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Below is a comparison of the Point Spread Function
of a f10 RC that is first focused on center, then focused to
make the on-axis near the size of the airy diameter and the third
example is focused to yield similar RMS sizes both on and off-axis.
Again, this is at the edge of a 2.0" diameter field. RMS
spot size (in microns) at the 2.0" diameter edge is given
for each PSF just below the image as well as the in-focus travel
amount needed to yield the below results. |
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23.6 |
Focused for on-axis. |
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Off-axis PSF. |
Best focus position for planetary or other narrow
field work. |
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17.0
0.23mm |
Focus of on-axis is optimized to be no larger than
the airy size. |
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Off-axis PSF. |
Best position for all around use, even planetary since
the on-axis is extremely seeing limited. |
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~11.7
0.39mm |
Focus optimized to give similar RMS spot sizes both
on and off-axis. |
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Off-axis PSF. |
Best position for wide field imaging, say with a 1.0"
x 1.5" CCD chip. |
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Continue on to the next page to see an analysis of
Classical, Ritchey-Chretien and Dall-Kirkham Cassegrains compared
at f12. |
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