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Sky Watcher 18" prototype

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#1 David Pavlich

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Posted 18 November 2013 - 05:45 PM

We spoke with the guy that's doing the work on this reflector and it's still in its rough stages, but you get an idea in which direction they're headed:

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This is Steve (Watermaster)in the picture for scale. He's over 6' tall.

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Note that there are no clips on the top side of the mirror holding it in place. The Teflon "clamps" actually protrude into the gaps between the mirror, the cells and the base to keep the mirror from shifting. Pretty novel approach!

David
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#2 dvb

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Posted 18 November 2013 - 07:38 PM

Wow - I hope they get the mechanicals right; that is, nice smooth bearing movements.

#3 David Pavlich

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Posted 18 November 2013 - 10:37 PM

Actually, the movement isn't bad now. What they are working on is gaining more rigidity. This is going to be a nice scope if they get a few of the rough spots fixed. They will be offering a shroud.

David

#4 Starman1

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Posted 18 November 2013 - 11:25 PM

A few things I noticed:
--no altitude stop (yet) so the scope can continue down until it comes off the bearing. I'm sure they'll add one in production.
--curved focuser board. I don't know why no one else thought of this. It really improves the rigidity of the focuser mount.
--All-metal ball/socket (similar to Moonlite) pole connectors with no-tools fasteners. Nice.
--1.25" poles (a lot more rigid than 1" in this size.
--Sandwich-construction mirror. Is this really lighter than a thin solid mirror?
Bottom supports were blocks at 90 degrees apart--better than a sling for this style of mirror.
--All metal construction. How light will overall scope be?
--secondary mirror looked to be over 4", perhaps 4.5", so they've paid attention to not making secondary too small.

Overall, a fairly impressive prototype. It'll create waves, I bet. If it comes in under $5K (ballpark estimate), it WILL create waves. And it'll pack in a very small car.

#5 skyward_eyes

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Posted 19 November 2013 - 09:30 AM

Don,

It was nice meeting you at the show and thank you for the input. It was very helpful to refine this scope into something people really want.

The primary mirror is 20 pounds for the 18". This is much light then standard 2" thick mirrors. We plan to have this technology for this new line of large dobs. I will be talking to the factory about the shroud and altitude stop. For the mirror cell, Howie Glatter has been a major help with its side mounting blocks. We use three of them to hold the mirror in place. The nice this is that the clips mount between the back and rear elements of the primary so there are no clips obstructing the mirror at all.

We are very excited about this scope. We hope to have a production model for next year. Once the 18 is done its onto the 20!

#6 dvb

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Posted 19 November 2013 - 11:19 AM

Very encouraging that Skywatcher is listening to people like Howie Glatter and Don Pensack in its designs!

#7 Starman1

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Posted 19 November 2013 - 02:09 PM

Don,

It was nice meeting you at the show and thank you for the input. It was very helpful to refine this scope into something people really want.

The primary mirror is 20 pounds for the 18". This is much light then standard 2" thick mirrors. We plan to have this technology for this new line of large dobs. I will be talking to the factory about the shroud and altitude stop. For the mirror cell, Howie Glatter has been a major help with its side mounting blocks. We use three of them to hold the mirror in place. The nice this is that the clips mount between the back and rear elements of the primary so there are no clips obstructing the mirror at all.

We are very excited about this scope. We hope to have a production model for next year. Once the 18 is done its onto the 20!

I really appreciated the bottom edge support design. Most sandwich mirror designs have suffered from improper support of the "bread" in the sandwich, inducing noticeable astigmatism. Your support should do a good job of solving that problem--an elegant solution.
I forgot to ask how many points of support there were for the primary mirror and whether you used a computer program like PLOP to design the support point locations.

#8 Roy McCoy

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Posted 19 November 2013 - 02:39 PM

Nine. Don't know if plop was used.

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#9 Markovich

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Posted 19 November 2013 - 03:37 PM

Ok, looking the picture above, I don't get something:
The 18" scope weighs 150 lbs?? Goodness, an 18" Obsession Classic weighs much less than that! Where is the weight coming from?

#10 cpr1

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Posted 19 November 2013 - 06:26 PM

Maybe it is made with steel instead of aluminium? Since it is a prototype. Hope they can lose the counterweights when the production model is done.

#11 bob irvin

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Posted 19 November 2013 - 06:29 PM

It would be nice to have an alternative to the Obsession.

bob

#12 David Pavlich

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Posted 19 November 2013 - 06:52 PM

A few things I noticed:
--no altitude stop (yet) so the scope can continue down until it comes off the bearing. I'm sure they'll add one in production.
--curved focuser board. I don't know why no one else thought of this. It really improves the rigidity of the focuser mount.
--All-metal ball/socket (similar to Moonlite) pole connectors with no-tools fasteners. Nice.
--1.25" poles (a lot more rigid than 1" in this size.
--Sandwich-construction mirror. Is this really lighter than a thin solid mirror?
Bottom supports were blocks at 90 degrees apart--better than a sling for this style of mirror.
--All metal construction. How light will overall scope be?
--secondary mirror looked to be over 4", perhaps 4.5", so they've paid attention to not making secondary too small.

Overall, a fairly impressive prototype. It'll create waves, I bet. If it comes in under $5K (ballpark estimate), it WILL create waves. And it'll pack in a very small car.


