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Are Ronchi eyepieces useful for checking mirrors

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#1 Wol

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Posted 23 April 2012 - 01:40 AM

I'd be grateful for any opinions. I was thinking of getting one to check my 10" dob. :question:

regards

#2 sopticals

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Posted 23 April 2012 - 02:55 AM

I'd be grateful for any opinions. I was thinking of getting one to check my 10" dob. :question:

regards


Yes. Recently I completed (first iteration) polishing and figuring a 22" f5.36 mirror using a ronchi grating 85 lpi, in a bench test, with grating and light, (LED) source at the radius of curvature. Seemingly when I compared my optic to another mirror I have with a known Strehl of 0.99 the side by side comparison, (after correction was made for mirror tilt), was very similar for the two mirrors, (both exhibited smooth, curved bars, indicating good correction edge to edge).But, when I carried out the test on the 22" (uncoated), in the scope with a 133 lpi eyepiece, I did notice the outer 1.5" of radius to show a gradually lengthening of radius of curvature outside the ideal. I conclude then from my observation, that the 133 lpi at the eyepiece is more sensitive than is the 85 lpi test on the bench at radius of curvature. Others may question my findings. I would be pleased to receive input on this count.

Yes, a Ronchi eyepiece,(very easy to use-just focus up on a bright star,with four or 5 lines in the image),will instantly show up zones, hills, hollows, TDE, TUE, roughness, astigmatism, etc. The test(used with a good 2x barlow), is sensitive enough to evaluate whether your optics are within 1/4 wave (for a f4.5 scope) or 1/8 wave (for an f6 optic).

Stephen.

#3 ahlberto

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Posted 23 April 2012 - 05:27 AM

Indeed,like stephen sayd,they are very useful and for mi,a must have.Check out this Gerd Neumann .10 lines per mm :p

#4 ausastronomer

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Posted 23 April 2012 - 06:51 AM

Hi Wol,

They are useful but you really need to get the 10 lines/mm or 250 lines/inch grating to have enough sensitivity to tell you much. Smaller surface errors can go undetected on lower resolution gratings

I have the Orion Optics UK 250 lpi grating and it works well.

http://www.orionopti...aneouspag.html.

When I bought mine some years ago the exchange rate was poor and it cost me $200 landed in Australia. With the more favourable exchange rate it would cost a bit over $100.

They are more expensive than the lower resolution gratings but like most things in life you get what you pay for.

The ronchi grating will in fact tell you less than what is detectable to an experienced star tester, and the stars are free. I bought the ronchi eyepiece to play with and see if it worked, as claimed. It did what it was supposed to do. However, if you dont know what you're doing it will not tell you a lot, and if you do, it will still tell you a lot less than a star test.

Here is a page to tell you how to analyse the results.

http://www.orionopti...petestpag.html#

Knowing how to star test and owning a ronchi eyepiece my advice to you would be to purchase a copy of Dick Suiter's book called "Star Testing Astronomical Telescopes".

http://www.willbell.com/tm/tm5.htm

This will cost you less money, tell you a lot more and teach you a lot more.

Cheers,

#5 Mirzam

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Posted 23 April 2012 - 07:35 AM

I'm curious whether the ronchi "eyepiece" is anything more than a holder (without lenses) for the ronchi grating?

If it is just a simple holder, could you buy a grating and make your own ronchi eyepiece by using an eyepiece with the lenses removed--just glue the grating over the field stop?

Cheap, but good gratings available here (near bottom of page):

http://www.willbell....TM_Supplies.htm

JimC

#6 David Knisely

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Posted 23 April 2012 - 12:58 PM

I'm curious whether the ronchi "eyepiece" is anything more than a holder (without lenses) for the ronchi grating?

If it is just a simple holder, could you buy a grating and make your own ronchi eyepiece by using an eyepiece with the lenses removed--just glue the grating over the field stop?

