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SkyWatcher’s 20" goto truss dob!

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#176 Pinbout

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Posted 22 July 2017 - 10:42 AM

Robert Reeves on Facebook has been doing some incredible lunar imaging with the 20" for his 365 days of the Moon.

The 18" has arrived at its reviewing location but we are not sure when they will be done with the review.

thanks for the link. tongue2.gif


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#177 MSWcdavis

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Posted 27 January 2018 - 11:43 PM

Been interested in the 18" goto Stargate skywatcher...

 

can't find anything really online

 

nothing really on cloudy nights

 

any experiences with these out in the wild?

 

i thought there were going to be some reviews or something...

 

 

this thread died

 

any life out there on this subject?



#178 IVM

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Posted 28 January 2018 - 01:20 AM

I received the 20" go-to for Christmas and have made one dark-sky run with it. The last post on my blog deals with the objects I saw. There is also a photo of the setup under the "Equipment" tab. It's too early for a firm verdict, but so far it seems to be an excellent scope.


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#179 vhinze

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Posted 10 April 2018 - 06:28 PM

Recently, a sale price of $7500 was offered at multiple Skywatcher dealers. On the last day of the sale, March 31st, 2018; I bit.  I followed up with a call to the dealer out in California and learned there was a six or seven month wait (meh!). Who knew you could have a big sale without stock?

A day or two later, a charge for $7500 shows up on my card. Thinking this can't be right (what happened to charge when you ship?) I call the dealer and protest. I am put on hold while cogitations and machinations transpire at the other end.

After a bit, the salesperson comes back on and sez: "We'll just charge you half as a deposit". Thanks but no thanks. I maintain that 10% (Yeah, $750) is a deposit. $3750 for six months is a free loan. None of these terms and conditions were apparent before I ordered, so I canceled.

Coming to my senses, I ordered an Obsession 20". Two thousand dollar deposit and the scope is to be delivered in no more than six weeks.  I live in Wisconsin, a couple hundred miles from the dealer. Obsession is not still working the bugs out of its product. The Classic is a known quantity. It weighs the same as the Stargate 20" and encloses and protects the mirror. No flimsy, blow-molded plastic lid. Options and accessories galore.

Costs more, lots more with go to, but, with Obsession, you can be confident you got what you paid for . Others leave you wondering...

Before you go for a Stargate, do some research, there are a very few posts here and there by early adopters working out the bugs in this scope. Almost nothing definitive has been said about optical performance, however.

This is an ambitious product, breaking some new ground in the commercial dob market and it can be expected that there'll  be growing pains.


Edited by vhinze, 10 April 2018 - 06:45 PM.

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#180 skyward_eyes

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Posted 10 April 2018 - 07:28 PM

For the record on this. We did have stock before the sale but these are large, high dollar items so we don't stock many of them. They sold way faster than expected pushing back those coming in late on the sale to a wait list. Nothing much we can do about it. Now you being charged the full amount before its ready, that's a different story to be discussed with the dealer handling the sale.

 

The Stargate is pushing new ground for what it is, we understand that. Obsession is a tried and true brand that's been around for years. I've used all their sizes and they have all been excellent. I just sold my 20" (moving to a 28") Obsession and it was always a dream to use. 

 

We know where we fit in the market and the Stargates reflect this as our upper level dob. 


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#181 vhinze

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Posted 10 April 2018 - 08:56 PM

You, Mr. Distributor, are welcome to carry on the discussion. I got nowhere, it was his way or the highway. 

 

For the record, the dealer's name wasn't Weatherwax.


Edited by vhinze, 10 April 2018 - 09:55 PM.


#182 Arctic eye

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Posted 11 April 2018 - 09:44 AM

Hi all.

 

I have been doing solar system imaging with the SW 16" flextube goto Dob for years and have been very happy with the results, so I upgraded into the 20" Stargate goto Dob little more than a year ago. I tried and tried and could not get a decent image. Actually after 10 Terabytes of imaging solar system videos, not even one sharp frame!

 

I still have the 16", so for several times I set the 16" and 20" side by side for Lunar imaging, using the same imaging train, and 16" gives sharp focus, 20" not. Realising there is something wrong, I started taking star test images, and the results were clear: The 20" mirror is having some sort of zonal error and a huge print through from the mirrors internal rib structure. And by huge I mean that you can actually see the mirrors structure in the defocused star when the seeing settles for a moment...

