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Building a 14" sub-f3 RFT

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#1 totvos

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Posted 12 October 2015 - 09:35 PM

In another thread, I am exploring sliding Newtonian ideas, with the goal of refactoring the OTA for my existing 8" dob. In THIS thread, I will be detailing the build of a new scope that is based on Mel Bartels' amazing (IMO) work on fast RFTs using meniscus mirrors. My goal is to replicate his 13.2" scope (detailed here), in a slightly larger, and faster format: 14" @ f/2.8. I am really excited by these scopes because, as I mention in my other thread, I am hampered by portability and with these very fast designs, I can have a highly portable instrument that is also of substantial aperture for (hopefully) excellent viewing.

 

For the last few weeks, I have been immersing myself in the design, trying to understand its nuances, and getting my head around the various stages I will need to go through. Fortunately, Mel has been extremely generous with his time and advice, and has been willing to answer any questions I may have. So at this point, it is full steam GO.

 

Of course, the scope all starts with the optics and, specifically, the primary. If you are familiar with these scopes, they feature very thin primaries and achieve their fast curvatures by being meniscus mirrors, like a contact lens with an aluminum coating. This enables them to have relatively deep sagittas, while retaining a constant thickness of glass, giving them surprising strength for their low mass compared to normal mirror blanks. Additionally, the mirrors use plate glass instead of pyrex, to make them substantially cheaper to fabricate.

 

So my story will start with the primary and note: this is an in-progress build thread. I am only just a few steps down the path, and will update as things change. I have a bit more to say now, but it is late from a long Thanksgiving weekend, so I will continue tomorrow on the fabrication of a meniscus mirror.


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#2 jtsenghas

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Posted 13 October 2015 - 08:42 AM

I have a bit more to say now, but it is late from a long Thanksgiving weekend, so I will continue tomorrow on the fabrication of a meniscus mirror.

 

There's a hint about your location; are you north of the border?

 

I'm also fascinated by these fast thin mirrors and am very impressed by the scope you are inspired by.  I've yet to join the ranks of the glass pushers and have no illusions about the challenges involved.

 

I do, however, have some experience on the mechanical sides of things and have been dabbling in ideas for back and edge supports for meniscus mirrors, and would like to participate in your thread--if to do nothing but "Oooo" and "ahhh!".

 

It is clear that a lot of the work on supporting thin meniscus mirrors is experimental and I have a few ideas along these lines for both analysis and supports.  Another Cloudy Nights member, prfesser, was working on an 8" meniscus mirror recently and I was sharing ideas with him via PM.  He unfortunately did not heed John Dobson's advice "If you drop a mirror, be sure to drop it early in the process".

 

Note that on Mel's smaller f/2.8" scope he offsets the altitude bearings quite a bit for balance.  The required Paracorr II and likely chosen wide angle eyepieces would put a couple of pounds pretty far from the optical axis and you wouldn't want to design such a portable scope to require a lot of ballast.  It would be well worth designing your rocker box for a bit more mirror box clearance than you may be planning on in order to achieve balance with a minimum of weight.

 

I look forward to this thread eagerly.



#3 totvos

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Posted 13 October 2015 - 09:41 AM

JT, I am excited you are excited, as it is clear you have a ton of mechanical expertise to offer.

 

For Mel's designs, the 13.2" (which is my template because it looks so freakin' cool) also uses a Paracorr II and 100 degree eyepieces. The benefit of going fast is that the optical path is substantially shortened, making the balance less persnickety. We'll see how it goes. I will also add however that my choice of 14" and f/2.8 wasn't exactly pulled out of thin air. I wanted to go slightly bigger...just because...but then I also shortened the FL a wee bit to come out roughly the same as Mel's scope. That way I should not have to tweak the design too much because the folding mechanism is a bit tricky to get right. So the way I look at it, I am beefing out the radius by less than 0.5", and keeping the FL close to the same (around 40"). I have mocked it up in balsa wood to get a better feel for it.

 

But I will caveat that depending on how my sliding experience goes with the 8", I may do a sliding design on this too. I can't decide right now, and it will largely be an aesthetics thing. I don't much like, for example, the holes that are drilled into the supporting structure, obviously to cut down on weight. I would rather use thinner material than the 1/2" plywood he is using. Could I, for example, get away with 6-8mm okoume. Dunno.

