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Stellafane Mirror Test Stand

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#1 Napersky

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Posted 10 January 2013 - 06:25 PM

My completed Stellafane mirror test stand for my new 15" flat.

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#2 Napersky

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Posted 10 January 2013 - 06:27 PM

Before the angle supports are cut.

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#3 Napersky

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Posted 10 January 2013 - 06:29 PM

Assembled before glueing.

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#4 Napersky

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Posted 10 January 2013 - 06:31 PM

Mostlu completed missing the top supporting rubber grommet.

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#5 DAVIDG

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Posted 11 January 2013 - 09:45 AM

Mark,
If your going to use the flat for autocollimation testing, beside the up/down tilt adjustment screw on the back you'll need a method to adjust the tilt from side to side.

- Dave

#6 kfrederick

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Posted 11 January 2013 - 10:06 AM

That flat would make a great"Fundy" telescope fixed eyepiece The planet is the mounting .Could also try EDs window Chief idea . Nice setup Maybe you could start a "Rent A Flat"

#7 Pinbout

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Posted 11 January 2013 - 10:13 AM

I ripped of Mr. Parkers AC to make a non perf AC.

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still on the list of things to do... :help:

#8 Napersky

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Posted 11 January 2013 - 10:27 AM

Mark,
If your going to use the flat for autocollimation testing, beside the up/down tilt adjustment screw on the back you'll need a method to adjust the tilt from side to side.

- Dave


Dave,

I have on the front of the tester two adjustible feet to move one up and the other down. Also from what I've read on the instructions for side to side I can just move the whole stand.

Mark

#9 Napersky

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Posted 11 January 2013 - 10:30 AM

I ripped of Mr. Parkers AC to make a non perf AC.

Posted Image

still on the list of things to do... :help:



Now that is cool. Also has the swivel for DavidG's "side to side" comment!

#10 DAVIDG

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Posted 11 January 2013 - 12:30 PM

Mark,
Trust me you'll need a way to precisely move the flat in both axis to get it aligned. Those rubber feet will grab and then let lose and you over shot and also get the up/ down alignement out of wack. Two adjustments screws for both direction will make life much easier. If the flat isn't aligned correctly, you see astgmastism which causes the Ronchi lines and interference fringes to bow which makes it difficult to determine if the errors are in the optics your testing or coming from alignment issues.
Dick Parker's portable setup is very well engineered. It takes about 30 seconds to get the alignment dead perfect since you can easily do tip/tilt and side to side adjustmenst using two threaded rods.

- Dave

#11 Gary Fuchs

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Posted 11 January 2013 - 01:17 PM

Here are some views of Dick's tester.

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The extra threaded rods (on the left below) allow the mirror stand to be adjusted at longer distances.
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For your setup you probably could have the tip/swivel controls on the inside and reach them from your viewing position.

I found what Dave G. described to be the case. If you don't have some sort of fine adjust for the swivel at least do something like a pin in the middle underneath and smooth bearing surfaces so you can push it smoothly to adjust.

The diagonal needs fine adjustability too...

Gary

#12 kfrederick

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Posted 11 January 2013 - 01:33 PM

Would not a Testing at the RC be more sensitive ??. I know it is a null test .Great test . Just ASKING a faint zone would the knife edge at RC be better to see?? You would know the zone was only on the mirror at the RC . Love to use the test only asking .

#13 Pinbout

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Posted 11 January 2013 - 01:44 PM

the AC doubles your error so they look twice as bad, but I heard Swayze say RoC is more sensitive.

You know that's a good reason to have the 3 test in your back pocket, to verify each other and rule out uncertainty, the KE, Ross-Null and AC.

the AC is so immediate with feedback, just opposite from what I'm use to so it can be disorienting.

#14 Gary Fuchs

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Posted 11 January 2013 - 01:53 PM

You would know the zone was only on the mirror at the RC .


