Re: be careful running cameras in "tracking mode"

[droege@snapmail.us]

Michael and all,

Thank you Michael for reminding me about the flat fields.  I have
programmed into my nightly run "do_darks", "do_flats" and "do_objects".

I will probably run "do_darks" every night as it is free.  Just take the
darks in twilight and possibly at dawn.  I don't plan to do "do_flats"
every night.  I think flats should be taken under good conditions and
while pointing at the area of sky to be observed.  I am thinking of having
flats for each Dec elevation.  Not sure yet.

In any case, the flats are taken by taking a small grid in RA and Dec. 
Currently this is 16 images taken on a 1 degree grid.  This is then stored
in a "current flats" directory.  I just point the pipeline at this
directory and tell it to make flats and then store the result in
/home/tom/mk4/flats under the current data.  Eventually I will study these
to see how much they caange from night to night.  You can see some current
flats in chipper/home/tom/mk4/flats/date  The V camera is particularly
ugly, but that is what you get for $1700.00  The I camera has a streak in
it that the master flat does not really remove.  If someone wants to look
at the flat that is produced and the master_flat files and think about how
to remove the streak it would be welcome.

My plan is to go through all the parameter files and think about each
parameter.  The present plan is to run 200 second exposures to try to get
a few new stars.  This requires rethinking some levels and thresholds.  I
have the I camera stopped down to 3".  It is still a little more sensitive
than the V camera but the sky background is now more like 5 to 4 instead
of 2 to 1.

I remind everyone about the "Andrew Bennett" variable stars.  This is a
quite spectacular dip in brightness caused when a star drifts over a dead
spot on the CCD.  I got one of these in the first test runs.  One can
easily recognize it since it is a big dip in one filter with nothing
matching in the other.

I also plan now to save the raw images and the dark and flat image files. 
By saving in a set of one tracking image, it only takes an hour or so to
reprocess the images in a set.  Probably less for the 200 second image
sets as it looks like there will be only 30 of them.

Keep the comments coming boys and girls.  Now is the time to get it right.

Tom Droege

>
>   Tom has written that his plan is to run the cameras in a
> "tracking mode" this season: pick one region of the sky and
> follow it for several hours as it moves from east to west.
>
>> 2) Look at where I can point the telescope.  Then look at a table of
>> tiles
>> to see which it the best tile to take.
>
>> 3) Point the telescope at the tile and take a set of images to make a
>> flat.  This is done by offsetting the telescope slightly in all
>> directions
>> to get sky flat data.
>
>> 4) Point at the tile and follow it as long as possible taking images.
>> For
>> tom1 this is two hours.  For tom2,3,4,5 it may be 3 or a little more
>> hours. This might produce a 30 image sequence at 200 second exposure for
>> tom1 and 45 images for the rest.
>
>   I just want to mention an effect that _possibly_ might appear
> when running the cameras in this mode, rather than the previous
> "look at different regions of the sky all the time" operation.
>
>   The reduction pipeline has several options for making master
> flatfield frames.  One option is to use every "object" image throughout
> the night, combining them via a median operation.  If you look at
> different
> regions of the sky, then a typical pixel will usually see the blank
> sky background, and only occasionally, in one or two frames, see a star.
> One can easily discard the high data values caused by the star and
> create a meaningful flatfield using the blank-sky values.
>
>   If, however, the camera stares at the same patch of sky over and
> over and over, then some pixels will see stars most of the time.
> Even a robust median filter will not remove the pixels with high
> values, and the resulting "master flat" frame will have lots of
> little star-sized residuals in it.  Using this malformed flatfield
> frame to correct all the raw images will add some noise to the
> results.
>
>   The pipeline _can_ be told to use only a subset of all the
> frames to create the master flats.  It sounds as if Tom is well
> aware of this issue, and is planning to do it.  Fine and dandy.
>
>   Since the season is just starting, and this is a new mode of
> operation, I thought I'd mention this.  Tom, if you glance at
> the master flatfield frames produced by the pipeline, you'll be
> able to see immediately if there are star-sized spots at any
> significant level.
>
>   By the way, I'm currently revising bits and pieces of the pipeline
> software.  No big changes, but some fixes to known problems, and
> a few enhancements.  I'm testing the new code on several sets of
> Mark IV data to make sure that it works properly.  I'll send out
> a message when the new versions of the software are ready for use.
> One of the new options will be to include "orphan" detections
> (stars seen only in one passband) in the final output.
>
>   Tom, I'll have a set of special instructions (no big deal, just
> a few lines) describing how you might most easily merge the new stuff with
> your existing code.  I know that you have figured out exactly which
> parameter values make the most sense for the current data, and I
> want to make it as easy as possible for you to upgrade without
> having to re-edit a bunch of parameter files.
>
>                                              Michael Richmond
>
>
>
>