 

Below is Dubbhe with ir-pass filter and ASI183MM camera + 1,5x Barlow. The 12 ribs of the mirror or 12 areas between the ribs create 12 bright blobs to the defocused star. And the focused star has 12 blurred and distorted star images around and on top of it.

 

_full.jpg

 

 

If this was not enough, when the star is defocused more, you can see the actual mirror structure in the image! Below is defocused Dubbhe and for comparison image of the main mirror I loaded somewhere from this forum (I havent removed my mirror at any point, it's as it was delivered from the factory)

 

_full.jpg

 

The scope performs like a 8" scope resolutionwise. I can use it for low power visual. More focal length I use, the worse the image gets... Stargate got a good review on S&T recently, but mine is basically useless...


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#183 dgoldb

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Posted 11 April 2018 - 10:05 AM

Hi all.

 

I have been doing solar system imaging with the SW 16" flextube goto Dob for years and have been very happy with the results, so I upgraded into the 20" Stargate goto Dob little more than a year ago. I tried and tried and could not get a decent image. Actually after 10 Terabytes of imaging solar system videos, not even one sharp frame!

 

I still have the 16", so for several times I set the 16" and 20" side by side for Lunar imaging, using the same imaging train, and 16" gives sharp focus, 20" not. Realising there is something wrong, I started taking star test images, and the results were clear: The 20" mirror is having some sort of zonal error and a huge print through from the mirrors internal rib structure. And by huge I mean that you can actually see the mirrors structure in the defocused star when the seeing settles for a moment...

 

Below is Dubbhe with ir-pass filter and ASI183MM camera + 1,5x Barlow. The 12 ribs of the mirror or 12 areas between the ribs create 12 bright blobs to the defocused star. And the focused star has 12 blurred and distorted star images around and on top of it.

 

_full.jpg

 

 

If this was not enough, when the star is defocused more, you can see the actual mirror structure in the image! Below is defocused Dubbhe and for comparison image of the main mirror I loaded somewhere from this forum (I havent removed my mirror at any point, it's as it was delivered from the factory)

 

_full.jpg

 

The scope performs like a 8" scope resolutionwise. I can use it for low power visual. More focal length I use, the worse the image gets... Stargate got a good review on S&T recently, but mine is basically useless...

How long did you let the mirror cool?  Use fans?  You could be seeing deformation due to irregular cooling of the mirror due to the support structure. 



#184 big_scot_nanny

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Posted 11 April 2018 - 10:08 AM

A recent S&T mag had a review of, i think, the 18" Stargate - they were very impressed. They seem not to see the issues you have with the mirror, which must be very saddening. 


Edited by big_scot_nanny, 11 April 2018 - 10:08 AM.


#185 Arctic eye

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Posted 11 April 2018 - 01:51 PM

How long did you let the mirror cool?  Use fans?  You could be seeing deformation due to irregular cooling of the mirror due to the support structure. 

Its not the cooling, both scopes had been out for two days in this test and temperature was stable. I have been comparing and testing several times in different temperatures. This is factory built print-through unfortunately. 

 

 

A recent S&T mag had a review of, i think, the 18" Stargate - they were very impressed. They seem not to see the issues you have with the mirror, which must be very saddening. 

 

It seems that at least in the earlier series there has been defected units. I have this huge print-through and in the French review they had also some, but also massive astigmatism which I dont have at all. My mirror is actually very stable also at horisontal position. I have made a reclamation and now waiting for an answer. If this is an early series quality glitch and they handle it, then thats o.k. But if there is this kind of a lottery all the time and they dont handle it...   Its saddening to watch this very expensive refrigerator size waste of space in my carage and wait. 



#186 Starman1

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Posted 11 April 2018 - 03:43 PM

This has been a problem with every brand of scope containing a layered or ribbed mirror structure.



#187 vhinze

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Posted 11 April 2018 - 06:44 PM

Looking at your star test images and comparing them to images in Harold Suiter's book ( fig. 7-5 and 13-5) turbulance and/or primary ripple (aka: medium scale roughness or "dog biscuit") are suggested. Honestly, I am a bit skeptical that what you have is actual "print-thru". Hard to say though, this is a novel mirror design and I'm no expert.