 

"Ballast" being the ATM term for a counterweight?

 

And yes, I am just outside Toronto.



#4 jtsenghas

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Posted 13 October 2015 - 09:53 AM

"Ballast" isn't really an ATM term, it's actually nautical.  I like to use it in the context of permanent low weights for helping to maintain balance much as a ship's keel or heavy weights in the bottom of a ship's hull does much the same thing.  Actually, I haven't seen others using it in that context but I'm one for playing with words that hint more at function. Yes, I am referring to permanent counterweights for maintaining a center of gravity close to the altitude axis for most eyepiece combinations rather than adjustable counterweights that may be added to compensate for the occasional heavy eyepiece..


Edited by jtsenghas, 13 October 2015 - 01:41 PM.

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#5 Pinbout

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Posted 13 October 2015 - 10:09 AM

I use the word cause I need to add weight on my thin mirror ultra light scopes.

 

 

anyway did you download his model

 

https://3dwarehouse....47a4848f4241ae0

 

I just downloaded it and scaled it to a 14in primary, wasn't that much of a difference is sizing.


Edited by Pinbout, 13 October 2015 - 10:16 AM.


#6 totvos

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Posted 13 October 2015 - 12:38 PM

anyway did you download his model

 

https://3dwarehouse....47a4848f4241ae0

 

I just downloaded it and scaled it to a 14in primary, wasn't that much of a difference is sizing.

 

Yes, I do have his SketchUp models and have played with them extensively, including generating some LayOut drawings from them. I would like to be better at SketchUp, but I am not (currently).



#7 totvos

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Posted 13 October 2015 - 10:13 PM

As mentioned before, the benefit of the meniscus mirror is that you can achieve deeper sagittas without sacrificing glass toughness by keeping it a constant thickness throughout. And because the glass thickness is much less than a flat blank, temperature issues are greatly lessened. The trick, then, is to get this meniscus shape. As Mel Bartels succinctly puts it, the enabling technology for these types of mirrors is computer-controlled kilns. They allow the glass to be gently slumped to the right shape, and then equally gently brought back to room temperature, annealing the glass.

 

My dilemma, then, is how to slump? Fortunately for me, Mel put me in touch with his master slumper, David Davis. Like Mel, David has been extremely generous with his time and advice which, in a nutshell boiled down to this:

  1. I could get David to slump my glass, paying only for his time, and the shipping of the glass. My net would be about US$250.
  2. I could build my own kiln and slump it myself, for about C$350.
  3. I could buy a kiln and slump it myself (retails for >$1500 new).

With the current exchange rate, it was almost a wash having David slump for me, or me building my own kiln (more on that in a moment). Aside from cost, though, one of the other factors was fun and learning: clearly I would get more out of doing it myself. And isn't that written down somewhere in the ATM Manifesto?

 

Making a kiln turns out to be surprisingly straight-forward, and David and I went back and forth on a couple of designs. You essentially need:

  1. A lot of insulation.
  2. Heating elements.
  3. Computer-controlled power.

The most expensive piece is (3), ranging from around $100-$200...eBay has them pretty cheap. The insulation, in the form of firebricks (made specifically for kilns), or ceramic wool (ditto), is easily obtained but adds up in cost as the kiln dimensions increase. I actually had a very local place near where I live, which helped with the USD/CAD thing, and shipping. The heating element involves a bit of electrical engineering, deciding how much coil and what gauge to use for the power you are set up to draw from the mains, and of course how hot you want your kiln to get. Luckily, for slumping we only need around 1300F, but your power needs are not only determined by the temperature you want, but also the volume of air you are heating. The push-pull is you need a big enough space to fit your blank with more than a few inches of air between it and the elements, but small enough to heat it with less power. And round is a more efficient use of space for a round blank than square or rectangular, but round is harder to do with bricks. More tradeoffs.

