Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 09/12/2020 - 15:34
Please forgive me if this has been asked an answered previously in this forum. I noticed a major difference in the magnitude uncertainty calculation when using VPHOT Time Series versus performing a photometry report on individual images. Specifically, using Time Series, I noticed the MAGERR for my AAVSO reports is 1/SNR while my photometry reports are substantially higher. Two specific questions:
1. How is the MAGERR calculated in photometry reports with individual images.
2. Using Time Series is there a way to have MAGERR calculated the same way as in the individual photometry reports?
Thanks
Chuck
Chuck:
I got just the opposite result for error!?
IF you use only one image and 1 comp (single image photometry), you cannot really calculate a standard deviation (STD), so VPhot reports only 1/SNR as the error. The photometry table shows that.
IF you use multiple comps (ensemble), VPhot can/does report a STD.
IF you run a Time Series, VPhot calculates and reports a mean and a STD.
I recommend that you always use an ensemble for your photometry OR that you always collect more than one image and run a short time series where a STD is calculated OR edit your report to insert a more realistic error on the basis of experience.
If you post some actual screen shots and aavso reports, we could continue this discussion.
Ken
Ken,
Thanks for your input. I am using an Ensemble in my Time Series analysis. I do see the STD error in the summary page, that's in agreement with what I get for an individual image photometry report. However, when I generate the AAVSO report from the Time Series summary page, the MAGERRs are reported based on SNR, which is simply 1/SNR. When I then use VStar to review my light curve, I see tiny error bars based on my 1/SNR MAGERR vs the STD error. Is it standard AAVSO practice to have observations only report MAGERR as 1/SNR? If not, is there a setting in my AAVSO report generation that I need to change to get STD error for MAGERR?
I will post a screen shot of my summary page and AAVSO report
Chuck
Chuck,
Does your time series ensemble sequence include a Check Star?
Phil
Phil,
Yes, I always use a check star and an ensemable of comparison stars
Chuck
Here are some screen shots from UU Aqr on 26 Aug 2020. As you can see the AAVSO MAGERR is reporting the SNR error. See highlighted observation line and then the photometry report for that observation.
Chuck:
We cannot attach screen shots. :-(
I tested my own images for another star using ensemble.
I think you are mistaken about STD. I think my Time Series Page STD AND my Time Series AAVSO Report STD AND my Single Photometry STD are not 1/SNR.
Some are close but not the same! Check out your results again. Email them to kenmenstar@gmail.com and we can discuss. I will share mine back.
Ken