 

I gave some thought to the nightmare scenario where I might take delivery of a sub-par scope when I decided to order the Stargate 20" (order since canceled). Recourse to the manufacturer, naturally.

But, I figured that I'd need to have the problem quantified in order to make progress against the anticipated "diffraction limited" headwind I could expect from that quarter.  An expense on the order of $400 from the likes of OWL. Ouch!  But put in perspective of the investment in time, treasure and tears, it might have been money well spent. That's my two cents.

 

Whatever the cause is, if atmospheric turbulence can be discounted and the disturbed wavefront remains, then, it appears, you have been ill-served by the manufacturer. I hope for the sake of all these back-ordered purchasers of said scope, later iterations of this scope are better figured. I am relieved I decided to go with Obsession.

 

The aforementioned review of the 18" go-to Stargate appears in the April '18 issue of S &T.


Edited by vhinze, 11 April 2018 - 07:11 PM.


#188 starman876

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Posted 11 April 2018 - 07:03 PM

when I  first saw pictures of the mirror structure I was wondering how that was going to work.  I suspected that star tests would end up looking like that in the above posts. 



#189 dgoldb

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Posted 11 April 2018 - 07:19 PM

I have to imagine if this is indeed integral to the design that they will be experiencing it on all/most of the units.  I am skeptical that they missed such a glaring issue.  



#190 starman876

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Posted 11 April 2018 - 07:44 PM

engineering is normally a compromise of desired values.  You give up some things to obtain others within the budget parameters of the project.   



#191 CrazyPanda

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Posted 11 April 2018 - 09:52 PM

I never understood the purpose of such a design. The actual mirror part is a thin disk already, so why not just bite the bullet and build a 27 point mirror cell to support it rather than the rib structure? Surely that will work better AND be less complex to get right than a perfectly manufactured rib structure has to be. You can practically stamp the parts out and then just needs a few nuts and bolts to put it together. 


Edited by CrazyPanda, 11 April 2018 - 09:54 PM.


#192 starman876

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Posted 12 April 2018 - 06:59 AM

You would think that the area where the ribs are would not cool at the same rate as the rest of the mirror.  And even after long hours there might still be a temperature difference because of the difference in mass at those points.  Cannot see how the mirror could expand and contract evenly constructed like that.  


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#193 vhinze

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Posted 12 April 2018 - 07:55 AM

The Vendor rep said in an earlier post that one of the issues holding up the release of this scope  were concerns about print-through. That said, the issue ought to have recognized and corrected by now.

 

The structure is composed of the same material and should have the same thermal coefficient of expansion throughout. If, after fusing the structure together, a successful method of annealing and stress relieving were carried out and the structure was uniformly supported during grinding and figuring, then I see no insuperable impediment to achieving a figure free of "print-through".

 

Has anyone compared the posted images to what can be seen in the Suiter figures I mentioned in my last post? It may be that there's another reason for the lobe-like appearance of the distortion. Count the "lobes" and bright spots. Are we all that sure there are twelve of them?

 

Speaking of the Vendor, perhaps he'll weigh in sometime soon. Recall dgoldb said this scope has been in his possession for better than a year. Maybe this isn't representative of what's going out the door going forward. We can certainly hope so.


Edited by vhinze, 12 April 2018 - 08:04 AM.


#194 starman876

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Posted 12 April 2018 - 08:28 AM

The Vendor rep said in an earlier post that one of the issues holding up the release of this scope  were concerns about print-through. That said, the issue ought to have recognized and corrected by now.

 

The structure is composed of the same material and should have the same thermal coefficient of expansion throughout. If, after fusing the structure together, a successful method of annealing and stress relieving were carried out and the structure was uniformly supported during grinding and figuring, then I see no insuperable impediment to achieving a figure free of "print-through".

 

Has anyone compared the posted images to what can be seen in the Suiter figures I mentioned in my last post? It may be that there's another reason for the lobe-like appearance of the distortion. Count the "lobes" and bright spots. Are we all that sure there are twelve of them?

 

Speaking of the Vendor, perhaps he'll weigh in sometime soon. Recall dgoldb said this scope has been in his possession for better than a year. Maybe this isn't representative of what's going out the door going forward. We can certainly hope so.

you would think that process of building a mirror like that would end up costing more than just a regular thickness mirror.


