Affiliation
American Association of Variable Star Observers (AAVSO)
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!

Arne

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Download times using ASCOM driver?

I just noticed in this review of the QHY 600 that the reviewer mentions slow (~20 second) download times when using the ASCOM driver, but not the native driver.  Are you using the ASCOM driver with MaximDL, and if so what download times do you see? 

(20 seconds isn't that slow, but definitely slower than one should get with USB 3.0.)

Thanks!

Eric

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Gavin and Eric

Hi Gavin and Eric,

Yes: mode 1 (also called High Gain Mode) and gain 0, offset 10.

I don't see 20 seconds download, using MaximDL V6.23.  There are a few seconds delay before the image gets displayed, but that is internal to Maxim and not due to the download.  I'm using a USB3.0 3m cable directly to the computer.  I've also used a Trip-Lite USB3.0 active extender with similar results, and have duplicated the rapid download from two different cameras and two different computers.  So I don't know where the slowdown was for the reviewer.

More coming soon on the QHY600 tests!

Arne

Affiliation
American Association of Variable Star Observers (AAVSO)
QHY 600 tilt issue

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.

Gianluca

Affiliation
American Association of Variable Star Observers (AAVSO)
Hi Arne,

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!)

best

gavin

Affiliation
American Association of Variable Star Observers (AAVSO)
Example bias frames to share?

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.

 

Eric

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Updates?

Now that some months have passed, I'm curious if Arne (or anyone else using CMOS for photometry) has updated experiences to share.  I still haven't taken the plunge on a new camera purchase, but need to do so soon, so I'd love to hear any followup experiences.

Affiliation
American Association of Variable Star Observers (AAVSO)
Re: Updates

I, too, would like to hear more from Arne. In the meantime, I'll offer my experiences so far with the Sony IMX571 sensor (the APS-C version of the IMX455 found in the QHY600). I've just shifted from an old SBIG ST-9 CCD to a QHY268M (both on my f/10, 10-inch SCT). Here's what I've learned so far as I've used this new CMOS camera for my BVRI photometry of Mira variables:

  • So many things about this new camera are dramatically better: QE, blue response, download time, FOV, overall linearity, ... I'm delighted with the camera.
  • Overall, my check star measurement scatter is slightly better than I achieved with the ST-9. One of my benchmarks is M67 residuals against the standard. These are now around 0.023 mag rms scatter (transformed, across about 175 stars) from a single observation set. I haven't yet had a chance to check Landolt fields. Within its linear range, my QHY268M's response curve (transfer curve) is almost an order of magnitude flatter than the equivalent curve for the ST-9; the QHY268M CMOS is much more linear than the old CCD.
  • Transformation coefficients seem well-behaved. So far, I have only used M67 for generating the transforms, but results have been consistent across multiple sessions.
  • Thermal management of the cooler and CMOS chip is very different than it was for the ST-9. This is mostly caused by all the support circuitry on the sensor chip, which is turned off during light integration. Thus, the thermal load varies. When you're obtaining a set of bias exposures, you have a high ratio of readout time to exposure time, so the "other circuitry" duty cycle is high and chip temperature can rise. Same for any set of short exposures. Plus, I'm not at all impressed with QHY's temperature-regulation firmware.
  • The bias offset ADU count is quite temperature-sensitive. If the chip temperature is different for your darks or biases than for your photometry images, it's easy to end up with negative pixel ADU numbers after calibration. (I'm currently ignoring the overscan region of the sensor; maybe I can use these pixels to help with this.)
  • Dark current is extremely low. However, darks and cooling are still needed as a way to manage "warm pixels".
  • Pixel size is far smaller than I was used to (3.8 micron vs. 20 micron). I'm using the camera binned 4x4, and this is working well with a pixel scale of about 1.2 arcsec/pixel (binned).
  • The QHY filter wheel has been my biggest headache. Its firmware is kind of crude; the interface between the camera and filter wheel isn't always stable. I ended up controlling the filter wheel directly by a separate USB connection to the computer. Precise filter wheel positioning is sloppy. This is particularly apparent comparing final filter position after a clockwise motion against final position to the same slot after a clockwise motion against final position to the same slot after a counterclockwise motion. Thus, I now move the filter wheel in a two-step process so that it always approaches each slot from the same prior position. This has resulted in much more consistent positioning of the filters.
  • I'm using 1.25" Astrodon filters. This, obviously, doesn't cover the full frame of the camera. I don't mind at all. I crop 600 columns off each side of every science image. (That, combined with the 4x4 binning, means that my final working images are 1270x1052.) I'm surprised at how quickly I've come to value the greater FOV compared with what I had in the ST-9.
  • Initially I used a plastic disk in one of the filter slots as a poor-man's shutter for making darks. It didn't work well, with way too much light leaking through the plastic of the disk. I attached a brass disk to the plastic disk, and now don't get any light leaks through the filter, but on long darks I can clearly see the effects of light leaking around the filter wheel (carousel) itself. I'm now very careful where the scope is pointed when I make darks.
  • I'm doing most of my photometry at the camera's "low-noise/high-gain" operating point: a gain setting of 56 with QHY's readout mode 1. Dynamic range is near-optimal at this setting. My typical exposure times range from 10 seconds to 60 seconds. Dynamic range is significantly better than what I saw with the ST-9, despite the ST-9's deeper full-well specification.
  • File size is potentially much bigger. I set a goal of being able to archive an entire night's photometry session onto one single-layer DVD, and have achieved that through a combination of binning, cropping the sides of the sensor that I can't use, and compression of my FITS files. The biggest challenge in all that has been making sure that any saturated pixels don't get "lost" during the binning process. I now test every pixel for saturation/non-linear-threshold prior to binning, and "promote" saturated/non-linear pixels through the binning/compression process so that I don't accidentally try to perform photometry on a star that contains saturated pixels.