Cheap, but good gratings available here (near bottom of page):

http://www.willbell....TM_Supplies.htm

JimC


Yes, all the Ronchi eyepieces are is a Ronchi grating mounted in an eyepiece-shaped holder that fits into the focuser of the telescope. With the eyepiece in the telescope, all you do is put the scope on a bright star and adjust the focus until you see just a few lines across the mirror from the out of focus light of the star. If the optics are good, the lines should be nice and straight with little bumpyness or irregularities. It is as simple as that. Clear skies to you.

#7 sopticals

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Posted 23 April 2012 - 06:11 PM

A 133 lpi grating eyepiece barlowed 2x, will be as sensitive as a 266 lpi unbarlowed.

Stephen.

#8 Wol

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Posted 23 April 2012 - 09:49 PM

Thanks all - much appreciated. I'll let you know how it goes.

Regards

#9 Alan A.

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Posted 24 April 2012 - 12:15 PM

Everyone I've talked with in my club who is experienced thinks the rhonchi eyepiece is a useful way to test an optic.

I just want to clarify one of the comments made by someone earlier in the thread - the rhonchi does NOT show astigmatism. That's not a big deal because that is usually easy to pick up on the regular star test.

#10 ausastronomer

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Posted 24 April 2012 - 12:59 PM

A 133 lpi grating eyepiece barlowed 2x, will be as sensitive as a 266 lpi unbarlowed.

Stephen.


Yes very true, that's why I use my 250 lpi ronchi grating in a 2.5X TV Powermate for an effective 625 lpi.

Cheers,
John B

#11 cjc

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Posted 24 April 2012 - 02:13 PM

A 133 lpi grating eyepiece barlowed 2x, will be as sensitive as a 266 lpi unbarlowed.

Stephen.


But you will be testing the combined system of morror plus barlow.

#12 sopticals

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Posted 24 April 2012 - 07:33 PM

Everyone I've talked with in my club who is experienced thinks the rhonchi eyepiece is a useful way to test an optic.

I just want to clarify one of the comments made by someone earlier in the thread - the rhonchi does NOT show astigmatism. That's not a big deal because that is usually easy to pick up on the regular star test.


Astigmatism CAN be seen with Ronchi grating at the eyepiece. If the system has "stig", it can be identified as "zigsage" in the observed bars, together, with a changing of the orientation, of the bars, as the eyepiece moves from insde, to outside focus.

Stephen.

#13 sopticals

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Posted 25 April 2012 - 05:43 AM

Excuse my spelling, in my last post.I meant "zigzag".Also as the grating/eyepiece moves through focus, the number of bars decrease,until a single bar,at which point, if astigmatism is present, then that bar (which will take on a very rough/chaotic appearance) will twists visibly, to take up the mirror image position after grating/eyepiece, crosses point of focus.

Stephen.

#14 dave brock

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Posted 26 April 2012 - 06:37 AM

Cheap, but good gratings available here (near bottom of page):

http://www.willbell....TM_Supplies.htm

JimC


I bought some of these recently and found that they count the dark lines as well as the gaps in between. This means their 200 line grating, for e.g., is really a 100 line.
Then got some from here http://www.ronchiscreens.com/ and they are as described.
Dave

#15 Mirzam

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Posted 26 April 2012 - 07:26 AM

I did not know about that source. Thanks!

(Project for the day--count the lines on my ronchi screen!)

JimC

#16 Pinbout

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Posted 26 April 2012 - 08:29 AM

I bought some of these recently and found that they count the dark lines as well as the gaps in between. This means their 200 line grating, for e.g., is really a 100 line.



yep. had the same *&* experience. it's the only place in the world that does that. I thought I was going crazy for a second.

did you look at the lines of the ronchiscreens.com under a scope since they are printed on a digital printer?

when i print them on a highend printer I get this under a loupe.

Posted Image

but under a 60x handheld scope it looks like this

Posted Image

not that great and you definitely can't use it for a star test.

