Quote:
Originally Posted by Stefan Buda
There is another factor involved in measuring the FWHM that nobody mentioned: the effect of the telescope used for the measurement.
Not only the quality of the optics and the thermal state of the OTA but even the size of the central obstruction will influence the measurement.
Take two instruments with perfect optics at thermal equilibrium, same aperture, but one with 50% central obstruction and have a look at the diffraction pattern of a star. The instrument with central obstruction will put more photons into the diffraction rings of the star therefore producing a larger spot.
I don't think the software that works out the FWHM takes into account the effect of the central obstruction.
Residual spherical aberration also has the effect of increasing the value of the FWHM reading.
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I am sure there are a lot of factors that will affect the FWHM measurement.
See the formula for Contrast Factor here:
https://alpo-astronomy.org/jbeish/Newt_Sec_Mirror.pdf
NOTE: an unobstructed telescope yields a CF of 5.25 : 1
There is also Strehl ratio:
https://en.wikipedia.org/wiki/Strehl_ratio
The accuracy of the mount too.
Adaptive optics will give a better FWHM.
There is no real way you can compare 2 different telescopes exactly
but a FWHM measurement with whatever software
is about as good as we're going to get.
cheers
Allan