Saturday, 11 July 2015

The Lagoon Nebula

This nebula, which first appeared to humanity in 1654, by Italian astronomer Hodierna, was catalogued by Charles Messier as his eighth entry. It appeared as an interesting fuzzy smudge, behind a sparkling little star cluster and 4 stars in a line. This is a modern-day picture of it using the following equipment: an iOptron equatorial mount, a Celestron 9.25 inch Schmidt-Cassegrain, a f/10 to f/6.3 focal reducer, an Atik 383L+ CCD camera cooled to -20ºC (which had the usual temporary icing problems), a 1¼" filter wheel with Luminance Red Green and Blue filters. There was no guider used with this, so we limited the exposure length to 30 seconds, which could have gone higher but the wind occasionally would spoil a frame by shaking the telescope. The dew shield was removed, due to the wind. The other operation problem in taking this was the observatory wall was located about 15 feet to the south of the telescope, and at 8 foot high, meant the lagoon nebula was shortly about to 'hit the wall'. Luckily colleagues stayed on until midnight enabling me to acquire at least 12 images through each filter. Given the altitude of the subject (about 10º above the SSE horizon), it's not a bad image. Processing this was difficult, due to there not being any time to take any calibration frames. This keeps the noise down but gives horrendous vignetting to remove, which is especially hard when nebula fills the frame. The 'hot pixels' that pop up, were removed using a clever stacking technique, called sigma clipping, and there was no dust on the sensor to spoil the image.

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