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My experience with QHY and ZWO might not be directly relevant to Roland's question, since my priority is occultations.
I have two QHY-174M-GPS cams that I use for occultations and they have been VERY effective in that application with SharpCap Pro. I don't think there is a better occultation solution out there at any price. The built-in GPS modules make occultation captures precise and painless. No more analog video streams and all of the problems and quirks that go along with them. I sure hope that QHY never abandons occultationists! ZWO, and everyone else, has nothing comparable.
Tolga from Tolga Astro helped me to learn that there is a hidden button on the QHY filter wheels that switches the control from the USB interface to the cam interface and back. Only one control method is active at a time. THAT explains why I was never able to control my filter wheel directly from my QHY-174M-GPS cams. And the QHY filter wheel is effectively too thin. Most filters are too high and will not load and clear the filter housing. I forget which filters I had to invest in to get the filter wheel working but Tolga knows all about the QHY cams, filter wheels and filters and can help you out. Tolga is a really nice guy and very knowledgeable. He would probably help you out, even if you were not his customer. However that would be tacky to do to him. If you want his help, I would encourage becoming his customer first. Even if you buy something besides a QHY or ZWO camera or accessory.
I have found that drivers and utilities for both ZWO and QHY leave a lot to be desired. Cruising the QHY website and looking for correct drivers and tools can be an adventure in itself, depending on the cam model. Once you get the right drivers installed, QHY and ZWO cams seem to work well and are stable.
Firmware updates on the QHY-174M-GPS is NOT for the faint-hearted and unless you are REALLY good with electronics and microcontrollers, I would leave it to someone like Tolga to help you out. It requires opening up the cam and using a special programming module connected to an internal connector.
I would say the mechanical build quality and mechanical design of my QHY and ZWO cams are both better than my SBIG STF-8300M. The round cam bodies are much more compatible with my Hyperstar adapters on my occultation SCT's too. My STF-8300M is infamous for getting damaged via its power connection and power issues. I have not had any power problems with my QHY and ZWO cams. One note, the QHY174M-GPS MIGHT work okay with just being powered over the USB3 connector, but in my experience, the cooler MUST have clean 12VDC power to work properly and I think the same goes for the GPS module. It will sort of work w/o 12VDC but not reliably. My suggestion is to ALWAYS have 12VDC securely connected to a QHY174M-GPS cam.
"My advice is always free and worth every penny!"
On Fri, Jul 2, 2021 at 1:43 AM Luca Marinelli <photo@...
I image with both ZWO and QHY cameras, as well as system accessories (filter wheels, OAG). As others have said they are more similar than different but here are my impressions:
Equipment I have owned or currently own: ASI1600MM Pro, ASI183MM Pro, ASI6200MM Pro, ASI294MM Pro, QHY 294M, QHY 268M, ZWO EFW2 and EFW3 filter wheels, ZWO OAG (old and new), QHY CFW3 medium ultraslim filter wheel, QHY OAG-M. (I have also used a FLI ML16200 with CFW2-7 filter wheel and Optec Sagitta OAG, for comparison to premium level products).
People often say that QHY cameras are built better. My experience is that the build is fairly comparable, neither is an FLI camera. I have never had hardware failures with any of my ZWO cameras. I image in the Northeastern US and have not had issues with frost or dew on the sensor. My new QHY268M does not connect reliably to the filter wheel with the direct cable connection and I will have to send it in when I switch imaging system on the pier later in the Summer. The good news is that I will only have to ship the camera to California and not to China, provided it can be fixed there. As Joel said, ZWO is rumored to be opening a service center in the USA as well.