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#195 CrazyPanda

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Posted 12 April 2018 - 08:59 AM

The structure is composed of the same material and should have the same thermal coefficient of expansion throughout.

But different parts of the mirror have different mass and surface areas. This makes the same thermal coefficient a bit irrelevant, as the parts supported by the ribs will cool more slowly than the parts that aren't. So maybe when the scope is fully thermally acclimated it will not cause a problem, but I can see it doing some weird stuff to stars as it's cooling or warming (beyond the usual under-correction of a cooling parabola with flat back)


Edited by CrazyPanda, 12 April 2018 - 09:00 AM.


#196 mark379

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Posted 12 April 2018 - 09:08 AM

Perhaps the addition of cooling fans would solve the problem?

#197 starman876

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Posted 12 April 2018 - 09:08 AM

But different parts of the mirror have different mass and surface areas. This makes the same thermal coefficient a bit irrelevant, as the parts supported by the ribs will cool more slowly than the parts that aren't. So maybe when the scope is fully thermally acclimated it will not cause a problem, but I can see it doing some weird stuff to stars as it's cooling or warming (beyond the usual under-correction of a cooling parabola with flat back)

my point also.  Thank you.  I am also wonder if the ribs act like cooling fins and they would always be a different temperature than the rest of the mirror. 



#198 skyward_eyes

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Posted 12 April 2018 - 09:12 AM

For all the owners I have spoken to here in the US no one has complained about the optics. The recent magazine review you can see the figure of the mirror, this is something the reviewer did. Print through was a major concern on these and we wanted to make sure it was done right.

Now, that being said, these are mass produced scopes. The factory usually does a good job at QC on these before they leave but I have seen strange things slip out.

From the images above it’s really difficult to get a feel of what’s going on. A 20” is a very large scope that requires very good seeing to utilize to its full potential. If this is how the scope has performed and has never provided what you expect then maybe speaking with your dealer about warranty on the optics should be discussed.

#199 vhinze

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Posted 12 April 2018 - 09:40 AM

"So maybe when the scope is fully thermally acclimated it will not cause a problem"

 

This is where the thermal coefficient comes into play. The main stated purpose of the ribbed mirror is to accelerate cooling. Reduction of weight may be minimal over, say, a two inch thick mirror. This mirror is said to be 3/4" thick, and with the ribs and secondary ring all the same thickness, probably equals the mass of a mirror 1 1/2 inch thick. So, a little less mass and more surface area should result in quicker equalization. Weird stuff occurs with any mirror that's used before it has equalized. What's more, the complainant states that the distortion he's concerned with is static, not changing. That conclusively rules out thermals.

 

But, all this emphasis on print through and thermals may be a red herring. I'd submit that the original images you print-through advocates are hanging your hats on support surface roughness much more strongly. I humbly suggest finding a copy of Harold Suiters book ( Star Testing Astronomical Telescopes) and referencing  the illustrations cited in the above post before drawing your conclusions.

 

"the ribs act like cooling fins and they would always be a different temperature than the rest of the mirror."

 

Equilibrium means just that, the ribs and mirror structure reach the same temperature as the surrounding air, heat flow and dimensional change ceases.

 

"For all the owners I have spoken to here in the US no one has complained about the optics"

 

Not that there's that many happy owners of this scope yet. At least, if there are, they're a close-mouthed group. In fairness, I'd also reiterate that the scope in question is over a year old and hopefully not representative of current production.


Edited by vhinze, 12 April 2018 - 10:24 AM.


#200 CrazyPanda

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Posted 12 April 2018 - 10:29 AM

This is where the thermal coefficient comes into play. The main stated purpose of the ribbed mirror is to accelerate cooling. Reduction of weight may be minimal over, say, a two inch thick mirror. This mirror is said to be 3/4" thick, and with the ribs and secondary ring all the same thickness, probably equals the mass of a mirror 1 1/2 inch thick. So, a little less mass and more surface area should result in quicker equalization. Weird stuff occurs with any mirror that's used before it has equalized. What's more, the complainant states that the distortion he's concerned with is static, not changing. That conclusively rules out thermals.

I guess my question is why they can't just use the 3/4" disc without the ribs, and provide a quality mirror cell to support it.

 

Maybe 3/4" thickness over 20" is not feasible even with a 27+ point mirror cell though. 




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