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SkyWatcher’s 20" goto truss dob!

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#51 sixela

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Posted 26 October 2016 - 06:18 PM

told me that the Stargate 450 (20") was reviewed in this month "Ciel et Espace" magazine and the verdict was quite positive. I'll try to get a copy of the magazine before I leave (out of curiosity, I'm not really interested in buying it).

They were quite positive about the structure, but less so about the quality of the optics, if I remember correctly (although it was still usable, especially given most of France ain't Florida). But it's a little hard to say whether that was well tested, whether that was typical, might have been a minor problem (one of the mirrors pinched?), or whether that will be indicative of final quality.

I'll try to dig up my copy and paraphrase.

Edited by sixela, 26 October 2016 - 07:01 PM.


#52 Pierre Lemay

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Posted 26 October 2016 - 07:20 PM

The shop owner told me that the Stargate 450 (20") was reviewed in this month "Ciel et Espace" magazine and the verdict was quite positive. I'll try to get a copy of the magazine before I leave (out of curiosity, I'm not really interested in buying it).  

If it's the same review I heard about (I did not read it but I think I could get my hands on it if someone wants to know more), it was not a very good review, at least for the optics. Here's a translation of the short text a French poster wrote on our local french Québec astronomy forum 10 days ago:

 

"There was a review recently of a 500mm Skywatcher telescope with a cellular mirror in a French astronomy magazine. The measured Strehl was only 0.5 whereas the diffraction limit is 0.8 (50% of the light dispersed in the diffraction rings) and that's in addition to very visible astigmatism". 

 

So: caveat emptor!



#53 TonyStar

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Posted 26 October 2016 - 07:30 PM

"There was a review recently of a 500mm Skywatcher telescope with a cellular mirror in a French astronomy magazine. The measured Strehl was only 0.5 whereas the diffraction limit is 0.8 (50% of the light dispersed in the diffraction rings) and that's in addition to very visible astigmatism". 

 

So: caveat emptor!

 

Merci bien, Pierre!

you just saved me 7.50 euros :)

I was indeed surprised the optics tested so well...

 

Sixela, usually Ciel et Espace runs interferometric tests and I don't see how the mirrors could be pinched since they are self supported. I would be more concerned that a mirror made with a bunch parts fused together behaves differently than a truly cellular, properly annealed glass....


Edited by TonyStar, 26 October 2016 - 07:38 PM.


#54 sixela

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Posted 26 October 2016 - 07:50 PM

I don't see how the primary can be pinched, indeed, but the secondary can still be warped quite easily by attaching it badly to its support. You'd think that by now you'd expect the Chinese manufacturers to know to attach secondaries with three small silicone blobs to the support, but no -- it appears there's no gap at all between the stalk and the mirror.

Also, it's quite possible that the centre support isn't behaving as it should -- you'd expect them to have that central cylinder supporting the mirror evenly in the central hole, but it's hard to tell whether that was actually OK.

If the mirror was still cooling, then there could also be some spherical aberration components, or really weird things if the mirror wasn't cooled symmetrically. That, too, is a bit of an unknown, and these mirror structures are more prone to these kinds of problems (it's really hard to heat up one side of a mirror by accident when it's a big slab of glass, but easier when it looks like that mirror.)

I don't remember the test protocol exactly, or whether they tested the primary separately (and if so, in what orientation).

I'd have to reread it, but it's sometimes hard to guess the exact protocol used, especially with these exotic mirrors.

Of course, the only proof of good optics would be a test delivering a good result ;-). There are many things that can go wrong, but no way to get it right with a bad mirror.

Edited by sixela, 26 October 2016 - 07:59 PM.


#55 TonyStar

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Posted 31 October 2016 - 01:49 PM

So, I was able to read the Ciel & Espace review of the SW Stargate 500 at the airport before I left. As Pierre and Sixela said, the review of the mechanics was essentially positive although it reported that the altitude movement was not very fluid. SW provides lubricant with the instrument to mitigate this issue. 

 

The optical test was done by the company Imagine Optic. Although it is not clearly stated I think that the whole optical train (primary + secondary) were tested. The verdict was lambda/7 RMS and lambda/1.2 PV with a Strehl of 0.58 (or 0.5 accounting for the 27% obstruction). The main reason for the low numbers was a pronounced astigmatism which the reviewers ascribed to the deformation of the primary under its own weight. A residual waverfront error of lambda/15.7 RMS was also reported, which showed various zones and a "print through" pattern of the mirror's 12 radial supports. This implies that even if the secondary alone were responsible for the astigmatism, the primary would barely meet the Marechal criterion...  


