Hi:
I recently acquired a camera that has a preflash readout mode, and this is the first camera that I've owned with this capability. I've run into a problem with the preflash readout mode however: I'm finding that preflashed dark frames don't seem to scale very well, at least in my trials so far. I used preflash to build master bias and dark frames, but I found that a slight "inversion" of the circular preflash pattern was visible in calibrated (preflashed) light frames that had a longer exposure (20 minutes) than the darks (10 minutes). Does anyone know if this expected, and if so, is there a workaround?
It's also not clear to me whether there is a real advantage to using the preflash readout mode. I know that flashing the chip is supposed to remove any residual bulk image (RBI), and while I've done only a very cursory inspection of calibrated light frames with and without preflash (using darks of the same exposure as the lights in all cases), I don't see any systematic differences, which might not be surprising given the low light levels of deep sky images. I've also read that preflashing induces extra noise. If darks don't rescale well with preflash, and if the advantages are truly marginal, then it seems more efficient to use a standard readout mode.
Any comments or suggestions in this regard would be greatly appreciated!
Howard.
