Quote:
Originally Posted by Bassnut
AND, Peter, am I right or wrong, OTA optical performance is generally measured by image quality over the whole illumination circle (I might have the terminology wrong, sorry), which is important with a large chip that is fully illuminated. If a small chip only captures a small proportion of a large aperture OTA illumination, then edge performance is far less critical?.
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Que? You asking me?
I suppose by definition Strehl looks at the whole wavefront.
But this is a bit moot, as is field curvature, if you just use the say centre 1/3 of the available field. That said, I suspect visual observers would not be happy with an optic that was only sharp in the centre 30 degrees of a 100 degree Ethos field!
But with imaging provided your optic pumps most of the available flux into the airy disks at the centre third..or what ever...and if your CCD is too small and can't "see" the rest you will have a high resolution solution of sorts. But wait there's more.
If you happen to image from within the atmosphere you are sky limited.
Lets say at 500mm you can get 1000 counts per pixel from the sky, plus 100 counts from the star sampled at 4 arc sec per 1 pixel. Total flux 1100 but less than 1/10 is signal
Let's now increase the FL to 2000mm . Our sky contribution over the same pixel has reduced by 4x. The object flux remains the same, Total flux 350, but now over 1/3 is signal!
Having more signal vs noise is pretty handy....de-convolution works better
plus when noise is pushed down to trivial levels the signal clearly and cleanly resolves above it.
Yes, narrow field rules
