After two years of DSO astrophotography with my Canon 7D Mark II, I decided it was time to upgrade and I recently purchased a QHY163M. The first commandment of astronomy (Thou shalt not buy a new astro-toy) was immediately enforced and the wrath of gods materialized as a thick layer of clouds, so i decided to run a full suite of (standard and non-standard) tests on my new camera to find out its real specs (the first light only came two weeks later).
There are many other reviews about the QHY163M and the closely related ASI1600MM-C (same sensor), but I tried to be original and to put myself in the perspective of a user upgrading from a DSLR (a very likely upgrade path for this camera), to answer some of the questions DSLR users are more likely to ask.
Just in case anyone is interested, I'm sharing here a short summary, while the full review (30 pages) can be downloaded from my website, in English (PDF) and Italian (PDF) (review page address: http://www.alessiobeltrame.com/qhy163m)
If you used to shoot DSO images with a DSLR and you're accustomed to think in terms of stops (mainly ISO stops), as opposed to gains and offsets:
- to some extent, you will feel at home with the QHY163M
- think of gain just like you think of ISO speed, but...
- in a DSLR, ISO speed is proportional to the amplification, but the gain of the QHY163 is not so, therefore...
- ADD 60 to the gain the get the equivalent of an extra ISO stop (e.g. from 240 to 300); in a DSLR you would DOUBLE the ISO (e.g.from 800 to 1600)
- SUBTRACT 60 from gain to obtain the equivalent of a reduction of an ISO stop
- at gain 0, the QHY163M behaves much like a DSLR at ISO 200; for the above rule, gain=60 is comparable to ISO 400, gain=120 to ISO 800, gain=180 (very close to the DSO preset in ASCOM driver) to ISO 1600, and so on
- the DSO preset (gain=174) in the ASCOM driver is a very good compromise between read noise (1.3 electrons only) and dynamic range (11 stops) - basically, it corresponds to ISO 1600 on a DSLR, a good starting point for many Canon cameras, so...
- if your used to expose for 3 minutes at ISO 1600 with your DSLR, you can keep the same exposure duration with the QHY163M at gain=180 - every other condition (focal ratio, filters, etc.) being unchanged. Used to work at ISO 800? No problem, just start with a gain of 180 - 60 = 120 (note: use this rule only as a starting point - the optimal exposure length for the QHY163M is likely to be different, depending on the readout noise of your DSLR, the working temperature and the filters you plan to use).
- Regarding the offset, just set it at 40% to 50% the value of gain. Doesn't need to be precise, just check that the histogram is detached from the axis.
- at lowest gain you get the best dynamic range at the expense of readout noise, at highest gain you get the lowest readout noise, at the expense of dynamic range. It's just like your DSLR
As regards main specifications, my measurements confirm they are in line with manufacturer's datasheet:
- excellent linearity throughout the entire dynamic range
- excellent readout noise, less than 4 electrons at higher gain, just over one electron in typical usage conditions
- excellent dynamic range, up to 12.5 stops at lowest gain, 11 stops in typical usage conditions
- full well capacity of 19800 electrons at lowest gain
- very low fixed pattern noise (checked with FFT)
Regarding thermal noise, I measured what follows:
- dark current is approx. 0,006 e-/s at -20°C
- dark current doubles for each increment of 8.50°C in temperature
- amp glow starts to be noticeable with exposures of some minutes - for short exposures, like 30 seconds, you don't even need darks.
- in any case, the effects amp glow can be canceled out through a good calibration of light frames
WARNING: due to amp glow, with this camera you should never use dark frame scaling (Optimization option in PixInsight ImageCalibration process - the guys at Pleiades are working on that, see https://pixinsight.c...g66770#msg66770). As pointed out many times elsewhere on CN, just use darks taken at the same temperature, exposure time, gain and offset. Using darks taken at different temperature may result in under-correction or over-correction.
My overall evaluation: the QHY163M is an excellent camera with an outstanding price/performance ratio, which you may also use for planetary work and/or in unconventional ways (due to the very low read noise, you're not forced to go with the very long exposures that are required by CCD cameras, particularly in narrow band imaging). Beside that, the use of a CMOS sensor could be an attractive feature for DSLR users searching for an upgrade. At the very least, the learning curve should be easier to climb.
If you find any error or flaw in my review or my procedures, please let me know. Hoping it may be useful to somebody.