#17 Alan A.

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Posted 26 April 2012 - 01:05 PM

Hi Stephen,

Roland Christen has specifically posted that astigmatism is not well picked up by the Ronchi test - that was the source for the comment I made.

Looking at some articles it appears that strong astigmatism can be picked up with the ronchi, but it looks like it may be hard to pick up mild astigmatism with this particular test.

#18 sopticals

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Posted 27 April 2012 - 12:54 AM

I bought some of these recently and found that they count the dark lines as well as the gaps in between. This means their 200 line grating, for e.g., is really a 100 line.



yep. had the same *&* experience. it's the only place in the world that does that. I thought I was going crazy for a second.

did you look at the lines of the ronchiscreens.com under a scope since they are printed on a digital printer?

when i print them on a highend printer I get this under a loupe.

Posted Image

but under a 60x handheld scope it looks like this

Posted Image

not that great and you definitely can't use it for a star test.


Using grating of 133 lpi in the eyepiece at focus,(when testing the 22" mirror I am currently working on), the image that I get, is of similar appearance,(not quite smooth),which I put down to some intrinsic roughness in the grating lines. Same when bench testing with 85 lpi grating,(grating and light source close to radius of curvature).As a check, carried out the same tests, on a "known optic", with a claimed RMS 1/110 cum Stehl 0.99 cum PV 1/25 wave.The viewed images being similar.(Have to admit that at the eyepiece with the finer grating the "known optic" showed marginally smoother lines, but in the bench test, and with the coarser grating line smoothness is very similar).So I wonder about using a barlow in conjunction with a Ronchi grating at the eyepiece. If grating has a "roughness" magnifying the image by 2x or more is not going to tell one more about the quality of the optic under test.

Stephen. :crazy:

#19 dave brock

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Posted 27 April 2012 - 01:47 AM

Hi Danny.
I have a 10+ year old 133 line grating from Willmann-Bell (from when they counted correctly) and the Ronchi Screen dotcom one is no worse than it for quality. When I test in the scope I generally use the star-test rather than the ronchi.
Dave

#20 cjc

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Posted 27 April 2012 - 01:58 AM

I see that that the Gerd Neumann Ronchi eyepiece that Orion Optics UK used to sell is now available directly for 32 Euros plus shipping plus tax:

http://www.gerdneuma...i-eyepiece.html

#21 dave brock

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Posted 27 April 2012 - 02:15 AM

I see that that the Gerd Neumann Ronchi eyepiece that Orion Optics UK used to sell is now available directly for 32 Euros plus shipping plus tax:

http://www.gerdneuma...i-eyepiece.html


Hi Chris.
Just went through the process to order one but the shipping to New Zealand is quoted as 40 euro making 72.10 total. No thanks.
Dave

#22 Wol

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Posted 27 April 2012 - 05:04 AM

That's what I got too! So I went to JSM instead. They cannot be serious with postage at that cost. I guess you could try to negotiate something less but it makes you go elsewhere first!

regards

#23 davidpitre

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Posted 03 May 2012 - 02:04 PM

Indeed,like stephen sayd,they are very useful and for mi,a must have.Check out this Gerd Neumann .10 lines per mm :p


Seems like a nice unit, but €40.00 (over $50. ) shipping to the US ? Seriously?

#24 ed_turco

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Posted 03 May 2012 - 02:51 PM

If I remember correctly, Suiter , in his book, had some serious reservations about testing with a Ronchi eyepiece. The accuracy you think you are getting may not be what you actually get.

note bene!

#25 ahlberto

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Posted 03 May 2012 - 06:16 PM

Ive made my ronchi eyepieçe with a printed circuit.You can see here a screen for a cell phne and that flexible printed circuits some times have more than 8 lines per mm and are very good for making a ronchi eyepieçe.Heres the pic of mi ronchi eyepieçe and it works really well :p

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