Well shoot! I wish I would have run into you, Don. :foreheadslap:

David

#13 GeneT

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Posted 19 November 2013 - 11:18 PM

Ok, looking the picture above, I don't get something:
The 18" scope weighs 150 lbs?? Goodness, an 18" Obsession Classic weighs much less than that! Where is the weight coming from?


:question: Good question.

#14 Markovich

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Posted 20 November 2013 - 07:52 AM

It would be nice to have an alternative to the Obsession.

bob


No argument there, but for a minimalist design and a sandwich mirror, one would expect it to be lighter, not significantly heavier than a comparable size from Obsession, or Webster,et al.....

#15 drago

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Posted 20 November 2013 - 07:58 AM

how about mirror in various temperatures?
i be reading, a synta 14" sandwich type mirror in low temperatures (0 to -30 celsium) get an out of "right" figure, and produce terrible images...?

#16 Fireball

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Posted 20 November 2013 - 09:13 AM

I am getting excited. I am looking for a reasonably priced dob in the 20" range with minimized design and weight (hopefully) to eventually replace my trusty 12" Lightbridge :jump:

#17 Starman1

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Posted 20 November 2013 - 10:11 AM

How about the mirror in various temperatures?
I've been reading that a Synta 14" sandwich type mirror in low temperatures (0 to -30 Celsius) gets an out of "right" figure, and produces terrible images...?

This would be true of any mirror that is still cooling. And once cooled, the sandwich mirror shouldn't perform any worse than it would 50C higher.

But my question concerns the cylinders between the upper and lower thin solid sections: they are filled with air which expands and contracts with temperature. Does that internal pressure change affect the surface figure of the mirror above the cylinders? The mirror could be perfect at +20C. Does it appear dimpled at -20C?

#18 skyward_eyes

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Posted 20 November 2013 - 11:09 AM

The 150 pound weight came from factory. This scope is entirely made of metal so the weight might be a bit higher as the Obsession uses wood. Once the scope arrives from the show I will be able to weigh it.

The scope is a 9 point flotation cell.

#19 Howie Glatter

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Posted 20 November 2013 - 11:31 AM

"Nine. Don't know if plop was used."

Don mentioned the edge supports. They are pivoted lever blocks, pivoted in the plane of the mirror's center of mass. That way the support force is divided over the two disks, and also, the mirror will not loose collimation when pointed low because there will be no tendency to tip forward or back.
Mike Zammit used something similar, but more elaborate with good reason, for a 32"(if I remember correctly)Horizon scope with a sandwich mirror. It's in the vendor forum.

I believe the mirror cells are individually vented by holes in the back plate, otherwise it would become a barometer read by Pickering.

Update: the below poster shows us that the cells are vented by cuts in the glass rings. I was wondering about that, because there were holes in the backplate on the
prototype mirror at NEAF this year, and I didn't see them on this one.

#20 Roy McCoy

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Posted 20 November 2013 - 11:40 AM

But my question concerns the cylinders between the upper and lower thin solid sections: they are filled with air which expands and contracts with temperature. Does that internal pressure change affect the surface figure of the mirror above the cylinders? The mirror could be perfect at +20C. Does it appear dimpled at -20C?



Don,

I noticed that there were small “V’s” in the bottom of the cylinders which I assume were needed during the fusing process. These openings should help mitigate the temperature differential a little.


Regards,

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#21 BigC

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Posted 20 November 2013 - 12:26 PM

I WANT one!

Probably won't be able to afford it though.

#22 Galicapernistein

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Posted 20 November 2013 - 02:09 PM

The 150 pound weight came from factory. This scope is entirely made of metal so the weight might be a bit higher as the Obsession uses wood. Once the scope arrives from the show I will be able to weigh it.

The scope is a 9 point flotation cell.


As long as the individual components arent too heavy I could live with a 150 pound 18" scope. I would much rather have a heavy, sturdy scope than a flimsy light one.

#23 skyward_eyes

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Posted 20 November 2013 - 02:18 PM

Yes, this is fully put together of course. I am not sure what the heaviest component will be at this time but it should be manageable. I am a dob user myself and wont let this thing out until its something I would want to buy.

#24 drago

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Posted 21 November 2013 - 02:20 AM

I WANT one!

Probably won't be able to afford it though.


why, for example, an "hubble optics" 18" f/4.0 Premium Ultra Light Dobsonian System not fit for you?
in what matters, you think, skywatcher variant will be better?

#25 Manousos

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Posted 22 November 2013 - 06:22 AM

Hi, this is very promising for a mass production company such as Skywather. However, IMHO this prototype needs improvements considering weight as well as design improvements. Truss dobsonians and particular this type of minimalistic design should be light and at the same time rigit. Rigidity can be acheived with lighter design and implementation. There are much lighter and more rigit scopes in the market.

I fully agree that the primary need is to consider rigidity but also weight reduction is mandatory. I'm amateur constructor of three truss dobsonian scopes. The first scope I was built was very heavy, the seconds was more lighter and the third was even lighter than the olders. Rigidity was the same on the heavier and on the lighter version of the scopes (for the same diameter and focal ratio).

I think that if the above issues will be addressed and with a logical compromise on the final cost, the telescope will be more competitive regardind the available models on the market today.






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