Re: [TASS] photometric vs. non-photometric

[Andrew Bennett <andrew.bennett@NS.SYMPATICO.CA>]

On Tue, 7 Sep 1999 10:08:41 -0400, Herbert R Johnson
<hjohnson@pluto.njcc.com> wrote:

>... 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.

What I actually wrote, following on a remark of yours:
>>... The accumulated Mark III
>>datasets offer an interesting opportunity for some QUANTATIVE considerations.
>>I myself don't think this will be "solved" in an email exchange: it will
>>take some homework.
was:
>And, I think, like much else that ought to be done,
>knowledge of what raw data sets there were and preferably
>access to the raw data. Otherwise one knows nothing about
>what somebody decided to throw away. A lot of the gaps in
>tenxcat are like this: was a star below some limiting
>magnitude (oops - I mean above; you optical types measure
>backwards) or was it not measured. It makes a big difference
>to the statistics.

You continued

> 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.

No, I don't intend to write a dissertation on the quoted
words and phrases, particularly as they are "quoted" only
in the sense that they have "" round them. Instead, I will expand
on what I meant by:

>A lot of the gaps in tenxcat are like this

A cursory examination of tenxcat shows that some sources
were measured once and some 40 times. More detailed
examination shows that this wide scatter is:
1) Not confined to weak sources.
2) Not confined to the edges of the survey area.
3) Not just the result of the uneven RA coverage.
Indeed the scatter persists even for small selected
areas well within the most heavily covered area and
with the exclusion of weak sources.

Naturally, things are better for moderately bright
sources than for the weaker ones. This was my reason
for bringing the matter up on this thread. If you
want to reduce it to a question, the question might
be:
"Are we too optimistic: are many of the analysed
images 'non-photometric' so that we have a wide
range in the actual limiting magnitudes from
image to image?"

But my personal opinion is that the scatter in the
number of accepted magnitude determinations is too
great and extends to sources which are too bright
to be accounted for by variations in seeing. If
you can think of a way to check on this without
going to the original images, please tell us.

Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.