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Do you use a waffle tree for bench testing?

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#1 stargazer193857

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Posted 16 February 2019 - 01:20 PM

If you do, have you ever observed high points where the supports are, on any cell?

Thanks.

#2 Bob4BVM

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Posted 16 February 2019 - 02:30 PM

Uhh, lets see, waffle tree, I go there for breakfast sometimes...  they make those "bumpy pancakes". Yeah there probably are high spots on them, though I've never measured them before eating. Cant see how it would matter much if you chew your food anyway 

:)  :)   :)

CS

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#3 Pinbout

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Posted 16 February 2019 - 02:38 PM

I use it on my 16" perforated flat for dpac but not my mirrors I'm testing...

 

but I intend to do better and use it on my mirrors also.



#4 Dale Eason

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Posted 16 February 2019 - 06:50 PM

I test mirrors using a Bath interferometer.  I have found the best edge support for my testing is the 2 point 90 degrees apart as shown at http://www.cruxis.co...ecalculator.htm.

When the mirrror rests on those supports at its edge wise center of gravity it will balance there with no back support needed.  Because of the geometry of the support it also induces no astig into the mirror..  When not aligning the mirror on those two points properly it always induces deformation that is easily visible with the Bath interferometer.

 

 

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#5 careysub

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Posted 16 February 2019 - 07:08 PM

I test mirrors using a Bath interferometer.  I have found the best edge support for my testing is the 2 point 90 degrees apart as shown at http://www.cruxis.co...ecalculator.htm.

When the mirrror rests on those supports at its edge wise center of gravity it will balance there with no back support needed.  Because of the geometry of the support it also induces no astig into the mirror..  When not aligning the mirror on those two points properly it always induces deformation that is easily visible with the Bath interferometer.

I see you are adjusting a threaded edge support to match the CG exactly as the Cruxis site recommends.

 

I had been wondering (and posted recently on a sandwich mirror thread) whether this can work for sandwich mirrors. Perhaps like this, but a separate edge support disk for each plate?

Or, as I proposed, linear supports that were free to move vertically (a Teflon split sleeve on a steel rod?, linear bearings?).

 

Wiffle trees set up for a fixed size is not that hard however, but would be more of a nuisance in a test bed intended for multiple mirror sizea.



#6 Pinbout

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Posted 16 February 2019 - 08:25 PM

I’ve seen several use slings

 

now for huge thin mirror Astatic rear support help potato chipping



#7 Dale Eason

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Posted 16 February 2019 - 08:30 PM

I see you are adjusting a threaded edge support to match the CG exactly as the Cruxis site recommends.

 

I had been wondering (and posted recently on a sandwich mirror thread) whether this can work for sandwich mirrors. Perhaps like this, but a separate edge support disk for each plate?

Or, as I proposed, linear supports that were free to move vertically (a Teflon split sleeve on a steel rod?, linear bearings?).

 

Wiffle trees set up for a fixed size is not that hard however, but would be more of a nuisance in a test bed intended for multiple mirror sizea.

Yes I saw you post about that.  I wondered to myself how I could support that on the CG and how important it might be for that particular mirror.  If it was me I could test with the bath to find out if it mattered.  I suspect it does and perhaps some sort of support on both plates is required.  I just don't know.



#8 Dale Eason

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Posted 16 February 2019 - 08:38 PM

I’ve seen several use slings

 

now for huge thin mirror Astatic rear support help potato chipping

I tried slings and found them very hard to get adjusted on the CG so that they did not create deformation as well as how much they should wrap around the mirror..  Then when you try to rotate the mirror like I do for interferometric testing there is so much friction with a sling it is very hard to impossible to rotate it easily without lifting it off the sling and ruining the edge alignment. It can be done but is much harder than what I have settled on.

 

If one is not going to do interferometric testing then almost any edge support will work because most other tests are nearly immune to seeing the astig. I'm not saying it can't be seen. It quite frequently is overlooked. It requires the correct orientation of the mirror and experience of the tester to see the small amount of astig that does matter.  Large astig is easier for them to see.

 

Dale



#9 gregj888

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Posted 16 February 2019 - 09:30 PM

Dale,

 

I've got one for you.  I've been working again on my 20" f/3.5 conical blank.  About 3/4" at the edge and 3" in the center... 

 

It's cored almost all the way through.  With the 90 degree support (not at COG), very definite astig. 

 

Thinking about cleaning some of the plaster from the cut and hanging from the center.  Another thought is to block a piece of "wood" to the back to make it easer to push on the lap then connect a cord and lift some to cut the weight on the support pins.

 

Thoughts or ideas (anyone)?

 

Greg



#10 Pinbout

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Posted 16 February 2019 - 10:38 PM

Make the cell it will live in and test that on th the stand 



#11 Dale Eason

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Posted 16 February 2019 - 10:47 PM

Dale,

 

I've got one for you.  I've been working again on my 20" f/3.5 conical blank.  About 3/4" at the edge and 3" in the center... 

