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The Ultimate Deep Sky Camera?: The QHY600M Reviewed

After getting good results with the QHY268M APS-C that I reviewed for AGT last year, I decided to try QHYCCD’s full frame offering, the QHY600M.

Below are the results of testing various modes, settings, and targets of the QHY600M over the last six months.

The QHY600M and QHY CFW3-L filter wheel. Credit: Rouzbeh Bidshahri

Models

There are several models of the QHY600 to choose from with a wide range of specifications and prices.

The QHY600PH-C is the only color option (also available in a short back focus configuration) but there are several monochrome options:

The Lite (QHY600PH-M L) uses a lower grade “consumer” version of the sensor, sports 1GB of DDR3 buffer memory, and costs a bit less than other models. The body is a bit shorter than most, too.

The Photographic (QHY600PH-M) uses the more robust “industrial” version of the sensor that is rated for more exposures. It also has more DDR3 buffer at 2GB. This one is the model most commonly used for general purpose astronomy.

The Short Back Focus (QHY600PH-M SBFL) has the same specifications as the Photographic but requires less back focus from the mounting flange to the sensor (12.5mm compared to 17.5mm).

The Pro Version (QHY600PRO) is aimed more at scientific or industrial applications. The price is significantly higher, and it requires use of a $1000 fiber optic data PC card. Download speeds are much higher, and the length of the fiber optics are not limited to the short lengths dictated by USB 3.0. It can also be water cooled.

Physical Attributes

The QHY600M is lighter and smaller than I had expected. My short back focus version is only 136mm long (compared to 141.8mm for the standard version) and 90mm in diameter. It’s surprisingly light as well, around 800 grams (1.7 lbs.). This is good for most users as the lighter load often translates to less flex and tilt of the focuser’s drawtube.

The compact short back focus version of the QHY600M. Credit: Rouzbeh Bidshahri

The build is solid, with nice fit and finish. Power connects via a screw-on 12v cable to avoid accidental disconnection. A proprietary 4-pin plug connects directly to QHY filter wheels. There are no auxiliary USB ports.

After thousands of exposures over the past six months, I have yet to see a single failed download. The drivers installed smoothly, and the camera works flawlessly with my imaging software, NINA. USB3.0 downloads take seconds.

The camera can bolt directly to QHYCCD filter wheels, or you can use the included M54 threaded connector to connect to other equipment.

Connections at the rear of the QHY600. Credit: Rouzbeh Bidshahri

Sensor

The heart of the camera is the Sony IMX455 sensor. This full frame sensor is a massive 36mm x 24mm with a diagonal of just over 43mm.

The imaging area is approximately 9600 x 6400 pixels, providing 61 megapixels! The images rendered are truly breathtaking in sheer size. It’s worth noting that each raw FITS file is 120MB, which forced me to upgrade my memory and computing power.

 

The large full frame sensor comparable to a small eyepiece. Credit: Rouzbeh Bidshahri

Cooling and Thermal Properties

As with all CMOS and CCD cameras, cooling the sensor reduces thermal noise (dark current).

These new generation IMX sensors have extremely low thermal noise. At -10° C, thermal noise from the QHY600 is 0.0046 electrons/pixel/second (eps), and half that at -20° C.

By comparison, the FLI KL4040 camera with the new Gsesnse 4040 sensor has 0.08 eps of thermal noise at the same temperature. While that seems small, for a typical 600-sec sub that adds up to 48 electrons per pixel. The same 600-sec sub with the QHY600 produces only 2.76 electrons of thermal noise per pixel.

I was amazed at how easily the QHY600 cooled the sensor to -10° C on a balmy summer night with ambient temperature of 20° C (68° F). Cooling power was stable at 60%, using just 1.65 amps. The cooler could easily cool the sensor further, but it’s not necessary.

QHYCCD advises using the standard version with more spacing between the sensor and window to reduce the chances of frost if you don’t need very short back focus.

However, I use my short back focus version at -10° C in my coastal location with over 90% humidity with no issues. The external window is kept frost-free by a built-in window heater.

What I REALLY like about QHYCCD cameras is the humidity sensor inside the chamber that lets me know when values creep up. Recharging the chamber desiccant is very simple using the supplied externally attached, refillable desiccant plug.

But I’ve had no need to dry the chamber for the past six months. Many other cameras require a trip back to the manufacturer or disassembling the camera’s clean chamber yourself to refill the desiccant.

Pixel Size

The QHY600’s pixels are quite small at 3.76 microns, especially compared to CCDs. This makes it a very good choice for refractors and short focal length instruments. For example, even a small 100mm (4inch) refractor can achieve relatively high resolution with an image scale of 1.2 arcseconds/pixel.

