D3X and D3S team up to nab a Nebula

Started Mar 8, 2010 | Discussions thread
tarnish Senior Member • Posts: 1,470
Auto guided photo example

MatjazVrecko wrote:

Can you show us that Hyakutake photo ?

I don't have a scan from the negative but a heavily damaged print was available. This image was shot under heavy light pollution from sodium lamps in San Jose, California, but the blue-green color of the comet comes through. (Anything that looks like a smudge but isn't green is in fact a smudge on the print.) The exposure time was about 10 minutes on ISO 400 print film.

Autoguiders are the original, and ultimate, image stabilization systems.

The comet's motion against the starry background was so rapid that it was apparent in the eyepiece, one of the highest angular rates ever recorded at about one degree per hour. Even with typical slower moving comets, manually guided exposures yield jagged star trails because the nucleus is a large featureless dot to the eye and it's hard to tell when it's centered in crosshairs. Autoguiders on the other hand can interpolate the position of the centroid to within 1/5 of a pixel. With bright stars exposure times of a few tenths of a second are enough to capture some light from the first diffraction ring and usually about 4 pixels receive a workable signal. Comet nuclei are non point sources so they also serve well as tracking targets.

The image is heavily vignetted because the focal reducer ("telecompressor") could not cover the full Pentax 6x7cm frame. (When the reducer is not used the image field is limited by the 2.75" internal diameter of the focuser. This is one of their small telescopes - the large ones come with 4" ID focusers.)

Nevertheless, it recorded more sky than a 35mm camera could. This was a challenging negative to print; extensive dodging was necessary to preserve detail in the nucleus region. A drum scan and relatively simple Photoshop work would provide a better end result. A little project for a future rainy day ...

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