[Aavso-photometry] Cosmic Rays

[Wolfgang Renz]

Hi Michael

Thanks for taking the time to answer my questions.

> ... The idea of subtracting the median image from  the Max,
> and then normalizing using the standard deviation image
> sound like interesting propositions. I am thinking that stars
> and galaxies will still be visible in all variations, as well as
> the transients like cosmic rays.
...
> Subtracting the "Middle" image (a Median or Mean combined
> frame) would seem to have a value in flattening the large-scale 
> trends in the background (like a slope or hump), if the background 
> is not already level. But that operation shouldn't make stars and 
>galaxies go away because the Max value scales with the signal
> level, so you just scale down the whole image by subtracting the 
> middle value. It is not a linear scaling, but things do get scaled 
> down according to their signal. The transients will also be there.
> Scaling the Difference image by the standard deviation image
> will increase the relative strength of the transients, but again, the
> "fixed" stars should still be there because the StdDev scales as
> the square root of the signal. Hence you will still see both objects
> and transients (e.g., cosmic rays) in a Max frame, a Difference
> frame, and a Scaled difference frame. But they will be of different
> relative strengths. Of course, the way these 3 cases (Max, 
> differenced Max, and Scaled Difference) are *displayed* will also
> affect how you see things, and maybe this causes some apparent
> differences in the way they are viewed.

I added the "some kind of scaling" to cut down the remaining
absolute diff values of stars, nebula and galaxies and to pronounce
the artifacts of interest. I also thought that they might be still visible
to some extent, but they should be due to the scaling at a much
lower level than without normalizing. I didn't state a single scaling
method because I wasn't sure which one should will work best.

> Wolfgang, first of all, I am not clear on your statement about
> the option of blinking images individually. Why is that bad?

Its not bad, just maybe time consuming for a large set of images.
If we finally end with an acceptable algorithm, that could be
(half-)automatable and integrated into the (photometry) software
(e.g. an dead/hot/unreliable pixel mask and an artifact mask),
it could help to detect outliers more easily and more quickly.

> What software are you using?

I just started to look at different software packages and for sure
not all features of e.g. CCDops and CCDsoft are satisfactory
for all users.
Just saw in the Mira product comparison matrix that the Rank Filters
Maximum value and Percentile Rank and the Image Combining
Maximum value, Rank statistic and some other stuff thats useful
for QA are just available in the high-end Mira versions and that
these also offer a cosmic ray filtering and noise reduction (based
on single images).


BTW, I tried to contact the former Axiom Research by email when
I bought my ST-10XME (it came with a rebate flyer for Mira) but I
didn't get any response. Is this offer still just valid in the US or now
usable with the new Mirametrics online purchase ?


Clear skies
  Wolfgang

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