- Mark

Affiliation
Vereniging Voor Sterrenkunde, Werkgroep Veranderlijke Sterren (Belgium) (VVS)
binning

Hi,

I am using a QHY 600 early bird CMOS camera. How you are able to get 4x4 binning? My QHY600 only gets 2x2 via the ASCOM driver. I have programmed the calibration in MAXIM to do a 2x2 averaging hence after the calibration in MAXIM I get a 4x4 binned file, which gives a decent result concerning photometry.

Josch

Affiliation
American Association of Variable Star Observers (AAVSO)
Re: binning

Hi, Josch:

I get 4x4 binning by using the QHY SDK software to get the raw pixels off the camera, then I do three interrelated things: binning, format conversion (to 16-bit integer, 32-bit integer, or floating point), and saturated pixel management (including setting the value of the FITS keyword DATAMAX). From there, I use NASA's HEASARC "cfitsio" package to compress it and stuff it all into the FITS file format. (I don't use the ASCOM driver.)

- Mark

Affiliation
American Association of Variable Star Observers (AAVSO)
I'm delighted to see this…

I'm delighted to see this thread re-started as I am just about to bring online a QHY600 in Chile on a 0.6m Planewave CDK.

And now we have a discussion ...!- Mark is suggesting mode 1, gain 26 (not sure on offset - perhaps he can add for completeness?) and Arni is on Mode 1, gain 10 offset 20...

Both of these appear to give the same dynamic range on the QHY600 spec charts on their website - the maximum that the charts show (around 13.6)

the discussion to date seems to affirm that for photometry maximum dynamic range is the goal... given that there is the tradeoff between full well depth and read noise, for those of us not batting at the same level as Arni and Mark is there any clue as to why one might chose gain 0 over gain 26 or vice versa? Does sky background/subexposure length come into play or is that all factored in?

and a follow-up if i may.  the spec page that we have all been studying with the various charts shows mode 3 in some but not all of the charts. specifically it does not show in the dynamic range chart... but if i take the other numbers at face value - ie full well of 80k+ and read noise of 6, does this not imply a dynamic range of 14 and hence suggest that mode 3, gain 0 should be considered as an alternative path?

thanks for all the teaching..

gavin

 

Affiliation
American Association of Variable Star Observers (AAVSO)
errorsorry i misspoke/typo…

error

sorry i misspoke/typo - Mark is at gain 56 - where the step is in the charts ...

but that was effectively a typo that does not impact the discussion/questions..!

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Re: I'm delighted to see this...

Yes, I'd like to see more discussion; I'm not convinced that I've thought this through properly.

I used a four-step process to come up with my photometry process for the QHY268M:

  1. Characterize my equipment and skies
  2. Clearly define what I'm trying to measure using scenarios (an idea borrowed from the world of software development)
  3. Predict performance for various alternatives and choices
  4. Select the alternatives that make sense given what I'm trying to measure

In particular, I identified 4 scenarios that describe the most challenging photometry that I care about:

  1. Blue photometry of a faint, mag 17(B), LPV using a mag 12 comparison star. My sky is bright with light pollution (Bortle 4-5), and this challenge is to collect sufficient photons from the star to overcome high noise levels from skyglow and internal camera noise.
  2. Ic photometry of a bright, mag 5(Ic), LPV using a mag 12 comparison star. This challenge is about avoiding nonlinearity on the bright variable. Skyglow is a non-issue.
  3. Exoplanet time-series photometry of a mag 12 target (with a mag 12 comparison star). I arbitrarily selected an exposure time of 60 seconds (no stacking allowed).
  4. Photometry of a Landolt field, a collection of approximately mag 12 stars. Nothing is unusually bright or unusually dim; the objective is to maximize SNR.

