Peter and Chris observed NGC6633 – an open cluster in the Galactic plane — on 26th June 2012. The aim was to calibrate the zero points and colour dependencies of all the filters.

Coordinates are RA = 18h 37m and DEC = +06 34 (J2000) — which goes more or less overhead. We observed very close to the zenith, so the airmass was very close to 1.0.

Image above shows the FOV for the IRCUT filter (which is very similar to V). About 80 well measured stars are identified in the cluster and marked with blue circles.

We got great images for all 5 filters (first time we got all 5!).

Johnson V and B data were obtained from WEBDA:

http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?dirname=ngc6633

Exposure times were:
V 25 sec
B 34 sec
VE1 12 sec
VE2 32 sec
IRCUT 12 sec

Counts were measured in the ~ 70 identified stars in a 2.5 pixel aperture (i.e. 7*2.5 = 20 arcsec radius aperture).

Transformations to V, B, VE1, VE2 and IRCUT were derived from ~ 70 stars, where the instrumental magnitudes for each filter are of the form:

Vinst = -2.5*log10(Vcounts/exptime)
Binst = -2.5*log10(Bcounts/exptime) etc…

V = Vinst + 15.07 – 0.05*(B-V)
B = Binst + 14.75 + 0.21*(B-V)
VE1 = VE1inst + 16.30 + 0.18*(B-V)
VE2 = VE2inst + 13.88 + 1.09*(B-V)
IRCUT = IRCUTinst + 16.43 + 0.16*(B-V)

The scatter in the derived relations is
V 0.02 mag
B 0.05 mag
VE1 0.04 mag
VE2 0.06 mag
IRCUT 0.06 mag

The transformed data for the stars are shown in this plot:


pdf

The V and IRCUT filters both transform to Johnson V, and the B filter transforms to
Johnson B, with relatively small colour terms (i.e. the dependency on (B-V) of the transformation).

Followup:

Firstly we compare these relations to those derived at the last (only partially successful) attempt to calibrate the filters using M41.

The report on the M41 data is here:
http://earthshine.thejll.com/#post112

The transformations from the two clusters are:

V = 15.15 + Vinst – 0.08*(B-V) : NGC6633
V = 15.07 + Vinst – 0.05*(B-V) : M41

B = 14.46 + Binst + 0.26*(B-V) : NGC6633
B = 14.75 + Binst + 0.21*(B-V) : M41

The colour terms are quite similar, but the zeropoints differ substantially, particularly for B. Since we were unsure about the quality of the M41 data, I think these these should be disregarded. We’ll do NGC663 a few more times over the next few weeks — it’s a very well placed cluster, and the stars are quite sparse — which is very good for us.

We need a set of images as the cluster goes to much higher airmass, so we can measure the extinction coefficients for each filter. I think we have at least some of these data already from 26th June 2012.

TO DO: apply these to lunar images to measure the colour of the brightside and earthshine light.