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sopticals
professor emeritus


Reged: 03/28/10

Loc: New Zealand
Re: Ronchiscreens.com still in bizness? new [Re: precaud]
      #5944213 - 06/28/13 04:40 AM

Quote:

Another couple reality-check questions for the experienced Ronchi testers. Last night, TDE was very visible in the outfocus, but not nearly so much in the infocus. Is that as it should be?

And if the lines start their "turn" say 10% of the radius from the edge, I can translate that proportionally into the size of edge mask required to hide it?




Yes, to both questions. Inside ROC gives more info on center mirror issues, while outside ROC does for outer regions.

Masking in proportion works well.At 10% the edge is more accurately called a "rolled down edge".

Stephen.(44deg.S.)

Edited by sopticals (06/28/13 04:42 AM)


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precaud
Pooh-Bah
*****

Reged: 12/05/12

Loc: north central New Mexico
Re: Ronchiscreens.com still in bizness? [Re: sopticals]
      #5944370 - 06/28/13 08:22 AM

Thanks Stephen. I was using the 10% figure just as an example, but the reality isn't too much less...

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Live_Steam_Mad
sage


Reged: 07/24/07

Loc: Moss Bank, St.Helens, England
Re: Ronchiscreens.com still in bizness? new [Re: precaud]
      #6271490 - 12/24/13 11:01 PM

I just ordered 2 large (6 x 6 cm) Ronchi gratings of 133 LPI and also 250 LPI from ronchiscreens.com. I'll be interested to see what results I get with my scopes when the gratings arrive. I hope to report back with some results.

Regards,

Alistair G.


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Live_Steam_Mad
sage


Reged: 07/24/07

Loc: Moss Bank, St.Helens, England
Re: Ronchiscreens.com still in bizness? new [Re: Live_Steam_Mad]
      #6273679 - 12/26/13 01:40 PM

Joe just e-mailed me to say my Ronchi screens are now in the post. I already have an 8" disc of completely stress-free (tested and verified with my polarizer, much better than my friend Jon's cross of death blanks) BoroSilicate glass I got a few months back, and next will be some grinding powder and then maybe I can make my own Newtonian, I'm thinking around F5.5 (don't like long tubes).

Regards,

Alistair G.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Ronchiscreens.com still in bizness? new [Re: Live_Steam_Mad]
      #6273756 - 12/26/13 02:21 PM

For star testing I'd use the 250. I cut a small piece and glued it with silicone into a 35mm film container.

http://www.cloudynights.com/photopost/showphoto.php?photo=26493&size=big&...


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Allan Wade
sage


Reged: 01/27/13

Loc: Newcastle, Australia
Re: Ronchiscreens.com still in bizness? new [Re: Pinbout]
      #6274329 - 12/26/13 08:07 PM

I just received my Ronchi eye piece and have had only a few minutes to use it so far. What was immediately obvious is that I have an over-corrected mirror. I believe that is a killer for seeing contrast on planets.

A question I have. I can see the shadow of the secondary in the middle of the lines. It takes up a large percentage of the FOV. I wasn't expecting this and it makes it a little harder to determine what the lines are doing. Is this normal, or am I doing something wrong?


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Ronchiscreens.com still in bizness? new [Re: Allan Wade]
      #6274353 - 12/26/13 08:19 PM

was the mirror totally cooled. if it is a good mirror while cooling it can read overcorrected.

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Allan Wade
sage


Reged: 01/27/13

Loc: Newcastle, Australia
Re: Ronchiscreens.com still in bizness? new [Re: Pinbout]
      #6274440 - 12/26/13 09:13 PM

I have a temperature gauge on the mirror, and it was close to ambient. And no, I've never thought this mirror was very good.

Any thought on the secondary shadowing the lines?


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Ronchiscreens.com still in bizness? new [Re: Allan Wade]
      #6274470 - 12/26/13 09:29 PM Attachment (4 downloads)

its the 70% zone out to the edge that really counts.

and thats the outer 1.75" of a 12" mirror.

this area contributes to more than half the light in the airy disc.


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Allan Wade
sage


Reged: 01/27/13

Loc: Newcastle, Australia
Re: Ronchiscreens.com still in bizness? new [Re: Pinbout]
      #6274503 - 12/26/13 09:48 PM

Thanks for the info.

The reference material that came with the eyepiece included centre of mirror defects. That is why I find this secondary shadow strange, because I can't see the lines going through the centre of the mirror, so can't determine whether any defects exist there.

I understand what you say about the outer edge being critical though.


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Live_Steam_Mad
sage


Reged: 07/24/07

Loc: Moss Bank, St.Helens, England
Re: Ronchiscreens.com still in bizness? new [Re: Allan Wade]
      #6274515 - 12/26/13 09:56 PM

Quote:

I just received my Ronchi eye piece




Where did you get the Ronchi eyepiece from?

Regards,

Alistair G.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Ronchiscreens.com still in bizness? new [Re: Allan Wade]
      #6274522 - 12/26/13 09:58 PM

We have to deal with that (@## central hole when testing in autocollimation also.

Heres another test at focus. lay some electrical tape across half the mirror use it as a knife edge and see if you see the classic donut pattern. If so thats sa. If its good it would totally null.

Remember tests at focus are not as sensitive as radius of curvature.

If you want to see the center pull the mirror and ke test or Ronchi test it with a 3mm led.


