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1.25" Ronchi tester -- the EASY-tester

Started by BABOafrica, 05/27/2012 09:29PM
Posted 05/27/2012 09:29PM Opening Post
A friend of mine told me about this tester for primary mirrors:

Looks interesting.

Anybody ever use one of these?


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 05/28/2012 11:19AM | Edited 05/28/2012 11:22AM #1
I have not tried this product but have used the Ronchi test. I am sure you know that its a well documented test method; see Star Testing Astronomical Telescopes Suiter and the ATM book books 1 and 2 for details. Works well but not as fool proof as the add implies.
Posted 06/04/2012 01:20PM #2
I have one. I wouldn't recommend it for critical testing any optic faster than about F/6. At F/6 or less it will only reveal gross errors (surface roughness, turned up/down edge, zones, etc.). The grating is not fine enough for critical tests of any short focus telescope IMO (faster than F/6). A 250lpi glass grating eyepiece is better for more serious testing. At F/5 or less I would recommend a 300lpi grating. The picture shows the result of using a 133lpi grating with a 8" F/5 mirror telescope at the focuser. Pretty straight bands at 1/4 wave smile

Many years ago I used to think that any grating was ok for testing any telescope. I was wrong. I used my Coulter "telescope tester" at star parties and thought I knew how good or bad some telescopes were. A 1/4 wave F/5 Newtonian and a 1/10th wave F/15 refractor look a lot alike with a 133lpi eyepiece grating :S I only found out later that the grating chosen must be fine enough work with the focal ratio of the instrument it is testing.

A 1/4 wave F/5 Newtonian and a 1/4 wave F/15 refractror look quite a bit different at the eyepiece using the same eyepiece grating. The view would cause you to praise the Newtonian as "nicely corrected" and condemn the refractor as inferior even though the wavefront of both would be identical.

As is well known, a 1/4 wave telescope at F/5 is much less forgiving in critical optical performance than a 1/4 wave telescope at F/15.


Attached Image:

klawson's attachment for post 144359


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