Mon, 06/29/2020 - 18:59

Since there seems to be considerable interest in CMOS sensors, I thought I'd give you a blow-by-blow account as I test a new QHY600 camera over the next week or so.  The OC61 (Optical Craftsman 61-cm) telescope node of AAVSOnet is located at Mt. John, New Zealand.  This telescope is a true Cassegrain, with a final focal ratio of f/14.6.  When we upgraded this system to robotic operation about a decade ago, Robert Ayers contributed his FLI PL09000 camera with filter wheel.  It has been a workhorse for us, but suffers greatly from Residual Bulk Image (RBI) effects.  These latent images require an LED preflash before every exposure to "wipe" the array of the RBI from prior images.  The preflash dramatically increases the noise level in the images, such that we are probably losing a magnitude on the faint end for photometry.  We needed this big sensor in order to get a reasonable field of view.  Even with this 52mm diagonal sensor, we only have a 14x14 arcmin FOV, and have to bin 2x2 to yield 0.55arcsec pixels.  I've seriously considered adding a focal reducer, but haven't found the right product yet.

Dick Post has generously funded the upgrade of the camera system to the QHY600.  We've also purchased the 7-slot 50mm square filter wheel to go with this, so that we can re-use the existing BVgri filters.  We now have 2 empty slots that need filters!  Over the next week or so, I'll test this camera in my lab, so that I know its characteristics before shipping.  Hands-on control works much better than remote testing!

While the QHY600 (IMX455 sensor) has 9576x6388 effective pixels, each is only 3.76 microns in size.  That means to yield the same 0.55arcsec/pixel scale, you would need to bin about 6x6!  The QHY600 only supports 2x2 binning in its ASCOM driver.  We will probably bin 2x2 to reduce the stored image size to 15 megapixels, and then transfer these images back to HQ for calibrating (dark subtract/flat field) using our pipeline.  Somewhere along the line, we'll probably do a further stage of binning.  The site can have 1-2 arcsec images, though most are 2-3 arcseconds.  How to handle this best in software will be a challenge.

Note that, while the pixel size is small, each pixel has a pretty decent full well depth.  QHY says you can get up to 80Ke-, but that depends on a lot of factors.  You have three different imaging modes, and can set the gain and offset for each mode.  You can also include or exclude overscan pixels, and change the USB transfer rate to lessen interference.  Lots of knobs to twiddle, and each affects the full well depth and read noise!  The mode we're choosing gets about 50Ke-, which is still pretty good, along with 3.5 electrons of read noise.  However, this means if you keep a 16bit FITS file output, something has to give when you bin.  If you do the binning entirely in software, say in MaximDL, you can store the output image in 32-bit floating point and not lose any precision.  This adds another layer of complexity.  For the time being, let's assume 2x2 binning and 16-bit FITS storage to keep things simple.

This sensor is only 42mm diagonal, so 50mm round filters could be used with it, but we're sticking with our original filter set.  However, the field of view is even smaller - about 14x9arcmin.  A focal reducer/field flattener/coma corrector would be really nice.  One big advantage with the IMX455 sensor is that it is back-illuminated and has greater than 87% peak QE according to QHY, compared with the 64% QE of the KAF09000.  Combined with the lack of RBI mitigation, we're expecting to obtain about 2 magnitudes of improvement with this camera, along with lots of other nice features, such as very short exposures and very fast readout time.

This is a USB3.0 camera that can take around 2 frames per second, so very high transfer rate.  Not all computers can handle this, and extending USB3.0 cables is tricky.  The Kepler KL400 camera from FLI is an example of a camera that doesn't work with all USB cables.  I've asked Nigel Frost, the Observatory Superintendent, to give me the needed cable length, and then I'll configure the system here and confirm that it works before shipping camera/filterwheel/USB3cables/etc. down south.

So at minimum, here are the upcoming tests:

- unpack the camera and filter wheel

- test basic operation with the computer and cable that will be used in New Zealand

- test gain and readnoise of desired operating mode

- test linearity and full well

- test dark current, hot pixels, and cosmetic defects at several different operating temperatures

- look for bias stability and whether to use overscan

- on-sky tests for RBI, squareness of sensor to optical path, etc.

- anything else that comes to mind

The next post will describe the unpacking and basic operations of the QHY600.  If anyone has any questions/comments/suggestions, please feel free to post.  I do not profess to know everything about the camera, nor assume that I will do every important test!



American Association of Variable Star Observers (AAVSO)

Hi Arne,

have you experienced any tilt of the sensor of this camera? I would like to get a 268 that is basically the same camera with just an APS-C sensor but I wonder whether these cameras need tilt adjustment as there is a tilt adapter that goes with them.


American Association of Variable Star Observers (AAVSO)

Hi Arne,

do you have any more to share on the QHY600 tests?

I have tried binning in Pixinsight and it averages the signal from the binned pixels but does show a drop in the noise and hence a bump in the SNR...

now i also have to do tests on time-binning batches of sub exposures and this all gets caught up in the gain/offset/full well depth/snr controversy and am dying to hear more sensible thoughts on that..! (Not least as all calibration frames and flats will need shot with consistent settings and I'd like to be as knowledgeable as possible before investing time on that!)



American Association of Variable Star Observers (AAVSO)

If anyone with this camera has some FITS files of example bias frames to share, I'd be very interested in taking a look at them and doing some statistics on random vs. systematic behavior.  Ideally I'd like pairs of biases to look at, with some taken at about the same time, and some taken at a later time but under similar conditions (e.g. temperature).  Looking at bias differences is a good way to sort out what aspects of the sensor performance are repeatable (even if non-Gaussian) and thus will subtract out well, vs. which are random noise. 

Thanks in advance for any help.  Happy to share my results here if I can find some files to work with.