Aperture photometry strategy for SN 2013

American Association of Variable Star Observers (AAVSO)
Fri, 01/24/2014 - 17:19

I'm interested how observers are dealing with the lumpy background "sky" when doing aperture photometry of the new SN.  I use AIP.

Phil Sullivan (SPP) 

sky background

hi Phil,

This is a case where you need to subtract off the underlying galaxy from inside of the measurement aperture.  Therefore, make sure that you use a concentric sky annulus, and don't go off to some blank sky area to determine the sky value.

That said, the lumpiness is a problem, but not a big problem with the current brightness of the SN.  Use a reasonably tight measuring aperture, and don't make a huge sky annulus (perhaps 5 pixels wide?).  Once the SN fades to ~14th or so, the sky will become more important and the photometry will be poorer.

As we've mentioned before with supernova photometry, the ideal method is to have an archival galaxy image in the same filter with your equipment without the SN.  Then, you can just scale that image so that the galaxy goes away when you subtract the archival image from the current image.  Then you can photometer as always.  If you don't have an archival image, then you can (a) wait a year or two until the SN is too faint, and get the image then, and back process your data, or (b) use some other smoothing/modeling technique to determine the "sky" background, or (c) ignore everything, and live with the poor photometry as it fades.


American Association of Variable Star Observers (AAVSO)
Another trick for photometry with lumpy background

Arne has listed several methods for doing photometry of an object immersed in a lumpy background.  As he says, the absolute best method is to use a template image taken before the object appeared.

If you don't have such an image, you might try the following technique.  Check to see if the image processing programs you are using allow you to make either artificial stars (via a simple mathematical function, such as a 2-D gaussian), or copies of real stars (by cutting out a box containing a star far from the galaxy, scaling it in brightness, then moving the copy to a different location).  If you can make an artificial star, and scale it in brightness, then consider this:

  1. make a completely blank image which has the same size as the image with the SN
  2. in this blank image, create an artificial star at exactly the same pixel coordinates as the SN
  3. scale the artificial star in brightness so that it has the same peak value as the SN
  4. subtract the artificial-star image from the original SN image
  5. look at the residuals at the location of the SN
    1. if the residuals are negative -- it looks like the galaxy has a "hole" in it -- then the artificial star was too bright.  Decrease the scaling factor
    2. if the residuals are positivie -- it looks like there's still a bright (or faint) star at the location of the SN -- then the artificial star was too faint.  Increase the scaling factor
    3. if the residuals look like the surrounding un-subtracted regions of the galaxy, the scaling factor was just about right.  Go to END
  6. go to step 4

END: you have created an artificial star with the same brightness as the SN.  Good!  Now, measure the brightness of this artificial star, and report that as your measurement of the SN

The human eye is pretty good at looking at a picture and deciding if too much was subtracted, or too little was subtracted, or just about the right amount.  This is obviously subjective, but it might be worth a try.  If nothing else, you might get a feeling for the uncertainty in your measurement.

Michael Richmond