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#56 wolfli

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Posted 02 November 2016 - 08:31 AM

So, I was able to read the Ciel & Espace review of the SW Stargate 500 at the airport before I left. As Pierre and Sixela said, the review of the mechanics was essentially positive although it reported that the altitude movement was not very fluid. SW provides lubricant with the instrument to mitigate this issue. 

 

The optical test was done by the company Imagine Optic. Although it is not clearly stated I think that the whole optical train (primary + secondary) were tested. The verdict was lambda/7 RMS and lambda/1.2 PV with a Strehl of 0.58 (or 0.5 accounting for the 27% obstruction). The main reason for the low numbers was a pronounced astigmatism which the reviewers ascribed to the deformation of the primary under its own weight. A residual waverfront error of lambda/15.7 RMS was also reported, which showed various zones and a "print through" pattern of the mirror's 12 radial supports. This implies that even if the secondary alone were responsible for the astigmatism, the primary would barely meet the Marechal criterion...  

If this is just due to a bad design, I don't understand why skywatcher should put all the effort in developing this scope that can never reach diffraction limited. It is hard to believe that they did not do similar tests and knew this issue.



#57 sixela

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Posted 02 November 2016 - 06:39 PM

It's not said how the test was done -- it's quite possible that the mirror only performs this badly when pointed to the horizon.

Nevertheless, the test (in very good seeing) on planets wasn't indicative of very good optics either.

But the actual wavefront in the magazine test is indicative of what is usually an incorrectly supported mirror. Sadly, they didn't appear to rotate the mirror to see if the astigmatism rotated or not!

It's quite possible that Skywatcher still has teething problems with the way the centre of the mirror is supported.

From the looks of it, there's a central hollow in the mirror on these Skywatchers, which is unusual; I can certainly imagine that if the mounting pole isn't inserted and glued with maniacal precision (or if the glue isn't playing along very nicely) very bad things can happen...

Edited by sixela, 02 November 2016 - 06:46 PM.

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#58 TonyStar

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Posted 03 November 2016 - 09:20 AM

.Nevertheless, the test (in very good seeing) on planets wasn't indicative of very good optics either.

Yeah, they said the scope could only take 500X on Mars from Pic du Midi.....

I agree they could have done a bit more in the test. In past tests they did rotate the mirror etc. 

 

The only other test of this kind of mirror I'm aware of is the russian review of the 18" on youtube. There the mirror was Ronchi tested in double pass against a flat, showing the same zonal errors but I don't think they found such a bad astigmatism....



#59 teimrgi

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Posted 12 January 2017 - 06:27 AM

Hi all,

 

any news about the stargate 14" and 16" goto? As far as you know, is there a plan to introduce them in the market?

Thanks/Marco



#60 skyward_eyes

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Posted 12 January 2017 - 06:56 AM

Not at this time.

#61 wolfli

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Posted 12 January 2017 - 07:53 AM

Not at this time.

Hello!

 

Is there any update on the 20''?



#62 skyward_eyes

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Posted 12 January 2017 - 10:20 AM

 

Not at this time.

Hello!

 

Is there any update on the 20''?

 

 

Both the 18" and 20" standard and GoTo are done. They are coming VERY soon, you may see them at a large upcoming show....


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#63 wolfli

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Posted 12 January 2017 - 10:37 AM

 

 

 

Both the 18" and 20" standard and GoTo are done. They are coming VERY soon, you may see them at a large upcoming show....

 

 

Sounds great! But I am a little concerned about the third party optical test results as mentioned by others in this thread. Do you have your own interferometer tests on these mirrors?

 

I know you've mentioned that star tests seems to be fine. But star tests are tricky to interpret and difficult to do for such big objectives. It might be more reassuring if as a company you can test your products with the most accurate method.


Edited by wolfli, 12 January 2017 - 10:41 AM.


#64 starman876

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Posted 12 January 2017 - 01:58 PM

lack of response from the vendor on the quality of the optics would be of concern to me



#65 skyward_eyes

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Posted 12 January 2017 - 06:25 PM

lack of response from the vendor on the quality of the optics would be of concern to me

I am sorry my response was not fast enough, I don't sit on CN all day waiting to respond to posts.

