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Modifying the annulus position

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TCB168
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Modifying the annulus position

Hi

I have had problems with a few images trying to measure a star with a close companion.

Would it be possible to add a feature where the target can be moved slightly to better centre the star.

See the image attached for the problem. Cleary I can not use the measurement made for this particular star.

Thanks

Terry

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HQA
HQA's picture
modifying annulus position

Hi Terry,

In the case of a close, nearly equal brightness, double like you've shown, the best aperture photometry approach is to use the Nemesis spreadsheet by Lew Cook:

https://www.aavso.org/software-directory

(near the bottom).  It does require knowledge of the companion star brightness, of course.  I think the original VPHOT did a "snap to" to the brightest star within a given radius, but that is usually the wrong approach (most target stars seem to be the faint star of the pair!).  You may be able to take the sequence that you create and save, and modify the coordinates; I know that you can change the magnitudes so maybe this next level of editing is available.

Close doubles are difficult.  The best measuring tools are either psf-fitting photometry like DAOPHOT or PSFextractor, or the complete image subtraction method pioneered by Alard.  My recommendation is to avoid close doubles and choose another, equally interesting, object.

Arne

TCB168
TCB168's picture
Close doubles

Thanks

I will try both methods described.

Unfortunately Arne sometimes close doubles are unavoidable. In this case it is a nova in the SMC and I need the photometry to flux calibrate spectra.

Terry

MZK
MZK's picture
Pixel mode and Settings

Terry:

There are a few things to try. Note that VPhot has a pixel/adu mode (above image). When you select it and click on the target of interest, you will zoom in on the two companion stars. You can move the aperture slightly to place the aperture on the correct target and only that star. Of course, the aperture needs to be small enough to exclude the companion. Give this a try. If the two stars overlap too closely as a result of the inadequate resolution achieved with your scope, you may need to give up BUT, give it a try and visually confirm the separation.

You can also modify (reduce) the aperture and sky annulus settings AND the centroid determination  under Tools and Tools/Settings. Be sure to confirm that the settings look appropriate on the image!

HTH, Ken

HQA
HQA's picture
flux calibrating spectra

Hi Terry,

If you want to VPHOT-share the images to HQA, preferably multifilter (but I can deal with a single bandpass), I'll go ahead and psf-fit them so that you have a magnitude for the companion - and then you can use Nemesis.

Arne

MZK
MZK's picture
Nice Experiment?

Terry:

This sounds like an interesting experiment. Could you also share images to me (MZK)?

It will be interesting to compare magnitudes obtained by both PSF and aperture photometry?

Ken

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