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Sat, 11/27/2021 - 10:07
Some exoplanet exposures take 5 seconds, others take 35 seconds.. In either case, how long do you wait before starting a subsequent exposure? A minute? Thirty seconds?, etc?
Ed
You want the shortest delay between exposures possible. Delays between images are simply wasted data gathering time. It is common that guiding is suspended while you download the science image, then the delay should be the shortest possible to allow guiding to re-center the guide star in your guider window withing the limitations of your seeing. The delay will depend on the length of your exposures and the number of exposures it takes with your equipment/software and observing conditions to re-center.
Brad Walter
Hi Brad,
I've never heard about guiding being suspended during download - why or how would that happen?
Regarding the length of any pause between exposures, it seems relevant to consider how frequent the data points 'need' to occur in order to achieve acceptable curve fitting. That number likely varies depending on where you are in the transit - perhaps more frequent images would be helpful at transit 'transition points', etc. I don't know what that frequency criteria might be but I'm sure someone has worked it all out. It would seem to me to be a bit unnecessary to be capturing 500-700 images during a 3 hour transit, to process, inspect and store later, if a lot fewer would be adequate for decent curve fitting.
Also, I wonder if, realistically, the systematic errors inherent in our equipment and our processes, don't' likely overwhelm any value gained from squeezing in a lot of added data points. Maybe it's a bit like my southern relatives sometimes say: "...at some point the juice just isn't worth the squeeze."
cheers
Gary
At least in my setup, Maxim DL stops downloading guider images while the main camera image is downloading. That's what the Guider settling criteria in the guiding options is about and its the main reason for delay between images. Another reason for delay is vibration from camera shutters but that is normally small and settles very quickly.
For a given signal to noise ratio, the more points you have to fit your transit model the better. The trade off you are trying to optimize in most situations is number of images (samples) vs. signal to noise ratio in the individual science images. If you have sufficient sample density to achieve adequate statistical power for whatever statistical test is used to select the maximum likelihood model for you light curve and have high probability (say 3 sigma level or better) that the detected effect (a transit) is real, then you would want to use any unnecessary delay time to increase SNR through longer EXPTIME. The exception, of course is if you have such high signal level that increased EXPTIME starts to introduce saturation effects. Then you would want to use unnecessary delay time to increase the sample size, which increases statistical power of your analysis improving the probability that your transit detection is real and your model is the maximum likelihood model.
Any delay between images that isn't necessary is just wasted photons. It isn't that you are trying to squeeze harder to get more juice, you are trying not to throw away some of the juice that you have already made the effort (telescope time, setup and calibration effort, equipment cost, and analysis time) to collect.
Systematic error is always an issue. However, it is much less of an issue for exoplanet transit observations where you are only concerned about changes in the light curve rather than absolute photometry. Sure there are slowly varying systematic errors from things like airmass changes and color differences, changes in seeing and focus but much of that is removed by the relative nature of the observations or can be modeled out using data available from your science images. It may take a lot of calculatin', but programs like AstroImage J or the new analysis program EXOTIC from the NASA Exoplanet Watch project do much of it for you if you gather the necessary data from your images and you have decent observing conditions.
Brad Walter
Thanks Brad. I appreciate your advice.
Ed