror...of course which is what you want to know. Don't we all. :) 

 

For me, personally, in and out focus images at about 10 waves is preferred to kind of gauge the star test. Also very close and in focus images are helpful. But, it's a low priority. You are working with your software, so I'd expect you to focus on what you need. But, if getting them is quick and easy and does not distract, great.



#67 RaulTheRat

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Posted 09 July 2019 - 10:12 AM

Okay, thank you. Interesting. 

 

Yea, keep some reasonable expectations. It might turn out to be pretty good, just do not know. Shadow differences are not far off as shown in the images, provided they are at least close to the same defocus either side. So, there is a good sign. I am not a Win Roddier guy, but I learn by looking at results in passing. See how it turns out. You may have a descent mirror...of course which is what you want to know. Don't we all. smile.gif

 

For me, personally, in and out focus images at about 10 waves is preferred to kind of gauge the star test. Also very close and in focus images are helpful. But, it's a low priority. You are working with your software, so I'd expect you to focus on what you need. But, if getting them is quick and easy and does not distract, great.

 

Well, I got the test files and the mirror does indeed look very bad.

 

The scope had been cooling (on grass) for 4 hours when the videos were taken and outside air temperature was stable (dropping around 1 degree at most during the hour or so it took to collect the test videos), collimation was very carefully done with a barlowed laser, the test star was at very high elevation (80 degrees or so).

 

I tested with two different cameras, using WRCALC to optimise the defocus distances. An ASI1600 with 3.8u pixels, and a QHY5III-174 with 5.86u pixels (both mono).

 

The first two WinRoddier screenshots (0042+0048 and 0055+0059) are both with the ASI1600, with 1ms and 2ms exposures respectively. 10000 frames were captured for each video, then stacked in AS2! in accordance with the recommendations in the WinRoddier documentation.

 

The third WinRoddier screenshot (0121+0125) is from the files that came from the QHY5III-174 this time with 5000 frames processed as above.

 

All the WinRoddier results show an unusable mirror, about 1.5 waves or worse PTV, and a Strehl somewhere between 0.02 and 0.04, it's reassuring that they all agree within a reasonable margin of error, even those from the different cameras, confirming the results.

 

Visually, the intrafocal and extrafocal images as analysed by WinRoddier also show all of the issues mentioned earlier - the very marked 12 radial spikes corresponding to the ribbed structure of the primary, and various zones. Also, this time, the astigmatism seems even more marked, showing a very obvious 90 degree rotation of the defocused donut between intrafocal and extrafocal.

 

I am also attaching images made from stacks of 2000 frames just inside and just outside of focus.

 

Lastly, there's 4 frames selected from a 2000 stack video of a star precisely at focus (carefully focused using the FWHM tool in sharpcap) and stretched to show the 12 radial spikes from the mirror structure print-through in a way that approximates what I see through an eyepiece when looking at a bright star in focus.

 

The vendor (TS in Germany) is working with me, and I have sent them all these results today after they requested that I give the scope a really long cool-down and test again to ensure we weren't dealing with a thermal issue. I think that's confirmed now, so hopefully I will hear back from them soon about a replacement. Due to other serious issues with the goto and tracking performance, which I won't get into detail about here as I'm mainly asking for input about the optics, I am going to be asking them for a replacement of the entire scope, or at least the entire lower assembly consisting of the drive base and primary mirror.

Attached Thumbnails

  • 0105 In focus star images.jpg
  • 0042+0048 WinRoddier.jpg
  • 0055+0059 WinRoddier.jpg
  • 0121+0125 WinRoddier.jpg
  • 0133 Just inside focus.jpg
  • 0135 Just outside focus.jpg


#68 dustyc

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Posted 09 July 2019 - 02:51 PM

Words fail me. shocked.gif

I guess the factory just builds and ships. QA/QC done by customer.

In the market for a big Dob. Guess I can cross this one off my list.



#69 RaulTheRat

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Posted 09 July 2019 - 03:54 PM

Words fail me. shocked.gif

I guess the factory just builds and ships. QA/QC done by customer.

In the market for a big Dob. Guess I can cross this one off my list.

