[Aavso-photometry] Re: Dark Sky Annulus

[arne]

Richard Huziak wrote:
> Can you give me a brief summary of what the 'optimal size' of the 
> dark-sky annulus should be in aperture photometry (in a 'normal' field)? 
> Is there some preferred ratio of dark annulus to aperture that is 
> 'best'?  I see a very small effect when resizing the annulus, except in 
> a really poor sky, where making the annulus smaller seems to smooth the 
> data.  This is contrary to what I thought - thinking a very large dark 
> sky annulus would give a nice quiet, average background.  I don't see 
> variations of more than 1 percent of so except on the worst nigths when 
> playing with the annulus.
> 
The "dark-sky annulus" (also just called the sky annulus) needs to be:
   (a) concentric with the target object, so that any gradient in the sky
background is handled;
   (b) large enough so that the noise in the sky measurement is reduced
to the point that it has little effect on the noise of the target measurement
(remember, you subtract sky from the target aperture);
   (c) inner radius needs to be large enough so that little of the target
object profile is included in the annulus

Setting the size of the annulus is a "black art" (so to speak).  I usually
set the inner radius to be about 5x fwhm and width about 10 pixels, but the
main idea is to have more pixels in the annulus than in the aperture.  Where
you run into potential problems are for faint stars (near the sky limit) or
crowded fields (where a bright contaminating star might be in the sky annulus).
The sky annulus size is not critical; you can use a large one on one object and
a small one on another.   You may not see any difference between different
annuli because the sky noise may be dominated by the Poisson noise of the
target object.

> Part 2 then becomes, if annulus size has little effect on the 
> measurement, then can you use zero diameter annulus for a measurement of 
> a supernova in a galactic arm, and a tight aperture to eliminate 
> background as much as possible (assuring, of course, you minimize 
> clipping of the image)?
> 
By zero diameter, I assume you mean a 1-pixel-wide annulus?  Not wise
because the noise in the sky measurement will be large.  Supernovae are
really hard to do right because the underlying galaxy is nonuniform, and
how you set the sky annulus will affect the final result.  When the SN is
bright, you can be pretty crude about the process, but as it fades, the
best solution is to remove the underlying galaxy before starting the measure.
This can be done with an image without the SN present (either precursor
or far post-cursor), by removing an analytical model, etc.  See
Richmond et al., 1995 AJ 109, 2121 for more details.  In general, though,
measurement of a supernova requires the smallest measurement aperture with
which you feel comfortable, and playing around with the sky measurement
until you get consistent results.  Watch out for the wierd colors when
the SN gets faint; transformations become very difficult.  I prefer to
leave SNe alone or only work on them when they are bright or isolated
from the galaxy.  SN2002ap was an exception; there I followed it for a
long time since it is a GRB SNe prototype.  Image subtraction is an
interesting technique that ought to work well, as long as you have
a good non-SN galaxy template.
Arne