The last 8 days' weather has been unusually clear for us in the Midwest, a golden opportunity to try out my new Rasa 8. As I get more familiar with the 1600MM camera, I'm finding a slightly lower gain helps in my Bortle Class 5 skies. I reshot Markarian's chain the other night at gain 76, but was disappointed in the way the flats seemed to be correcting the image. The centers of the stars had small black dots just off center in each of the stars and there was a general unevenness to the image, as if the flats weren't doing the job. I wasn't sure if this was my processing in this image or something to do with the flats themselves.
Normally, I'd set my Spike-A panel atop the dew shield to take my flats. At F2, the Spike-A panel has to be set very low in brightness in order to get something like 28K ADU. So low, in fact, that out of 1026 brightness levels, I have the panel turned down to 1 with an exposure time of just .04 seconds. Since the images with flats at this setting were giving me issues, I thought I'd try a different approach. I had read in a recent Jon Rista post that he's using sky flats again. Something about collimated light that seemed to be better than using a flat panel.
I wondered if I couldn't do the same thing, but use the night sky itself as the light source. Since my Spike-A panel was so dim as to be almost black anyway, I figured something like parchment paper over the top of the dew shield might provide a sufficiently diffuse surface to act as a panel, lit by light pollution, if you will. The other night I made up a test flat panel, made of cardboard and tape. A cardboard ring holds the slips over the dew shield, while a flat ring of cardboard on top forms a drum head, sort of like a tambourine. I set this atop the dew shield and started taking pictures, monitoring the ADU in TheSkyX. At 15 seconds I was getting 7800 ADU. I had spoken to andysea about this subject and he said that that was the number he was shooting for in his flats. I then took two sets of flats; one the normal way, with the Spike-A, the other with Nightflats, as I call them.
I process the night flats and in Pixinsight using manual calibration, and got this. The Nightflat is on top, the Spike-A flat on the bottom: