The ASI071MC-cool camera

The ASI071MC-cool has a large, 16-megapixel color sensor, comparable to an APS-C still camera. It represents an alternative to DSLR imaging, with decisive improvements such as regulated cooling, a vast choice of possible, standard filters, very low noise, an anti-reflective protection window with no undesired filtering of near ultraviolet and near infrared, and a relatively high frame rate, thanks to the USB3 port.

One could think that the necessity for a computer, at the contrary of a autonomous DSLR, is a drawback. In the field, many DSLR users do have a laptop to precisely adjust the focusing by previewing the image on the computer screen rather than on the tiny display of the camera, not to speak of the precious help of DSLRfocus, BackyardEOS/Nikon, ImagePlus, MaxDSLR or other comparable software. A DSLR has some other weaknesses for deep-sky imaging: it is originally uncooled (cooling boxes exist, either DIY or commercial products, sometimes at the price of the ASI071MM-cool, on top of the expense for the DSLR itself). At the exception of some costly, astronomy models (e.g. 20Da), a DSLR needs de-filtering to shoot nebulae then re-filtering for proper color balance or light pollution rejection. The filters are specific in diameter and mount and cannot be used on a astronomy camera. Live focusing with LiveView is only possible on very bright stars. The display is very small. Handling the tiny, mostly unlit buttons of a DSLR at night is hard, especially with a refractor aiming at the zenith. Apart from very experienced users, planetary imaging provide disappointing results due to the limited frame rate and compression of the AVCHD movies.

I’m not an ennemy of astronomy imaging with a DSLR and I have three of them. Once we have considered the whole equipment and respective qualities and weaknesses of a DSLR facing a cooled camera with the same sensor, the expense, including de-filtering and DIY or commercial cooling box, is somewhat equivalent. But the results are not.

The ASI071MM-cool is primarily intended to wide-field, deep-sky imaging in color with a unprecedent ease of use relative to multi-band imaging with monochrome sensors. It is also capable of color, planetary / lunar / solar imaging. The camera is not compatible with too small, C or C/S all-sky lenses, nor it has an autoguider port, but it directy accepts full-frame or APS-C photolenses, in addition to apochromatic refractors and reflectors of all kinds as long as the correct image field produced by the optics is at least 28-mm large.

Housing and connectors

The housing is comparable to previous cooled ASI (refer to the review of the ASI178MM) with some differences:
Above: connections of the ASI071MC-cool for long exposures, with an ASI1200MM as autoguider with its guide refractor in parallel. Cables are not present for clarity of the image. (1) USB2 cable to be plugged into one USB2 port of the hub (5) of the ASI071. (2) RJ11 cable to be plugged into the autoguider port of the mount (other possibilities exist). (3) USB3 cable to be plugged into a USB3 (possibly UBS2) port of the computer. (4) Power supply cable for the cooler (the camera also may operate with no cooling). The ASI120MM was rotated to show the connectors.

An innovative, very welcome feature is the presence of three screws on the front side of the housing, to adjust possible tilt of the sensor – or to adapt the camera to a peculiar assembly suffering from tilting. This is the garantee for a planar (at least orthogonal) image field with sharp stars on the entire, large sensor of the camera. Other cameras require a optional tilt corrector which lenghthen the back focus. Since the screws are intentionally very close to the border of the housing, we do not need to unassemble the whole imaging train to adjust the tilt ! It is important to verify the centering of the image field and the tilt of the possible corrector of the telescope prior to blaming the camera itself by rotating the camera and taking successive images.

Two spacers come with the camera, 16mm and 35mm in length, to achieve the proper distance from the sensor to the optics equipped with a field corrector / focal reducer. Such parts are designed to fit a DSLR, with a back focus of about 55mm (this depends on the brand). In my case, the required back-focus is only 51.66mm. You just have to add the length of possible filter wheel and off-axis guider, the depth of the sensor (17.5mm), and possible adapters (e.g. M48 to M42), to choose or pile the spacers. Various optional spacers are available in case the required back focus differs from the optimal distance.

If the optics is exploited with no corrector (e.g. non-edge HD Celestron, simple doublet/triplet apochromatic refractor or Ritchey-Chrétien reflector with no focal reducer/field corrector), the camera may be directly inserted into the drawtube of the eyepiece holder with the enclosed, 2-in/50.8-mm adapter, or screwed into a M48 thread or a M42 thread with the help of the enclosed M48-to-M42 ring adapter, with no concern at all for the back focus.

Sensor and possible applications

The IMX071 was initially released in 2011 ; this is a very peculiar choice because ZWO always prefer very recent ones, but we will immediatly see that, it is still a high-end sensor. It equips renowned DSLRs such as the Nikon D7000 and D5100, or Pentax K5. The sensor has a high saturation level (potential well depth) of 46000e-, that is more than a Kodak CCD KAF16200. By comparison, the CCD KAF11000 is only 25% better than the CMOS IMX071. This is the best performance of all cameras released by ZWO as of March 2017. A photosite with a deep potential well is able to record a vast range of shades; this is a determining asset for deep sky. In practice, bright stars do not saturate and inflate in a raw image even if the exposure is long enough to get the pale arms of a galaxy or a faint diffuse nebula. This also applies to lunar landscapes comprising both low levels (a dark Mare near the terminator) and high levels (a fresh crater rim), or Uranus and its moons. The potential well depth has no significant effect while imaging the surface of planets. For deep-sky, the capacity and the low readout noise lead to a dynamic range of 83dB, by far the best of all ASI cameras as of March 2017.

