[Aavso-photometry] Cosmic Rays

[Wolfgang Renz]

Hello Ben

There are some other kind of QA that might also catch the large ones:

- Plot the star aperture ADU values against the time and you will see
  the effect of the extinction, clouds/cirrus, ... .

- Plot the sky annulus ADU values against the time and you will see
  the effect of dusk, dawn, moon rise and set, light dome, horizon
  brightening, clouds/cirrus, ... .

- Plot the FWHM against the time and you'll get some idea of hight
  dependant scintillation and temperature dependant focus shift
  changes.

If in these plots a single measurement falls out of a smooth trend,
its for sure worth to take a look at it (and in many cases rise the
error value or even reject it).
Most of the outlier images will work well for differential photometry
(see e.g. http://www.bav-astro.de/rb/rb2005-4/217.html) but not for
all-sky photometry

You should also take a look at your darks and exclude hot pixel.
And at the flats too and exclude pixel that are dead or shadowed
severely (e.g. due to dust on the CCD chip or its cover slip)

Clear skies
  Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Ben Davies" <ben at davies.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Saturday, January 28, 2006 6:54 AM
Subject: Re: [Aavso-photometry] Cosmic Rays

> If a cosmic ist falling into the sky annulus, is often not too serious 
> asmost (advanced) photometry packages have implemented some 
> kind of outlier rejection.
> 
> Hi Woolfgang,
> 
> I believe that you are right about this in most cases.  However, a 6 x 
> 42 pixel cosmic ray is not uncommon and this will fit nicely into 
> annulus rings set at 15 and 24.  That is 25% of the pixels being 
> changed.  Here, there is surely there is some scenario of pixel values 
> that will screw up any ourlier rejection scheme.
> 
> But I guess I would say, if you don't have to take a chance, why would you?
> 
> Thanks for pointing out the usefulness of the method in detecting 
> asteroids.  I believe it will find anything in the images that varies in 
> time or in space.  And thanks for pointing out last week the links to 
> the studies on cosmic rays.  I don't know how you always come up with 
> these things.  You either have one great database of links, or you have 
> a gift for choosing the right search terms.
> 
> Ben