Without binning, the QHY600’s pixels would be too small for my 14-inch CDK telescope with a 2500mm focal length, or 1700mm with the reducer, yielding 0.31 or 0.46 arcsec/pixel, respectively. But since binning the pixels is so easy I can synthetically create larger pixels. I found 2×2 binning (Bin2) worked well with the native focal length, giving me a high-resolution image for galaxies at about 0.6 arcseconds/pixel. With the reducer, Bin2 yields a scale of 0.9 arcseconds/pixel, which is excellent for nebulae.

The standard QHY driver adds pixels for binning in the camera, which isn’t the best method. I prefer to bin or resample after image capture using processing software like Pixinsight. This often yields better results.

Image scale at different focal lengths with Bin1 and Bin2. Credit: Rouzbeh Bidshahri

Performance

Now for what really matters, the images.

Modes: QHYCCD cameras allow users to alter characteristics of the readout. High gain mode lowers the read noise for low-signal narrowband imaging. The 2CMS mode offers very high full well capacity while maintaining good read noise levels (more on these later).

Amp Glow: The camera has ZERO amp glow. I was unable to detect even the slightest trace of amp glow in a 600-second dark frame. This means the sensor is very well behaved, with no need for dark flats. Bias files are enough.

A raw 600 second dark frame showing a very uniform frame with no amp glow. Credit: Rouzbeh Bidshahri

Bit Depth: The camera’s native bit depth is 16 bits, which means you get 65,536 shades of grey. This is helpful for very faint areas of nebulosity, making brightness transitions smoother.

Quantum efficiency (QE): While QHYCCD reports the measured peak efficiency of the sensor at more than 90%, some independent tests measure it in the mid-80s. Regardless, the QE is extremely high compared to the typical 50% to 60% of many cameras of the past. This is partly due the sensor’s back-illuminated technology (BSI), which allows better photon capture. The greater QE effectively makes your system faster, with much less wasted light. I get very clean images with relatively short integration times.

The high quantum efficiency shows in this image of M82 captured from Bortle 6 skies with 6 hours of LRGB and 6 hours of H-alpha data. Credit: Rouzbeh Bidshahri

Very High Full Well Capacity (FWC): FWC is a measure of how many electrons each pixel can hold without saturating. The FWC of the QHY600’s pixels is unusually high, reducing star bloat from over-saturation.

The QHY600’s very high full well capacity and dynamic range keep very bright areas in this image of M81 from oversaturating, while still showing very faint wisps of gas and dust. Credit: Rouzbeh Bidshahri

At the highest FWC setting (2CMS mode with gain=0), each of the 14 square-micron pixels can hold 83,600 electrons. By comparison, the far larger 81 square-micron Gsesnse 4040 pixels hold only 70,000 electrons.

To put that in context, the QHY600 can hold SEVEN times more charge per unit area than a camera costing four times as much!

The QHY600M sensor in 2CMS mode tested with Sharcap. Note a FWC of 83,611 electrons with a low red noise of 5.7 electrons. Credit: Rouzbeh Bidshahri

Low Read Noise: The read noise of the QHY600 can be set to fantastically low values when needed. In high gain mode (gain=56), read noise is only 1.58 electrons per pixel, one of the lowest currently available to amateurs. Even at these high gains, it maintains a respectable FWC of 21,000 electrons.

For faint narrowband imaging, I find 600 second sub exposures are enough, while my old CCD camera required 1200 seconds.

A test with Pixinsight showing the very low read noise of 1.58 electrons in high gain mode. Credit: Rouzbeh Bidshahri

The low read noise provides a higher signal-to-noise ratio (SNR), allowing you to capture much more faint narrowband detail. Below is an example of a narrowband image with about 14 hours of integration time. The level of faint detail and the clean data are very satisfying to work with.

Narrowband image captured with the QHY600M showing very faint detail with high SNR. Credit: Rouzbeh Bidshahri

Final Thoughts

After several months with the QHY600M, I can’t find a fault, or even something extra I wish it had. The hardware and software are reliable, and the price is very reasonable.

The sensor is very large, but the camera body is still compact and light. The read noise is very low, while maintaining a very high full well capacity. The dark current is very low, and the cooling system is very efficient.

The combination of these factors, coupled with very high quantum efficiency, allow the user to sample large swathes of sky with high resolution and high signal-to-noise ratio.

I often find something to nitpick to justify upgrading to something bigger or better, but in this case I really can’t find any excuse to want another camera for the foreseeable future!

 

MSRP: $4600

Website: www.qhyccd.com

 

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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