Testing De-focusing for use in Photometry

Discussion of using SharpCap for Deep Sky Imaging
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oopfan
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Testing De-focusing for use in Photometry

#1

Post by oopfan »

Clear skies last night to test the theory that de-focusing stars raises the signal-to-noise ratio and therefore the accuracy of magnitude measurements.

The answer is a definitive YES.

In the screenshot below I make reference to "HFR" which stands for "Half-Flux Radius", similar to FWHM. Typically I can achieve HFR values of 1.0 and lower on steady nights. Last night came in at HFR 0.8 with perfect focus on a magnitude 10 star and 1-second exposure.

I captured 10 frames at perfect focus (HFR 0.8). It required a 55-second exposure in order to bring the maximum pixel value of the comparison star to 50,000 ADU. Then I de-focused to HFR 1.8, roughly twice the star radius. I captured 4 frames that required a 180-second exposure.

For calibration I captured 50 flats, and 30 darks each at 55s and 180s. This morning I processed all in AstroImageJ and then ran differential photometry for the target star with respect to the comparison star.

The attached image tells the full story.

Thanks to Robin for his expert advice!

Brian

By the way the field is AAVSO's Standard Star Field SA38 in Lyra. The stars are constant brightness and measured to an accuracy of one-thousandths of a magnitude by professionals and advanced amateurs.
Photometry De-focus Composite Presentation.jpg
Photometry De-focus Composite Presentation.jpg (174.85 KiB) Viewed 1109 times
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Re: Testing De-focusing for use in Photometry

#2

Post by admin »

Hi Brian,

Glad to hear that it worked in practice as well as in theory :-) Hope you get some clear nights soon to put it to practical use on a real target.

Cheers, Robin
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oopfan
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Re: Testing De-focusing for use in Photometry

#3

Post by oopfan »

I need more aperture!
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Re: Testing De-focusing for use in Photometry

#4

Post by admin »

You can't beat a Newtonian in terms of value for money when it comes to collecting photons. Of course they are temperamental and high maintenance beasts, so if you put any value on your time then it may not be quite such the bargain that you first thought!


Cheers, Robin
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