Re: [TASS] photometric vs. non-photometric

[Tom Droege <droege@WWA.COM>]

Because of the drift scan, it is a little more complicated than this.  Each
exposure is taken over roughly 370 seconds ending at the time the line is
taken.  So line n+1 is taken 0.9 seconds after line n, but overlaps the sky
conditions for 511/512 of the time of exposure n.

This can produce some bizarre effects.  Clouds, in general, cause the
background to go up.  Possibly this is different at a dark location.  A
thin haze can make fuzzy stars with a high background.  Fast moving well
defined clouds will produce sharp images sitting on top of a higher than
normal background.

There is a scheme to detect sky conditions.  I have allowed a channel for
this and possibly can include it later.  One takes a thermal electric
cooler and glues it between two plates.  One side is allowed to look at the
ground, or anything of constant temperature.  One could water cool one
side.  The other side looks at the sky.  One can use optics, or just a
plate exposed to the sky.  Sky without clouds looks "colder".  So one uses
the TEC to measure a wide area of the sky for its ability to absorb radiation.

Tom Droege

At 02:47 PM 9/2/99 -0400, you wrote:
>For the Mark III there is a fairly simple metric called the background
>variation.  Since the Mark III image was a drift scan image each row was
>taken at a different time.  If the cloudiness changed during the exposure
>the background would change also.
>
>This is one of the characteristics that StarPost uses to filter the images.
>
>
>Mike G.
>
>
>                -----Original Message-----
>                From:   Stupendous Man [mailto:richmond@A188-L009.RIT.EDU]
>                Sent:   Thursday, September 02, 1999 2:13 PM
>                To:     TASS@LISTSERV.WWA.COM
>                Subject:        photometric vs. non-photometric
>
>                  Peter Mount asks:
>
>                > What does everyone thing the criteria are that determine
>what
>                > type a particular
>                > night are, and how would you deal with either a night that
>starts as
>                > Photometric quality, but deteriorates and visa versa?
>
>                  Terminology: "photometric" is an astronomical term which
>means
>                "skies are clear and stable".  In practice, it often means
>                "Fred went outside once an hour and never saw a cloud."
>
>                  Technically speaking, it's complicated.  I do this sort of
>work
>                for a living, so I'd be happy to go into MORE gory details
>than any
>                sane person would want to know -- just ask.  But let me give
>                two succinct definitions, and skimp on the details for now.
>
>                  Rock-solid method to see if night is photometric: collect
>observations
>                of stars with known magnitudes at a range of positions
>around the sky.
>                Run the observations through a procedure which checks to see
>if they
>                obey equations of the form
>
>                     instrumental mag  =  known mag +  A  + c*(color)  +
>k*airmass
>
>                where A, c, and k are constants, one per passband, and
>"color" is the
>                color of the star, calculated from its known magnitudes in
>two passbands.
>
>                  If the observations obey the above equation, then the
>night was
>                photometric; if not, it wasn't.
>
>                  Quick n' dirty method, which can be done during a night:
>
>                         Step 1: carry out rock-solid method over a few
>weeks
>
>                         Step 2: calculate mean values for constant A, c, k
>in each passband
>
>                         Step 3: during a night, use the mean values to
>calculate the
>                                 expected instrumental magnitude for stars
>as the telescope
>                                 takes images.  If the expected magnitude
>matches the
>                                 instrumental magnitude, then the night is
>photometric.
>
>
>                  So, if the plan is for a user to be able to specify,
>"Please take
>                this action only if the conditions are photometric", one
>must
>
>                             a. take pictures
>                             b. reduce pictures to "clean" versions
>                             c. measure and identify standard stars in the
>images
>                             d. compare measured magnitudes to a known model
>
>                all during the night!  It's a tough task.
>
>                  A better idea is to have a VERY simple program scan each
>new image
>                and look for "cloudiness" -- Peter McCullough finds that
>Stardial
>                images during cloudy weather are easy to distinguish from
>those taken
>                during clear conditions, by several metrics.  If any image
>shows
>                hints of "cloudiness", then declare the night "not
>photometric".
>
>                  One might consider classifying two halves of the night
>(early and late)
>                separately, or even each hour .... but that's overkill, to
>my mind.
>
>                                                          Michael Richmond
>
>