 

Indeed. I have another SkyWatcher scope (a 190MN) which couldn't be more different. It seems these have some real QC issues, as I've extensively researched them prior to purchasing, and found that while some people get a lemon (and I seem to be one of the unlucky ones), others seem to get a good one (for example there's been a good S+T review, a good review by a russian site on youtube including optical tests, and a number of forum posts from people with good ones).

 

Frankly, if it wasn't so difficult and expensive to get an Obsession or a Teeter in Europe, I'd probably have gone that route, albeit the cost is significantly higher. I didn't want an ultralight due to collimation shift, but I wanted something light enough to be transportable with at least 16" of aperture and goto or at least push-to and tracking. These are about the only game in town as far as I can tell. All the 16" models with goto and conventional monolithic mirrors are waaaaaay too heavy to transport, although they seem to be a lot less of a crap-shoot on the mirror quality and optically seem to reliably be good to excellent.

 

I am going to maintain hope that the replacement will be good - as I said, it seems these definitely can be good based on a number of accounts, but it seems to be a bit luck of the draw.

 

How on earth this mirror was ever allowed out of the factory, I have no idea.



#70 Asbytec

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Posted 09 July 2019 - 06:49 PM

Raul, I am sorry to see and hear that. What's the likelihood of demanding a replacement? 



#71 RaulTheRat

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Posted 09 July 2019 - 07:28 PM

Raul, I am sorry to see and hear that. What's the likelihood of demanding a replacement?


Oh I've really no doubt I'll be getting a replacement - it's from Teleskop Service who I trust and believe to be a reputable dealer, and is brand new, only having arrived with me a few days ago.

It's certainly inconvenient having to box this massive thing back up again, having to wait to have a working scope, and having the worry that another bad example like this might arrive when I'm out 5000 euros on this.

So yes, I'm concerned, and not happy that it was allowed through QC like this, but I have to give the dealer a fair chance to fix the problem and I'm sure they'll do so.
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#72 RaulTheRat

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Posted 10 July 2019 - 12:08 PM

An update on my faulty 450p - the dealer has said the plan is to replace the entire scope, and is going to update me in the next couple of days on exactly when that will be etc. They suspect faulty glue between the mirror front plate and the supporting rib structure has failed causing the issues.

I'll update further here as this progresses and hopefully will be able to post some decent star test results soon enough.

For now at least it seems that, although the problem is obviously very annoying and disappointing especially on a scope this expensive, the dealer has responded promptly and is providing good support.

They've told me that the replacement will be tested before shipping, but I didn't get into the details of what that testing will be - I'm not exactly expecting an interferogram, but equally I'm hopeful that the testing won't just consist of putting a 10mm eyepiece in it as Arctic Eye indicated his replacement mirror was "tested" !!!

Presumably the dealer will also want to make sure it's properly tested anyway as this rather large and heavy item was shipped internationally to me, now the dealer has to bear the cost of it's return and the shipping out of the replacement, so they'll want to make sure the next one is right as much as I do hopefully.

Edited by RaulTheRat, 10 July 2019 - 12:09 PM.

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#73 Lukes1040

Lukes1040

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Posted 16 July 2019 - 08:42 AM

Does anyone know if these are the same mirrors as in the Hubble Optics telescopes? They appear to be.

#74 Ivan Maly

Ivan Maly

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Posted 16 July 2019 - 10:51 AM

Hubble Optics are traditional sandwiches - two equal disks with many cylindrical supports in between. They are meant to be mounted in traditional-type cells with distributed back and edge support. Skywatcher 18-20" are two unequal disks (the one in the back is about 1/2 the size) with radial ribs in between. They are mounted simply on a stalk that passes through the middle (though not through the top surface).


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#75 RaulTheRat

RaulTheRat

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Posted 15 August 2019 - 07:57 AM

OK, I'm the guy above with the 450P that had the delaminated mirror and the terrible WinRoddier results. The dealer (TeleskopService) have been very helpful, and I got a prompt replacement (considering a new one had to come straight from SkyWatcher, and they had to test it before sending it out), so well done to them and thanks for the help.

 

I now have WinRoddier tests for the new scope, and the mirror is much better. In visual use, it seemed OK to me at least for a deep sky scope. I did have a quick look at Saturn as well, and it gave a nice view, but seeing was not great last night (about 1.7" - 2") and I was testing at my house with streetlights around and a full moon, so contrast wasn't easy to assess.

