Ronchi-gram advice

[FraserClarke]

Hi All,

Attached are through-focus Ronchi-grams of a 6-inch mirror I'm almost finished making. Couder tests and FigureXP show the correction is pretty good (best fit conic is actually slightly uncorrected at -0.91, but that is probably within the errors of my measurements), and gives a rating of lambda/15 for a parabola -- which I don't believe for a minute!!

I would appreciate your thoughts on the ronchi-grams though, to judge the smoothness of the figure. The turned down edge is obvious and pretty severe. I've worked on it a fair bit, and not improved it much, and I'm happy to just mask the outer 2--3mm of the mirror after coating. What I'm more worried about is what I perceive as a slight zone at ~70% radius. Possibly also a slightly high centre region? Can anyone else see this? Is it enough to worry about? Am I missing anything else obvious?

Contrast isn't great -- better by eye -- but I hope good enough to show what might be going on. Scan goes from outside ROC to inside, probably in about 5mm steps (though I didn't measure it precisely). 100lpi grating (printed) and a 6" f/6.1 mirror.

Many thanks,

Fraser

[Glasspusher]

Fraser, nice to see these ronchigrams, thanks for sharing them. Generally I would say that the mirror looks good and would probably perform quite well in a telescope. If you look at a ronchigram of any mirror the edge of the mirror will show a diffraction effect, this can give the impression of a turned edge. In some of your images a turned edge is in evidence, in others for example image 6, the turned edge is less evident. I would say that what you are seeing at the edge is in part due to this diffraction effect. Looking at the mirror with the Knife edge test would reveal the extent of the edge problem. If is important to use more than one test method if at all possible. Image 3 is interesting, with fewer bands visible more detail is revealed. There are hints that a diffraction ring is present round the edge of the mirror…this would indicate a clean edge which is desirable. The right hand side shadow shows some asymmetry indicating the presence of astigmatism, but this could be due to localized air currents or poor alignment in the test set up. Before testing time must be allowed for the mirror to reach ambient temperature and there should be as little air currents as is reasonably achievable in the test area.

If you have a tube assembly for this mirror I would perform a star test, with the uncoated mirror on Polaris, look to see if the intra and extrafocal images appear similar as would be expected from a good mirror.

Looking forward to hearing your first light report.

John

[FraserClarke]

Thanks for the thoughts John. TDE is clearly visible in a knife-edge test, which I've been using for most of the figuring. I don't have any images to these, but I will try to see if I can get them (even harder to align than the Ronchi images though!). I think the test setup is to blame for the uneven illumination, but I should do a point-source test to check for astigmatism.

No tube yet - next on the list to do. No major hurry to coat the mirror though; it has taken me 15 years to get it to this stage (not continuous working, I hasten to add!!)