Telescope Reviews: P Cygni & comparison stars

I've recently started monitoring P Cygni using my "high precision" DSLR technique. For a change I'm trying to extract "DSLR B" as well as "V" magnitudes.

The AAVSO reccomendation for comparison stars is to use 22 Cyg and 36 Cyg. This works well for the visual range

P Cyg Sep 20.09 4.927V +/- 0.014 ref 22 Cyg = 4.936V
P Cyg Sep 20.09 4.903V +/- 0.014 ref 36 Cyg = 5.573V
P Cyg Oct 07.94 4.863V +/- 0.018 ref 22 Cyg = 4.936V
P Cyg Oct 07.94 4.866V +/- 0.016 ref 36 Cyg = 5.573V
P Cyg Oct 16.88 4.842V +/- 0.014 ref 22 Cyg = 4.936V
P Cyg Oct 16.88 4.843V +/- 0.008 ref 36 Cyg = 5.573V

but there are serious discrepancies in the "B" measures

P Cyg Sep 20.09 5.038B +/- 0.009 ref 22 Cyg = 4.850B
P Cyg Sep 20.09 5.103B +/- 0.011 ref 36 Cyg = 5.629B
P Cyg Oct 07.94 4.980B +/- 0.011 ref 22 Cyg = 4.850B
P Cyg Oct 07.94 5.080B +/- 0.012 ref 36 Cyg = 5.629B
P Cyg Oct 16.88 4.990B +/- 0.010 ref 22 Cyg = 4.850B
P Cyg Oct 16.88 5.082B +/- 0.011 ref 36 Cyg = 5.629B

It looks to me as though the measures against 22 Cyg are consistent but that there is a problem with 36 Cyg - as though it it varying in the B band but not in the V band.

I would honestly have expected the comparison with 36 Cyg to be better as it is much closer to the variable.

The other explanation is that my reduction is faulty - perhaps I clicked on the wrong star sometimes when trying to measure 36 Cyg - it's a crowded field & this is a possibility but I don't think it's likely. But I'd be very grateful if someone could monitor 36 Cyg in "B" and/or follow it spectroscopically just to see if something odd is going on.

Incidentally the equipment used is Canon 40D camera with EF 50mm f/1.4 lens on a static tripod, ISO 400, 8 seconds at f/2.8 unfiltered, the R G & B images are seperated out from the .CR2 "raw" frame using AIP4WIN V2.3.0. Each measure above is derived from the average of 25 frames taken at 30 second intervals, the error indicated is the "error of the mean" derived statistically from the actual individual measures. Correction for differential atmospheric absorbtion has been applied depending on the actual altitude of the star at the midpoint of the individual exposures.

Edited by brianb11213 (10/20/09 04:46 AM)