[ATM] Ross Null tester
gary at gfphoto.com
Fri Mar 11 03:03:45 JST 2011
I appreciate the warning about the laser and the measuring tips.
From: Paul A. Valleli [mailto:valleli at rcn.com]
Sent: Thursday, March 10, 2011 11:57 AM
To: 'GFPhoto', atm at atmlist.net
Subject: Re: [ATM] Ross Null tester
Nice workmanship, Gary.
I especially love the clothespin holding the attenuating filter. I am glad you found a good use for an OSTI lens. Most were made to an accuracy of 1/8th wave P-V.
A Class 2 laser can be very dangerous, so be careful not to focus the expanded beam onto your eye. An optician friend was knife edge testing an F/1 lens and stared too long at the focal point and burned several small blank spots onto his retina.
Remember to keep track of the radius of curvature of the mirror under test as well as the spacing from Ross lens to mirror vertex. I frequently use a wooden dowel, rounded at both ends, as a radius rod to keep checking the spacing. This is to assure that you don't leave a small residual of over-, or under-corrected spherical aberration. The radius can drift slightly during long polishing or figuring operations.
---- Original message ----
Date: Thu, 10 Mar 2011 15:47:14 +0000
From: "GFPhoto" <gary at gfphoto.com>
Subject: [ATM] Ross Null tester
To: atm at atmlist.net
>I've been working on a Ross Null tester to complement regular knife edge Foucault and autocollimation.
>The light source is a Melles Griot class II HeNe laser I found at the scrap yard, powered by a 15.6V DC power supply from Surplus Shed.
>The PCX lens was a really nice swap table find at Stellafane--part of a commercial order from the early '80's (Thanks Paul). A little over 6" diameter.
>My son made the holder so the front is level with the lens surface so measuring to the mirror is easy.
>The base is also a scrap yard find from I guess some piece of lab equipment. A bit of overkill but it weighs about 20 lbs so nice and stable and the movements are smooth. I machined the prism holder from a piece of plastic rod so that it would be a friction fit on the front of the laser and be able to turn for alignment.
>There's a 1mm thick diffusing ground glass (both sides) after the prism and then the screen.
>With the ground glass and screen removed there's a dot for alignment.
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