My guess would be that the AC flat (or Ross lens) can't introduce a zone; but I'd like to hear a definitive answer.

Gary

#15 MKV

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Posted 11 January 2013 - 08:11 PM

You would know the zone was only on the mirror at the RC .


My guess would be that the AC flat (or Ross lens) can't introduce a zone; but I'd like to hear a definitive answer.

Gary

AC flats can depart from flatness by many waves, but their surface finish must be impeccably smooth. One way to check if the zone is introduced by the flat is to rotate the mirror. If the zone remains unchanged in the same location on the wavefront, then it must originate in the flat.

#16 Napersky

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Posted 12 January 2013 - 04:09 PM

Dave,

I need a mirror test stand for my interferometry. I do like Dick's mirror stand. I wonder does he have specific measurments and plans available such as for the Stellafane test stand I built?

Thanks,

Mark

#17 DAVIDG

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Posted 12 January 2013 - 04:37 PM

Mark,
I understand that you need the stand to hold your flat for interferometry, but the setup to test a mirror or lens using a flat and your interferometer is exactly the same as autocollimation just instead of using a Foucault/Ronchi tester it will be replaced with your interferomter. So your going to need to able adjust things with the same or better precision as what we do when doing autocollimation since you'll be looking at fringes.
I don't know if Dick has set of plans available.

- Dave

#18 MKV

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Posted 13 January 2013 - 02:24 AM

My guess would be that the AC flat (or Ross lens) can't introduce a zone; but I'd like to hear a definitive answer.

Gary, you are not looking at any optical surfaces but at a returning wavefront, whose quality is a sum-total of all the errors/deformations caused by individual optical components.

It stands to reason that a small, say, 1/30 wave valley on the flat will reflect off of it as a 1/15 wave local deformity on the wavefront and that it should be visible to an observer as a "zone".

Mladen

#19 Pinbout

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Posted 13 January 2013 - 09:29 AM

I wonder does he have specific measurments and plans available



I did another model more like his, the threaded rod ends in the dowel nuts so the stand can rotate as the screw is turned.

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#20 Gary Fuchs

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Posted 13 January 2013 - 11:18 AM

My guess would be that the AC flat (or Ross lens) can't introduce a zone; but I'd like to hear a definitive answer.

Gary, you are not looking at any optical surfaces but at a returning wavefront, whose quality is a sum-total of all the errors/deformations caused by individual optical components.

It stands to reason that a small, say, 1/30 wave valley on the flat will reflect off of it as a 1/15 wave local deformity on the wavefront and that it should be visible to an observer as a "zone".

Mladen


Thanks Mladen,

I'm in over my head so please bear with me; I'm thinking of a "zone" as an annular region of the mirror; and having trouble understanding how, say that valley on the flat, could cause a zone to show up if it wasn't there in the first place.

Gary

#21 Napersky

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Posted 13 January 2013 - 03:05 PM

Aligning the test elements is a Bitc*!

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#22 Napersky

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Posted 13 January 2013 - 03:06 PM

To be able to return the test beam isn't easy.

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#23 Gary Fuchs

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Posted 13 January 2013 - 04:51 PM

Aligning the test elements is a Bitc*!


That was my experience too...but it gets easier.

Gary

#24 DAVIDG

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Posted 13 January 2013 - 05:06 PM

Mark,
You need to start with the just the flat and the laser. Adjust the flat so the beam falls back on laser. Then put the lens in place and position it so the laser is passing thru the center of the beam. You'll see a bunch of spots on the flat from the beam coming off all the surfaces in the lens. I can see some of them in the pictures you posted. Adjust the tip and tilt of the lens so all the spots line up with the major bright one from the laser passing thru the center. The lens is now aligned.
This is why you need adjustment screws that are easy to get at while your setting things up. The fun is yet to come when you try to get things lined up to show fringes !

- Dave

#25 Napersky

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Posted 13 January 2013 - 06:46 PM

Thanks Dave will try that.






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