Improving sharpness with image selection and short exposures

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timh
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Improving sharpness with image selection and short exposures

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Post by timh »

For any other folk interested in the topic of 'lucky imaging' a 2009 Royal Astronomical Society publication which I thought quite useful and accessible..

Using Strehl ratio (rather than FWHM) as an measure of image quality it demonstrates - inter alia - that selecting out only the best ( 1-10%) images for alignment results in significant (> 2 fold) improvements in image sharpness over a range of exposure times from 4 right up to 640ms (the shorter times resulting in much greater improvements of 6- 7 fold). The factor of two chimes pretty well with what I have seen myself with a 25 cm telescope and exposure times of 300-500 ms -- and within a range practical for the brighter deep sky objects.

The paper provides some useful background/ links to the theory and analysis of a range of variables including telescope D.
https://academic.oup.com/mnras/article/ ... 069/983416

TimH
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Re: Improving sharpness with image selection and short exposures

#2

Post by admin »

Hi Tim,

interesting stuff - unfortunately I think that the amount of info needed to calculate the Strehl ratio (and possibly the amount of calculation needed) means that it is not something that can be used directly in SharpCap. I would note that SharpCap strictly uses HFD (half flux diameter) when measuring star sizes for focus etc. HFD suffers much less from the undesireable behaviour that can affect FWHM in some circumstances and is easier to measure consistently. SharpCap continues to use FWHM in the UI as it is more familiar to end users.

Did you find any processing tools that can evaluate the Strehl ratio of captured images/movies?

cheers,

Robin
timh
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Joined: Mon Aug 26, 2019 5:50 pm

Re: Improving sharpness with image selection and short exposures

#3

Post by timh »

Hi Robin,

In the methods description part of their paper Smith et al (2009) say "We used the Strehl ratio as a measure of the quality of the resulting star images. Another possible measure is the full width at half-maximum (FWHM) of the image. However, this is not, in practice, a good measure of image quality. Images resulting from frame selection generally have a core-halo structure, i.e. a diffraction-limited core surrounded by an extended halo the size of the original seeing disc. The core can give rise to a small FWHM even though most of the energy is in the halo. Strehl ratio is a much more demanding measure of image quality, since a high Strehl ratio can only be achieved if most of the energy is in the diffraction-limited core."

However --as far as I can understand (not very far) Strehl ratio measurement requires simulation of 'equi-bright' diffraction-limited PSF's for each star and is probably not a practical proposition for quickly selecting from 'normal images' having many pixels, lots of stars and noise -
and in any case nowhere near diffraction limited ? see https://www2.keck.hawaii.edu/optics/aoc ... meter2.htm

In the meantime, while image selection based on SNR, FWHM and eccentricity is not the Strehl gold standard it does at least seems to works well enough to realize a significant level of improvement. I have now set up the darks and flats for my fast 10 inch reflector so that once the clouds clear I will hopefully be able to find out if SC livestacking 0.5s frames along with hard FWHM and brightness filter selection provides a relatively easy way of getting sharper autostacked images of some of the brighter deepsky objects?

Tim
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