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star aperture

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star aperture

Probably old subject matter, but newbie photometry observer here. Just looking for clarification from experienced observers on this topic.

The AAVSO Guide to CCD Photometry says regarding the selection of star aperture..."make the star aperture 3-4 times the approximate average FWHM of all stars to be measured".

The written V Phot tutorial says..."use 2 times the FWHM of the approximate average for the star aperture".

The Photometrica video says..."use 1-1/2 to 2 times the FWHM of the average for the star aperture. For bright stars 2X and for dim stars 1-1/2X."

Slightly confusing, so I've been using 2 times the average FWHM of my measured stars and all seems to be well. What would be considered the "normal" setting for the star aperture with V Phot?

Jamey (JJLB)



Eric Dose
Eric Dose's picture
Watch out for aperture

Watch out for aperture measured as radius vs measured as diameter. The VPhot guide (p. 6) is ambiguous, for example.

You won't go too wrong with an aperture diameter of 3.5 x FWHM if your star profiles tend to be quite round, perhaps 4 x FWHM if they tend to be a bit elongated.

What out for aperture

Thanks for your input Eric!! My policy is to only measure images with round stars, and reject any with trailing issues, which do indeed occassionally pop up. So, using 3.5 x FWHM would be preferred so as to most likely include all star light to be measured as well as a slight bit of surrounding sky? The surrounding sky component then being removed during the reduction process of VPhot?

Just trying to make sure I'm understanding the reasoning behind selecting apertures.


spp's picture
trailed images


Don't be too quick to reject all trailed images.  You might be surprised to find that you can often get pretty good measurements from slightly trailed images.  The downside of trailed images is that you must use a larger aperture to get all the signal from the star, but this means you will also include more sky background (more noise). 

Yes, the increased sky background signal form the enlarged aperture is removed in the photometry process, but the increased noise from that signal remains.

If you use the SNR curve of growth in the in the VPhot Meaurement Details plot which Ken mentions, you can find the optimum aperture for your trailed image measurement.  If the star is relatively bright (high enough signal), then the additional noise from the sky brackground may not be a big problem. 

Comparing measurements done on trailed images with meaurements of the same field with round stars could be an interesting experiment.


trailed images

Hmmm...yes, that would indeed be an interesting experiment. May explore that question in the future.

Thanks for your input Phil!!


MZK's picture
VPhot Aperture

Hi Jamey:

Rather than just accepting what the guide recommends or someone has told you or a "rule of thumb", I recommend that you look at the VPhot Star Profile graph in Measurement Details for the target and some comps in your image. Consider using an aperture (radius in VPhot) where the star profile begins to return to the background level so that you include most/all photons for your target but negligible background.

After doing this many times, you will discover that for your system, the aperture radius may be quite stable (most of the time BUT not every time!). At that point, you may consider using this "normal" aperture radius. Calculate what factor of FWHM this turns out to be for your system/sky (probably close to 1.5x - 2x?).

However, since VPhot provides the graphical displays, why not just regularly check to confirm this?


PS: As Eric noted, some documents report aperture as a radius and others report aperture as a diameter. So, they differ by a factor of two, but it is the same aperture.

VPhot Aperture

Hello Ken,

Logical and analytical way to approach my question. Obviously, and I've noticed in some instances with my own images that a "one size, fits all" approach doesn't always work. Best way is to analyze my images and discover if a trend exists to discover a "good starting point" for future observations then adjusting as necessary. I can do this while paying attention to the terminology used, that is radius or full easily missed attribute.

Thanks for the advice fellows...sounds good!!


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