Re: [TASS] photometric vs. non-photometric

[Herbert R Johnson <hjohnson@PLUTO.NJCC.COM>]

On         Tue, 7 Sep 1999 01:15:58 GMT, Andrew Bennett <andrew.bennett@NS.SYMPATICO.CA> wrote:
*>On Fri, 3 Sep 1999 16:22:27 -0700, Chris Albertson
*><calbertson@LOGICON.COM> wrote:
*>
*>>If the star is in an area of the sky that was covered by the survey
*>>and it is not in the tass_catalog then it must have been below
*>>TASS' detection limit for any of a number of reasons.  To dim,
*>>to bright, to close to another star, bad luck with clouds, and
*>>so on.
*>>
*>>There is lots of raw data to look at.  I think it you asked you could
*>>get a reasonable sized sample.
*>Which, being interpreted into the vernacular means:
*>"Bennett - put up or shut up!"
*>Yes. This looks like something I could undertake to
*>fill up those long winter evenings ...
*>
*>gigabytes. I would propose to look at a VERY small
*>area. Or better, two very small areas.
*>
*>I would propose to look at every star in the TASS
*>database in selected very small subareas, on every
*>image provided and attempt to build up statistics,
*>for those stars that don't appear in all cases, on why
*>they are not there.
*>
*>I think this would tell us a lot about the reliable
*>magnitude limit for the Mk III and the information
*>should carry over (m.m.) to the Mk IV.
*>
*>To do this, I would need:
*>1) Raw + Dark + Flat (preferably) or processed FITS
*>files for all processable images...that overlap my chosen analysis areas.
*>I am perfectly willing to accept somebody else's
*>decision that a particular image is not processable -
*>a few marginal ones would be of interest.
*>2) Information as to precisely where the image is on
*>the sky
*>3) Lots of help when I start finding what the real
*>problems are.

My impression from your original question was that you were interested
in an explanation for why some "surveyed" parts of the sky did not make
"the TASS catalog". As you've stated the problem here, you propose defining
a section of the sky, asking for ALL the images taken and processed of
that section, and then somehow "comparing" those images and processed
data to the corresponding section of the TASS "catalog" to see what was
missed.

I've put a number of words in quotation marks, because I think the issues
are not quite as you have implied in your proposed methods. Before you
request a lot of data, you might want to refine your proposal or your
question; and to do that I think you may want to consider those quoted
words and what they could actually mean to the Mark III camera owners.
All this is of course my opinion, and I'm not a TASS observer, but I've
written about them and I have some sense of these issues.


FIrst, the areas of the sky actually under POTENTIAL survey are those
which the Mark III cameras are pointed at: specific declinations for
each site. Also, given the longnitudes of each site, given areas of
RA are not visible at various seasons, and some seasons provide fewer
nights of observation.

Next, not all the observations made were processed, and some that were
processed were rejected by the site's observers for various reasons.
My casual knowledge suggests the skies may have been too hazy or patchy
for "good results" (whatever that means). INformation about rejection
criteria, however ad-hoc, would be informative. - That is what I THOUGHT
your question was about.

Finally, not all the processed observations made the TASS Mark III database
as compiled by Richmond. Michael recently released another version of
TenXcat, and noted explicitly that some data remains to be "entered", but
first it must be color corrected - there is a TASS Tech Note on that
process.

That is the short version of what *I* thought you were asking.

If you were asking something like "what is the limiting magnitude of
the TASS Mark III survey, and what impacts that?", the simple answer is
"sites and the weather". If you want the dimmest magnitude obtainable, that
would be site-dependent. Sky background may be the limit, which would
depend on weather
and local lighting, but each site can probably give you some figures
that you could check against their actual observations in TenXcat or
the TASS Mark III Richmond database.

Other limits are the consequences of the optics and tracking of the
mark III camera in use. Stellar images in the image file are not
dots but blobs of varying shape: the selection of PSF sets some limits
to detectability as a "acceptable" stellar source. A review of the
software, its parameters, and typical image files would be informative.
I *think* the bottom line is that we use a large PSF to get around
the lumpy images, so some dimmer objects will be "seen" but not
"detected" as they will not be distinguishable from variations in
background, moving objects, and other crud. If you want RELIABLE
detectability, you have to set some tough standards.

(As usual I'd appreciate any corrections to gross errors in my
considerations.) Andrew, I asked somewhat similar questions about two
months ago; my Tech NOte 57 represents the background work I did to
get a handle on what I'd need to know to answer my questions. Maybe that
TN will help you.

Michael R., a side note to this: do inquiries to the database produce
results that also reference the *camera ID* of the observation? I realize
that TenXcat has "neutralized" data, with site dependencies removed. But
there is still site-intrinsic information, like limiting magnitude or
simply "who saw what" information that would be useful to know from any
report of "databased" observations. My recollection is that camera ID is
not a returned field but I did not verify this.

Also: it would be nice to have a "worked example" of data processing,
from raw image to star list, to database entries. If the APL site
does not have this already, I wish the TASS Web site would. A "worked
problem" is always useful. I'd like one myself.

HErb JOhnson


Herbert R. Johnson              http://pluto.njcc.com/~hjohnson
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