Re: Pipeline and Introduction

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Rich asks:

> In looking at the collation file for hra2011797 I noticed that the minimum
> magnitude is 16.701.  This is termed as the Raw Magnitude, and I'm wondering
> how seriously to take this number.  The point is, when I look at the frame
> with DS9, there seems to be some pretty respectably bright stars there and I
> wonder whether these numbers are right. 

  That is the correct value.  The confusion arises because you are
trying to interpret a _raw_ magnitude as a _calibrated_ magnitude.
I'll explain the difference in a moment, but first, let me provide
an analogy.

  Two mountain climbers reach the summit of Everest.  One turns to
his companion and asks how high they are.  His companion says, "About
fifteen hundred feet."  "What, are you crazy?" says the first.
"No," says the companion, "we've been climbing at about 500 feet
an hour for three hours since we broke camp, so we're 1500 feet up."

  See, altitude is a relative measurement.  We are conditioned to
interpret any value as "above sea level," but, as the mountain climber
points out, sometimes it's easier to measure altitude relative to 
another point.

  Okay, back to magnitudes.  The "raw magnitude" values are determined
with an arbitrary zero point:

         raw magnitude  =  25  -  2.5*log10(counts)

where "counts" is the number of data numbers above sky in all the 
pixels belonging to a star.  These magnitudes have nothing to do 
with the standard Johnson-Cousins magnitude systems -- but they are
easy and quick to calculate.

  In order to compare the magnitude measurements to the standard 
scales, one must adjust the zero point so that stars with known
magnitudes end up at their standard values.  That is the job 
of the "collate" and "photom" tasks:

          http://spiff.rit.edu/tass/photom/

  The first, "collate", collects all the raw measurements for a single
star in different passbands.  The second, "photom", matches up by
position stars from a catalog with stars measured in a set of images,
and figures out how to adjust the zero point so that the raw
magnitude values are converted best into the standard system equivalents.

                                      Michael Richmond