I am comfortable with single exposures of up to 2 minutes. I find that stacking multiple exposures generally gives me better photometry than averaging multiple measurements from each individual exposure, so with the exception of scenario (c), stacking is permitted in my scenarios.

I define photometry performance in terms of SNR. So, for example, in scenario (d) I know that I will collect a total of 686,676 electrons within my photometry aperture for a 2-minute exposure (signal). At a gain of 56 (readout mode 1), my noise budget looks like (each number is a total across the entire photometry aperture):

  • Target shot noise: 829 e-
  • Read noise: 78 e-
  • Dark noise: 52 e-
  • Skyglow noise: 258 e-
  • Total noise: 873 e-

And that gives an SNR of 686,676 / 873 = 787. Further, selection of readout mode and camera gain has almost no affect on the SNR, which is dominated by target shot noise.

Scenario (c) gives almost the same result (SNR = 547), pretty much independent of readout mode and camera gain.

Scenario (b) becomes more interesting. If you stick with readout mode 1, gain 56, then you have to shorten the exposure time to 4 second to avoid non-linearity within the LPV's aperture. But when you do that, the comparison star SNR drops to about 53, driven by read noise. The noise budget (total within the photometry aperture) for the comparison star looks like:

  • Target shot noise: 78 e-
  • Read noise: 78 e-
  • Dark noise: 9 e-
  • Skyglow noise: 29 e-
  • Total noise: 114 e-

If you choose a different mode/gain (for example, mode 1, gain 0), things change. You can now go to 8 second exposure time and stay linear. SNR rises to about 59 on the comparison star, even though read noise has doubled.

And, finally, scenario (a) requires 120 second exposure times with a readout mode of 1 and gain of 56. This will give an SNR of 25 on the variable and 700+ on the comparison star. The noise budget is dominated by skyglow. Because of that, the readout mode/gain really doesn't matter. For example, mode 1, gain 0 gives an SNR of 20 on the variable and 650 on the comparison star, not tremendously different.

My conclusion:

  1. SNR is all about counting electrons, not ADU. For this reason, the effects of altering gain tend to be secondary, and relate to gain's influence on readnoise and where nonlinearity starts.
  2. Photometric accuracy (SNR) is relatively insensitive to selection of readout mode and gain in environments that are dominated by skyglow, as long as you avoid the modes with relatively high readnoise (e.g., mode 0 gain 0 for this camera).
  3. It is important to understand where nonlinearity sets in for whatever mode/gain combinations you use, and adjust exposure times to avoid nonlinearity.
  4. Only one of my scenarios is particularly stressful to the camera's dynamic range (the LPV bright in Ic).

- Mark M

(And, by the way, choosing an offset value is an entirely different thought process that I'll save for another post.)
 


 


 

Affiliation
American Association of Variable Star Observers (AAVSO)
Re: Choosing an offset value

I've been using an offset of 5 with my QHY268M camera. Overall photometric performance does not seem particularly sensitive to the selected offset, as long as you stay away from extremes.

Understanding the purpose of the offset is best done by starting with a mental image of the histogram of pixel values in your images. Imagine a number line running from zero on the left to 65,535 on the right (the largest 16-bit value that the camera's A/D converter will generate). Photometry images are mostly background sky; the stars themselves cover a relatively small number of pixels. So, the histogram will have a distinct peak around the sky background's median value. Around that peak you'll see a gaussian drop-off on both sides. The camera will only generate non-negative values from the A/D converter, so some of the gaussian drop-off on the low side of the peak will be truncated (clipped) at a value of 0.

Of course, if you're making bias or dark images, the peak will be at a lower ADU value than the peak for sky background, because neither bias nor dark will include skyglow (including light pollution) counts. The histogram's peak will be lowest for bias exposures.

Changing the value of the camera's offset will shift the entire histogram left or right. Smaller values of offset will shift the histogram to the left; larger values will shift to the right.

Many image processing algorithms make an implicit assumption that the statistics of the image's background regions are statistically "well-behaved," with a roughly symmetrical histogram shape on either side of the peak. If the offset value is set too small, a significant part of the histogram's shape to the left of the peak can be shifted below the zero ADU level, chopping off some part of the histogram.

On the other hand, if the offset value is set unnecessarily high, the histogram will be nicely preserved, but some pixel values at the high end of the histogram (i.e., in the core of the brightest stars in the image) will be shifted toward the magic number of 65,535. If the shift is sufficient to push a valid star pixel value beyond 65,535, then that pixel has effectively saturated, and you've lost linearity on that bright star. For that reason, you don't want to crank up the offset too much. (Put another way, raising the offset will slightly reduce the dynamic range.)

