[Aavso-photometry] Differential transformation

[Walt Cooney]

Wow.  There is really a routine that does that automatically?  Can you tell us
any more about that routine Chuck?

For me working out transformation coefficients and calibrating other fields is a
long and painful exercise -- lots of human intervention.  As a matter of fact,
I've really never done it all the way through.  I have calculated my
transformation coefficients from M-67 and NGC 7790 several times and used them
to correct differential photometry where Arne calibrated the field, but I have
not had the guts to take that to the next step and assume I could successfully
calibrate an unknown field myself.  It's something I would like to learn how to
do.  I even have delusions of being able to help calibrate AAVSO fields someday.

Clearest skies,
Walt

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org]On Behalf Of Chuck
Pullen
Sent: Wednesday, December 29, 2004 10:26 AM
To: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] Differential transformation


It's pretty crowded for me too!  I use a number of stars on the edges, and
get about 12-15 usable ones in my 15 by 15 arc min field.  But at the heart
of the "dipper" it's pretty hopeless.

For us IRAF users, the PHOTCAL task makes this all pretty easy.  If you
shoot Landolt '95 fields at high and low airmass, then your unknown
field(s), it calculates extinction and transformation and gives you
corrected magnitudes for the unknown fields all in one shot.  The catalog
is even in the software.  But, it is a bit of a black box, albeit a very
well documented black box.  And you still have to be careful, as Landolt
stars are not all equal in quality.

Chuck

At 10:19 12/29/04 -0600, Michael Koppelman wrote:

>For us wide-field guys M67 is pretty crowded. I have had good luck
>observing Landolt fields. I made a database and imported it into TheSky so
>I can easily see which fields are transiting.
>
>I wonder how you guys with small FOVs do it. I can get 5 to 15 Landolt
>stars in a single field a lot of times. It would be painful to get only 2
>or 3 stars in a given field.
>
>I do ignore extinction and just observe everything when it is at 45
>degrees, which is where the Landolt fields are for me. This eliminates one
>source of error but drastically restricts what I can observe when.
>
>Cheers,
>Michael Koppelman
>http://www.lolife.com/astronomy/
>
>
>
>On Dec 29, 2004, at 10:09 AM, Chuck Pullen wrote:
>
>>Gier - Note that M67 has been extensively studied and there are a number
>>of stars available in it that should meet the quality test and Arne's
concerns.
>
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