Free Mirror Testing: Is your scope not performing as good as it should with fuzzy images? There could be many possible causes; seeing, alignment, tube currents, etc. as well as the optics quality. If you think all the other possibilities have been addressed then you might want to have me check you optics for free. I use a Zygo GH with a modified Offner null system and can check mirrors up to 20 inches.
Secondary Mirrors: The most common cause of an astigmatic image in Newtonians is from a poor diagonal mirror. I can check diagonal up to 4 inches major axis on my Zygo and for larger diagonals I can check them using the Raleigh water for any size diagonal you might have or optical flats up to 12 inches or more with no danger of scratching.
Bad mirrors that I have have checked: I thought I’d post a few bad mirror folks have sent me to check. Here is a recent one from a major name brand with a bad case of astigmatism. In this check the fringes should be perfectly round about the center dot but these look like footballs, about the worst I’ve see, obvious in the 90 degree rotation.
I was going to refigure this one but after stripping the coating I checked it with polarized light in this pic. in front of my monitor:
With polarized light an annealed blank should appear black. With stress it will show some white areas, a little worse will show a Maltese cross, worse yet are bands and finally colored band (like this) are highly stressed. This was the cause of the astigmatism.
But this one takes the cake. This is a 16 in. sandwich mirror sent because of bad images. The fringes were extremely tight and apparently it has become delaminated and with no apparent cause for the delamination. It’ll be hard to top this as the worst mirror ever!
Contact: Please do not send me you optics for testing before contacting me. Do not send your mirror packed with peanuts, bubble pack or styrofoam; please use hard or closed cell foam as in this video: https://www.spectrum-coatings.com/packing Contact me just before you ship (to be sure my laser is still working, they can die unexpectedly), enclose a return shipping label and a flash drive is handy for images and video but not required. Send your secondary as well as it is more often the cause of poor performance. I will send you interferograms of your mirror done with my null setup and my opinion. They are easy to interpret as a perfect mirror has straight parallel evenly spaced fringes where each fringe is one wave.
1071 Terrydel Lane Cincinnati, Ohio 45245