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Astronomical Observatory: Cool Images

Astr384 Photography Projects, Spring 2004

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Pelican Nebula, Andrew Vache

Pelican Nebula

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.

Image Reduction:
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 bright-red glow.

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.





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