Workaround for QHY(2) CCD on slow ARM board

[je-lamiaud]

This is not an "issue" per se, at least not an Indigo issue, but could be useful to other users facing the same problem : a QHYCCD camera hanging upon image capture (the restless can jump at the end 😀).

I'm using a QHY5III178 for guiding with PHD2.
Although it is running quite fine connected to my MacBook laptop, I wanted to connect it to my Beaglebone Black, which already controls the telescope mount and focuser, in order to guide / image from a warmer environment 😀

With indigo (at commit 8ae90de) installed on the Beaglebone, the camera was correctly detected, but image capture did always fail.
Logging QHYCCD driver and libusb debug traces pointed at the CPU reading USB too slowly for the camera to be happy, even with bit depth set to 8 and USB traffic parameter at 60 maximum value.

Eventualy, I discovered that the USB traffic parameter can be set at higher values than the camera reported maximum. Setting the value to 255 (and bit depth to 8), the frame rate is awfully slow (enough for guiding anyway), but image capture does succeed on the Beaglebone Black. When read back, the camera reports a USB traffic value of 60, but the behaviour is completely different.
If your need is telescope guiding, and your darn QHYCCD camera hangs, this workaround may help you.