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Meniscus primary mirror

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#1 Two shoes

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Posted 03 August 2020 - 07:16 PM

I'd like to build a fast light weight 16" Newtonian telescope based on a meniscus mirror. Using the saggita calculator, I would like a f3 which brings the overall facial length to approximately 48". I would build the tube out of carbon fiber (I have a vacuum pump) to also keep the weight down.  Not sure of the mirror cell design, but I have a vertical mill and lath in my shop, and could make something appropriate. My goal would be under 60lbs. So components would be.... 3/4"  thick 1 6"  primary, aluminum mirror cell 18 point? Carbon fiber tube,  aluminum end rings,  3.1or 3.5 secondary?  Light weight focuser manufacturer?  I wouldn't need a cooling can with meniscus. Coma corrector?  I would be interested in any suggestions for any aspects of this build. I did for a time have a kiln behind my shop but I sold it. So my list of questions. Where to acquire a mirror, mirror cell design, secondary size. Light weight focuser, coma corrector for f3, spider design,  most likely, I'm not the only one out there kicking around the big  light weight Newtonian light bucket idea. If this was your build, what would your newt look like?



#2 cuzimthedad

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Posted 03 August 2020 - 07:46 PM

Moving to ATM



#3 Augustus

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Posted 03 August 2020 - 07:52 PM

I wouldn’t bother with CF, it’ll cost you hundreds of dollars and save a few pounds and most.
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#4 don clement

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Posted 03 August 2020 - 08:18 PM

I wouldn’t bother with CF, it’ll cost you hundreds of dollars and save a few pounds and most.

However with CF you would have a very low coefficient of thermal expansion so little changes in focus with temperature changes

.

 

Don


Edited by don clement, 03 August 2020 - 08:18 PM.

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#5 arcainemachinest

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Posted 03 August 2020 - 08:33 PM

Where would you get the mirror blank?



#6 TOMDEY

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Posted 03 August 2020 - 08:42 PM

Nice project! 16-inch is today's mid-sized deep-sky scope. Have you made a mirror or built a scope before? If this is your #1, I'd suggest not going too exotic right out of the blocks. Stick with a vanilla standard Dobsonian design and you will finish it quickly and reliably. Normand Fullum has made premium 16-inch mirrors for me, excellent performance. At F/3 a Coma Corrector is imperative. TeleVue Paracorr Type 2 "It is ideal for mirrors as fast as f/3". So, I'd say go meh conservative old-school to get observing soon. FeatherTouch is a nice ultralight Dobsonian focuser. Here >>>

 

http://starlightinst...t&product_id=62

 

Can always consider upgrading to exotic materials, geometries, ultralight... later on.

 

[A convex back mirror needs a special whiffle tree for that, and also thoughtfully-different edge-support. I have that on mine, but have been into telescope builds for many decades. Picture of that elegant mechanicals here. >>>]    Tom

 

With 16-inch you can't go wrong with a standard build. Might also reconsider something slower than F/3.

 

~click on~ >>>

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#7 Two shoes

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Posted 03 August 2020 - 09:05 PM

Tomdey, thanks for the info. I like the cell, simple, with many points. I counted 3 on 3 with 3 for 27 and assume the center of the first 3 had a touch point. I have a light bridge 16 that I like a lot except for moving, I picked up a small 10" Orion dob and liked everything but the bending over. Now a fast Newtonian on the other hand is a lot more arch  friendly, vertical to horizontal. I know a dob would be easier, but I'm looking to possibly use it for ap, being that fast should speed up exposure time substantially. Does Norman Fullum have a web site? What would it run for a 16" ? Is this finished or a blank. 



#8 TOMDEY

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Posted 03 August 2020 - 10:47 PM

Tomdey, thanks for the info. I like the cell, simple, with many points. I counted 3 on 3 with 3 for 27 and assume the center of the first 3 had a touch point. I have a light bridge 16 that I like a lot except for moving, I picked up a small 10" Orion dob and liked everything but the bending over. Now a fast Newtonian on the other hand is a lot more arch  friendly, vertical to horizontal. I know a dob would be easier, but I'm looking to possibly use it for ap, being that fast should speed up exposure time substantially. Does Norman Fullum have a web site? What would it run for a 16" ? Is this finished or a blank. 

Normand makes finished premium mirrors. 16-inchers are traditional solids... and most sensible in that modest size. Premium 16" are a bargain.

 

https://www.optiquesfullum.com/

 

He no longer lists prices on his site, but good browsing there to see his offerings. He has a contact # there. Premium mirrors are about double the price of the ultra-cheap ones found in the production line scopes, and well worth the upgrade!    Tom



#9 Pinbout

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Posted 04 August 2020 - 08:04 AM

I wouldn’t bother with CF, it’ll cost you hundreds of dollars and save a few pounds and most.

they could build it in a sonotube 1st to get things spaced correctly then consider if they wants to continue with a CF tube...


Edited by Pinbout, 04 August 2020 - 10:10 AM.


#10 Pierre Lemay

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Posted 04 August 2020 - 11:34 AM

Normand has a 32 inch diameter slumping mold with a 120 inch radius of curvature. You could ask him to slump a 16 inch dia x 3/4 inch thick borofloat disc on it. This would result in a 16 inch f/3.75 blank, ready to fine grind. However he normally works with 17mm thick borofloat (0.67 inch). Would be less costly if he has a piece lying around than ordering a full sheet of 19mm (3/4 inch) which costs about 900$USD.


