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Old 21-11-2013, 10:15 PM
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alpal
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Modulation Transfer Function or MTF is difficult to understand:

http://www.telescope-optics.net/mtf.htm

That may be why spot sizes & Strehl ratios are more often used.
Any telescope with an obstruction is going to worsen the MTF
due to the reduction in contrast.
The trouble is that if you want aperture i.e.
something bigger than a normal refractor -
then you end up with an obstruction.

Still - the pics are better once processed when using a large non -refractive telescope.

To me spot size does matter -
pics done with high end RC telescopes & the Hypergraph etc
mentioned at the star of this thread do look a lot better than
ones taken with SCTs & Newts.
The caveat is that the pictures I linked to were all taken at high altitude dark sites
& maybe that's where the real improvement was.
Seeing conditions are the greatest limitation.
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