[ngcles]

Hi Satchmo & All,

Originally Posted by Satchmo

Les, Yes this does look like a sure fire way to become unpopular ...

Yep, and I only mention it because I have actually seen it happen, many moons ago (back in the early-days) at a well-known star party you and I have attended many times. No names, no pack drill; it wasn't a member of this forum (that I know of) but he is a highly experienced amateur and an experienced telescope maker who has made at least a dozen mirrors and other optics -- maybe even twenty. I look up to him a great deal ... and he is no fool. He has a reputation for being both "candid" and quite "direct" (in the same sense that an exocet missile could be described as "candid" and "direct") in his comments. I think you'll probably work out who it is from that all that.

He had a home-made ronchi eyepiece at the star-party and was doing precisely that. It was fascinating to see the muttering, cursing trail of disgruntled individuals he left in his wake, proffering all manner of sundry advice touching upon the subject of other darkened places he ought to consider inserting his ronchi eyepiece. From memory, there wasn't a single Schmidt-Cassgrainian that he gave a passing-grade to. He did it to me too ... on my old 10" f/6 (AOS mirror) and pronounced it excellent (whew ...). But I already knew that.

Originally Posted by Satchmo

I would advise anybody new on this path to do lots of testing and understand the way telescopes behave out in the 'wild' , before telling anyone that their telescope is has genuine problems . Collimation , internal seeing , atmosphere and thermal inertia can all make a telecope look defective in terms of surface smoothness, and spherical aberration and astigmatism. I know some large dobsonians can show significant over and under correction while cooling depending on their design and some nights will never reach their equilibrium state. I've seen the same optics perform superbly on other nights.

Yes very much correct -- I agree for what it's worth.

Originally Posted by Satchmo

Ronchi gratings at focus in the presence of inclement seeing make it particularly hard to judge any correction defects better than 1/2 to 1/4 wave for faster than F5 scopes, though will certainly weed out lemons. Even then the bowning of the lines can be very subtle, and very difficult to determine the differnce between a mediochre one and a superb one. Seeing and tube currents usually limits the ditance from focus to four to six bands bands. In the workshop with good seeing I find a single band is about right for seeing correction errors that seperate the average from the excellent.

Re; "Bowing of the lines can be very subtle ..." Yep, but this is for a Ronchi used approximately at the centre of curvature as in an optical bench-test, not at the focal-plane like this device. Used at the focal plane a ronchi ep examining a good optic shows ruler straight lines, not curved ones -- you don't have to make an assessment of whether a curve "looks right". All curves, any curves are wrong! The ronchi will be an easier-to-read test in mediocre and average seeing that a full-on star test that requires very good to excellent seeing, though the star test, in the right conditions, is undoubtedly a more demanding examination.

As you say "... will certainly weed out lemons". I don't think it really pretends to be a high-precision optical test but as you say will certainly show a dud and pretty quick. Personally, I'd generally be lothe to condemn an optic based on a 10-second assessment with a ronchi eyepiece -- but it is an excellent indicator that further investigation at least is warranted if the result looks consistently dodgy. Re the comment about f/5 yep certainly less accurate at f/5 than say f/8, but if you have a barlow of known excellent quality, you can effectively make an f/5 an f/10 and get a more meaningful reading assuming good seeing and an optic at or near thermal equilibrium.

Originally Posted by Satchmo

There are ways to increase sensitivity like barlow lenses or higher frequency grating , but these can also amplify the swimming effect of seeing conditions . So tread very carefull before declaring a mirror either a gem or a lemon with a casual ronchi test under the real sky , it may take prolonged observation and further star testing on a few nights before making any meaningful declarations particularly with faster mirrors . At a casual glance most telescopes will show rudimentarily straight bands ...

True, but as I said personally I don't think it is a be-all and end-all. It is a very good indicator if used properly, and for just a few bob (in context) will tell you a good deal about the state of the optics.


Best,

Les D