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Folded 12" Alvan Clark

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#26 NinePlanets

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Posted 16 April 2024 - 02:55 PM

You have a lot of experienced, helpful guys rooting for you, jragsdale. You'll do fine.

Forward march!

We may all learn some good stuff from this project!


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#27 jragsdale

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Posted 16 April 2024 - 04:23 PM

Ok, here's a good question regarding flats and optical substrate. Would a standard pyrex flat work well as a shallow tilt "steering flat" like in this folded refractor project when it comes to thermal equilibrium. Thermal plumes will obviously be an issue with any mirror, but glass substrate changing figure with a standard reflector is a big problem and a reason people shell out big bucks for fused silica and zerodur newtonian mirrors. My question is, does this same distorting of figure affect flats in the same way? Will it change the spherical abberation of the wavefront? Or add astigmatism? Or will it just make the focal plane move a tiny bit by adding a minute convex or concave shape to the flat?

#28 Max Coe

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Posted 16 April 2024 - 05:13 PM

I think that you will have the same design issues with mounting the flats that one has with mounting parabolic mirrors. Do you have access to a copy of Paul Yoder's book on mounting optics, by any chance? You have many options, there. Also, I wouldn't lose any sleep over the scatter issue. All optical systems have losses somewhere. Yours will be small, and mostly associated with the objective, I think. 


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#29 jragsdale

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Posted 16 April 2024 - 06:23 PM

I think that you will have the same design issues with mounting the flats that one has with mounting parabolic mirrors. Do you have access to a copy of Paul Yoder's book on mounting optics, by any chance? You have many options, there. Also, I wouldn't lose any sleep over the scatter issue. All optical systems have losses somewhere. Yours will be small, and mostly associated with the objective, I think. 

Not to mention all the striations and bubbles that were common in glass in the mid 19th century that this lens has.


Edited by jragsdale, 16 April 2024 - 06:25 PM.


#30 RichA

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Posted 16 April 2024 - 08:31 PM

Looking at the size of the single-folded 10" f9, and given your much longer focal length, your idea of an N (Z) fold using two mirrors would be better. Dave Trott's site has this one... which I assume represents the direction you are looking

 

https://davetrott.co...lded-refractor/

 

Six-Inch-Folded-Refractor-Schematic-Smal

Could fold it into a Newtonian shape, two mirrors at right angles at the lower part of the scope, reflecting up to a standard focuser and a in-focuser diagonal.  Wouldn't be as short as a Z-fold, but it would be easier to mount, the mount would be low to the ground, a Dob style.  A 20 inch spacing on the low end of the scope between the reflex mirrors would mean the focuser would be around 70 to 80 inches from the ground, approx.


Edited by RichA, 16 April 2024 - 08:37 PM.

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#31 Oregon-raybender

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Posted 16 April 2024 - 08:32 PM

I think until you test the lens, I would not be concern about

any thermal issues of the flats. The lens will have more issues

than the flats with temp ranges. Plus you add the tube currents

they would be bigger than changes in the flats. The large flat

would be on concern. But Pyrex or if you can afford it, FS or Zerodur

would be fine, but Pyrex will be cost effected. Again scatter is of

low concern. The issues you will have is the thermal issues with

the type of mounting plate you will use. You may need to cover it

with a foam to disconnect it from adding to the currents.

 

I would just add up are the possible issues, first. I think optics

would be somewhere in the middle or lower.

 

Check with Roger C on the ATS site and ask him what he did and

his issues and how he solved them.

 

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#32 jragsdale

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Posted 16 April 2024 - 08:58 PM

You may need to cover it

with a foam to disconnect it from adding to the currents.

 

Check with Roger C on the ATS site and ask him what he did and

his issues and how he solved them.

Yeah, it'll probably be trial and error for the thermals but I'll probably include fan mounting and electronics in my design. Then not sure if I should do open truss + shroud or full cover + insulation.

 

I had a great hour long conversation with Roger C on the phone a few days ago, very helpful on many fronts!


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#33 Guntram Lampert

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Posted 17 April 2024 - 02:11 AM

Hi jgragsdale.

 

 

Years ago, I set out to make an 8" unobscured reflector that uses four (yes) powered mirrors to form the final image.

I was advised against doing it because "of all the scatter and light losses".

I made it anyway, and it serves me very well since more than ten years. All mirrosrs use plain overcoated aluminium coatings.

 

So, no need to be discouraged from using two or three folding flats.Their impact on the image will be absolutely minimal if well made with ok coatings.

The advantages of a compact setup far outweigh any optical disadvantages that might arise from the fold flats.

 

The flats should just be as low - mass as possible, to speed up thermal equilibration and thus reduce the thermally induced effects on the wavefront.

 

Best regards, and good luck,

 

Guntram Lampert


Edited by Guntram Lampert, 17 April 2024 - 02:43 AM.

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#34 jaliteuk

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Posted 17 April 2024 - 04:03 AM

Many years ago I built a 4” F20  Red Lens (Ha) Solar scope folded in  N.

format . But the best tube I found to use was a section of a Yacht mast with oval form. Maybe you might find a short piece of a suitable diameter in a yacht yard vor mast builder?  Just a thought. John.    All the best with the build.


