Re: quick note on DS23 positions

[Tom Droege <tdroege2@earthlink.net>]

collect_stars.pl uses Michael's subroutine with the match radius set to 12 
arc seconds.  This is actually a box of 24 arc seconds square, so we could 
get two stars 34 arc seconds apart in the worst case.  As I pointed out 
before, I settled on 12 arc seconds based on plotting the number of stars 
found using different values for the match radius.  At 10 there is little 
decrease in the number of stars found.  It does not start decreasing again 
until 25 or so.  OK, I guess I need to put this curve in a tech note.

I used the flat fields and dark fields distributed with DS 20 for the 
reductions of DS23.  The attempt has been to run at constant temperature 
since last November or so.  This is one thing to check.  What do the 
covered pixels do over time.  What is the plot of chip temperature over 
time.  Mow that Michael has told me how to use some of his nice tools I can 
start to investigate these things.  The projects tend to take some time 
since there are 400 odd CDs to load if you want to look at the full DS23 
raw data.  If any of you want to do some analysis work, I stand ready to 
make up suitable data sets.  Meanwhile, I will just keep plodding through 
tests trying to understand the data set.

Tom Droege

At 03:19 PM 6/21/02 -0400, you wrote:

>   On the other hand, Tom Droege wrote that when he created the
>"collected.big" file, he placed together all objects within 12 arcsec
>of each other.  Hmmm.  How could one not include as a match a star
>which was within "a few arcsecs", given a 12-arcsec matching radius?
>I dunno.
>
>   Anyway, my candidates for creation of twins:
>
>         a. two stars _just_ the right distance apart (around 3 or 4 pixels)
>                that they are detected separately one night, but
>                together on all the others (due to changes in seeing and 
> focus)
>
>         b. a honking big cosmic ray in the outskirts of a stellar object
>
>   I put at a much lower probability -- without any evidence --
>
>         c. a faint variable object (e.g. CV) around 3 or 4 pixels away from a
>                bright steady star; the faint object usually isn't
>                detected, but one night it flares up and overwhelms
>                the steady star.  The photocenter of the object moves
>                to the fainter star's position, for this night only
>                (I think this is the situation John Greaves was
>                 mentioning as a possibility)
>
>   Anyway, that's a brief status report.
>
>   Arne also wrote:
>
> > I expect the
> > final Mark IV product to be ... 0.01mag photometry.
>
>   Preliminary results are that the V-band data may be in this
>ballpark, but the I-band data definitely are NOT.  There are clear
>indications of systematic errors of up to 0.10 mag as a function
>of position on the chip.  Tom, it would help if you would explain
>how you chose to do flatfielding for the DS23 measurements:
>
>          - used the night-sky images for each night to create
>                     flatfields for each night individually
>
>          - used the night-sky images for one or two good nights
>                     to create flatfields which you applied to
>                     other nights
>
>          - used a flatfield lamp to create flatfields which
>                     you applied to many nights
>
>   Back to the salt mines ...
>
>                                         Michael Richmond