An un-cooled, high resolution experiment was carried out using a Stella Mira 66mm ED APO refractor with a field flattener. The scope was fitted with a ZWO EAF motor focuser and an Altair V2 magnetic 2” filter holder containing an Altair Quadband filter. A 12 bit ZWO ASI585MC OSC uncooled camera was attached. AstroDMx Capture for Linux x86_64 running on an Ubuntu Linux mini computer was used to capture 78 minutes worth of 3 minute exposures of the Eagle nebula (centred on the star HIP89743) as FITS images with matching darks, flats and dark-flats.
An SVBONY SV165 guide scope with a natively connected QHY-5II-M guide camera was used for PHD2 multistar pulse auto-guiding via an INDI server running on a Fedora Linux mini computer. The auto-guiding was controlled by a separate Linux laptop indoors. AstroDMx Capture sent the scope/mount to the star Arcturus which was used to focus the scope with a Bahtinov mask. The ZWO EAF was controlled by AstroDMx Capture via the INDI server. AstroDMx Capture then sent the scope to the star HIP89743 within the Eagle nebula to optimally place the nebula in the field of view.
The equipment used
The ZWO ASI585MC uses an IMX585 sensor which is BSI, and built on Sony's STARVIS 2 technology. It has a full well depth of 40k to 47k electrons and a Read noise of 0.7 to 5.5 electrons depending on gain. It has a high QE of 91% at 500nm and 80.9% at 656nm.
The ADC bit depth does not determine the physical dynamic range which is calculated as:
Dynamic range = Full well capacity / Read noise
A 12-bit ADC offers 12 stops 212 = 4,096 of digital container. If one shoots at the optimized HCG gain setting where dynamic range increases and read noise falls to extremely low values every digital step represents genuine, photon data.
The ADC bit depth determines how much charge CAN be stored, but as long as the full well capacity is not met by the incoming photons producing charge, a 16 bit ADC depth has no advantage over a 12 bit ADC.
What the 12 bit ADC does limit is the exposure time. The exposure must be limited so that the full well capacity is not reached or the centres of stars will be saturated and blown out.
Stacking Nullifies the 12-bit Limitation. While a single 12-bit sub-exposure might technically show minor step-like gradients in very smooth nebulae, stacking images permanently breaks the 12-bit ceiling. When deep-sky stacking software averages many 12-bit sub-exposures the mathematical integration recalculates the data into 32-bit floating-point files. The fractional differences created by the noise floor create very smooth gradients that can equal native 16-bit files. The advantage of the 16 bit ADC is that much longer exposures are possible before the stars become saturated.
This thinking is behind the practice of EAA which frequently uses cameras such as the ZWO ASI585MC and short exposures with live-stacking, to build up an acceptable image, often for outreach but also as an observational technique not using an eyepiece.
Another point to note here is that if a number a short exposures are captured with the aim of preserving the fidelity of the star colours, then the stars from the stack of these images can be used to replace possibly-saturated stars in the longer exposures. These short exposures might ideally be made through a UV/IR cut filter so that the star colours cannot be influenced by a filter such as a quadband or dualband. This is a technique that we shall explore in the future and is analogous to the standard technique of using RGB stars in narrowband images.
AstroDMx Capture saving 3 minute exposures of the Eagle nebula
The data were Stacked and part processed in PixInsight and further processed in SetiAstroSuitePro, Fitswork4 and Gimp3.
The Eagle nebula
The experiment showed that it is possible to get acceptable deep sky results from a 12 bit OSC camera such as the ZWO ASI585MC which has an 8.29 MP sensor of dimensions 11. x 6.3 mm and a resolution of 3840 x 2160. When the ZWO ASI585MC is paired with a Stella Mira 66mm ED APO refractor with a field flattener, it produces high resolution detailed images.
We look forward to working more with this setup and exploring the use of short star exposures as discussed above.
Meanwhile Nicola continues the work on Version 3 of AstroDMx Capture whilst still maintaining the code-base of Version 2.
