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Don Reed | all galleries >> Galleries >> Astrophotography > The Rosette Nebula
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7-Feb-2015 Don Reed

The Rosette Nebula

The Rosette Nebula is about 5000 light years from Earth, seen from here in the constellation Monoceros. At the center of the nebula is the star cluster known as NGC 2244, which was discovered by John Flamsteed about 1690. Flamsteed was unable to see the nebulosity surrounding the cluster, which was first described by William Herschel almost a century later. The stars in the center of the nebula were formed from the material that makes up the nebula.

The process of producing the final image of a faint deep-sky object like this one with a DSLR involves "calibrating" multiple long exposure images of the subject by subtracting a "bias" frame from them to reduce the fixed pattern noise that is always present in DSLR images, then subtracting a "dark" frame to reduce the noise that accumulates during long exposures. Finally, a "flat" frame is divided into each individual subject image to negate the effect of vignetting (dark corners) and dust particles on the sensor. The master bias, dark and flat frames are each created by combining multiple exposures - the bias exposures are very short exposures made with the camera body capped, the flat exposures are made by photographing an evenly illuminate field, and the dark exposures are made with the camera body or the telescope capped, and are usually of the same length and ISO setting as the subject exposures, and made at about the same ambient temperature.

Then the calibrated images are aligned so that the stars are in exactly the same positions in each image (registration), and the registered images are combined into a single image (integration). This integrated image was prepared from 16 eight minute exposures of the nebula made with my Canon EOS Rebel T2i digital SLR, through an 8 inch f/4 newtonian reflector telescope. A Baader MPCC coma corrector was used between the camera and the telescope. Sixteen flat frames were also acquired and combined into a master flat frame. I used a master bias frame created on a previous date, as the fixed pattern noise contained in the master bias frame doesn't change much over time. For this image I did not use any dark frames, relying on the number of exposures, the relatively cold temperature and dithering during acquisition (moving the telescope a little between frames so that the fixed pattern noise doesn't accumulate in the same position in the frames) to reduce the dark current. This process seems to have worked well - although the nebula was barely discernible in the individual, raw light frames, the noise in the calibrated light frames was quite acceptably low.

All raw images were acquired with BackyardEOS software, and all image processing was performed in PixInsight. The exposures were guided with a separate guide scope and camera, controlled by PHD Guiding V2. The telescope mount was an Atlas EQ-G, controlled by the EQMOD driver through Cartes du Ciel planetarium software.

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