I purchased the QHY268M instead of the ASI2600MM Pro for two main reasons: sensor tilt and read-out modes. I did not have space for a tilt adjustment unit in my imaging train and the common lore is that QHY sensors are more likely to be square to the optical axis. I don't really know the statistics of how many units are shipped with tilted sensors by the two manufacturers but my N=1 experience was that the ZWO ASI6200MM Pro required a tilt adjustment unit on the FSQ106 to get good stars throughout the field of view (it should be said that my scope had to be sent in for collimation so I can't with 100% certainty assign the blame to the ZWO camera until the scope comes back and I see if it still needs the tilt adjustment or if it was a way to tune out some of the effects of optics misalignment over the large field of view). The QHY268M is on a 10in f4 newtonian telescope and the sensor gives perfect stars all the way to the edge of the field of view, without a tilt adjustment unit.
For the new SONY APS-C and full frame sensors, QHY offers several readout modes. The standard high-gain mode with very low-noise is common to both the ZWO and QHY cameras. This is the primary imaging mode for deep-space targets. The QHY camera also offers extended full-well modes that can be interesting for broadband imaging. The idea is to have higher read noise but also much deeper full well capacity so fewer deeper subs (a la CCD imaging) can be acquired, with comparable dynamic range to the low-noise readout mode. Given the large file size, this was appealing and I was curious to try it out. In practice, I never did for a simple reason: I can change gain dynamically in the imaging software but I cannot change readout mode outside of the QHY driver. I almost always image multiple targets in one night and pretty much never only broadband. I need the high-gain readout mode for narrowband imaging and in practice i find myself choosing two different gain setpoints for RGB subs and narrowband subs (higher full well or lower read noise), which can be controlled on a per filter basis from inside the imaging software.
The cooler appears to be slightly better on the ZWO camera. The QHY with the APS-C sensor maxes out at about 32-33C below ambient, while the ZWO with the full frame sensor is able to reach 35-36C below ambient. That difference allows me to have only two sets of dark libraries for the ZWO camera (-20C for Winter and -10C for Summer), while the QHY camera needs three sets of dark libraries (-20C for Winter, -10C Spring and Fall, -3C hot Summer nights). I haven't processed any of the -3C data yet but if thermal noise is not an issue, I may just eliminate the -10C set point. Dark noise in these cameras is so low that constant sensor temperature for proper calibration is more important than deep cooling.
Drivers on the ZWO camera are completely transparent and straightforward. No special tricks are necessary for a successful installation. QHY is a different story. I disagree with Dale that driver robustness with QHY cameras is a thing of the past. The QHY294M and 268M after more than six months after introduction are still running on beta drivers. The installation procedure is not well documented and sensitive to the local environment. I was not able to get the QHY268M to connect successfully to my desktop computer, I gave up and brought into the observatory where it connected to the imaging computer after a few tries of installing, uninstalling drivers, cleaning registries, and other incantations. As I mentioned above, the direct 4-pin connection between my QHY camera and the QHY filter wheel doesn't work, so I have resorted to using the USB connection on the filter wheel. The USB driver for the filter wheel several years post-introduction is still buggy and requires a precise set of steps to ensure correct operation of the filter wheel or the imaging software will give up and halt the sequence (the filter wheel doesn't respond, it's not the imaging software fault). I really with QHY did a complete overhaul of their software, test it broadly, and publish reliable installation notes. This to me is one of the biggest drawbacks of the QHY camera.
For the QHY600, which I do not own, I agree with Dale that the most appealing option is the Photographic version with the short backfocus option. This is the same front mount used on the QHY 268M and it offers a rock solid connection to the filter wheel with no opportunity to introduce tilt in the optical train. I never rotate the camera independently from the filter wheel, to minimize need to reshoot flat frames, so the bolted connection is welcome to me. The short backfocus is also more versatile and allows the unit to be used with camera lenses, for example.
The old ZWO OAG was very flimsy and not really comparable to the QHY. The new one is much more substantial. Neither the QHY nor the ZWO OAGs are comparable to the Optec Sagitta in smoothness, quality of manufacturing and materials, are size of the prism. However, they also consume much less optical length and are significantly less expensive. The QHY OAG has M54 opening while the ZWO OAG has bigger clear aperture (M68). The newly announced ZWO OAG will also be available with a bigger prism, which is a welcome change. Overall, both OAGs do what they are supposed to.