The offset that I chose, 5, puts the peak of my bias exposure histogram into the range of about 80-90 ADU when I run at mode 1, gain 56. The standard deviation of the background distribution is around 6.0, giving at least 10 standard deviations on each side of the peak before zero-clipping happens. This is way more than enough to preserve clean statistical behavior, even though I often see something on the order of 17,000 pixels that "want to go negative" but have been clamped to zero.

To add another wrinkle, the offset is temperature-sensitive. The offset is actually determined by the value of the reference voltage fed to the A/D converter. The precision voltage source that is used for the A/D reference is located on the CMOS chip itself (this isn't true for CCD devices), so the temperature of the reference voltage circuit changes with the temperature of the sensor. I see a dependence of about 1 ADU per degree C variation in sensor temperature (at this gain setting). This temperature sensitivity is particularly relevant when making a long series of bias exposures. The circuits on the sensor with the highest thermal dissipation are the circuits used for the readout cycle; QHY keeps these circuits idle except when a readout is needed. When making a series of really short bias exposures, the circuits have a high ON time duty cycle, and I frequently see chip temperature fluctuations during bias sequences. Sadly, this means that the act of measuring the location of the zero-exposure histogram peak will itself shift the peak's location due to transient heating of the sensor.

In summary:

  • The offset value of 5 preserves (to at least 10-sigma) the bias histogram's shape (at readout mode 1, gain 56) for my camera. (Your mileage may vary.)
  • Choose a single offset value and use it for all of your exposures. In particular, bias, dark, flat, and science images should all use the same offset value.
  • Poor temperature management can easily lead to bias or dark frames that are brighter than the background of your science images. Get to know your camera and the amount of reserve cooler power you need to handle a series of short-exposure images that raise the CMOS sensor's thermal load.
  • Don't spend a lot of time agonizing over the precise offset setting. Its effect on dynamic range is quite small, and it's pretty easy to keep your pixels away from zero-clipping.

- Mark M

Affiliation
American Association of Variable Star Observers (AAVSO)
re the question one the…

re the question one the downloads i am seeing a bin 1 read/download cycle of 17 seconds for the qhy600 to voyager.

best

gavin

my qhy600 ph monochrome camera-looking for settings suggestions

I am using it with an RCOS 16" at a dark sky site. My seeing averages about 2.5 arc per sbig seeing monitor and I get about the same fwhm stars per maximdl. I am using it for photometry and exoplanet work, not pretty pictures.

I am currently binning 3x3,readout mode1-high gain, offset 10 and gain 0.

I love the camera especially when it comes to sky flats and low noise without artifacts. The camera is quite sensitive and I  can saturate quite quickly  even using my current settings.

I guess I could bin 2x2 but I would be at .46 arc-sec pixel which I understand would be oversampled for exoplanet work. I would like to maximize my dynamic range. 

I understand that if I'm binning that I am losing my full well depth with this camera. I am using maximdl 3.23 with ascom and the 10.12.21 sdk software. Does anyone know if there is any reason for me to update to the 2.02.17 sdk software?

Any suggestions for my setup would be greatly appreciated. 

thanks,

jeff

Affiliation
American Association of Variable Star Observers (AAVSO)
QHY-600 settings and binning

Jeff,

I have a QHY-600M on a 20" f/8 RC and use the readout mode, gain, and offset numbers that Arne suggested as you do.  I bin 2x2 rather than 3x3 though, even with typical seeing of 2.5 to 3.5" FWHM.   I tested binning vs. S/N with the system last summer and found that there was less than 5% S/N improvement binning 3x3 vs. 2x2 for S/N values above 100.  At S/N below 100, I found that S/N actually dropped with 3x3 binning.  I do a lot of cataclysmic variable photometry where you trade higher cadence for lower S/N (within limits).  That made the choice of staying with 2x2 pretty straightforward.  For your exoplanet work, the greater well depth of 2x2 might be worth more than the small S/N improvement of 3x3 binning.

Clearest skies,

Walt (cwt)

Affiliation
American Association of Variable Star Observers (AAVSO)
Hi all

 

just wondering if…

Hi all

 

just wondering if there are any updated thoughts on the best qhy600 setting for photometry?

i have been using a different camera for a while and now getting back to the 600 - and to start off with have taken mode 1, gain 0 offset 10 as starting point but am sure lots of folk have more experience with the camera over last 2 years (!) since we were debating here...

 

best

gavin

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Any updates to the QHY600 evaluation

I am using a QHY600M. I am new and have uploaded some M67 V & B data to VPHOT using the QHY600M.

 

Steve Hoffman - HSTG