I imaged the very faint Abell 85 for two nights and, to my surprise, I found several light streaks, from different directions showing in the integrated image (175 x 1 minute at -10C), that I cannot explain. Attached is a downsampled, stretched image, made from the best frames only, with StarNet applied just to make it obvious (there are many stars otherwise. I don't intend to use StarNet so ignore the artifacts).
Things I know:
- I was alone at a closed dark site with no local source of light
- there are no bright stars just out of the frame in those directions (I loaded the image into SkyTools which matched the orientation so I was able to check with certainty in the atlas)
- the moon had already set
- the streaks are visible in the light frames (barely so, they are faint, but once I knew where they are in the integrated they are) so it wasn't a passing place or something. That would have been eliminated at integration time anyway. I used PixInsight's NormalizedScaleGradient script prior to integration, with Generalized Extreme Studentized Deviate for pixel rejection).
- the streaks are not visible in the dark frames or the master dark frame (out of 36 1-minute exposures at -10C).
- there is no way light can get into the frame. The OTA was a Tak TSA-120 with the dew shield extended. Everything was tight (I checked).
- temperature outside was around 10C, so the cooler was working but not particularly hard. The camera's dew heater was on.
The master light was fully calibrated (with a master dark and a master flat + flat darks, everything at -10C as well), as I did in the past.
Is this some kind of very faint amp glow that only becomes visible after hours of integration? Or is there any other explanation? I had another problem with the camera last week (the grease leak, documented elsewhere) but I'm not sure these are related.
I sent an email to ZWO as well, but they are in the mid-autumn fall festival until the 21st.
Thanks for any suggestions.