[Aavso-photometry] Cosmic Rays

[Wolfgang Renz]

Hello Micheal

You are right, just pickíng the maximum pixel value at each
location will be sufficient for bias, dark and flat frames as
these images have a pretty even ADU count level. All you
might have to do, to see what you want is a simple contrast
strech. In light frames it will also make transient objects
appear as streaks or a series of dots and will pronounce
the (single image) artifacts. It will do that much better than
every median/sum/average combination algorithm.

But for light frames (that have widely varing ADU counts) a
"maximum pixel" image will also show the stars and nebula
in the image. This might be sufficient for blinking against
single and differently combined frames depending on the
personal preferences and on what one wants to see.

Subtracting a median combined, sigma rejected or similar
image should get rid of any constant stuff and the artifacts
and transients will remain. If you don't want to blink every
image manually, you need something like an artifact mask
that can be used in further steps by software.
There might be also weaker optical artifacts like filter/lense
reflections and diffraction spikes. If these stay at the same
position, they should disappear in the subtracted image. If
they move and/or rotate a bit between the different images
(depending on the mount type, accuracy of polar alignment,
dithering, ...), they should get visible as some kind of
contured variing gradient area in the subtracted image.

Light frames will have widely varing photon noise levels due
to their widely varing ADU counts. To not show this as artifacts
in the subtracted image (especially with stars), some kind
of stddev, mean-min, median-min, meandev scaling should
help to get these differences down to a more even level. Of
course this will affect the absolute ADU count detectable in
these areas.

I'm not common with Mira. Do you have a link to a description
of the different combination algorithms implemented in Mira ?

Normalizing the image set is essential to get a valid median
value. How do you do this in Mira ?
Can it handle moving gradients due to dusk/dawn and moon
rise/set ?

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>
Sent: Saturday, January 28, 2006 5:59 AM
Subject: Re: [Aavso-photometry] Cosmic Rays

> Thanks---I think I understand better now what you mean. Believe me, your
> English is far better than my German!
> 
> Once you pick the maximum pixel, I don;t see how scaling or subtracting
> the median helps you get further detection. This happens in Mira at least,
> because the transfer function is automatically adjusted relative to the
> median. You just change the stretch. One of the capabilities of Mira Pro
> is rank combining.If you set the rank to 100% then you get the maximum
> pixel value at each location. It makes transient objects appear as streaks
> or a series of dots.
> 
> Is that what you are describing?
> 
> Michael Newberry
> 
> 
> 
> ----- Original Message ----- 
> From: "Wolfgang Renz" <w_renz at onlinehome.de>
> To: "Michael Newberry" <mnewberry at mirametrics.com>
> Cc: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>
> Sent: Friday, January 27, 2006 9:34 PM
> Subject: Re: [Aavso-photometry] Cosmic Rays
> 
> > 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/