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#11 Two shoes

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Posted 04 August 2020 - 07:14 PM

Pinbout, i like the idea of building it out of a sonotube first, then the tube could be used as the mold.



#12 Two shoes

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Posted 04 August 2020 - 08:08 PM

Pierre, I will email Norman tomorrow, should I mention your name as a reference, or just say a member of the cloudy nights community.



#13 Pierre Lemay

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Posted 04 August 2020 - 09:11 PM

Pierre, I will email Norman tomorrow, should I mention your name as a reference, or just say a member of the cloudy nights community.

Feel free to mention my name. He and I spoke about this just a few weeks ago. 


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#14 stargazer193857

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Posted 06 August 2020 - 08:39 AM

I'd like to build a fast light weight 16" Newtonian telescope based on a meniscus mirror. Using the saggita calculator, I would like a f3 which brings the overall facial length to approximately 48".

...
My goal would be under 60lbs.
...
3/4" thick 1 6" primary, aluminum mirror cell 18 point?


...
3.1or 3.5 secondary?
...

Why do you want f3? Prefer a camping chair vs a tall astro chair? An 18-point cell would work fine even at f4.


If you use low density wood such as poplar or light ply, 0.4, I bet you can make it a hair under 40 pounds. Big advantage of going fast.


An 18-point should be fine with 16" f3 0.75", from what I've read, although some people want even less reflection.


At f3 and a low profile focuser that puts the focal plane 4" outside the on axis light path, you would need a 4.5" secondary.
If you make the focal length 56", you could use a 4" secondary.
If you use a helical focuser like Augustus bought (very low profile), you can get the focal plane even closer to the on axis light path and use a smaller secondary, but the coma corrector will protrude into the light path.
It is recommended that a 1" gap be present between the inner UTA and the on axis light path.

Edited by stargazer193857, 06 August 2020 - 08:43 AM.


#15 stargazer193857

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Posted 06 August 2020 - 08:54 AM

I thank Pierre for telling us that 17mm is Fullum's standard size. Sounds like you already did a search for Fullum optics or mirrors.

17mm is thick enough for a fast 16" and 18 point cell. It should cool even faster. I also suggest taller nylon pads on your triangles, for a better air gap between the mirror back and triangles.

#16 Pinbout

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Posted 06 August 2020 - 09:25 AM

Pinbout, i like the idea of building it out of a sonotube first, then the tube could be used as the mold.

you could just CF/epoxy right on it. leave it for the inside flat black surface. not sure but the cardboard may have better thermal properties... just saying


Edited by Pinbout, 06 August 2020 - 09:25 AM.


#17 Two shoes

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Posted 12 August 2020 - 12:50 AM

So I've been emailing Normand @ optiquesfullm and the cost for the mirror is fairly hefty, of course you get what you pay for. To figure a mirror that thin and fast is a daunting task. The problem is if I want to stick with my fast newt on an EQ mount, I'll need a fast F 3 mirror and his mold is a F 3.75. That puts my focal length at approximately sixty inches. I'm considering getting another kiln and going the refractory/slump, plate glass route. If I do I'll have to build a rotating table with a rpm control and do a epoxy negative to make the refractory mold.  I do like the epoxy negative idea for the added benefit of sending the negative off with the mirror too both support and protect the mirror in shipping, but also I'm wondering if it could be used to support  the mirror during the figuring.



#18 stargazer193857

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Posted 12 August 2020 - 11:46 AM

However with CF you would have a very low coefficient of thermal expansion so little changes in focus with temperature changes
.

Don


I think that is more important for imagers doing a long exposure. Visual users can just use fine adjustment as needed, assuming all tubes cool symmetrically. Warm air rising out might affect that.

#19 stargazer193857

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Posted 12 August 2020 - 11:58 AM

So I've been emailing Normand @ optiquesfullm and the cost for the mirror is fairly hefty, of course you get what you pay for. To figure a mirror that thin and fast is a daunting task. The problem is if I want to stick with my fast newt on an EQ mount, I'll need a fast F 3 mirror and his mold is a F 3.75. That puts my focal length at approximately sixty inches. I'm considering getting another kiln and going the refractory/slump, plate glass route. If I do I'll have to build a rotating table with a rpm control and do a epoxy negative to make the refractory mold. I do like the epoxy negative idea for the added benefit of sending the negative off with the mirror too both support and protect the mirror in shipping, but also I'm wondering if it could be used to support the mirror during the figuring.


If you do build the mold and kiln, there will be at least a few other imagers who will want to buy slumped blanks from you. Might want to make kiln large enough for an 18". Sure is a big investment for just 1 blank.

Why epoxy for your negative, instead of plaster?

#20 Two shoes

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Posted 12 August 2020 - 06:22 PM

The epoxy wou!d be the starter mold, I could cut a radius of cement board and chicken sure for strenth, and then make a concave and covexed refractory mold from the epoxy. I did see a kiln for sale locally with an inside dimension of  twenty two and a half inches.  I think it I invest in a kiln I would want something closer to twenty six to twenty eight inches, then make all my molds size appropriate to the kiln itself, I could always slump smaller mirrors. I'm always looking for ways to offset the cost of my (always expensive) hobbys.




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