 

I'd like some input from people on the WinRoddier results below, I took 3 pairs of videos, each of Altair at around 60 degrees elevation - collimation was done fairly carefully, but I do see a hint of bad collimation in the stacks below so that may put the results out slightly. The other factor in favour of giving the mirror a bit of the benefit of the doubt is that I tested on my concrete driveway, only about 3 hours after nightfall, and it had been hot as usual (35 celsius / 95 fahrenheit) during the day, the scope had been out the whole time since nightfall but still, I expect putting it on grass and a proper cooling may improve the results, but no way to say yet.

 

The video pairs taken were all of 3 minutes duration, at about 90fps, so about 16000 frames stacked for each.

 

The pairs of intrafocal and extrafocal stacks are as below:

 

0101 + 0105 = 2ms exposures, gain 250, around 95px defocused star diameter, approx 2.3mm defocus, about 30 waves of defocus.

 

0111 + 0115 = Same as 0101+0105 above except I dropped gain to 200 in case I was overexposing, as it was hard to assess the histogram with the star image shimmering etc

 

0120 + 0124 = 2ms exposures, gain 240, around 120px defocused star diameter, about 38 waves of defocus.

 

In accordance with convention, in each image the defocused donut on the left is the intrafocal stack, and the one on the right is the corresponding extrafocal stack.

 

Gratifyingly, the 3 tests all agree fairly closely that we're looking at about a half-wave mirror (at the wavefront, so more like a quarter-wave at the surface if I understand correctly how WinRoddier gives the results, so if anyone can confirm for sure that when it says "Loss on the wave" it does indeed mean that, but it's what I expect).

 

Strehl in all 3 tests comes out somewhere around 0.55 as compared with an Smax (max possible with the obstruction I assume) of 0.81, so the S/Smax (ie. the ratio of achieved performance vs a theoretically perfect scope with this obstruction) is somewhere around 0.67

 

The inferred/derived ronchigrams and foucaultgrams also seem to agree closely between the tests, as well as the derived PSF which looks OK to my untrained eye. They seem to show a fairly smooth mirror, with some undercorrection.

 

A half-wave mirror is not what these things are billed as having (SkyWatcher say they are diffraction limited and will give you a 1/4 wave at the wavefront), so objectively, if we're going to talk numbers, I can't confirm that performance, however it's possible that the undercorrection is a thermal thing, I've only had the new scope a couple of days and last night was the first opportunity to test it. Can anyone comment whether mirrors normally become more corrected as they cool, or more undercorrected? If the mirror hadn't equalized properly, should I expect to see some undercorrection that might improve with a better cool-down?

 

Just looking visually at the stacks, I see no real print-through on this mirror, perhaps I can see a hint, not sure, if there's any though it's minor. I don't see much if any astigmatism either.

 

Overall, I didn't expect I was buying a planet-killer. If I wanted a 1/10 wave mirror, I'd buy a smaller one. This is primarily a deep-sky scope for me with plenty of light gathering, so I think, given the various stories of QC problems, that if I was to try to get it replaced again, I'd just as likely get a worse mirror as a better one! I don't really want to play that lottery.

 

Mechanically, this scope also gave a better impression straight out of the box with the general standard of construction seeming higher - the holes were tapped better, stuff fits a bit more square and tight etc. That was confirmed with the goto performance which, while it wasn't great (I hadn't levelled the base super carefully, and didn't even use a reticle during alignment as I didn't have much time) was massively better than the previous scope. Objects were in the central quarter or less of my finder, and probably in a wide field eyepiece, so with better levelling and alignment I'd expect the goto to be totally acceptable. Another reason not to mess with it.

 

Overall, I guess since I don't know a great deal about star testing, I'm looking for some confirmation here - I am inclined to call this good enough for an 18" goto dob at this price (5000 euros / 5500 USD). I am concerned that if I sent it back to try to get a quarter-wave one, I could end up with another lemon. It means shipping it back to another country here in Europe and several weeks delay to have it replaced, plus it obviously costs the dealer a lot in shipping, which although it's not really the biggest factor for me I'd obviously like to save them the money unless it's necessary.

 

If it was yours, would you be happy with this mirror?

Attached Thumbnails

  • 0101 + 0105.jpg
  • 0111 + 0115.jpg
  • 0120 + 0124.jpg



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