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Estimate of the visual magnitude of variable star T Cephei for JD 2453747.4

The comparison stars are marked (a) to (c) and (e) to (i) together with their visual magnitude as indicated on the AAVSO (a) chart dated 12/97 (note that the decimal point has been omitted, so that 6.7 is written as 67). The labelling for these stars is the same as in previous observations. T Cephei is marked as T in this full frame image (which has been modified in Photoshop to brighten up the fainter stars). Some other bright stars in the constellation of Cepheus have been marked. The area of sky covered is approximately 12º.7x8º.5. South is towards the top right-hand corner of this image.

The b values are the sum of the counts obtained from the original frame in 20x20 pixel boxes (this corresponds to about 3.5x3.5 arcminutes). Between 3 and 4 measurements were made for b_sky and the value after the = sign is the average and the standard deviation. Note that the measurements of the star brightnesses are well above the average of the fluctuation of 44 counts in the sky.

The average value for c is:-

This gives an estimate for the visual magnitude of T Cephei as:-

T Cephei has therefore dimmed by 1.0 magnitude since the last observation in December 2005 and this is a little bit brighter than the light curve for this star over this period from the AAVSO. The accuracy of this observation has been a surprise given the conditions under which it was made. The sky background is about 20 times brighter than the last observation in December and about twice as bright as those made in September and October 2005, yet the fluctuation in the comparison stars magnitude estimates is approaching a respectable 0.1 magnitude.