Re: [ap-ug] Which Camera?

Dale Ghent

I have the QHY600M (the Pro version, but it's not likely you'll want that), and a bunch of my colleagues have the ZWO ASI6200MM. Both use the IMX455 sensor. I have other ZWO cameras, and have direct experience programming for the software stacks of both companies. We also all sit in a chat together so we always talk about our current camera war stories.

On the whole, QHY and ZWO are equivalent in a broad sense. I prefer some design aspects to the QHY cameras over ZWO's habits, but then I also feel the same about certain ZWO design aspects. QHY have cleaned up their driver situation over the past two years so gripes about that are a bit dated, I feel. This camera functions fine.

The QHY600M comes in 3 main flavors. It's likely you would be getting either the Lite (QHY600L-M) or the Photographic (QHY600PH-M) versions. The obvious difference between the L and PH is price. The L uses the "commercial grade" variant of the IMX455, and the PH model uses the "industrial grade" variant of the same. According to QHY, the differences are chip lifetime, with the industrial variant having some higher MTBF. Accordingly, the QHY600M-L is cheaper.

The PH itself is available in 2 sub-model: the normal one with a 17.5mm backfocus and a "short backfocus" (SBFL) variant that consumes 12.5mm of backfocus. The attachment of the two variants are different. The normal backfocus version attaches to the rear of the QHY filter wheel via circular dovetail that is secured by 3 screws placed around the circumference of the saddle. This saddle is bolted to the rear of the QHY filter wheel. This saddle consumes 6mm of backfocus.

The SBFL variant (12.5mm) of the camera has a different mechanical interface. In addition to the sensor being 5mm closer to the front of the camera, the camera bolts directly onto the rear of the QHY filter wheel. This eliminates the saddle and saves another 6mm of backfocus, bringing the total backfocus savings to 11mm. This SBFL variant is not available on the L model.

In most cases, I would suggest going with the QHY600PH SBFL because of the non-trivial amount of back focal length that's conserved, leaving room for whatever paraphernalia you want between the flattener and filter wheel. It's also better for faster optics because it places the sensor closer to the filters. The "normal" version is useful because that circular dovetail makes rotating the camera quick and simple. If you have a rotator or prefer to rotate the imaging rig via the focuser, then it's obviously not any use in that case.

Outside the mechanical aspects, the QHY600 has 4 readout modes. QHY uses the readout mode facility in a different way than it has been traditionally utilized for on CCDs. These CMOS sensors can be driven in a myriad of different ways which can yield various combinations of pixel well depth, read noise, and dynamic range. QHY details the 4 modes they implement here:

The modes are intended for specific applications, with them being divided more or less along the lines of well depth and noise. The QHY site explains them with graphs. I personally shoot in mode 1 ("High Gain mode") and just inside the High Conversion Gain domain of the sensor.

As for the ZWO analog, it consumes 17.5mm, and I think that can be reduced slightly by removing its front push-pull tilt plate. It does have a 2-port USB2 hub built into the rear, but its intent is mainly for running the filter wheel and an OAG camera. It does not have multiple readout modes, and it drives the sensor in a way that is equivalent most to QHY's mode 1.

There are a bunch of other differences between the two cameras. QHY puts humidity and air pressure sensors in the sensor chamber so you can know when it's time to attach the desiccant tube. Stuff like that.

In the end, you won't go wrong with either choice; the decision comes down to technical specifics, such as needing to conserve backfocus distance or the desire for the different operating modes. If you want, I can throw you some raw frames from my 600.

On Jul 1, 2021, at 16:09, Roland Christen via <> wrote:

I've been looking long and hard at full frame mono CMOS cameras as a next purchase for our AP observatory. So the question is - which camera - QHY or ZWO?
Which has the better accessories such as filter wheel, off-axis guider etc...
Any and all thoughts welcome.


-----Original Message-----
From: Dale Ghent <>
Sent: Thu, Jul 1, 2021 12:44 pm
Subject: Re: [ap-ug] Questoin for Roland: Waited too long to buy 175TCC (discontinued) for my QUADTCC is as good of a solution.

On Jul 1, 2021, at 01:54, ROBERT WYNNE <> wrote:

Based on what I've read on this and other venues I remain unconvinced in CMOS chip technology's ability to capture an equal amount of photons compared to an CCD, in focus and without tilt at the pixel well level with 0 or near 0 noise in reasonable time frames. I would not underestimate my desire to obtain the largest Sony CCD available if the notion caught my interest nor a CMOS chip if the advancement in CMOS quality presents itself in the next year which could easily exceed a 67 mm circle. Thus my push.
With all due respect, but none of this makes any sense.

Current CMOS sensors have QE in the high 80's, pushing 90%, and approach 1e- read noise in typical operating modes. 5, and even 3 minute long narrowband exposures at circa f/5 are normal... no more 15-20 minute exposures that can be ruined by a passing cloud or other interference. On top of that, you don't need to endure the comparatively glacial readout speeds that CCDs have, which eats into total integration time when you sum up the 15-20 seconds it takes to pull each frame off a CCD camera.

You sentiments would have made more sense perhaps up to 2 years ago. But the current generation of CMOS sensors, namely the IMX533 (1", color), IMX571 (APS-C color+mono), IMX455 (FF, color+mono), and IMX411 (150mp medium format, mono), have performance characteristics that make choosing them over CCD almost a no-brainer. They also have such low dark current that chilling them below -10C is very firmly in the territory of diminishing returns, making these sensors more warm-climate friendly top operate.

I will say that there seems to be a lot of sentimental or emotional attachment to CCDs; perhaps more so now that stocks of them are running on fumes. There are *plenty* of compelling reasons to adopt modern tech, however.

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