 

So I had mentally made the leap to DIY my kiln when, as luck would have it, a suitably sized kiln came up for sale on Kijiji (a Craigslist thing that they have here in Canada, also owned by eBay). I could get not one but TWO kilns for $150. Bingo. All I would need to do, then, is build the computer control since these kilns were old-school cone and timer jobs. But in parts alone, these were worth over $250, so I think I got a good deal. My thinking is that after I am done, and have computerized the bigger one, I could probably resell it at a profit because, of course, I am only going to make one mirror.  :lol:

 

As of right now, I have two kilns in my garage, and the necessary bits to computerize it arrived this morning. I'll get that part going before another write up. Oh, and the glass is ordered (again from a very local source) and should be ready to be picked up next week. Stay tuned.



#8 Pinbout

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Posted 13 October 2015 - 11:12 PM

Once the glass is slumped and annealed properly the trick is to grind and figure it without any stig. Not an easy task as i saw a couple of 16"f2.9's 3/4" thk at delmarva. They made a total of 6. This is the only one i saw that had virtually  no stig.

 

 

20140323_093642.jpg


Edited by Pinbout, 13 October 2015 - 11:33 PM.

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#9 allardster

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Posted 14 October 2015 - 09:25 PM

Interesting stuff Tom. I am looking forward reading more about the kiln and slumping. I am in the next town east of you. There is a small active community of glash pushers and scope makers here. One of us just finished a Chiefspiegler there are a few more in the making. Someone else in our groups was looking to slump some glass. Come and join us for one of our next informal meetings / show and tells.


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#10 kfrederick

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Posted 15 October 2015 - 05:59 AM

Interesting stuff Tom. I am looking forward reading more about the kiln and slumping. I am in the next town east of you. There is a small active community of glash pushers and scope makers here. One of us just finished a Chiefspiegler there are a few more in the making. Someone else in our groups was looking to slump some glass. Come and join us for one of our next informal meetings / show and tells.

    A  CHief has been finished   Great   How big and how it doing ?


Edited by kfrederick, 15 October 2015 - 06:32 AM.


#11 Pierre Lemay

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Posted 15 October 2015 - 11:26 AM

In THIS thread, I will be detailing the build of a new scope that is based on Mel Bartels' amazing (IMO) work on fast RFTs using meniscus mirrors. My goal is to replicate his 13.2" scope (detailed here), in a slightly larger, and faster format: 14" @ f/2.8. I am really excited by these scopes because, as I mention in my other thread, I am hampered by portability and with these very fast designs, I can have a highly portable instrument that is also of substantial aperture for (hopefully) excellent viewing.

How much mirror making experience do you have? What are the fastest mirrors you have made?



#12 totvos

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Posted 15 October 2015 - 11:47 AM

How much mirror making experience do you have? What are the fastest mirrors you have made?

 

I have not done any glasswork for well over 35 years. And certainly nothing at this f-ratio. I understand this is a challenging project.



#13 allardster

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Posted 15 October 2015 - 03:01 PM

@kfrederick: got to see the scope last night. It's just finished and collimated. Images of the moon are coming out crisp!


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#14 Pierre Lemay

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Posted 15 October 2015 - 04:21 PM

 

How much mirror making experience do you have? What are the fastest mirrors you have made?

 

I have not done any glasswork for well over 35 years. And certainly nothing at this f-ratio. I understand this is a challenging project.

 

Well if it can be of any encouragment, I hadn't ground and polished a mirror in 30 years as well when I finally got around to working my 20 inch f/3.9 mirror four years ago. The biggest I had ever done before was a 12.5. f/5.2 and that was not a resounding success. The 20 inch turned out OK, usable (1/6 wave according to my optimistic measurements but probably closer to 1/3 wave p/v). It will need to go back on the polishing machine in a year or two to get a smoother and better surface but, for the moment, in spite of the so-so surface, I get a lot of fun out of it.

 

Your F/2.8 will be quite a challenge. My intention is to regrind the 12.5 inch I just mentioned above into an f/3 (full thickness pyrex Mirror) sometime next summer. Also inspired by Mel's work (and, more recently Dany (Pinbout)), the idea is to make a very fast mirror to get wide views, but which will also double as an astrograph since this will be mounted in a very solid equatorial mounting. 

