September 30, 2007
==================

The version in this directory is functional but may have some new parts still in
development.  

This version  should probably be used instead of older ones because the flaws I
am aware of have been corrected here.  

A few things to note, however, about this "beta" edition:

There is almost no error checking on the operation of the filter wheel.  It
would be a good idea to check at some point at least once, to be sure that the
wheel is responding.  I have seen instances on our STL6303 and STL1001 where a
newly installed wheel did not respond either to XmCCD or to SBIG's CCDops. A
procedure that seems to work reliably for us when installing a new filterwheel
is to check it for binding first, and press lightly on the motor to engage the
O-ring.  Make sure that a white spot on the ring is directly over the pickup
before closing the case.  CCDops has better error checking than XmCCD, so if you
have a Windows computer available, check your camera with CCDops when you use it
for the first time.  That said, we obtain images with XmCCD that are are just as
good as those with commercial Windows programs that also rely on the SBIG
libraries for Windows.  

The routines and user interface  for filter control are new.  In this version
the filter will increment with each button press.  I have a built-in fixed delay
to inhibit multiple requests too fast for the wheel.  You will have to change
the filter labels for your camera in the source code.  Look at the top of
xmccd.c for the notes on how to make your own filter set appear.  The finished
version will have a provision for a user file.

I am working on an implementation of time delay integration (TDI) or "drift
scan" imaging and pieces of it are in this beta version. It works as is, but you
will probably need to trim the delays in the line read to get perfect star
images.  The inherent minimum line read time will vary from one camera type to
another.  The values here are for an STL6303.  I hope to refine this part of the
program and expand the TDI user interface if the preliminary encouraging test
results hold up to detailed scrutiny.  

On the positive side, TDI offers exposure times with the STL6303 of about 100
seconds on the equator, increasing as the field is moved toward a pole.  In
effect, it uses electronic shift of charge along a column on the CCD to track
while the telescope remains fixed.  For this reason, the columns must be exactly
parallel to lines of constant declination.  This requires both careful initial
adjustment of the camera rotation, and exacting polar alignment to maintain the
parallelism in different pointing directions.  There are two inherent advantages
over conventional mechanically tracked imaging:  the motion is very smooth and
therefore the image quality may be quite high; and, averaging over columns
greatly reduces pixel-to-pixel variations.  


 
