I use the program FotoDif, to make my photometric measurements. Using it since february 2014, lot of clear nights after that day. I make time series ...and have send some 17 000 measurements to AAVSO database after february 2014.
Today i was examining my last night TT Ari measurements, and I noticed that the air mass value was over 27. Airmass 27 sounds to me quite a lot.. And I checked out what went wrong.
I found my FotoDif observation location was longnitude 27 06 30W , which should be 27 06 30E. No idea if that fault config have been since i start use FotoDif...
The question is, should I remove all my observations since february 2014, an remeasure them..? How critical error this is??
In attachment pics you can see difference of airmass from lpngitude 27 06 30W and 27 06 30E
Hi Timo,
Most programs that do differential photometry do not use airmass in their calculations; I don't know if that is true for FotoDif. When you add the extra step of transformation, then airmass would be used.
If you report your observations to the AAVSO using the extended format, then airmass would be included in each star's record. Any airmass value between 1 and 40 is stored without further checks from WebObs. Anything less than zero or greater than 40 will be flagged as an error. Since your observations gave airmass of 27, they were stored without further comment. A researcher, however, will question your observations because no one observes at that airmass. Further, they cannot properly transform your data at a later time using typical mean extinction values, as the photometry will give wildly discrepant values even for close comparison stars.
I highly recommend deleting all of your incorrect submissions and resubmitting. It is difficult to do this easily through WebObs, so if you want to do this deletion, I recommend that you contact Sara Beck on staff (sara at aavso dot org) and have her give you a hand.
Arne
Thanks for the reply Arne.
Fortunately, my observation list contains only a few star:
TT Ari, BL Lac, SS Cyg and UX Uma
I've now corrected some 4500 measurements -TT Ari, where airmass was the most wrong ,28 vs. 2.5, and BL Lac (I removed incorrectly calculated, and downloaded the new correctly calculated measurements) .
SS Cyg is easy to fix, because the measurements are from recent 20 last day - easy to find. Try to work this out in this week.
UX Uma is a little trickier, because the measurements are from last spring, and the pictures have been somewhere in external hard-disk - this take about 2 weeks. I do not want to have removed these UX Uma measurements before I'm sure I can find the matching pictures which i can make new measurements.
Just cheked difference of calculated airmass when the UX Uma was at its lowest, beginnin of my observation airmass was 2.05 (27 06 30E) and 2.49 (27 06 30W) 2456686.25 - 2456686.73 JD
Timo
Airmass 40 sounds like an extreme of the extremes! I'd suggest something more realistic - say a flag for anything lower than 9 degrees elevation (X ~ 6). Anyone doing observations lower than THAT could be accused of spying on the neighbors.