plot of polaris (C5-B) night average from 6/3/04 to 11/3/04
This is not good news. Polaris appears to jump around by 30 percent during the year as observed at Wise. Lior's plot (above) is the WO counts always taken at the same sidereal time of night. And Lior tells me that this WO data is the most steady of all the CONCAMs!
Oy vey! And I have previously seen plots from CI data that look similar.
In the worst case, we have found that NSL photometry is only good to 30 percent between random nights. Yuck. If so, we can still concentrate primary science on meteors and transients.
Being a member of the eternal optimists club, however, I think that further investigation will show that the situation is not as bad as it looks here.
What we need is a check star. If the 30 percent fluctuation occurs not only for Polaris but also for surrounding stars simultaneously, then we can correct for most of it by comparing relative fluctuations between stars and their comparison stars. Actually, this is the standard method of doing photometry.
Now the comparison star method works best for a star of similar color that is close by in angle. Exactly how similar in color and close in angle needs more study. There are also ways of using the stars in the entire field to determine how they are all changing simultaneously.
I envision that to do long term (more than a few days) photometry, there needs to be several "correction factors".
The "sidereal correction factor" corrects for the star's photometric measure coming at a different sidereal time of day, so that the star appears to be at a different place on the measuring CCD chip.
The "opacity correction factor" corrects for the star's photometric measure coming though different opaque material, be that material aerosols in the air, volcanic dust in the air, dirt on the lens, etc. This can be determined from surrounding stars that are known to be constant. Possibly some sort of fuzzy-logic interpolation can work here.
I believe once these two correction factors are estimated, long term photometry will suddenly become much more accurate, and the 30 percent fluctuations will become much smaller.
Or I will eat my "eternal optimist club" membership card.
- RJN
Oy vey! And I have previously seen plots from CI data that look similar.
In the worst case, we have found that NSL photometry is only good to 30 percent between random nights. Yuck. If so, we can still concentrate primary science on meteors and transients.
Being a member of the eternal optimists club, however, I think that further investigation will show that the situation is not as bad as it looks here.
What we need is a check star. If the 30 percent fluctuation occurs not only for Polaris but also for surrounding stars simultaneously, then we can correct for most of it by comparing relative fluctuations between stars and their comparison stars. Actually, this is the standard method of doing photometry.
Now the comparison star method works best for a star of similar color that is close by in angle. Exactly how similar in color and close in angle needs more study. There are also ways of using the stars in the entire field to determine how they are all changing simultaneously.
I envision that to do long term (more than a few days) photometry, there needs to be several "correction factors".
The "sidereal correction factor" corrects for the star's photometric measure coming at a different sidereal time of day, so that the star appears to be at a different place on the measuring CCD chip.
The "opacity correction factor" corrects for the star's photometric measure coming though different opaque material, be that material aerosols in the air, volcanic dust in the air, dirt on the lens, etc. This can be determined from surrounding stars that are known to be constant. Possibly some sort of fuzzy-logic interpolation can work here.
I believe once these two correction factors are estimated, long term photometry will suddenly become much more accurate, and the 30 percent fluctuations will become much smaller.
Or I will eat my "eternal optimist club" membership card.
- RJN
-
nbrosch
- Ensign
- Posts: 78
- Joined: Tue Jul 27, 2004 1:45 pm
- Location: Back at Tel Aviv University after a sabbatical
Photometry with NSL hardware
Actutally, one should not try to re-discover astronomy but learn from almost a century of experience doing stellar photometry. What Lior's plot shows is probably clouds. In order to obtain absolute photometry, one compares unknown stars with standard stars. A few stars are standards, and the primary one has been Vega since the monumental work of Hayes & Latham. They compared Vega with an artificial star, if I remember well traceable to a black body, that was located on a distant mountain!
In differential photometry, when one compares a star with a standard and knows how to take out atmospheric effects, it is fairly easy to reach an accuracy of 0.05 mag. A lot of work will get the errors down to a few thousanths of a mag (mili-magnitude level). I have seen a couple of days ago some first results from the HAT (Hungarian Automated Telescope) operating at our Wise Observatory. These were light curves of stars in a field, similar to what MTU people are trying to do. The variability level that was detectable by eye was about 0.02 mag at 13th mag.
Finally, it may be worth taking a look at this Master's thesis that is available on the web: http://www.atmos.umd.edu/~bobe/downloads/musat.pdf
Noah Brosch
In differential photometry, when one compares a star with a standard and knows how to take out atmospheric effects, it is fairly easy to reach an accuracy of 0.05 mag. A lot of work will get the errors down to a few thousanths of a mag (mili-magnitude level). I have seen a couple of days ago some first results from the HAT (Hungarian Automated Telescope) operating at our Wise Observatory. These were light curves of stars in a field, similar to what MTU people are trying to do. The variability level that was detectable by eye was about 0.02 mag at 13th mag.
Finally, it may be worth taking a look at this Master's thesis that is available on the web: http://www.atmos.umd.edu/~bobe/downloads/musat.pdf
Noah Brosch