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#35 jragsdale

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Posted 18 April 2024 - 07:03 PM

2nd flat received, the 6" one. Still looking for a 10" flat now.

Attached Thumbnails

  • 20240418_120029.jpg

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#36 Oregon-raybender

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Posted 18 April 2024 - 09:43 PM

That should do. Nice find

 

Starry Nightswaytogo.gif


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#37 JonH

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Posted 18 April 2024 - 11:17 PM

Hi Jordan, congratulations on securing this wonderful objective! John Wall's Zerochromat uses a folded lightpath as discussed here and elsewhere on CN. I think Zerochromats were available with apertures up to 12 inches so this could work for you. You wouldnt require the corrector at the eyepiece as the zerochromat used a large singlet as an objective. 

 

Good luck!

 

Jon

 

Zerochromat.jpg

 

From Dark Star astronomy


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#38 jragsdale

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Posted 05 May 2024 - 01:04 PM

I've located a 10" optical flat (Fused quartz, 2" thick, 1/10 wave wavefront (0.000001") in excellent condition that I've purchased. Will evaluate then send off to be coated.

 

In the meantime I'm acquiring and designing the mirror cells. I want to 3D prrint the holder for the 160mm flat because it's so thin it's more like a secondary mirror. I want to incorporate the correct angle of incident into the design to allow a fairly flush install and collimation screws will be working on a flat surface as well. Am I correct in my calculations that the angle of tilt required in an N design for the 2nd smaller flat is half the tilt of the first flat? Or is it the same? The angle of incidence reflection calculations are a little fuzzy to me.


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#39 Oregon-raybender

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Posted 05 May 2024 - 01:38 PM

I would suggest making a simple optical layout in full scale maybe

on a sheet of plywood, like a train layout on a table.

You can make simple cardboard cutouts of the all the optics.

 

Then use a laser pointer to move the real mirrors about until you

get the right angles. Allow for extra tilt just in case, set the mirror

at you mid range, then +/- about a 1/2 degree (which will give you 1 deg )

Like the image in #30.

 

Starry Nightswaytogo.gif


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#40 macdonjh

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Posted 05 May 2024 - 01:58 PM

Many years ago I built a 4” F20  Red Lens (Ha) Solar scope folded in  N.

format . But the best tube I found to use was a section of a Yacht mast with oval form. Maybe you might find a short piece of a suitable diameter in a yacht yard vor mast builder?  Just a thought. John.    All the best with the build.

That is a cool idea.  I never would have thought of that.  I'd have ended up building my own OTA using fiberglass/ epoxy lay-up.  Messy, messy.

 

I've located a 10" optical flat (Fused quartz, 2" thick, 1/10 wave wavefront (0.000001") in excellent condition that I've purchased. Will evaluate then send off to be coated.

 

In the meantime I'm acquiring and designing the mirror cells. I want to 3D prrint the holder for the 160mm flat because it's so thin it's more like a secondary mirror. I want to incorporate the correct angle of incident into the design to allow a fairly flush install and collimation screws will be working on a flat surface as well. Am I correct in my calculations that the angle of tilt required in an N design for the 2nd smaller flat is half the tilt of the first flat? Or is it the same? The angle of incidence reflection calculations are a little fuzzy to me.

After doing just what Oregon-raybender suggests, I'd hope to find my thought experiment result is correct: the same (but opposite-sign) angle for both flats.  Your incoming light cone is traveling at some reference angle, I'll call it 0o.  Your first flat directs the light cone at some angle, but your second flat probably has to "straighten" the light cone back to 0o for your eye piece or diagonal.  At least, that's the way I thought it through.

 

Good luck with your project.  


Edited by macdonjh, 05 May 2024 - 02:00 PM.

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#41 jragsdale

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Posted 05 May 2024 - 02:15 PM

I'd hope to find my thought experiment result is correct: the same (but opposite-sign) angle for both flats.  Your incoming light cone is traveling at some reference angle, I'll call it 0o.  Your first flat directs the light cone at some angle, but your second flat probably has to "straighten" the light cone back to 0o for your eye piece or diagonal.  At least, that's the way I thought it through.

Yeah, I think you're right, for some reason I was thinking if the mirrors are parallel to each other, it would just bounce the light right back out the front, so the tilt angles couldn't be identical, but I think I'm looking at it from a single mirror perspective, not a double reflection.



#42 Oregon-raybender

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Posted 05 May 2024 - 03:51 PM

You can see in #30, the angle tilts are very small, a few degrees.

The issue is the size of your bouncing mirrors, making sure

the cone from the 12 inch is clear, with the right amount of

offset, then tilt the mirror to that. You can do all of this on

the plywood with a large sheet of paper, pencil, rule and protractor.

 

Show us images

 

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#43 jragsdale

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Posted 08 May 2024 - 03:57 PM

Got my 10" 1/10 wave (wavefront) quartz flat today. It's in near perfect condition and here's a partial fringe test with an existing 4" 1/20 wave flat that I already had. Tested with a 3nm Ha filter.

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  • 20240508_145105.jpg

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