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photometry on overlapping stars

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ssrfo59's picture
Joined: 2010-09-10

Hi everyone:

I'm interested in doing photometry on a pair of stars that are very close, overlapping one another, and are both very bright (mag10) for the equipment I use.

Assume that both stars are varying.

AIP seems to be able to determine the centroid of the individual stars in this pair.  How should the annulus be set, specifically the middle and outer radii?  The outer ring (normally set to contain just background sky) will of course contain the companion star's light, so I'm unsure of how much/how little should be included.  My previous photometry has been on isolated stars so the annulus settings were straightforward.  


Very short exposures separate out the stars better, but then there's the problem of low SNR and flux issues.

Or--should I treat the pair of stars as single entity and then have the photometric data parsed out later--have software analysis determine the variability of each?


Steve Smith (SSTB)




TCB168's picture
Joined: 2010-07-26

Why should the outer ring contain the nearby star?

Why not make the inner ring just contain one star, place the second star in the gap and then place the outer ring well out. AIP4WIN will measure the flux of the star in the inner ring and then subtract the hopefully empty outer ring. The gap between the outer ring and the inner ring is ignored.



Photometry of Overlapping Stars
jji's picture
Joined: 2010-07-24

Hi Steve

What do the two stars look like.  Do they meld together into one longish object?  Do they look like a figure 8?  Can you see sky between them?  Sometimes expanding the image to 2x or greater will give you a better idea of how close they really are.  You can also examine the PSF of the pair to see how much they are actually overlaping.

If the stars are very close, you may find that while the software seema to find a centroid, that centroid will wander from image to image and cause noise in your results.  Also if you are doing a time series, your seeing may vary during the night causing the stars to overlap even more and causing the target star to "outgrow" your aperture.  Choosing a very tight aperture can increase the effect of this.

If they are far enough a part, you can put the offending star in the gap.  Keeping in mind that the tail of the unwanted star may extend into the target star.

You may have to choose an aperture that includes both stars and remove the effects of the unwanted star with something like Lew Cook's Nemesis Star spread sheet.  Of course it can't remove the effects of variability of the second star.

In the end you may need to find another target.  I have any number of stars that I have had to abandon without submitting any data because in the end I had to admit that they just were not a good fit for my equipment and capabilities..  Sometimes after multiple night attempts.


Jim Jones


Steve I left out one
jji's picture
Joined: 2010-07-24


I left out one option.  You can include both stars in your aperture and add a note to your AAVSO submittal describing what you did.

Jim Jones

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484