Comet Machholz


Image Processing:  There was a fair amount of light pollution at the site, so 6 minutes at f/4 on 400 speed film was about the limit.  Later exposures of 10 min at f/2.4 on 400 speed were definitely too dark (on the negatives).  Here is the original uncorrected scan of the first 6 minute exposure:  .  This approximates the appearance of the 4x6 machine print from the drug store.  Sky fog has significantly lightened the background and the vignetting of the wide open lens  is evident from the hot spot in the center of the photo.  All of this is readily corrected digitally, as for instance, has been described in some of the AP articles in S&T these past few years.  Specifically:  the bright stars and comet are cloned out of the picture.  Then the remaining image is thoroughly Gaussian blurred until only the sky fog is visible.  This background is then subtracted from the original scan.  The result is free of sky fog but flat.  This can be reasonably fixed using the sliders in most photo programs for darkest, lightest, and midpoint densities.  However a much better result for this negative is obtained by hand tailoring a brightness curve for the faint tail of the comet and the Pleiades nebulosity.  Essentially, the contrast of the faint part of the image has to be dramatically boosted.  This brings out the comet tail and nebulosity at a cost of emphasizing the grain and any imperfections in the film base.  This is all outlined in the following panel (for one negative only).  For the two negative stack, the sky fog was subtracted, the negatives stacked, and then the contrast boosted.   The limiting magnitude near the center of the field was checked against Guide 8 yielding m = +14 as shown in this full resolution panel near the center of the field and in the corresponding Guide 8 field .  The bright star, center right in the photo, is GSC 1803 1582.  This photo also demonstrates the tracking accuracy of the Kenko mount for 6 min at 300 mm focal length.  Kenko suggests a maximum exposure time of 15 min at 300 mm without guiding.  This has not been tested yet.  The quality of this lens for star images is shown in the full resolution, center panel (from above) and in the following upper right corner panel . (This may not be the absolute upper right as a little cropping was done.)  I judge these pictures to be a hair out of focus.  Slightly tighter star images should be possible.  For my old eyes, a 12x magnifier on the focusing screen is not quite enough.  Finally, in case you think my drug store processor is better than yours, I typically spend 15 to 45 minutes per negative digitally cleaning all the little specks, scratches, and crud from the images.


fw kleinhans 2005