 

It's cored almost all the way through.  With the 90 degree support (not at COG), very definite astig. 

 

Thinking about cleaning some of the plaster from the cut and hanging from the center.  Another thought is to block a piece of "wood" to the back to make it easer to push on the lap then connect a cord and lift some to cut the weight on the support pins.

 

Thoughts or ideas (anyone)?

 

Greg

I think supported by the center is the best option.


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#12 Pierre Lemay

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Posted 16 February 2019 - 11:30 PM

I had been wondering (and posted recently on a sandwich mirror thread) whether this can work for sandwich mirrors. Perhaps like this, but a separate edge support disk for each plate?

That’s how Normand Fullum supports the large, 36 to 65 inch Technofusion sandwiched mirrors he makes. If you look at the picture I took of Normand standing behind a 50 inch mirror he was working on last spring, you can see, just right of his right hand, behind him, one of the two large, aluminium double whiffletree support he uses to support the mirrors in the test stand. 

 

B6155663-C607-4904-B0B2-D56E1FDC0516.jpeg


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#13 Pierre Lemay

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Posted 16 February 2019 - 11:38 PM

Dale,

 

I've got one for you.  I've been working again on my 20" f/3.5 conical blank.  About 3/4" at the edge and 3" in the center... 

 

It's cored almost all the way through.  With the 90 degree support (not at COG), very definite astig. 

 

Thinking about cleaning some of the plaster from the cut and hanging from the center.  Another thought is to block a piece of "wood" to the back to make it easer to push on the lap then connect a cord and lift some to cut the weight on the support pins.

 

Thoughts or ideas (anyone)?

 

Greg

Supporting from the central perforation is how I tested my 20 inch f/3.9 conical mirror. It’s very easy to mount this way. I didn’t see any particular distortion due to supporting the mirror like this  but, unlike Dale, I didn’t have a Bath thus I might have overlooked some forms of astigmatism. 


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#14 stargazer193857

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Posted 17 February 2019 - 12:03 AM

If the star test shows stig, and other tests don't, what eyepiece focal length is likely needed to fail the star test?

Has anyone tested the mirror on a padded flat back and then again on their intended wiffle tree and seen sag between the support points?

#15 Dale Eason

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Posted 17 February 2019 - 12:59 AM

If the star test shows stig, and other tests don't, what eyepiece focal length is likely needed to fail the star test?

Has anyone tested the mirror on a padded flat back and then again on their intended wiffle tree and seen sag between the support points?

There is no correlation between eyepieces and tests that don't show astig.  Some test can't see it.  It is magnification that lets you see astig if present on the mirror in the star test.  Not focal length directly.  

 

Yes I saw  astig on the padded flat back as support using the Bath interferometer.  Testing with it at 45 deg on the wiffle tree was prettry much the same as PLOP predicted with no primary astig from the 2 90 degree edge supports on the COG.  

 

Padded flat back creates astig but that is alleviated by rotating the mirror during it's use.  If done properly it will not create astig due to manufacturing process.

 

Dale


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#16 Pinbout

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Posted 17 February 2019 - 06:26 AM

That’s how Normand Fullum supports the large, 36 to 65 inch Technofusion sandwiched mirrors he makes. If you look at the picture I took of Normand standing behind a 50 inch mirror he was working on last spring, you can see, just right of his right hand, behind him, one of the two large, aluminium double whiffletree support he uses to support the mirrors in the test stand. 

 

attachicon.gif B6155663-C607-4904-B0B2-D56E1FDC0516.jpeg

 

He’s a crazy Canuck lol.gif

 

go montreal 



#17 careysub

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Posted 17 February 2019 - 11:00 AM

That’s how Normand Fullum supports the large, 36 to 65 inch Technofusion sandwiched mirrors he makes. If you look at the picture I took of Normand standing behind a 50 inch mirror he was working on last spring, you can see, just right of his right hand, behind him, one of the two large, aluminium double whiffletree support he uses to support the mirrors in the test stand. 

I may not be understanding this correctly. 

 

I see a cylindrical post in the picture, which is my right of his right hand (his left), not a wiffletree. Is that a support upon which the actual wiffletree goes?



#18 Pinbout

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Posted 17 February 2019 - 11:16 AM

It’s under the yellow bar way in the background 

 

8 rollers support the two plate edges



#19 Pierre Lemay

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Posted 17 February 2019 - 02:17 PM

It’s under the yellow bar way in the background 

 

8 rollers support the two plate edges

 

Exactly. Unfortunately I don't have a picture of a mirror on the test stand but Normand explained the procedure to me: he picks up the mirror off the polishing machine turntable with suction cups after a figuring session, moves to his test rig just behind him, lowers it on the test stand which is in a horizontal position at this pont and pushes the double edges of the mirror against the whiffletree supports, one of which you see in the picture. He then puts in place mirror clips for safety and pivots the test stand with the mirror now on it from a horizontal to a vertical position. The mirror is then ready to be tested inside the test tunnel.




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