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two kinds of subs with the uncooled ZWO ASI 178MC/MM

ZWO ASI178MC Equipment CMOS Artifacts
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dkamen avatar
dkamen avatar
Hi,

I am using the uncooled ASI178MC with a Rpi4 and have noticed that it delivers consistently two kidns of subs:
-A "normal" one
-A more glowy one with a very visible dark band at the top.

Attaching two subs that were taken consecutively (less than a few seconds apart) for you to understand what I am talking about:
"normal" sub:

"bad" sub:


The behavior is consistent in the sense that it appears to be happening regardless of sensor temperature (seen it from 2.5 degrees up to 35), sub duration, gain, offset, cabling, USB speed and bin level (although the band is a significantly wider at bin 2x2, that is it consumes a bigger percentage of the frame). It happens with the mono and the osc version of the sensor. It is pretty much random whether a light will have it or not. It does seem to be completely absent from darks which is very unfortunate as it makes it impossible to calibrate away. Both the dark band at the top and the glow at the bottom.

Anybody seen this before?

Thanks,
Dimitris
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dkamen avatar
dkamen avatar
All right, incredibly I think I've just sorted it out after several months of fighting against it (I guess it helped that I put everything in writing). It is the USB bandwidth setting in the Indi driver. If set to 40, the subs are incredibly noisier than e.g. 60. And at 80 all glows go away completely.

Wow it's like I've just bought a new camera!
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dkamen avatar
dkamen avatar
After some additional debugging:

- The USB speed setting has a profound impact on how "glowy" and noisy is the sub. I would say under 60 it is non practical, despite the INDI driver page recommending to set it to 40. 80 is best.

- The dark band is caused by the ASI 120MM guiding camera. Its "random" appearance has to do with whether a guiding sub will coincide with an exposure, although it is not clear at which stage (whether they both have to be read simultaneously or not). But when I stop looping the ASI120MM (or when I stop guiding), the subs from the bigger ASI are clean.

- There is no such problem if I use a ASI 178MM and a ASI 178MC together, as imager and guider. It is only the ASI 120MM that is causing the issue. If you look at dmesg on the pi, it's all errors about a USB device being reset when the 120MM is plugged in.

Conclusion:
- When you have a RPI4, the  recommended speed for an ASI camera on the INDI driver settings is 60-80. Not 40.
- It is not recommended to use the ASI 120MM as a guider alongside another ASI on the RPI (it is probably still okay to use it with a DSLR).
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