Hi All,
I want to download ASAS V Band photometry for a bright variable but I'm a bit confused after re-reading the AAVSO Sequence Team ASAS Tutorial. In the Tutorial Sebastian Otero (in an extract from a forum post dated Tuesday, July 29, 2008 12:11 AM) stated that there were problems with the "new" reduction of the ASAS data and recommended using the "old" catalog at http://www.astrouw.edu.pl/~gp/asas/asas_old.html.
When I go to that URL I can search for the variable but the light curve is not plotted and the data can't be downloaded (I get a "Internal Server Error" message).
The tutorial is quite old, 2008 was the last update. So my questions are:
1) is the "old" catalog no longer available?
2) have the problems mentioned by Sebastian in the tutorial been corrected in the current catalog (at http://www.astrouw.edu.pl/asas/?page=aasc)?
3) does anyone have a more up to date ASAS tutorial?
Thanks in advance,
Mark
Hi Mark,
As you said, the tutorial is outdated and should be replaced and simplified. I'll work on that.
1) It's been years since access to the data in the old catalogue (the good data...) has been discontinued.
2) No. There were 3 or 4 re-reductions of the whole data but the results were never the same as the first reduction. You have now some "false eclipses" or scattered datapoints that were just normal points in the first reduction of the same image. So we have to live with this, these are the only available observations. As a whole, ASAS-3 are still a very good source of photometry. You only have to be careful to come to wrong conclussions .e.g. in the case of long period eclipsing binaries, assuming eclipses from single faint datapoints.
Besides those false eclipses, the zero point changed a bit compared to the first reduction. Data from the first reduction was = V but now it is usually slightly fainter than V by 0.01-0.05 depending on the field. It looks like they used every available Tycho star for the reductions instead of only bright stars and that introduced errors from the faint subset of Tycho stars where the photometry is not reliable.
Also, aging of the optics seems to have caused a degradation of the photometry quality that is quite apparent around HJD 2455000 when there are lots of D-flagged datapoints (usually with magnitudes slightly brighter than avergae).
3) I'll work on a new tutorial
In the meantime, ask any question about the use of ASAS-3 data here.
What star are you checking? Depending on its magnitude, there will be saturation problems or not at the early dates.
Remember to use the proper aperture result depending on the star's brightness. If the star has a very large amplitude, you may even need to change columns to get the best result. The ASAS data page usually shows the correct aperture column first but in those cases, you have to check.
MAG_0 = <12.0
MAG_1 = 11.0-12.0
MAG_2 = 10.0-11.0
MAG_3 = 9.0-10.0
MAG_4 = >9.0
Cheers,
Sebastian
Thanks Sebastian,
the star is QZ Car (V mag 6.2 to 6.5) so saturation was a serious problem before the exposure time was reduced, I'll ignore the early data. Later observations seem to have suffered saturation occassionally too so I'll weed those out manually. The largest aperture (MAG_4) is the right one for this star, but at 90 arcsec contamination will have to be checked.
Is the zero point error of a particular field consistent from night to night? That is, will a constant star always be 0.05 mag too faint or does the amount change from night to night?
Can you suggest a way to correct the zero point error in the current database? Would it be appropriate to download the photometry for several bright constant stars in the same field as QZ Car and determine the average zero point error relative to their Tycho magnitudes then apply that offset to the variable?
Cheers,
Mark
Ah yes, I see, Stan told me about that a couple of days ago.
The best way to get rid of saturated measures is to compare the results between MAG_4 and MAG_0, if they are more than ~0.05 diferent (usually much more), there is saturation.
Observations between JD 2452950 and 3300 are the better ones due to the change in exposure time (from one 3 minute exposure to three 1 minute exposures).
Yes the zero point offset is constant for a particular dataset.
BUT the same star may have several datasets (coming from adjacent fields) and each one has its own zero point (there are different stars used in the calibration) so they have to be corrected individually.
You can usually see if there are differences checking the ASAS light curve, where every dataset is plotted with a different color.
You can't correct the zero point using other ASAS stars. Just use GCPD or HIPPARCOS stars in the field. A couple of stars as close to the variable as possible.
Compare the difference between the GCPD/HIP values of those constant stars and the ASAS values, that will give you an idea of the difference we are talking about. It is not a big deal but it pisses me off because this difference did not exist in the early years (first reduction).
Cheers,
Sebastian
Thanks again Sebastian,
I'll try your suggestions. Cheers,
Mark