When I saw the QHY miniCAM8 camera with the integrated filter wheel for the first time, I was really impressed. My only concern was the imaging sensor used, which is IMX585, because I did not know much about this. But apparently, this very sensor seems to be even better than the well-known family of IMX533/571/455 sensors. If you compare the sensitivity (quantum efficiency) of both sensors, you can easily notice how the “maximum QE” term may be misleading. Both sensors’ maximum QE is similar, around 90%. But IMX571 reaches this maximum in the narrow range around 450-500nm. And IMX585, which is present in miniCAM8, has almost flat 90% QE from 400nm to 550-600nm range. That is a noticeable difference in capturing the data.
Sensitivity
When we compare it for a specific filter, we will have the following differences:
- For the B filter in the range 400-500nm, IMX585 is about 7% more sensitive
- For the G filter in the range 500-600nm, IMX585 is about 10% more sensitive
- For the R filter in the range 500-600nm, IMX585 is about 39% more sensitive
- For the L filter in the range 400-700nm, IMX585 is about 16% more sensitive
These synthetic calculations above assume a uniform distribution of the light across the visible part of the spectrum. In a real-life scenario, the more data we collect in the red part (where the hydrogen alpha line shines), the larger the difference will be.
For narrowband filters, we will have:
- For the OIII filter at 500nm, IMX585 is about 1% more sensitive
- For the Ha filter at 656nm, IMX585 is about 41% more sensitive
- For the SII filter at 676nm, IMX585 is about 50% more sensitive
So it is quite obvious that the imaging sensor in miniCAM8 is a very sensitive device. It is also not as small as I thought before. The total area is still smaller than IMX533 for example, but it is the same size league. It is not a large format sensor, but also not a guiding chip size sensor 🙂
Filters
The compact size of IMX585 is an advantage of the miniCAM8 device. The whole imaging setup with cooled monochromatic camera and 8-position filter wheel (LRGBHOS filters plus dark frames metal plate) is packed into a compact 105mm diameter body with 44 mm thickness (plus radiator fan). Of course, no regular 1.25″ filters are applicable there. QHY, in cooperation with XiMei and Optolong, developed a new set of 19×11 mm filters that fit perfectly the IMX585 sensor. Filters available currently are:
- LRGB wideband filters for color imaging
- HOS narrowband filter for narrowband imaging
- LPF light pollution, HLP heavy light pollution, FCE 4 channel enhancement, and UV/IR cut filters for the color version of the camera
- UGRIZ Sloan photometric filters set
Filter installation is quite straightforward, but a bit of time needs to be booked for this process. Each filter has a tick mark on the edge – that tick needs to point towards the telescope tube.
Cooling
Due to the relatively small size of the 585 sensor, the cooling is very quick. QHY claims the miniCAM8 is able to reach a -45°C temperature difference. I did not check it yet, but at 15°C ambient temperature camera cools to -20°C within 2 minutes, and the cooling power is about 25-30% for this -35°C difference. So this -45°C seems to be achievable. However, I do not think that such extreme cooling of modern CMOS sensors gives any significant benefit. They work well already at -10°C, even for narrowband imaging. miniCAM8 is also a zero-amplifier glow camera, plus it has a heated front window of the sensor chamber, which prevents dew condensation. Sensor resolution is 8.3 Mpx (commercial 4K UHD standard) in a panoramic format 16:9 – you need to keep it in mind when framing your favourite astrophotography targets. Pixel size is 2.9um, giving a 12.8mm diagonal that should not be a problem for any field flattener or focal reducer.
IMX585 itself is a BSI (back-side illuminated) sensor of the Sony Starvis II family. Due to this fact, the dynamic range and pixel capacity are quite remarkable. Native ADC in this sensor is 12 bits, but QHY developed a process of merging high and low gain data to extend the data to 16 bits. This so-called Linearity HDR mode ensures smooth transitions and richer color representation. In this mode, the full well is 46 ke, while the read noise is 1e only, so the dynamic range is then 15.5 stops. I have made several images already in this mode, and no problems occurred – neither during capturing data nor during calibration and processing.
Installation
QHY miniCAM8 is quite a versatile camera. It can be used with a short FL telescope to capture a significant area of the sky. It can be attached to long focal SCT or Mak telescopes to capture planetary and lunar details. It can also be used with a medium-sized telescope to capture a specific part of the sky. I plan to use it mostly in my travel setup with an 80mm refractor at a focal length of 384mm. The effective field of view for this setup is quite useful. It is okay for a group of galaxies, even for distant galaxy clusters. It is also okay for a large number of medium-sized nebulae. Extended nebulae can be mosaiced. Small nebulae and single galaxies can be imaged in a context.
Camera installation was easy – I just replaced the current IMX533 color camera with the miniCAM8. The modest sensor diagonal makes this setup not very sensitive to the precise setting of the backfocus distance, so I did not adjust it to a fraction of a millimeter. Also, driver installation in my mini PC was an easy catch, and both the miniCAM8 camera and integrated filter wheel were immediately visible in N.I.N.A. software and connected successfully.
First light
First light tests were performed in my suburban backyard, where the sky brightness during the transparent nights is about 19.5 mag/arcsec2. The camera was attached to an 80mm triplet with 384mm effective focal length. I played a little with camera modes, but after a while, most of the data was captured in Linearity HDR mode. The first impressions are all good:
- Driver installation, connection, and capturing data with N.I.N.A. were all good without any problems or disconnections
- The camera cools quickly
- The filter wheel is probably one direction only
- The camera is as sensitive as the data sheet says
- Calibration and stacking were performed correctly, there are no issues with the offset/gain settings
The fan attached to the radiator is not very large and works all the time at 100% speed – that is a bit noisy. But that is so far the only small inconvenience I found. There are, of course, other attributes someone may not like, but they are by design, not by mistake. Why sensor so small? To keep the device size compact. Why not a regular filter size? There is no such small filter standard yet available. Why is the fan noisy? Because it is small 🙂
In my opinion, QHY miniCAM8 is overall a great device. It is versatile, seems to be well tested, does not cause issues during work, and has limitations, like everything, but these limitations are the result of the project assumptions. And you cannot complain the wheel is round, right? 🙂
