Quote:
Originally Posted by Satchmo
There is no mystery about this. As far as I know those figures are for primary and secondary combined, as a functioning instrument. The Star Instruments web site , who make RC Optical instruments clearlys state that these wavefront figures are Root Mean Square, not P-V .
So assuming the surface figure is smooth, 1/24 RMS would equate to 1/6 wave PV wavefront. In the situation where they are measuring points all over the surface it makes sense to stick to talking in RMS as simply picking the highest and lowest point on the wavfront to define its quality simply makes no sense.
A heavily obstructed instrument like this has its Strehl ratio knocked down into 60's regardless of how good the mirrors are because of the large obstruction, so any wavefront better than 1/4 wave , 1/20 RMS is fine for the job of prime focus.
I did ray traces once that showed perfected optics with a 45% obstruction producced an EER ( Strehl taking into account the obstruction ) of 0.72 and system with 1/4 wave spherical aberration of 0.8 ended up at EER of 0.68 obstructed.
Its the guys doing planetary photography at 4X-5X their native focal length that will pick the difference between 0.95 and 0.8 Strehl wavefronts.
Clearly the lesson is to keep the obstruction to a minimum which generally visual observers, and planatary photographers have the luxury of doing.
Mark
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i didnt understand any of that ....
but now i feel strangely aroused
