First, I determined the stellar limiting magnitude of each telescope at each site for objects at various different altitudes,
as explained in
Telescopic Limiting Magnitude. (Click on bar at left if this link does not work.) I then used the star-charting program Sky Map Pro 8 to print background
stars down to those limits on 8.5x11 paper for field inspection, and in PDF format for use in this document.
For open clusters with no apparent nebulosity, like the pictures of M38, this was nearly the end of the story; the stars
generated by Sky Map Pro are essentially all that is shown in the pictures. However, below a limit of roughly 11.5, Sky Map
Pro's charts are based on the Hubble Guide Star Catalog (GSC), which is notoriously unreliable. Therefore, I wrote a computer
program which uses heuristics to eliminate spurious GSC stars, and adjust the magnitudes of the remainder. The program also
eliminates stars outside the field of view and fiddles the dot sizes produced by Sky Map Pro.
Even after this, some spurious stars remained, and some real stars were missing. To the extent that I could, I corrected
the results based on field inspection, but I cannot promise that that pictures are complete and accurate with respect to all
of the fainter stars.
When nebulosity or apparent nebulosity was present, I sketched it on the 8.5x11 charts, with supplementary notes. I then
used the program Corel Photo-Paint to replicate on the computer what I had noted in the field, and superimposed it on top
of the background stars.
Final compilation was done with Corel Draw.