With the usual lousy winter weather I have had no opportunity to image for the past couple of months, but this has afforded me time to take measurements to characterize my QHY600M-PH camera.  The QHY website provides much of this information, but I realized there were some things missing, and I wanted to check the characteristics of my particular camera against their results.

Here I post several graphs showing various camera measurements vs. the Gain Parameter setting, which I vary from 0 to 100 (in arbitrary camera units [AU]) for each of the four camera modes.  The four modes are defined as folllows (using QHY's terminology).  For each mode I also list a fixed Offset Parameter value (also in arbitrary camera units [AU]) that I used for all the Gain Parameter values tested:

Mode 0 - Photographic - Offset = 100
Mode 1 - High Gain - Offset = 250
Mode 2 - Super Fullwell - Offset = 75
Mode 3 - Extended Fullwell 2CMSIT - Offset = 75

The acquisitions were taken with Sequence Generator Pro.  Sensor cooling was at -5C.  I may have been able to go cooler than that in my workshop, but experience has taught me that the TEC would have to labor unnecessarily to maintain something cooler than this.  For these measurements, I required at least 2 bias frames and at least 2 flat frames for each data point.  I attached the camera on my TS RC10 scope in my workshop and used the luminance filter.  Flat frames were exposed for 4 or 5 seconds each with varying brightness from my Artesky 550mm Flat Panel.  This set of tests did not include dark current measurements, which I may do at a later time for favored Gain and Offset Parameter settings.

To quickly compute System Gain and other camera values, I used the equations from R. Berry and J. Burnell's book The Handbook of Astronomical Image Processing, particularly from Sec. 8.2.  Since I planned to vary the Gain Parameter and camera modes, I did not take sufficient images to produce full-fledged photon transfer curves.  This could be an exercise to do at a later time with particular Gain and Offset Parameter selections.

Berry and Burnell's Eq. 8.2 computes the camera's System Gain in units of [e-/ADU] as follows:



For each mode's Gain Parameter value, I took 4 bias frames and 4 flat frames, the latter of which were exposed to place the histogram peak at approximately the 75% illumination position.  Above this point, the sensor is increasingly in its non-linear response region, and I wished to illuminate within its more linear response region.  All frames were taken at 1x1 binning.  For each frame, I extracted a 500x500 area in the middle of the field.  From this sub-frame, I calculated an average value, shown above as F1, F2, B1, B2, etc.  For a pair of flats and a pair of bias frames, I took the difference of the two frames and computed the variance of this difference.

With the results listed in Excel spreadsheets, it was an easy matter to perform the calculations to get the System Gain results g from the above equation.  The plots are shown in Revision A.  As I had 4 flat and 4 bias frames for each data point, I built up three pairs each and took the average of the results.  With 4 frames of flats and biases each, I could mix and match them into as many as 6 pairs, but it was easier to limit the calculations to 3 sequential pairs from each set of 4 frames.  I automated all these calculations in Mathematica, which I also used to compute the results that follow.  Mathematica has functions to read the metadata from FIT files, and I could easily parse these to extract camera settings as well as the field data.  I selected out the 500x500 region in the middle, computed the signal averages, took the frame differences and computed the variances.  The results were written to Excel spreadsheet for the rest of the processing.

For the Read Noise, this was a simple matter of dividing the bias frame difference variance by 2 and taking the square root.  Multiplying by the System Gain put the results in units of electrons [e-].  The results are shown in Revision B.  (See Berry and Burnell's Eq. 8.3.)



Full Well Capacity can be quickly estimated by taking the number of digitized levels multiplied by the gain.  As this camera has a 16-bit A/D converter, the Full Well Capacity can be calculated from the following.  The results are shown in Revision C. (See Berry and Burnell's Eq. 8.5.)



One of the main goals of this whole exercise was to get the Dynamic Range of my camera and express it in various ways to examine the differences of the various modes of the span of Gain Parameter values.  Revision D shows the linear Dynamic Range in number of levels or [steps], which is simply the linear ratio of the Full Well Capacity to the Read Noise:



The Dynamic Range is often expressed as a logarithm in [dB].  The formula, obtained from various sources on the internet, is the following, and the results are shown in Revision E:



The Dynamic Range is also often expressed in terms of [stops] or [bits] as is done on the QHY website.  I had to figure this one out myself, but it wasn't too hard.  The Dynamic Range in [steps] is expressed as a value 2 to the power x, and solving for x gives the Dynamic Range in [stops].  The results are shown in Revision F:



The results in the plots mostly follow the values published on the QHY web pages, although some of my values differ at the higher Gain Parameter values, possibly due to encroaching on the more non-linear regions of the sensor response.

My results show more of the behavior of the newer Mode 3, particularly highlighting the extension of the Dynamic Range over that of Mode 2 due to its lower Read Noise.  This is most apparent in the linear Dynamic Range plots of Revision D.  For Mode 3 and Gain Parameter 0, the highest Dynamic Range is achieved.  A close second is Mode 1, Gain Parameter 0 and then similar value at Mode 1, Gain Parameter 56.

My initial analysis suggests that going for highest Dynamic Range ought to be the goal in best operating this camera for astrophotography, however I also ponder if there are other considerations that need to be properly weighted.  For example, in choosing a mode and Gain Parameter setting, how does the desired SNR, guiding conditions, light pollution, etc. come into play for a particular target and its brightness with respect to background, the filter being used, etc.?

I hope these results will be of interest and of use to you, particularly if you have this camera or are contemplating purchasing it.  It has a truly impressive Dynamic Range, good performance and a myriad of configuration choices.  In fact, it is this latter point that makes using this camera more complicated as it's not entirely clear (to me at least) what settings to use and when.

I'd like to ask a special request of those more in the know than I to please leave some comments below about the various implications of this camera's characteristics and to better detail what settings to use and under what circumstances to use them.

Thank you for your attention, and I look forward to your comments.