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Earthshine blog

"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Instrumental zero points

Observation ResourcesPosted by Chris Flynn Jul 01, 2012 03:47AM
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:

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:

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).


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:

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.

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MOON table for 2012

Observation ResourcesPosted by Peter Thejll Jan 01, 2012 10:39AM
The table gives times and positions when the Moon is suitable for observing from MLO. The columns are: JD, SEM angle, illumination fraction, Moon altitude, Sun altitude and UTC date. An asterisk at the end indicates the special SEM angle near 42 degrees. PDF table - click on the icon:

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New category

Observation ResourcesPosted by Peter Thejll Jan 01, 2012 10:34AM
Let us collect various tables, plots and links under the new category so that it becomes easier to find them when observing sessions need to be planned.

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