Blog Image

Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

PSF profiles from the NGC6633 images

Post-Obs scattered-light rem. Posted on Apr 07, 2013 09:38

The PSF of the VE2-band Arcturus data deviated markedly from the other bands (next to previous post). We check on this further by looking at the PSF profiles of the stars in NGC6633, which were used to calibrate the filters (in this post).

We plot the central flux normalised PSFs in B, VE1, VE2 and IRCUT versus V (shown in green in each panel). The PSF is formed from 95 different stars spread over the frame in each case: e.g. this shows the stars in the IRCUT image.

The PSFs look like this:

Same plot as above but in log-linear form:

None of the PSFs are as sharp as the V band one — they all have more
flux at greater distance from the center. Interestingly, the VE2 band
profile is not discrepant, so the conclusion is that the IRCUT data for
Arcturus may simply have been out of focus.

It would seem the lesson from this is that the PSF can vary quite a bit — but note well that this is in the core only — as this is not a test of the extended power-law wings at all, as we are only probing to a radius of about 4 pixel (~30 arcsec) with these data.

V-VE1 colour map

Post-Obs scattered-light rem. Posted on Apr 07, 2013 03:59

In the previous post, we noticed that the VE1 and IRCUT filters have very similar profiles to the V band — at least for the data we obtained pointing at Arcturus.

Here we show a colour map (V-VE1, on an arbitrary magnitude scale: properly calibrated colours will come later).

It’s clear that the deep artifacts around the edge of the moon are much smaller now, compared to the B-V colour maps we have been producing to date.

Data used:

2456015.7558321MOON_V_AIR_averaged.fits 2456015.8108611MOON_VE1_AIR_averaged.fits

This shows that the core of the PSF of the V and VE1 filters are quite similar — and the power law tails as well — not just for Arcturus but for lunar images too.

Profiles of Arcturus in different bands

Post-Obs scattered-light rem. Posted on Apr 07, 2013 00:53

This is a followup to the previous post on the V and B band PSFs.

Here is the stellar profile of Arcturus, shown in B (blue) and V (green). Clearly the B band light has a different core to the PSF than V.

Arcturus data were taken on the night of 2011-03-22. The files used are

align_stacked_2455643.4800437Arcturus-B-FILTER.fits align_stacked_2455643.4891739Arcturus-V-FILTER.fits

Oddly, the V band data are affected by what we called “shutter bounce” — a non-axisymmetric feature to the right (along crows in the CCD) of the PSF, but the B band data show no sign of such a bounce (the PSF profile can be exceedingly cleanly centered, unlike V band). No explanation for this for the moment!

Here are the other bands, all compared to V (green symbols):

PDF version here:

Here are the same profiles, this time with the data averaged into radial bins. The solid green line is the V band profile in each case.

VE1 and V are very similar — but this is what we would expect. IRCUT is not too far off V either.. the B and in particular the VE2 filters differ significantly from V. The difference between VE2 and all the other filters is huge, so we need to look into that next!