STREAKS IN IMAGES
September 21-24, 2003 Lately
I've noticed streaking of my fainter images
particularly those made with the HA filter or
when objects present weak signal using one of
the RGB filters. That said, the streaks are
apparent even on fairly strong LUM data.
Below, LUM and RED image on the left are dark
subtracted, aligned and combined in CCDSOFT.
The images on right are aligned and combined
with no dark subtraction.
Clearly the streaks are image
noise spread along the diagonal by the
alignment of the images. As a side issue I
need to reduce the amount of shift seen here.
I may have been out of balance for these
shots. The images were guided with my 201xt
autoguider.
It is also clear that the
subtraction of dark frames before combining
helped to reduce, but did not eliminate, the
effect. The streaking in the weaker Red data
makes the final reduced, aligned and combined
image unusable.
An experienced imager I spoke
with guessed that the problem was with the
temperature of the dark frames. He suggested
that the temps of the darks and the images
didn't match or that the camera was having
trouble holding a steady temp due to an
overly agressive temperature set point. To
address this issue, I set the temperature
point to that recommended by CCDSOFT (-5c in
this case), and took a new set of darks
immediately following the imaging session. I
checked the FITS headers to determine the
temperature at which each image was taken. In
all but 4 images, the LUM, Red, and dark
frames were taken at -4.8c. The 4
discrepancies were at 5.2c, with 2 dark and 2
red frames exhibiting the difference. This
strikes me as being pretty close to identical
temperatures, and yet the streaks remain.
Examples:
I received more
on-line advice telling me to try using Sigma
combine software. The author of one of these
utilties, Ray Gralak, contacted me and got me
going. His software can be found at Sigma Home Page.
It worked wonders! Just using it to recreate
my dark frames produced much smoother images.
I'm grateful to Ray for his help and his
Sigma software! I found I still likes CCDSOFT
to combine my bright LUM images, but SIGMA
for the fainter and noisier color shots. I
followed his on-line instructions for
adjusting settings for both the new combined
dark frame and the color light frames, they
worked well. Here are the re-done LUM and Red
images. Both reduced with the new dark, LUM
combined with CCDSoft and the red combined
with SIGMA:
Major
improvement! It was great to complete my
first color galaxy image, and in fact, my
first galaxy since May 4, 2003, a full 4
months! Here's the final image: gallery-galaxies-ngc6946-001.htm
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