Accuracy of the Tracking Error Graph in Time Series Report

American Association of Variable Star Observers (AAVSO)
Thu, 10/19/2023 - 13:56

Hi All

I’ve recently processed 5-6 EB time series observations. Each one of these reports includes a graph showing the migration of the target image across the sensor in X and Y. These are intended, I believe, to be an indication of the quality of tracking and therefore the quality of the photometry as targets wander across multiple pixels, each with slightly different quantum responses to incoming photons. 

I was concerned about moderately large ‘graphed’ tracking errors shown in one of these time series reports so I checked the underlying data of the tracking graphs using X(Fits) and Y(Fits) locations on spreadsheet data from Astro Image J.  I found the X and Y movement documented by AIJ was far less than what was shown on the Time Series Result Report. For example, one time series report indicated movements of about 120 and 80 pixels in X and Y, while the AIJ data indicated only a movement of 29 and 19 pixels respectively. 

I’m posting here to ask whether others, more familiar with how the time series results reporting is calculated, are aware of an issue with the graph or whether I’m somehow misunderstanding what’s being displayed and/or how the tracking error data is being calculated. What could be a useful indicator of observation quality, seems to be depicting incorrect tracking performance. 

Hope to hear your thoughts….



American Association of Variable Star Observers (AAVSO)
VPhot tracking graph

Problems with the VPhot timeseries tracking graph have been reported in the past by Arne and others.

And the problem you are seeing is real.

Its been on my list of things to fix for a long while. Its still TBD.

I'd also like to add a feature: it should be possible to comment on the scopes polar alignment based on a clean graph over a reasonable base time line.

Anyone want to investigate the computation to do this?