Quote:
Originally Posted by Shark Bait
Aside from human error, is there normally a means to explain the difference between photometry results and careful visual magnitude assesments? 
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Good question, Stu. When Terry and I talk about "V Mag" we are talking about an image taken through a Johnson V filter. This is essentially a green filter with a passband centred on about 5300 A - the dead centre of the human visual range. So for average stars the measured V mag will be quite close to the estimated Visual mag. Terry and I also took B (Blue) measurements, which were around 4.4. So this nova has a B-V value of 0.2 at the moment. B-V is a shorthand and quantitative measure of the colour of a star: typically B-V around 0 is a fairly blue star, while B-V of 1 or more us a fairly red star.
The problem is, this nova is neither average nor typical! A lot of the light is coming from the Ha line in the red part of the spectrum, and a fair bit from the Hb and other lines in the blue. These are both outside the main passband of the V filter. So I'd expect Visual estimates to be a notch or two brighter than the V measurements. But it is very hard for even experienced visual observers to get closer than 0.1-0.2.
The AAVSO gas an excellent manual for visual observers.
http://www.aavso.org/visual-observing-manual. I hope you are reporting your observations to them!
Cheers
Jonathan