I've owned a STF-8300C for a month now, and my imaging efforts have been frustrated by persistent, characteristic gradients that are in all my images. They are bright bands in the upper left and on the right side. I didn't have that problem previously when I was using a DSLR. My OTA is an Orion 8" f/3.9 Newtonian. My first guess was stray light, and by taking lights in daylight with the cap on on the front of the scope discovered that I did have that problem. Stray light was entering through the back and the focuser. I carefully shielded those areas until I was seeing an even dark field even when the scope was in full sunlight. Unfortunately, the gradients essentially remained. Those pictures above were taken after I shielded the areas where stray light could intrude. I did the processing with Pixinsight, using darks, biases and flats. I now think the problem lies with my flats. I have been using the spike-A flat fielder to create them. Unfortunately, even at minimum (25%) illumination, the signal is too bright for my system. Only at minimum (0.09 sec) exposure do I get down to about 50% saturation or less. So I took my flats with 0.09 seconds. I have seen some forum posts that are already a few years old that suggest that with the shortest possible exposure times, the shutter gets partially in the way, and that flat exposures with the STF-8300 C or M should be taken for a minimum of several seconds to minimize that effect. My master flat looks reasonable enough at first sight- a radially symmetric brightness distribution centered in the middle of the image. However, when I do an automatic background extraction in Pixinsight (degree 6) to subtract the "regular" part of the signal, here's what remains: To me, that seems to confirm that there is something wrong with my flats, and that they are responsible for introducing the gradients I see in the calibrated images. I'm thinking of buying a tinted plexiglass sheet to put between the flat fielder and the scope to attenuate the intensity enough so I can take 2-3 second flats. Before I do that though I'd like to hear from the more experienced people here whether my thinking is accurate. I'd be grateful for any advice on how to best fix this problem.
The STF shutter is designed for even illumination, but only to a very good approximation. An extremely short exposure time will reveal that limitation. Before you do anything fancy, put a piece of typing paper over the flat field device (or maybe several) to dim it down. That will be quite effective, if not durable, and will allow you to verify that you have the correct solution before you do something fancier.
