Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Fri, 04/26/2024 - 19:20
Having participated in the recent AB Aur campaign where we were asked to use the sequence:
104 comp and 120 chk, I have a few queries.
1. Why were these selected given that they are a lot fainter than AB (about 7 V mag)?
I understand the need for standardisation in a programme like this, and I see they are quite close to the target.
2. Before the campaign I was using what I thought was a better sequence with an ensemble of comparisons much closer in brightness.
Given that (unless persuaded otherwise) that I would prefer to revert to my previous sequence, should I submit both reductions?
Kevin:
Do you have a good comp selection that was available in your FOV?
Ken
I was observing AB Aur…
Ken,
I was observing AB Aur a few times before the HST programme started, so yes I had a sequence all ready with comps nearer in brightness
to AB which I thought (all other things being equal) is best practice to keep things near the linear section of one's system response.
I'm not wedded to them though and I went back and deleted my old reductions (and transformed them) and re-submitted using the HST sequence.
It was also a good exercise in comparing my results with others.
I think my main difficulty was getting a decent SNR for the check star whilst keeping AB away from saturation.
From memory it was sometimes below 50 in B filter.
The comp and check…
Kevin,
The comp and check stars were chosen specifically for standardization, closeness to target since field size is an issue for many, and also simplicity. For a collaborative effort amongst a large number of observers, these are critical factors. Please submit your data reduced in the way that the campaign has asked for so that it can be best combined with everyone else's work. Definitely don't submit data from the same images reduced two different ways.
Clearest skies,
Walt
Yes I have…
Thanks Walt,
Yes I have done what you suggested. Please see my reply to Ken
Regards
Kevin
The comps for the AB Aur campaign were selected based on the assumption of rather small fields-of-view. For a mag 7 star, where a small, short focal-length telescope/camera would be best, and if you have an FOV of something like 1.5x1-deg (say), my opinion is "of course use more appropriate comps!" But this does mean it is incumbent upon you to get results concordant with the rest. I would be sure to look up each star in SIMBAD and VizieR and possibly elsewhere to find 'best' values for the stars you adopt, and to convince yourself that those stars are not problematic. Then you _must_ show that your system matches the AAVSO scale by knowing your zero-point and color terms, preferably determined each night --- don't assume your set-up is stable. The obvious thing to do as a start is to measure the 'official' AAVSO stars as unknowns against your comps. Do you recover the 'official' mags/colors for them? How close? Why are they different?
The reason for having a standard system is so that one can measure any combination of reference stars on different nights, and you get the same result within the various uncertainties (small, one hopes). So there is no reason to insist on the selected stars if they're inappropriate. But you then have the responsibility of convincing the rest of the world that you have matched that standard system.
As an aside: did anyone get well-calibrated data for AB Aur during the campaign on 'many' nights? Can that lightcurve be posted somewhere so that others can compare their results to see how they match?
\Brian