Deepsky with a planetary camera
Quite often I considered to add a telescope with a larger focal length to keep me busy during the, so called, galaxy season. On the other hand I never had a clue if such an invest does make any sense with my local seeing conditions as these small objects require a decent angular resolution in order to get some details. As I needed to buy another guiding camera for a second setup anyway, I went for a color camera with an IMX664 sensor utilizing a pixel size of only 2.9µm.
To directly exaggerate my experiment, I mounted this camera on my 580mm APO including a 2 times barlow lens and pointed this setup one last time before winter at the Orion Nebula (Messier 42):
This result was quite surprising, as I did not expect this level of detail, knowing that this setup is highly oversampled. The relative small aperture of 100mm can not provide the necessary angular resolution.
Physics calculate an optical resolution for this setup of about 1.25" (arcseconds, not inches), while each camerapixel resolves about 0.5". If you like to calculate the optical resolution for your own equipment, check out Dylan O'Donnell's Astro-Calculator: https://byronbayobservatory.com.au/astronomy-calculator/
As the ASI664MC is a one shot color camera, the optical resolution is slightly larger than one Bayer group (2x2 pixels), so we have to expect some softness. Instead using a barlow lens, we may use a camera with even smaller pixels to obtain basically the same results, maybe without additional issues introduced from the barlow optics.
So I also purchased an ASI715MC, the cheapest color camera available from ZWO. With a pixel size of only 1.45µm this camera has the same optical resolution per pixel as the previous combination. If you do not own a barlow, this camera may be the cheaper way to go.
This image of Messier 106 was created using the ASI715MC with the APO:
But even if these images look promising, do not get too excited. This approach does not trick physics and do come with some penalties. First of all we have to expect much longer exposure times. These tiny pixels may only collect a fraction of photons, compared to regular deep sky camers. Comparing the 1.45µm pixels of an IMX715 sensor with the usual 3.76µm of an IMX571 sensor, the small pixels only collect a seventh of the photons. In addition these tiny pixels do have a significantly smaller full well capacity, so care must be taken to not overexpose too much.
And as a consequence of this small full well capacity, these kind of cameras do not come with high resolution ADCs, 12 Bit ADCs are pretty common which results in a smaller dynamic range.
These cameras are also optimized to image bright objects like planets to capture as many subs as possible with considerable short exposure times to reduce seeing effects. But even the brightest objects from the Messier catalog require longer exposure times with such small pixels.
As these cameras are not equipped with active cooling, we also have to deal with lots of noise in each sub, which gets worse with rising sensor temperatures, which can easily be 10 degrees C above ambient temperature:
I performed a series of test exposures with the ASI715MC while it was placed in the freezer. Starting from an ambient temperature of about 20°C, the sensor already warmed up to 32°C and produced a significant amount of read noise (variable pattern per sub). In the center the sensor temperature reached 10°C and went down to 0°C towards the bottom right.
While the readout noise decreases significantly towards 10°C, we do observe another issue with long exposures. While the camera does not expose any amp-glow (which is good), it does have some sort of "pixel-glow", a fixed pattern of brighter pixels. These are no defect pixels (the bias is clean), but their intensity increases with exposure time and temperature. To minimize this effect the subs must be properly dithered. I additionally use darks with this camera to get rid of these pixels before integration.
To counteract this noise issue I am currently using a "Black Shark FunCooler 4 Pro" as an active cooling solution. Its intended use is to cool down your smartphone while gaming, but with a 3D printed adapter (Thanks Daniel Nimmervoll) it mounts perfectly on these round planetary cameras:
One big disadvantage is the lack of temperature control. Even at its lowest setting ice will build up at the camera over time and the sensor goes down to about freezing temperatures. Dew will be an issue with this setup. Probably because I am using the 1.25" adapter with a UV/IR cut filter the sensor glas may be isolated enough, that I did not encounter a blind camera yet. But it is probably a good idea to seal the electrical connectors of the camera to avoid dew there.
From the camera's manual it may operate at temperatures as low as -5°C, so cooling probably should be controlled to not get too low. I did not experience a fault yet, though.
In addition to various sensor related challenges, you obviously do need a mount capable to follow the stars with the same precision as the angular resolution of the pixels. And if that's still not enough, do not forget to book an excellent seeing for great results, which probably never happen at my location.
This image of starcluster Messier 3 the ASI715MC was mounted to my fast 130mm Hypergraph telescope. This combination is a nearly perfect fit, as the pixel resolution of 0.8 arcseconds can be achieved by the telescope with its 130mm aperture. As a consequence the stars are nicely resolved, even inside the core
This image of Messier 97 and 108 was taken with the same setup and looks pretty convincing to me: