Help me read an OMI mirror report?

I feel a bit naked doing this but I thought I would put OMI's specs on the mirror I just purchased (used) up for review:

http://tinyurl.com/yele62q

The link takes you to a pdf of the report (or at least it should). This is a ten inch f/6. My take is there is some TDE but maybe not enough to bother worrying about. I have read it is difficult to eliminate. Club members who look at these things say they are puzzled by the number of lines in the interferometer report, that usually there are fewer lines.

The Strehl of .96 looks pretty good to me, I don't know what to make of the RMS and I don't know how to translate this into "wavefront error" etc.

I'm sure a lot of people here can read this in their sleep.

OMI puts all its mirror reports on the 'net which I think is a great commitment, helps prevent the kind of nonsense that Dick Nelson did with Optical Craftsmen back in the 60s (all of that is in two threads lower down).

I'm hopeful that this is a very very good mirror without necessarily being the peak of the peak. I bought it here on the 'mart and got a good price.

OMI has a good relationship with Obsession and is one of the more conspicuous names in the upper end market these days, I think possibly with higher output than some of the other zen masters. Some people may for that reason feel shy about posting a critical opinion, so PM me if you want to say something private. But as y'all may gather from my having posted the Optical Craftsmen Foucault, I'm pretty hang loose. Things are what they are.

regards
Greg N

I think you've read it about right.

The large number of fringes is not a problem. Using a large number
lets them detect small or narrow features like the TDE.

The RMS and Strehl are the important numbers. The RMS or Root Mean Square
is mathematically computed as

square root( average over mirror ( [height - average height]^2 ) )

This is a good indicator of quality. Small bumps or features like the TDE
are appropriately down-weighted, since they only affect a small area of
the mirror.

The Strehl can be computed directly from the RMS number.
The reported Strehl 0.96 is an excellent mirror. Strehl 0.80 would
correspond to "diffraction limited" so you can say this one is "much
better than diffraction limited."

The TDE looks to be about 0.1 wave on the surface, or 0.2 waves
on the wavefront. This is from the contour plot at the top right
where is seems to roll-off by 2 contours, or 2 x 0.05 waves/contour.

They do not quote a PTV number. You can roughly estimate it by
placing a ruler on the interferogram. The TDE seems to be the
low point. The high point seems to be a central hill. The
range from peak to valley is about 1 fringe or about 0.25 waves on the
surface, or 0.5 waves on the wavefront.

Altogether is seems like an excellent mirror. I would trust this
much more than something measured with Foucault that said 0.999 Strehl.
I've seen too many Foucault tests that missed huge astigmatism
errors, etc. Some people also badly botch interferograms --
important thing is to get it in a scope and try it out. Do some
star tests, etc.

Attached Image:

>>I feel a bit naked doing this but I thought I would put OMI's specs on the mirror I just purchased (used) up for review:
http://tinyurl.com/yele62q>>

It shows a synthetic interferogram, so basically it doesn't give you the actual mirror data. No way to verify the results, I'm afraid. You will have to take their word for it, the data is not complete.

The attached image shows a real interferogram of a 1/10 wave .98 Strehl Ceravolo mirror. Note how smooth it is with no wiggles (zones) and essentially no edge zone.

Rolando

Attached Image:

There is a definite TDE but the magnitude is difficult to determine from the graphs (at least difficult for me). I think it would be very instructive to see if you could detect any improvement in viewing fine planetary detail when the mirror is stopped down to 9.5".