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Allan Wade
sage


Reged: 01/27/13

Loc: Newcastle, Australia
Re: Ronchiscreens.com still in bizness? new [Re: Pinbout]
      #6274552 - 12/26/13 10:22 PM

Alistair, it's a Gerd Neumann 250lpi. I bought it from OPT.

Danny, thanks for the info. I will have to sit down and consider it all.


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Live_Steam_Mad
sage


Reged: 07/24/07

Loc: Moss Bank, St.Helens, England
Re: Ronchiscreens.com still in bizness? new [Re: Allan Wade]
      #6292717 - 01/04/14 08:55 PM

Well my Ronchi gratings (both 6x6cm) (133LPI and 250LPI) arrived today (quick considering Boxing Day etc and New Year inbetween LOL) and they look good to my eye and are both pieces of film (from a large format film camera I assume) and mounted in slide projection thick plastic frames, rather sturdy looking.

The 133LPI grating lines can just be seen by my eyes under domestic lighting. I looked using a 4x and 8x magnifying Loupe and the lines looked straight and of high contrast and sharpness and blackness. The 250LPI grating lines were invisible to my eyes and I saw by the loupe that they were very much finer (thinner and more high densely packed) but still good blackness / high contrast, however looked slightly "wibbly" at the edges under bright light but my hands and eye moving around seemed to cause some wibbling as well so it was hard to tell.

I placed my Hubble 5 stars Artificial Star with the smallest pinhole visible about 30 feet away from the 'scope (at the end of the kitchen near the back door) on top of a 2 inch high box on the floor, then put the Meade SN102 4" F8 right up near the front door, on the floor (normally this setup shows the 1/4 wave under correction for SA that my optics have got, quite well, i.e. outer ring is about twice the brightness inside focus and about half the brightness on the other side of focus with about 12 waves defocus i.e. about 3.5mm defocus), and shows a nice sharp Airy disc at 100x with my TV Radian eyepiece .

First I tried putting the Ronchi over the eyepieces, nope that doesn't work So you take out any eyepiece you have, (until there is just the objective lens in the front of the tube of my refractor) then hold the Ronchi at the eyepiece holder exit and take a look.

The 133LPI showed basically 4 patterns, 1st was about ten VERY sharp lines, bright and VERY thin light blue, they looked just about dead straight and showing only a small hint of a turned edge in the bottom left quarter of the image, then as you pull the grating towards my eye I get the 2nd pattern, of several dark bands, quite sharp and pretty high contrast, when 3 bands were showing there was either no visible turned edge or maybe the slightest hint of it on the bottom left, and dead straight lines. Then the number of bands decrease to zero (at the "focal point" I assume) and then as you pull the grating towards you again I once more got the 3 or 4 dark bands with the same result of dead straight lines and then as I pulled it very close to my eye I once more saw the 1st pattern again.

So 1/4 wave undercorrection for SA seems to be impossible for me to detect handholding the grating as the lines looked dead straight to me in all 4 patterns . The stuff inbetween the 4 patterns was blurred and low contrast due to diffraction effects I think.

Then I tried the 250LPI grating. This time I saw slightly wibbly lines in the 1st pattern of 10 or more lines but same appearance as with the 133 LPI (sharp, very fine bright very thin lines) but the lines were once again generally dead straight (if you see what I mean) despite the wibbly look of the lines. Then I tried to see the 4 or 3 dark bands but they were lower contrast than the 133 LPI pattern and quite challenging to make out also due to the increased sensitivity to my hand vibration, never mind see any bowing of them (I saw maybe a hint of bowing), same on the other side of focus for the next 2 patterns as for the 133 LPI one.

I must say the 133 LPI with the Hubble Artifical Star looks MUCH easier and better to use and higher contrast than the 250 LPI. However it looks like Suiter is correct in his Star Testing book that the Ronchi can't show the SA sensitively enough to be visible easily. Pity. Also the conclusion is that if you can see ANY bowing or turned edge with 133 LPI Ronchi then you have quite questionable optics of not great quality, IMHO!

Fascinating test to perform though, and sort of confirms that I have a pretty fair quality SN102 from Meade (it has minor Astigmatism also that the Ronchi didn't show either, and quite a bit of Chromatic Aberration).

Next test will be with Polaris. I'll try and take some pictures also with both these tests (I like to repeat the tests multiple times).

Best Regards,

Alistair G.

Edited by Live_Steam_Mad (01/04/14 09:10 PM)


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Ronchiscreens.com still in bizness? new [Re: Live_Steam_Mad]
      #6292788 - 01/04/14 09:31 PM

cut a small piece of the 250 and glue it to the inside cover of a 35mm film container...of course after you had drilled a 3/8 hole.



you can also lay a piece of white artist tape across half the focuser and use it as a knife edge.


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Live_Steam_Mad
sage


Reged: 07/24/07

Loc: Moss Bank, St.Helens, England
Re: Ronchiscreens.com still in bizness? new [Re: Pinbout]
      #6293079 - 01/05/14 12:49 AM

Quote:

cut a small piece of the 250 and glue it to the inside cover of a 35mm film container...of course after you had drilled a 3/8 hole.
you can also lay a piece of white artist tape across half the focuser and use it as a knife edge.




Thanks Danny, I might well give that a try, it sounds like the best thing to do. If I do, I'll try both the 133 and 250 LPI like this.

Best Regards,

Alistair G.


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