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Samir Kharusi | all galleries >> Galleries >> Autoguiding a C14 at f11 > Beach-house-M45
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17 Nov 2004 Samir Kharusi

Beach-house-M45

Yiti, Oman

OK, heaved the equipment to the beach-house at Yiti, set it up on the roof and had massive light pollution from numerous BBQs :-( It was a long holiday weekend. Anyway the sky was perceptibly darker than my home site. I guessed it was around VLMag 4.5 near zenith and I decided to try imaging without any filters. Quite nice to see some nebulosity even in the 5-minute Jpeg captures :-) A 5-minute exposure at my home with a UHC filter and the same f5.6 optics, and the same exposure at the beach without a filter both reached roughly halfway up the back-of-the-camera histogram. Actually this measures the skyfog at slightly darker than Mag 19.5/sq arc-sec, two stellar Magnitudes darker than my home site, VisLimMag 5.3 or thereabouts. The above is a stack of 31x5-minute RAW exposures at ISO 1250, using a Canon 600mm/4 lens with a 1.4x tele-extender, thus yielding an 840mm focal length at f5.6. This is just over half the total integration time of the previous image, yet the increased visibility of the nebulosity is quite obvious. Theoretically, for each 1.0 improvement in VLMag one can expect the Signal-to-Noise ratio to improve by about 2.5x. Two stellar Mags and the integration time becomes 6x more effective. By the way, I used this particular framing to include that red star towards the top (North). Just wanted to see how the colours would come out.
This grouping/asterism of stars, now often identified as M45 or Messier 45 on current maps, has been recognised by numerous cultures from antiquity. E.g. it's also called the Seven Sisters, the Pleiades, Subaru (in Japanese) and Thuraya (chandelier) in Arabic. The image above has North up.

Canon EOS 1Ds ,Canon EF600mm/4.0L + 1.4xII
ISO 1250, unmodded camera, 31x5min, 840mm/5.6 full exif

other sizes: small medium large original auto
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