 

A secondary objective is to improve my mirror making skills in preparation for the reparabolizing of the 20 inch. Among other things this will be a very good occasion to re-visit the more rarely used measuring tools I've built over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I might even try my hand with a Bath IF test altough I have doubts about it's reliability with a 12.5. f/3 paraboloid.

 

So best of luck with this exiting project. Many of us will be cheering you on.



#15 totvos

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Posted 15 October 2015 - 05:00 PM

Thanks for the encouragement, Pierre. I know I have a bit a tough road ahead, but it is clearly not impossible. Equally clearly, Mel is a master mirror maker and I am not, but I am very inspired by the amount of detail he provides in his process, and I believe that perseverance and methodical work will prevail in the end. One very interesting point about Mel's process, and I am inclined to trust his experience, is that he figures his mirrors solely through Ronchi and start testing.

 

As an engineer, I dig precision and methodical work. So this might all come together as planned. As a side-effect, I am pulling along one of my boys (10 years old) who seems to have a "maker" bent to him. He'll learn a lot, starting with basic electrical work on the kiln.


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#16 Pinbout

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Posted 15 October 2015 - 06:16 PM

[[over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I ]]

Get it up to a full polish and bring it to delmarva next march.
The AC is the most qucikest way to test precisely


Edited by Pinbout, 16 October 2015 - 09:57 AM.


#17 totvos

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Posted 16 October 2015 - 06:26 AM

<p>((over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I ))</p>
<p>&nbsp;</p>
<p>Got it up to a full polish and bring it to delmarva next march.</p>
<p>The AC is the most qucikest way to test precisely.</p>

Sorry, but I have tried to parse this several times now, and I really don't know what it means.



#18 Pierre Lemay

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Posted 16 October 2015 - 07:47 AM

 

<p>((over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I ))</p>
<p>&nbsp;</p>
<p>Got it up to a full polish and bring it to delmarva next march.</p>
<p>The AC is the most qucikest way to test precisely.</p>

Sorry, but I have tried to parse this several times now, and I really don't know what it means.

Don't worry if you don't understand. It happens sometimes on this forum. It's a New Jersey dialect that Danny sometimes uses to speak in code to other NJ ATMs which, I believe, only they can understand :)



#19 rik ter horst

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Posted 16 October 2015 - 08:45 AM

Interesting thread this is! You obviously like challenges ;-) 

 

Also inspired by the work of Mel Bartels I started building a 16 inch F/3 dobson with a 19 mm thick, or thin ;-)  slumped mirror (plate glass) as an experiment. This mirror now is in its final stage and needs some last corrections. Despite the mirror not being aluminized yet I only can say that I'm excited about the views it delivers when mounted in the test-scope built earlier by Geert Kwast, a fanatic local telescope maker. Equiped with an ES coma corrector it delivers a wide, totally coma-free field of view. I was quite surprised to be able to separate each individual component of Epsilon Lyra not only in the middle of the field but also almost right to the edge with a 13 mm Ethos ! And all these observations were done comfortably seated.... 

 

Below the mirror in its test-scope. I'm working on hardware now and will post some more pictures of it later.

 

Have fun with this project Tom! 

 

 

Attached Thumbnails

  • 20151011_164702 small.jpg

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#20 radicell2

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Posted 16 October 2015 - 09:01 AM

A


 

  1.  
  2. I could build my own kiln and slump it myself, for about C$350.
  3.  

With the current exchange rate, it was almost a wash having David slump for me, or me building my own kiln (more on that in a moment). Aside from cost, though, one of the other factors was fun and learning: clearly I would get more out of doing it myself. And isn't that written down somewhere in the ATM Manifesto?

 

Making a kiln turns out to be surprisingly straight-forward, and David and I went back and forth on a couple of designs. You essentially need:

  1. A lot of insulation.
  2. Heating elements.
  3. Computer-controlled power.

The most expensive piece is (3),

 

 

 You really need to look into the cost factor and the types of controller and the very subject of how to anneal glass.Plus you need to make a mold to make a mold over which to slump.If Davis is still using a kiln that only has coils in the side walls,stay away from his stuff.Perhaps Mel may have mentioned the Yahoo group "red hot mirrors" - sort of dead now but has a lot of good stuff still.

