Most deep sky imagers need to guide their mounts for long-exposure imaging. Large telescopes with long focal lengths and fine image scale are especially sensitive to guiding errors.
Using a small guidescope on reflectors is not usually advisable, especially on Schmidt-Cassegrain (SCT) telescopes with moveable primary mirrors. In my article on AGT on off-axis guiders (OAG), I described why they’re preferred to guidescopes on most reflectors.
With an OAG, the guide camera is working with the focal length of the reflector, typically 1000mm to 4000mm, and needs to be chosen appropriately.
Good choices for OAG guiding among IMX174 chip-based cameras are the ZWO ASI174MM Mini and the QHYCCD QHY5III-174. ZWO also uses the IMX174 sensor in its ASI174MM, which is larger and faster, but it’s not as well-suited for OAG guiding. More on that below.
At about $400 to $600, they cost a bit more than typical small guide cameras, but less than the Starlight Xpress Lodestar family that have been popular for decades.
The large sensor size of these cameras is a big benefit for OAG guiding. The Sony IMX174 CMOS sensor they use is a generous 11.3mm x 7.1mm chip. By comparison, the low cost ASI120 has only a 4.8mm x 3.6mm sensor, barely one-fifth of the larger sensor’s area. The IMX174 sensor is also substantially larger than the very popular Lodestar’s sensor.
As focal length increases, the field of view becomes proportionately smaller. That can make finding guide stars a frustrating task with a tiny guide camera, requiring moving the telescope to search for them.
With a larger guide camera, though, it’s important to ensure that the pick-off prism in your OAG is large enough to take advantage of the larger detector. Many OAG manufacturers are now offering larger prisms to accommodate these sensors.
Choosing the appropriate pixel size is important, especially with longer focal lengths. Large pixels are preferred for longer focal lengths as they will better match the larger image scale, avoiding oversampling that leads to a low signal to noise ratio. You can always bin (combine pixels to effectively make larger digital pixels) for other purposes if need be.
The IMX174-based cameras have large 5.86-micron pixels. While not the largest available – some Starlight Xpress Lodestar’s have 8.3-micron pixels – the IMX174’s pixels are much larger than the 2.9-micron 3.75-micron pixels typical of most CMOS sensors.
Increasing the focal length not only makes the field of view smaller, but it also renders a smaller image scale (higher resolution). This means the light of any given star is spread out over more pixels, thus lowering your signal to noise ratio. That also means you’ll have less of a chance of finding a suitable bright guide star since the faintest stars are no longer recorded.
Staying below 3.0 arcseconds/pixel is recommended, but you don’t need too fine an image scale, either. The best image scale depends on several factors we won’t go into here. Popular guiding software like PHD2 works well with a large range of image scales.
My current setup guides very well at 0.95 arcseconds/pixel. Below is a table showing the scale of the IMX174 over a range of focal lengths. The user can image without binning (Bin1) with lower focal lengths and bin 2×2 (Bin2) with longer ones.
Low Read Noise and High Quantum Efficiency
Another advantage of the IMX174-based cameras is the sensor’s low read noise, typically between 3.0 and 4.5 electrons/pixel. With these low-noise imagers, dark frames aren’t necessary.
I’ve never had any issues with the guider picking up a hot pixel instead of a star, either. Below is a typical frame from a IMX174-based camera vs. the CCD-based Lodestar. Since the IMX174 sensor is CMOS, there are no column defects as with CCDs.
The peak quantum efficiency of the sensor is a very healthy 77%, which also helps in detecting dimmer stars.
USB Speed and Frames per Second
The ASI174MM Mini uses USB 2.0 for data transfer, while the larger ASI174MM and the QHY5III-174 use USB 3.0. This determines the number of frames that can be transferred per second.
The ASI174MM Mini can deliver up to 18 frames per second over USB 2.0, which is more than you’ll need for guiding. You’ll rarely need more than 1 frame per second, and usually less.
The QHY5III-174 and ASI174MM are capable of an amazing 138 and 164 frames per second, respectively, at full resolution. This is useful for solar or planetary imaging, but you pay a premium for the USB 3.0 interface in these cameras that you won’t need if you only use it for guiding.
Most of these cameras offer the ST4 guide port option, but I suggest connecting them via USB only. ST4 allows the guide software to control the mount, but not the camera.
Both the ASI174MM Mini and QHY5III-174 are the 1 ¼ inch “lipstick” type. The ZWO ASI174MM is larger and installs using M42 threads but isn’t always suitable for off-axis guiding. In my setup, the larger M42 camera would physically bump against the filter wheel.
The lipstick cameras can also be inserted deeper into off-axis guiders, allowing the sensor to get closer to the pick-off prism, when necessary. There also may be times when there isn’t sufficient space to squeeze a larger M42-size camera into your imaging train. Those using guide scopes sometimes prefer the ASI174MM for other reasons, though.
I must have used about a half-dozen guide cameras but have found the Sony IMX174 based cameras to perform the best so far.
They have very good characteristics that tick almost all the boxes for a guide camera and have a good pixel size that works with a very large range of focal lengths.
The generous size of the sensor coupled with high quantum efficiency and low noise means the user is almost always able to find a number of suitable guide stars.
For guiding, I think you won’t be disappointed with the performance of either the ZWO ASI174MM Mini or the QHYCCD QHY5III-174 for anything from an 8-inch SCT to a 20-inch CDK.
MSRP: ZWO ASI174MM Mini: $399 ; QHYCCD QHY5III-174: $629