I usually bracket my optimum exposures to combat airmass variable sky etc. I guess everybody does. So I might take a number of say 30s exposures and then 10 s exposures. I calibrate and stack these separately using ASTAP. Then upload the two stacked images to VPhot.
Is it ok to stack these two images in VPhot?
Just bumping this up to hopefully get some advice. Meanwhile I keep stacking.
Hi Kevin,
Generally speaking, I wouldn’t recommend stacking images of different exposure times because this can cause issues with the photometry that are difficult to detect.
For example, at the high SNR end, if you have a bright star that saturates in the 30 second image, but not in the individual 10 second images, the profile of the star in the stacked image could either end up being saturated or have a “shelf” depending on an implementation detail of the stacking software. If the images and internal buffer used for stacking all have the same underlying data type, the resulting stacked image should have a saturated star in all cases. However, if the internal buffer uses a data type with more bits, you’ll end up with a shelf. This might be detectable as a non-linearity for bright stars in an instrumental vs. catalog magnitude plot.
In this situation, there are also some artifacts that could occur for objects with low SNR or if you were using very short (few millisecond) exposures; however, if you are targeting a SNR ~100 for your target star, you probably won’t observe these.
Brian
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Thanks Brian,
I do check Co, Ck and any targets for saturation in every image as I encountered a problem where a saturated star could be averaged away from saturation by the non saturated images in the stacking.
So each stack in the calibration (10s 0r 30s) is a reliable unsaturated stack prior to the VPhot stack.
The shortest exposure I use (reluctantly) is 1s, as I read that scintillation can cause problems.
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