 

Hint- don't run the kiln in an unheated space during winter.- controllers go "odd"Always have a second temp display unit active at the same time as the primary.You may be making only one mirror but used kilns and coils have been "used up"  so a coil can go anytime.Watch out for the drunk driver that "knows" you're annealing glass that week and finds the nearest hydro pole.

 

Ric

 

http://www.rokoszoptical.yolasite.com


Edited by radicell2, 16 October 2015 - 09:18 AM.


#21 totvos

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Posted 16 October 2015 - 09:09 AM

Interesting thread this is! You obviously like challenges ;-) 

 

Also inspired by the work of Mel Bartels I started building a 16 inch F/3 dobson with a 19 mm thick, or thin ;-)  slumped mirror (plate glass) as an experiment. This mirror now is in its final stage and needs some last corrections. Despite the mirror not being aluminized yet I only can say that I'm excited about the views it delivers when mounted in the test-scope built earlier by Geert Kwast, a fanatic local telescope maker. Equiped with an ES coma corrector it delivers a wide, totally coma-free field of view. I was quite surprised to be able to separate each individual component of Epsilon Lyra not only in the middle of the field but also almost right to the edge with a 13 mm Ethos ! And all these observations were done comfortably seated.... 

 

Below the mirror in its test-scope. I'm working on hardware now and will post some more pictures of it later.

 

Have fun with this project Tom! 

Do you have details written up on your work anywhere, Rik? Right now, I am most interested in the slumping, but anything about your process from slump to test would be helpful.



#22 totvos

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Posted 16 October 2015 - 09:48 AM

Plus you need to make a mold to make a mold over which to slump.

What Ric is pointing to here is something I have not written about yet, partly because I am not there yet and partly because I have not firmed up how I am going to do it. But essentially, the mold (or what I would call a form) over which the glass is going to be slumped must be "refractory", meaning able to withstand kiln temperatures. Making a precision refractory mold can be done a variety of ways, and one way is to create a "mold for the mold" using materials that are easier to work with. I believe different folks have different philosophies on how accurate that mold has to be, just like there are two options for slumping: over a convex form, or into a concave mold.

 

My current thinking is to start by casting two plaster disks, about 16" in diameter. Then I would do standard ATM grinding to make spherical surfaces of precisely the right sagitta. My goal is to be *very* precise in order to get my glass to where I don't have to do any rough grinding. Once the plaster is correct, I would use the concave one to provide the mold for a form that I would cast out of it, using a formula that Mel and David seem to find effective, and the resulting form would go into the kiln. The remaining convex piece would become the base for the tool to do fine grinding.

 

That's the plan. For now.



#23 Pinbout

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Posted 16 October 2015 - 09:59 AM

 

<p>((over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I ))</p>
<p>&nbsp;</p>
<p>Got it up to a full polish and bring it to delmarva next march.</p>
<p>The AC is the most qucikest way to test precisely.</p>

Sorry, but I have tried to parse this several times now, and I really don't know what it means.

 

 

my phone did that. pretty freaky. and when I went to fix it, I couldn't even see my post...

 

till now


Edited by Pinbout, 16 October 2015 - 10:04 AM.


#24 Pinbout

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Posted 16 October 2015 - 10:03 AM

 

 

<p>((over the years (particularly Caustic and Ross null tester) but also make better use of the star test. I ))</p>
<p>&nbsp;</p>
<p>Got it up to a full polish and bring it to delmarva next march.</p>
<p>The AC is the most qucikest way to test precisely.</p>

Sorry, but I have tried to parse this several times now, and I really don't know what it means.

Don't worry if you don't understand. It happens sometimes on this forum. It's a New Jersey dialect that Danny sometimes uses to speak in code to other NJ ATMs which, I believe, only they can understand :)

 

 

moudre votre miroir pour un vernis plein.

prendre votre miroir pour delmarva Mars prochain

en utilisant l'auto-collimateur, vous serez terminé en trois jours


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#25 kfrederick

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Posted 16 October 2015 - 10:11 AM

Why a curved back?  How many top mirror makers do that ? Why you wont the most important part of the mirror the [outside area] To be thinner than would be otherwise .?   I see no big advantage and see it be harder to support on the table . No crime to try 




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