Image of the day

From the
ATWB Customer Gallery

M8 & M20 Wide Field

My Account

New to Astromart?

Register an account...

Need Help?

PLOP cell designs

Started by dsummers, 09/02/2002 07:51PM
Posted 09/02/2002 07:51PM Opening Post
I've been looking at PLOP cell designs vs. standard commercially available designs. Now that I'm convinced that PLOP designs are the only way to go, I'd like to find a shop that makes them. Anyone know of a good machine shop that's familiar with ATM work and PLOP cell designs in particular? Thanks!
Posted 09/02/2002 10:43PM #1
Hi Doug:

No one commercially uses PLOP for cell design. David Lewis himself said it wasn't necessary. Everyone uses Chandler, based on Hindle's idea's. David Lewis (creater of PLOP) wrote an article for S&T a year or two ago describing PLOP and it's features. He confirmed that Hindle/Chandler designed cells were entirely adequate and further they more than complied with Lewis' suggested criteria for a good cell.

I just ran a couple of cells in PLOP for a 22"x2" f/3.7 mirror. For instance, an 18 point Hindle cell will produce and error of 1/59 wave p-v at the surface of the mirror, or 1/392rms. A PLOP optimized 18 point cell is 1/67p-v and 1/458rms. Either cell blows away Lewis' suggested minimum criteria of 1/32p-v and 1/128rms at the mirrors surface.

Lewis' confirmed Hindle's ideas are fine, further there is literally NO GAIN AT THE EYEPEICE of this PLOP cell. Why then retool, redesign, and risk unhappy customers for unproven PLOP cells if there is absolutely no gain in performance for the user. PLOP will not make a bad mirror good, no matter how hard we try!

I think there's better places to worry ATM projects. For instance the few companies that user interferometer data to certify thier mirrors use 632nm red light for the wavefront calculations. If a mirror is 1/4 wave p-v on the wavefront in red it will be ~ 1/3.5 p-v in yellow green (550nm) which is the standard for visual use. A similiar reduction in RMS is promised as well.

Couple that with the error of the secondary mirror, the whole system has NO chance to comply even with the Rayliegh criteria let alone more stringent standards, at the itsy bitys diffraction limited central spot before coma takes over, Paracorr or no Paracorr! A Paracorr will not make a bad mirror good either, wish as we might!

This alone will have a much bigger influence over the image at the eyepiece than the literally unpercievable difference between PLOP vs Hindle or just about anything else. Except maybe poor baffling and crappy collimation It's easy to get caught up in numbers and penciling out stuff, but at the eyepiece is where the rubber finally meets the road.

Steve Vegos