Re: Calculation of mag errors

[droege@snapmail.us]

Welcome Data Munchers,

On matt and mark in the directories:
/data/0/2004
/data/1/2004
are all the TOM3 Mark IV images from 2004.

These are the flat fielded and dark subtracted images, but there are a lot
of them.  About 40,000.  Doug Welch has set up a search engine so that you
can search the available images to find those covering a particular point
in the sky.  These images cover 50 to 90 degrees in declination.  I can
load others but at the moment it is a pain to do so. You should feel free
to ssh to matt and run any programs that you want there.  It is a dual 2
GHz processor machine with a fair amount of memory.  Or you can copy any
of the images that you want.

I will set up accounts for anyone that needs them.

/data/0 and /data/1 are pretty full.  /data/2 and /data/3 have lots of
room so pick a spot to work.

I now have 2 DSL lines working so I should be able to find a way for you
to get at your work.  At the moment (until I figure out how to connect my
network through the new router) the IP is dynamic so I have to update it
at dyndns.  I think you want to ssh to tass-survey.dyndns.org  I will look
it up for anyone who wants to use it.

I can state for sure that all the images in the "engineering run" have
been                   taken at 100 second exposure.  The lenses are
refocused and cleaned from time to time.  This has not been recorded.  I
will do a better log book job for the coming "official" runs.  Mainly I
will try to put things in the fits header.

Just let me know what you want and I will try to set it up on matt.

Tom Droege

>
>   Andrew Bennett went through the calculations of
> uncertainty in the magnitude measurement of a star
> of V=12 on one particular image.  He found
>
>> Magnitude s.d. = this x 1.0857 = 0.06698 mags
>
>   I took his numbers for integrated flux from the star
> and for the background, did the calculations by hand,
> and end up with roughly the same overall uncertainty
> of about 0.07 mag.  So we agree.
>
>> The values 0.019 to 0.022 mags in the database are wrong.
>
>   They certainly don't seem appropriate for a magnitude
> V=12 star from this particular image.
>
>   I looked at the sky values for other nights close to the
> image Andrew mentioned (I looked in TR1 dataset, which has
> dates JD 2452606 - 2452658, in Nov-Dec 2002).
> The V-band background sky values are typically 1900 counts per
> pixel at a minimum, so they image Andrew picked doesn't
> have a higher-than-average sky value.  The TR1 images have
> exposure times of 100 seconds.
>
>   I happen to have some data from the Mark IV database on my
> machine at the moment, which spans the time range from Nov 2002
> to the most recent, late 2004.  When I look at the "uncertainty"
> values for stars in the range V=11.5 to V=12.5, I see several
> groups, as Andrew mentioned: one group at about 0.02 mag,
> another at 0.04 mag, a few up to 0.08 mag.
>
>   In this particular dataset (which covers 1 square degree near the
> equator), I picked out all the stars with V=11.5 to V=12.5.
> I then plotted the uncertainty versus JD.  Aha.  I see
> that the values fall into clear groups by date: between JD 2452200
> and 2452548, the values are around 0.04-0.05 mag.
> There's a clear change somewhere in a gap of data
> between 2452548 (Sep 30, 2002) and 2452608 (Nov 29, 2002):
> after 2452608, the values drop to about 0.02 mag.
> Over time, they slowly rise, so that the most recent
> measurements have uncertainties estimated at 0.03 mag.
>
>   Tom may be able to tell us if the nature of the observations
> changed between Sep 30, 2002 and Nov 29, 2002.  Perhaps
> the exposure time increased, or the lenses were re-focused,
> or something like that.
>
>   In any case, I would like very much to have a single sample
> image which corresponds to some measurements in the database,
> so that I could go through the analysis just as Andrew did
> and compare the numbers with those in the database.  Unfortunately,
> I don't think I have any of the raw or processed Mark IV images
> from this period in my possession at the moment.  If anyone
> does have sample Mark IV images from any time in the past three
> years, and could send one to me, I would be happy not only to
> do the checks, but also to put the image on my web site so
> that it would easily be available to others, too.
>
>   It's a good thing that Andrew is bringing this up -- thanks,
> Andrew!
>
>                                   Michael
>
>
>
>