[Aavso-photometry] Cosmic Rays

[Wolfgang Renz]

Hi Micheal

In begin of October Dean C. Rowe sent an image of NGC 660
with two brighter minor planet streaks ((13494) 1985 RT and
(3580)) to the SBIG list:
http://deanrowe.net/images/ngc660_asteroids.jpg
In the JPG that must base on some kind of mean/sum combined
image (as the very very frequent, bright cosmics are still visible)
just these two minor planet are visible.
But I discovered a third, much fainter one (2005 SN1, ~ 20.1
magV) in his "raw image" annimation:
http://deanrowe.net/images/ngc660_asteroids_animation.gif
Its coming out of the core of NGC 660 and is moving to the top.
In the animation its pretty obvious due to its movement. In a
single image of the animation its close to the background.
But in the mean/sum combined image is not visible at all.
BTW: He missed its discovery by just about a month.

Having a tool to find even this one would be helpful for minor
planet hunting and for sure would be helpful to detect weak
cosmics in images for photometry.


For the "detection of cosmics" sequence of Ben I meant of
course a) but on a per pixel base.

But such a detection could be easily incorporated into a median
combine (or more complex algorithm) that requires the pixel
at the same position in the different images to be sorted anyway
(by just picking the largest value). By comparing the difference
of the largest value to the mean(/median) of the pixel and maybe
even scaling it by the std(/mean) dev of the pixel one should get
an acceptable estimator on how deviant the outlier is.
But for weaker cosmics very close to the core of brighter stars,
probably just a PSF-based estimator will give sufficient good
results.

Such an "higest absolute deviant combine" algorithm will require
well registered images. And with undersampled images (without
explicit dithering and without increasing the resolution by real
drizzling), one might get issues due to intra-pixel sensitivity
variations or a high share of non-sensitive inter-pixel areas of
some CCDs. The same might be possible for short exposures
that are seeing limited.


Clear skies
  Wolfgang

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



----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "Wolfgang Renz" <w_renz at onlinehome.de>; <aavso-photometry at mira.aavso.org>
Sent: Saturday, January 28, 2006 3:14 AM
Subject: Re: [Aavso-photometry] Cosmic Rays

> Wolfgang,
> 
> Can you elaborate a little on what you are describing?
> Are you wanting to
> a)  combine the images having the largest deviant, or
> b) reject the largest  deviant from the combination?
> 
> Michael Newberry
> 
> 
> 
> ----- Original Message ----- 
> From: "Wolfgang Renz" <w_renz at onlinehome.de>
> To: "Ben Davies" <ben at davies.net>
> Cc: <aavso-photometry at mira.aavso.org>
> Sent: Friday, January 27, 2006 5:06 PM
> Subject: Re: [Aavso-photometry] Cosmic Rays
> 
> > Hi Ben
> >
> > If you average/sum combine the differential images, you'll lower
> > the detectability of the artifacts as they will just appear in a
> > single image. If you combine 10 images, its just 1/10 of the
> > difference in the affected original, if you combine 100 images,
> > its just 1/100.
> >
> > Using something like a "higest absolute deviant combine"
> > instead, should lead to much better results. I don't know if
> > such an algo is implemented anywhere, but it should also
> > be helpful in finding moving minior planets that are just barely
> > above the background.
> >
> > 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: Friday, January 27, 2006 10:07 PM
> > Subject: Re: [Aavso-photometry] Cosmic Rays
> >
> >> Here is a method I've come up with to analyze cosmic ray hits in
> >> photometric images:
> >>
> >> - Make a median of all images.
> >> - Subtract this median from each of the images to get a set of images
> >>   that contain all the artifacts.
> >> - Average combine (or add, depending)  the artifact set.  Now one image
> >>   contains the defects and another contains the signal..
> >> - Blink the averaged image against the median.
> >>
> >> If no artifacts fall on or near the stars you are interested in, the job
> >> is done.
> >> If an artifact does coincide, you just track down the offending image.
> >>
> >> I'm probably just stating the obvious here, but it took me a while to
> >> figure it out
> >>
> >> Ben Davies
> >> http://ben.davies.net/