foucault considerations

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bobsorenson
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foucault considerations

#1

Post by bobsorenson »


Hello!
I have two big mirrors I want to work on. This whole project is kind of off the wall and I am quite the noob at all of it.

The mirrors; 24" f3 solid cast Al 80 lbs each. Probably made for high power lasers or for IR so I do not know what the specs are.
Of course, concurrent with the work on the mirror surface, I need to test and am constructing a foucault/ronchi tester and of course a stand for the mirror.

Question 1. when this mirror is vertical for the testing, will there be distortion problems? Remember, 80 lbs. It may be required to support using more than the customary two pegs...?
Question 2. are there temperature considerations when testing? My preferred workspace has uneven temperatures and sometimes is quite cold. I have other spaces I could work in but prefer my big barn/shop with wood heat.

Note: I am aware of the difficulties this entails but am determined to proceed. The mirrors were basically free so if I do not succeed then all will be fine and I'll have learned something.
Thanks in advance for advice.
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Re: foucault considerations

#2

Post by notFritzArgelander »


Tough. I'd rough out a design for support first since flexure is likely. Some wooden rings would be inexpensive and adequate without committing to a finished design. I'd also test the mirror flat with the tester above it. Uniform temperature is a very good thing. The index of refraction of air is quite temperature sensitive.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: foucault considerations

#3

Post by OzEclipse »


Bob
I'd figure out your mirror cell design and support the mirror the same way for testing.
Joe
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Re: foucault considerations

#4

Post by bobsorenson »


Thanks to you both.
I've read of a program called plop (?) Used to design cells. If this is so, can it do calls for a solid Al mirror and might I get a more experienced party to help with that?
It looks to me that the mirrors were hard mounted originally which I take as a clue that they were not made for optical.
Attaching a pic showing the back of the mirror.
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20191109_164610.jpg
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Re: foucault considerations

#5

Post by OzEclipse »


Bob

They are what is called a conical design. Used extensively by R.F. Royce to make high quality glass mirrors. I purchased a 12.5in Royce conical mirror some months ago when it came up second hand. The advantage is much lighter weight and faster cooling. Royce is out of business, retired. But he has left his website operating with info about how to mount the conical mirrors

http://www.rfroyce.com/conical
and
http://www.rfroyce.com/conical/mount%20CFB.htm [This page has a drawing of mirror cell design]

In addition to the flexure problem the other two problems I see you'll have with Al mirrors is
thermal coefficient expansion. In the vacuum of space, temperature can probably be kept quite constant. Not so on Earth.
In the vacuum of space, there is no oxygen, no surface oxidisation

You wrote:
Question 2. are there temperature considerations when testing? My preferred workspace has uneven temperatures and sometimes is quite cold. I have other spaces I could work in but prefer my big barn/shop with wood heat.
Yes there are two temperature issues when Foucault testing. air currents of varying temperatures are seen as heat shimmers across the surface similar to heat shimmer you see on a long flat road on a hot sunny day. Temperature fluctuations will change the figure of a pyrex mirror, it will have a much greater effect on an Aluminium mirror.

Given the mirrors were free, it is certainly worth pursuing this a bit to see what can be done. A 24 inch f3 mirror costs a lot of money.

kind regards

Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
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bobsorenson
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Re: foucault considerations

#6

Post by bobsorenson »


Joe, great info.
Thanks so much.
I am in hopes that the much greater thermal conductivity of Al will compensate somewhat for the thermal expansion problem.

First step is to very carefully clean the oxidation and polish the surface without mucking up the figure of the mirror so that I can test and see if I am even in the ballpark.
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Re: foucault considerations

#7

Post by bobsorenson »


Hello,
Can anyone recommend what type(s) of Ronchi screens (LPI) would be best to test my 25" mirrors?

TIA
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Re: foucault considerations

#8

Post by bobsorenson »


20191211_140248.jpg
Tester complete!
Here is the bottom stage. I used maple for the knobs and stops and sch k copper for the ways.
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Re: foucault considerations

#9

Post by bobsorenson »


And here is the front showing the wood battery box.
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Re: foucault considerations

#10

Post by bobsorenson »


Now I am unable to get the mirror face illuminated.....I think because it needs to be polished, the surface is quite matte.
So, 25 micron, 12 m and some 5 m aluminum oxide, then we'll see.
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Re: foucault considerations

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Post by OzEclipse »


bobsorenson wrote: Thu Dec 12, 2019 10:34 pm Now I am unable to get the mirror face illuminated.....I think because it needs to be polished, the surface is quite matte.
So, 25 micron, 12 m and some 5 m aluminum oxide, then we'll see.
Bob,

Suggest you first apply the finest polishing compound gently with cotton balls. Depending on the extent of the surface damage, this might gently polish the oxidised surface sufficiently to allow a reasonable optical reflection without changing the figure too much. Depending upon the depth of the surface oxidation, changing the figure may be unavoidable.

I like your tester, I can see its design definitely has some Jean Texereau DNA in its genes ;-)

Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
Memberships Astronomical Association of Queensland; RASNZ Occultations Section; Single Exposure Milky Way Facebook Group (Moderator) (12k members)
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