I've been measuring some old and new images of KT Eri (Nova Eri 2009) and am using both the AAVSO sequence in the VSP (chart id 10300HXK) and Seqplot for comp star magnitudes. Most of this photometry is from BSM and APASS, with some measures from AAVSONET telescope Wright 30cm. After comparing these sources, I've noticed that there are three distinct zero points for this field. Also, the errors for most of the comp stars are typically 0.05 to 0.10 for stars of magnitude 9 to 14. Many of these stars have been measured on 4 to 12 nights. These errors are much larger than I would expect for such moderately bright stars. When I measure a set of my images for this field, the s.d. of KT Eri is usually 0.01 to 0.02 mag. When I apply the zero point using the VSP or Seqplot comp star mags, the total error is doubled or tripled. I've seen this for other fields as well. Maybe I've been spoiled by the generally excellent field photometry from the USNO 1m over the years, but I'm rather disappointed by some of the newer photometry from BSM and APASS. I'm not sure what to do about this and would appreciate any advice on this matter. Thanks, Bob
Hi Bob,
Without knowing exactly which stars have you checked it's not too easy to answer your question. Did you take into account that APASS starts to saturate above ~10.5 magnitude? That can be one of the causes of the problems you have noticed.
As I can see in VSP the old BSM photometry had quite small error (mostly <0.02) as well as the Wright 30 photometry. I'll check them for zero point and saturation as well. But maybe Arne can give you more advice later.
Robert Fidrich (FRF)
Unfortunately the Wright 30 photometry for this field are not available in Seqplot, so I could check only those Wright 30 stars for wich photometry are availably in VSP.
I checked BSM and APASS photometry for this field agains Tycho-2 magnitudes (Seqplot) and ASAS-3 photometry (using the ASAS tutorial on the AAVSO Sequence team website [1]).
These data show that:
a) 'comp 97' and 'comp 99' are clearly saturated in APASS - as I expected.
b) BSM photometry is in quite good agreement with ASAS-3 and Tycho-2 data
c) APASS data are ~0.02-0.05 magn fainter than BSM or ASAS-3 data (although mostly withing their quoted error bar)
d) APASS data are mostly ~0.05-0.08 fainter than Wright 30 photometry
e) 'comp 128' photometry data are more or less in good agreement both in BSM, ASAS-3 and Wright 30 (even APASS photometry is only 0.02 mag fainter than Wright 30 data)
f) Wright 30 photometry of 'comp 134' is ~0.1 brighter than BSM, ASAS-3 and APASS photometry.
g) Wright 30 photometry of 'comp 124' is ~0.05 brighter than BSM, and ASAS-3 data and 0.09 mag brighter than. APASS photometry.
More details in the attached file.
I hope this helps.
[1] https://sites.google.com/site/aavsosequenceteam/asas-tutorial
This field highlights some of the issues in calibration. KT Eri was nova Eri 2009, and gained significant attention from the professional community. I started observing it with BSM near the peak, and used 20-second V exposures so as to properly measure its brightness. With BSM, that means a saturation around V=6.5 and a faint limit of about 12.
In January and February, I resumed monitoring of this field with W30 and SRO50. Here, the exposures were much longer as the nova had faded. For example, SRO50 was using 90sec V exposures, with a saturation around 11mag. Finally, APASS covered this field during the course of its survey, with a saturation magnitude around V=10.
If you use Seqplot, you gain access to all of the calibrations from these various sources, as well as many additional sources like Tycho2. (Robert, I bet the W30 calibration was not restored when we uploaded APASS DR7 into Seqcat; I will make sure it gets added back in during the DR7 update). Each calibration HAS to be different than the others, not only due to the different exposure/saturation levels, but because of the nature of statistics. All of the calibrations, however, should fall within their respective uncertainties.
The Sequence Team has a pretty good handle on where each calibration set should be used; you will see this in the KT Per VSP sequence, where BSM is used for the bright stars and W30 for the faint stars. If you use Seqplot on your own, you will see differences and will have to make educated judgement as to which is best, or whether perhaps an average is a better choice.
Right now, there is no single survey that has consistent saturation/faint limits. APASS comes very close, and will only get better.
As far as KT Eri is concerned, I think the VSP sequence is pretty good, but I'll look at it in more detail once I get back from the AAS meeting. Thanks for mentioning this field - it brings back some good memories!
Arne
Hi Arne,
Not only W30 but SRO50 photometry data are also missing from the Seqplot data base for the KT Eri field. Please could you work on restore it too?
Thanx,
Robert Fidrich "Fidusz" (FRF)
Robert,
I checked BSM and APASS against ASAS also and can confirm the same offsets for this field that you noticed. I figured the 9th mag stars were saturated but wasn't sure about 10th mag.
Arne,
Thanks for providing the saturation limits for BSM, APASS and W30. This explains some of the discrepancies. Although W30 is 0.06 to 0.10 brighter than APASS, which is outside their mutual uncertainties. Was the W30 field photometry done all-sky with Landolt standards on photometric nights? If so, how many nights for the KT Eri field? Does the software used for BSM, APASS or W30 have the ability to flag measurements of saturated stars?
Thanks,
Bob
W30 certainly has the fewest calibration nights between the various systems. The KT Eri field was only calibrated on three nights: 100130, 100213 and 100214. I consider this marginal, and so its errors may well be the limiting factor. For BSM, the field was observed on 24 nights, most of which were photometric, and so the bright calibration is quite good.
All BSM, W30, SRO, etc. calibrations use nightly Landolt standards observed over a wide airmass.
Arne
Arne,
Thanks for the W30 info. Even though there are only three nights of photometry for this scope, it would be a good idea to have this available in Seqplot to better check against APASS. As of last week, Seqplot only had BSM and APASS data for this field. Seqplot only showed 12 nights of BSM data and 4 for APASS.
Bob
Hi Bob,
I assume there are only 12 nights of BSM data only available in Seqplot since these were the photometric nights at the time of the latest BSM data uploaded into Seqplot. Maybe the rest will be uploaded after the APASS DR7 release.
Clear skies,
FRF