Nebula, Andrew Vache
The Pelican Nebula is a gaseous
emission nebula in the constellation Cygnus. It is about 1800 light years
across, and is most famous for its Ionization Front, or area around the
“neck” where cold gas is being turned into hot gas by forming
stars in the region. The interest in this nebula comes from the ability
to do mosaic compilations, and the ability to take long exposures in rather
homogeneous sky conditions at Rehoboth.
I had the telescope in Rehoboth, NM take a series of 3 images, at 5 minutes
per. I chose 5 minutes based on the saturation level of the region, and
the filter used (H-alpha). I chose H-alpha because emission nebulae emit
at a vast range of visible wavelengths, but the H-alpha range is particularly
bright, due to the first-level transition of Hydrogen, the most common
element in nebulae of this type. The image was to be a compilation of
18 fields, roughly 6 x 3. As it turns out, the telescope had a few problems
shooting with the H-alpha filter, and did not want to take all requested
images. I ended up with two images of fourteen fields, and only one image
of the remaining four fields. Nevertheless, the single-image frames were
of decent quality. To find the positions of each mosaic piece, I used
TheSky, and utilized the “user-defined position” function.
This function allows the user to click on the screen (while holding CTRL)
and make little red “x” marks on the screen. I used the pre-defined
ST-10 bounding box to mark off sufficient rectangles with the “user-define”
function. I put marks on the edges of each field, because the frame, as
defined in TheSky is actually smaller than the actual camera field by
about 2 arc seconds on all sides.
Once I retrieved my images, I reduced them. I first subtracted the “ghost”
image from each field, applying the ghost as a dark, and doing a dark
subtract. I then saved each frame as a new image (sans ghost) and applied
the flat, bias, and dark reductions. I then combined the frames where
appropriate, and saved every frame as a completed reduction/combination.
I then created a new image, 3500 x 3500 pixels, in which to start the
mosaic. I placed the first image in the middle bottom, as the images started
in that order. I placed and merged the first image, then overlaid the
second, matching star patterns, and ensuring the overlap was smaller than
the blend width. I did this for each frame, until the mosaic was complete.
I utilized the auto-equalize function for background, and ended up with
a very well balanced image. I then applied a Gaussian transfer function,
and worked on pseudo-colors. I picked a red to white pseudo-color, so
the stars would still appear white, but the nebula would have a strong
The finished product looks great, and shows off both the spectacular structure
of the nebula, but also shows the extreme precision and capability of
the new robotic observatory. The image was only possible through flawless
tracking, and pin-point accurate pointing. If the fields were even a few
arc seconds off in one direction or another, the mosaic would be nigh
unto impossible to successfully assemble. The image reduction was done
on MaxIm 4, using a trial version in one of the labs.