I'd also like to know what minimum wavefront is represented by straight lines at f/10. I read somewhere that the ronchi test wasn't good for checking astigmatism, so you'd still need to check that on a star.
I believe that sensitivity of Ronchi tests depends greatly on type of scope being tested. The "defects" you are seeing here may well be within tolerance limits for high quality mirrors. You need to talk to someone who knows how to interpret these tests for a given scope.
E.g. my Dobs Ronchi test just about perfect, but the 4" Mak shows wavy fringes. But for its small aperture the Mak performs very well for its aperture at high powers.
That's good then. Your initial post implied you still needed someone to makes sense of the results. If Don says he can do it then I'd take his word for it.
Ok. I'm no expert either Matt but I've made mirrors before & tested them with a ronchi grating. (a long time ago)
I also recently made another one to test my C9.25 after reading about your scope & the general state of affairs re Celestron lately.
I assume your scope was at ambient when these shots were taken. If not, you will get a false test result (as I found out from practical experience).
My first test was conducted with the scope way above ambient & the result was images similar to your pics, but no where near as severe as yours.
The following night I let the scope sit outside for 5 hrs to cool & this time my lines were perfectly straight.
If your scope was at ambient simply disregard most of the above. According to your pics (& the link) its either under or over correction, depending on what side of focus your shots were taken. As to the severity of it, I guess you'll find out.
Hi Matt and everyone,
Being a beginner mirror maker and having just gone through all the figuring process I say that looks rather bad..Main Mirror or the secondary is undercorrected and one of the mirrors has a few ring zones in it as well by the wriggles in the lines. I presume the pics are inside focus..
Make sure they give you a replacement mirror at least.!!!! Must have slipped through there quality control somehow?
You can see my ronchi tests from COC and they don`t look that bad....and my mirror works out about 1/10 PV..
I cant find the Ronchi Nebula on my software...what's the NGC number cuz the pics you posted look like a cool ring nebula...
frank
(trying to inject some humour here cuz I know how frustrating the saga has been for you). I hear you're happy with the replacement gear.
frank
Hi Matt. I'll throw my two cents worth in here. The mirror is no good Period. For a company of this type to release a mirror like this is inexcusable. Yes there are tolerances but not to this extent. Most beginners can make their own mirror much better than that. We tested some GS scopes at a star party last Christmas and a few at my local instrument making club and the GS ones were all correct. Nothing like this one. It would take a "while" to correct this mirror by hand. Glad to see they took it back.
Still got any doubts within reasonable tolerance limits?
I don't think temp would distort the test enough to give these images?
thanks cristian
Frank - only had first light last night. Seeing not flash, but the signs are promising. Sorry if I'm not laughing about all this at the moment. The wound goes very deep!
Matt, those posts are for you to read. I have no interest in buying any sort of SCT. You're the one who wrote:
Quote:
Originally Posted by matt
I'd really welcome your opinions/diagnosis on what these images reveal.
I was only suggesting to get expert opinion before jumping to conclusions. So it turns out you had gotten expert opinion already from Don who says he can interpret the Ronchi patterns. Then I say trust his diagnosis.
Quote:
Originally Posted by matt
OK. No more comment.... please
Sorry, I'm having a little trouble figuring out where you're coming from.
Hi guys regarding my previous post I am assuming that the ronchi test was made through the eyepiece of the telescope, not with the mirror on a stand. If it was made with the mirror on it's own then it may be within tolerances. A ronchi test will show straight lines when done through the focuser looking at a star (Artificial or real). If it is done with the mirror on it's own then outside of the radius of curvature then you would expect the lines to be bent like that. Where the error may lie is to what extent the lines turn in relation to the F ratio of the mirror. The accuracy of the bend and the smoothness of the bend will give the mirror it's quality. A mirror can be 1/4 wave and look like that. Most amateur mirror makers aim for 1/20 wave mirror or better. That is why some of us prefer to make our own because most production telescopes have an error in the mirror somewhere (once they are subject to testing). Only quality mirror makers can produce outstanding mirrors (Royce, Mark etc) and most large telescopes companies can only produce reasonable ones, It's the nature of mass production. I may have been a little too harsh in my previous post if this mirror was tested on it's own so I apologise if that is the case. But regardless of which way the mirror was tested I would not be happy specially if I paid good money for it. i hope i haven't added fuel to the fire.
