Negative observations

I understand what you are saying but it goes against the grain to throw away information.  Here’s my reasoning.

Suppose that on the night in question SV Ari was at V=19.45 and that I had easily enough SNR to measure it to an accuracy of 0.01 magnitude based on an extrapolation of the sequence magnitudes down below 169. I would report a positive result.

Suppose that a field star was also measured on exactly the same image at, say, V=19.61, also to an accuracy of 0.01 magnitudes.  I feel I would be justified in recording it as such, if only in my own records.  Note whether or not that second star is a variable is irrelevant because it is being measured at a specific point in time.

Now, a week or so later, SV Ari has faded to a true magnitude of, say, V=22.0 which is way below the detection limit.  However, that same field star is still measurable on an image taken at the later date.  For the sake of example, let´s say it is now measured at V 19.62 with accuracy 0.01, again using only the official sequence. It is quite irrelevant in this particular instance whether that star has truly faded slightly or whether the difference between the two measurements arises for SNR reasons.  It is quite clear that SV Ari at this date is significantly fainter than V=19.6.  It seems wrong to me to throw away the additional information about the limit on the brightness of the variable.

Please note, in the latter case, I would NOT be using the V=19.6 star as part of the sequence to determine instrumental magnitudes and their errors.  All of that is still being done exclusively with the standard sequence through a lengthy extrapolation.

Yes, I´m quite prepared to work with Jeremy and/or the AAVSO to extend the sequence in this case and others.  However, prospective additional sequence members will need to be checked that they do not vary significantly on timescales ranging between hours and years before they can be used with confidence. (This issue has already bitten me: I discovered that one of the  AAVSO comparisons for V3721 Oph is an EA with minima 0.025 and 0.010 magnitudes.) My suggestion, on the other hand, requires no assumption of constancy, only that the limiting magnitude can be measured at the time of observation.

Thank you. I’ve read that paper in the past, and imaged M67, but it’s good to read it again. Section 4 (p77) is particularly relevant, especially the comment about the difference between visual and CCD estimates of a 17.4 object when the image goes down to 20 or so. Another apposite comment is on page 79: This magnitude-bridging technique is common in the professional world, as most of the standard stars are too bright for large telescopes.

Please note that for present purposes I am emphatically NOT trying to detect the faintest possible object on the image. I am trying to measure the magnitude of the faintest object which has an error smaller than a specific limit, 0.1 magnitude say. In this case the SNR is way above the 5-sigma limit mentioned in the paper.

As I pointed out earlier, if I used a sequence which ends at 16.9 to measure a variable at say, 19.5 +/- 0.1 that estimate would be accepted without question. Why should a measurement to the same accuracy of an equally bright nearby star be rejected purely because it is not a (known) variable?

Behind all this is my firm belief that one should not throw away data. It should be preserved for later scientists to re-analyse if they wish. For my part I store every image which is not too badly corrupted by focus errors, guidance errors, passing clouds, etc. In only 18 months I already have more than thirty thousand images, together with their metadata in a SQL database. All can be retrieved and re-examined for whatever reason — pre-discovery observations, perhaps, or searching for previously unknown variables.

Don´t misunderstand me: I will continue to play by the rules as they stand but it seems to me that the present rule is extremely conservative to use Arne Henden´s phrase.