Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 03/07/2019 - 23:39
I've been updating my photometry spreadsheets with magnitudes from the latest APASS data release. I notice that the errors for all magnitudes have increased substantially even though there are more nights of photometry. Errors in the B band in some cases exceed 0.2 magnitudes (for mag 11-12 stars)! Why is this?
Bob
As a follow up on this subject, I just examined the APASS magnitudes in the V906 Car (Nova 2018) field using Seqplot. I found errors here of up to 0.9 magnitudes! These were for stars in the range of mag 9-10V. Is there saturation involved here? I would think the reduction pipeline would exclude such measures. The APASS documentation states that the photometry is supposed to be good for mag 7-17V. I would appreciate some guidance.
Thanks,
Bob
Bob,
Known Problems in DR10
"DR10 is not perfect. We only have sufficient staff resources to keep up with the observing and are only minimally processing the results until the image acquisition phase is completed. The image quality is high; we know which are the good and which are the poor nights. We will reprocess all images in about a year to improve both the astrometry and the photometry, especially in crowded fields.
The photometric errors appear to be worse in DR10 than in DR9. We feel this may be due to the automated transformation method; the mean values appear to be ok. Or, it may be a bug in our calculation. We'll investigate this in the next release." Arne
https://www.aavso.org/apass
If you proceed to the above URL and scroll down you will find a lengthly list of known problems; the above is just a small portion.
As a sequence team member, I would not, for example, be inclinded to update your exampe of V906 Car from DR9, or eailer, with DR10. Simply leave well enough alone.
Historically, I have found APASS data to typically have a brighter limit of ~ 10.2 V; but there are exceptions as Arne sucessfully was able to process some FOV's several magnitudes brighter. As a sequencer I have to take in the whole of the data presented for a specific APASS FOV (or any survey for that matter) and use best judgement by examing all the brighter comps presented to determine whether or not data brighter than ~ 10.2V is useable. You can use seqplot to accomplish this.
Always let the data speak for itself. Each and every Survey and each and every specific fov data within that survey are unique. The data can be of high quality or can be of low quality for any given fov, within any given survey for any given filter.
Tim Crawford