It seems that many of the miras I follow are faint atm and so I’m reporting a few ‘fainter than’ observations (more than I like). In doing so I’ve encountered an issue a few times. For example, last night I checked T Phe but couldn’t see it. The faintest comparison star on my chart was 11.0 so the obs should be recorded as <11.0. However an hour earlier while observing RT Sgr I had glimpsed an 11.9 star and had seen 11.6 and 11.7 a few times through the night. The seeing hadn’t changed so I was confident that T Phe was probably below 11.7 and certainly below 11.5.
So the question arises: can I use a previously determined limiting magnitude when reporting a ‘fainter than’ obs or must I only report it as fainter than the faintest star plotted on my finder chart?
Great question! I generally just use the faintest comparison star I can see on the chart for that particular variable. I wonder what other observers do.
One tries to make an observation rather than reporting a “confident guess”. It appears you are visually observing, maybe with a small telescope, so it would be best to report at the time of your observation the faintest comparison star that you are sure you can see from the AAVSO sequence for the variable star you were trying to observe.
Use the ‘chart’ version. Chart dates can vary (no pun intended) from one to another, for various reasons. Your T Phe sequence could have been made at a different time to your RT Sgr one. The source from which the sequence values come could be different, and so on. Using the ‘chart’ values, especially in conjunction with the ‘photometry’ option and the chart reference code enables us (the sequence team) to determine if there are any inconsistencies that may need addressing.
Thank you for that. I’ll follow your recommendations. I understand that there could be inconsistancies between sequences though I don’t know how you determine them from variable star observation. However I don’t need to understand so that’s OK.
For my part I was trying to use all the information available. I understand about ‘making an observation’ but there is also the consideration of calibrating your instrument. Unless conditions change the lim mag should be consistent. The three factors which I can see that would change lim mag are 1) transparency, 2) amount of light pollution and 3) relative location of the comparisons star and the variable. Of these only (1) is any sort of judgement call and that is based on over 50 years as a DSO observer and knowing the meteorological conditions. Factor two is not a problem because light pollution falls during the night at my site. Neither is location as T Phe was 15 degrees higher than the calibration star. Of course this all just conjecture now as I will be basing all obs on what can be seen on the chart at that moment.
Hi David, on the basis that ‘more information’ is always better than less…
the ‘zero point’ may vary from one chart, or source, to another - though hopefully the differences are not as large as they were. One crucial factor is the date of the chart (strictly, the sequence used to make it). A personal example - for many years I observed T CrB using a chart dated 1928!! Quite rightly I was given a (polite) telling-off by HQ for not updating the chart! In those days I suppose sequences were established using a wide variety of sources such as purely visual estimates, or old-style photographic determinations - presumably there would have been a zero point for these but the inadequate surveys at that time could not be compared with those we have today. The bottom line is therefore that even today you shouldn’t “shit bricks” over whether a limiting magnitude is (say) 11.3 vs 11.7 because several factors, some of which you cite, come into play to determine those values.
Just make the observation, fainter or positive detection, at the same time using the AAVSO chart sequence.
There are many other factors at play actually when making fainter than visual observations.
In visual observing one must also consider dark adaption factors. It is good practice to allow the eye to dark adapt at least 15 minutes after coming outside from an illuminated place before beginning your observing for the night.
If I remember it was Steve O’Meara who wore an eye-patch over his dominate eye during the day so he could make super faint visual observations with big optics. I tried the patch over eye thing a few times but decided that looking like a pirate during the day wasn’t worth the effort!
A supplementary question if I may. What is the situation when the variable is visible but is fainter than any of the comparison stars? This has happened a couple of times. Of course no one wants to extrapolate but in the case of RV Sgr I have comparisons at 11.1 and 11.4. If I estimate that the 11.4 is half way between the 11.1 and the variable it seems valid to estimate the variable at 11.7. On the other hand for T Phe there are two stars at 11.0 and the next faintest is 9.7! Clearly extrapolation is out of the question. So what do I write? Is ‘fainter than’ appropriate or is that only for cases where the variable was not seen?
In the case above, the solution is to submit a CHET report to the Sequence Team so they can add a fainter comparison star to the sequence. It’s something easy to fix. I do not recommend extrapolating.
The same in the case of T Phe. They will add comp stars to fill in the gap.
Cheers,
Sebastian
PS: of course, check that the you are using a chart with a faint enough limiting magnitude to include potential fainter comp stars.