 

The samples of Stargates we currently have passed our optical standards and did not show anything to worry about. We have gone through several generations of these mirrors before receiving our most recent product samples. Early samples of mirrors showed zones and other issues. The newer samples have show the issues have been corrected.

 

We had several experienced observers use these scopes during Texas Star Party last summer without any complaints as well. 


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#66 wolfli

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Posted 12 January 2017 - 06:51 PM

 

lack of response from the vendor on the quality of the optics would be of concern to me

I am sorry my response was not fast enough, I don't sit on CN all day waiting to respond to posts.

 

The samples of Stargates we currently have passed our optical standards and did not show anything to worry about. We have gone through several generations of these mirrors before receiving our most recent product samples. Early samples of mirrors showed zones and other issues. The newer samples have show the issues have been corrected.

 

We had several experienced observers use these scopes during Texas Star Party last summer without any complaints as well. 

 

Thanks for the response! That is good to know!

 

Just to be clear, quantitatively, what is your optical standard?



#67 skyward_eyes

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Posted 12 January 2017 - 08:06 PM

All optics need to meet 1/4 wave.
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#68 havasman

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Posted 12 January 2017 - 09:28 PM

I spoke with a friend of mine who's based far outside the US and he was telling me about his new 18" Skywatcher. He & his compadres had been to their dark site with it and a couple of other big Dobs in the 20 - 24" range. He came away very pleased with his scope and called the optics "perfect" though, of course, as a sophisticated engineer he knows better. Still, he's pleased with his scope.


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#69 wolfli

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Posted 12 January 2017 - 11:10 PM

All optics need to meet 1/4 wave.

 

Thank you! Sounds good!



#70 starman876

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Posted 13 January 2017 - 12:01 AM

All optics need to meet 1/4 wave.

is that 1/4 wave at the eyepiece or just the mirror? Also, how are the mirrors tested and do you have a strehl measurement?


Edited by starman876, 13 January 2017 - 12:04 AM.


#71 starman876

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Posted 17 January 2017 - 02:10 PM

Guess we will not get an answer to the above posted question.



#72 wolfli

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Posted 17 January 2017 - 02:48 PM

Well, both are great questions. I would also add that:

 

Do the numbers (either strehl or wave front) change when the mirror is rotated?

 

It will be very helpful if the vender would like to provide these information. It will certainly dispel a lot of uncertainty associated with an expensive new product.


Edited by wolfli, 17 January 2017 - 02:51 PM.


#73 starman876

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Posted 18 January 2017 - 12:44 PM

I believe that the primary is  1/4 wave . That with a secondary most likely the same would mean you are getting a 1/2 wave scope.  OK for deep sky, but not so great for planets or double star viewing.  Also, not such a great deal for the price.  When spending that kind of money I would want to make sure the mirrors are from known good mirror makers.  



#74 wolfli

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Posted 18 January 2017 - 03:15 PM

I believe that the primary is  1/4 wave . That with a secondary most likely the same would mean you are getting a 1/2 wave scope.  OK for deep sky, but not so great for planets or double star viewing.  Also, not such a great deal for the price.  When spending that kind of money I would want to make sure the mirrors are from known good mirror makers.  

Well, I guess one has to be realistic about this. A 20" f/4 Zambuto mirror along would cost $6600, comparable to the price of the entire scope that skywatcher is proposing. 



#75 dgoldb

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Posted 18 January 2017 - 05:24 PM

I believe that the primary is  1/4 wave . That with a secondary most likely the same would mean you are getting a 1/2 wave scope.  OK for deep sky, but not so great for planets or double star viewing.  Also, not such a great deal for the price.  When spending that kind of money I would want to make sure the mirrors are from known good mirror makers.  

You can get a secondary of very high quality for a relatively low price, in the event the secondary is poor.  1/4 wave is plenty good for planets or double star viewing - and that is if you get a rock-bottom sample.  I believe 1/4 wave is in line (or better than) other mass produced dobs - which are just fine for planetary unless you are unlucky and got an exceptionally poor one that escaped through QC checks.  Compared to the cost of a premium mirror, this is a very good value and should provide spectacular images.  This is especially true when you consider the fact that an equivalently priced premium mirror would be of far smaller aperture (~12-14 inches), and would thus offer less resolution than the larger mirror.  Any difference in the figure of the mirror would almost certainly be compensated for (or even outweighed) by the increase in aperture.    


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