Another determining capacity for deep-sky imaging is the precision of the analog-to-digital converter, or ADC. The 14-bit converter is able to make out four more distinct shades than the ASI224, ASI120, ASI290 or ASI1600. As of March 2017, only the ASI178 shows a comparable accuracy and only deep-sky CCD sensors and newer ASI such as the ASI2600mc-Pro and ASI62000mm/c-pro are better with their 16-bit ADC. This is discussed in the review of the ASI178MM. The converter delivers a little more than 16 000 possible different shades, at the moment of its release the camera stands exactly between the best contemporary, planetary CMOS sensors and deep-sky CCD sensors.

Being a color sensor, the IMX071 has a Bayer matrix, like the ASI224MC’s IMX224. The presence of the filters of the Bayer matrix implies a loss in sensitivity relative to monochrome sensors; anyway this APS-C sensor has not been designed with a monochrome version in mind. The photosites are grouped by four (one red, two green, one blue). A peculiar feature is that the Bayer matrix voluntarily becomes transparent in large wavelengths, that is near infrared (NIR), pretty much like the ASI224; this noticeably enlarges the scope of the camera.

The camera offers binning modes to group photosites by four or more, to get a much higher sensitivity at the price of the reduction of the number of pixels and a more rough sampling rate. The purpose is to more easily detect faint galaxies and nebulae on the screen before switching back to binning x1, or to shorten the exposures by shooting with binning x2 or more.

Noise management

The noise is extremely low for a CMOS sensor and this probably is one of the reasons for why ZWO picked it up. With no cooling, gain set to 60% and 60-s exposures, uncorrected images show some colored pixels brighter than others. This is normal, especially for a CMOS sensor, and the dispersion in sensitivity remains very low relative to a EOS 450D DSLR –one of the most appreciated still cameras for astronomy despite its readout noise is about tenfold the ASI071’s. I initially choosed to cool the camera down to -15°C in most cases for some practical reasons: Like the ASI178MM-cool, the two-stage, regulated cooler reaches the desired temperature surprisingly quickly, within in a few minutes, then stays steady with a reported precision better than 0.3°C with no detectable variation in thermal noise. I’ve ever seen frost in the sensor protection window, but, in case dew appears on the window, I just set the «anti-dew» feature on in the image acquisition software. The feature is also available in the ASCOM mode: this garantees a total compatibility with third-party, ASCOM-compatible, image acquisition software such as MaximDL, APT and many others. This only applies to the FIRST BATCH of ASI071mc-cool (ZWO also provides a anti-dew heater strip kit with a handy, Y-power cord for the 12-V / 0.35-A power supply). The ASI071 has limited thermal noise indeed, thus a temperature of 2-3°C above zero is sufficient for relatively short exposures of, say, 30-120 seconds: this saves power and prevents possible dew to form in case of extreme ambiant humidity. The NEWER ASI071mc-Pro has an integrated, heating resistor to definitively prevent dew (in addition of high-speed RAM and others improvements).

Since the camera has a regulated cooler, ensuring stable results in same conditions (gain, temperature, exposure, binning), dark frames may be taken at daytime, camera removed and covered to avoid possible parasite light beams entering a filter wheel or an open-air mirror.

Planets and Moon

The ASI071 can image planets and the Moon the same way as a DSLR, with superior ease of use, especially focusing, much higher frame rate (134 FPS in 320x240, a sufficient format to image Jupiter or Saturn) and uncompressed images (SER, TIFF, FITS...). A deep-sky, CCD camera is unable to offer the same results because of its low frame rate, not to speak of its higher readout noise in most cases. To be honest, the ASI290 or the ASI224MC are more suited to planetary imaging than the ASI071 for specialized, planetary photographers. Nonetheless, the ASI071MC-cool is a deep-sky camera capable of planetary and lunar imaging. The real advantage of the 071 in planetary imaging is its exceptionnally large sensor at moderate price to frame conjunctions while keeping excellent accuracy, in addition to its 14-bit converter and deep potential well, in case the contrast between the two bodies is strong. The deep potential well is a serious advantage to image earthsine, too. I shot the large-separation, Mars-Uranus conjunction with background stars at the end of February 2017 with a 300-mm telelens; unfortunately passing clouds severely damaged the images, but even the ASI1600 could not have framed the conjunction with same optics. Another excellent use of the 071 is wide-field, lunar imaging in color. It is best to add a near-infrared rejection filter since the sensor is voluntarily unfiltered; if not, the color balance is altered – this may be fixed by empirically decreasing the red layer but the accuracy somewhat suffers from infrared.


Above: first quarter, 300 frames, 10-in/254-mm Newton at F/4.7 (due to the absence of coma corrector, the South pole was somewhat blurred). The original image was pinky because near infrared was unfiltered, leading to a manual correction of the color balance. Anyway, by increasing the saturation, the natural colors of the Moon were easily emphasized (colors are related to the age and composition of the soil, especially titanium oxides and iron oxides, while ejectas from fresh craters show a bright, bluish tint). Image by the author and Rabah Bouzidi.

Deep sky


Above: simulation of the field of view of different ASI cameras with the same focal length, images not to scale. From left to right: ASI224, ASI178, ASI071.

My early tests were devoted to open clusters, a subject that I traditionally shoot with a DSLR because this requires a large field and preferentially a color sensor. I’ve already mentionned the unprecedent ease of use of the camera facing a DSLR at nighttime. The other advantages are the low noise and the cooling. Cooling was on and anti-dew was off. My goal was only to get aesthetic images and I voluntarily oversaturated the histogram (signal was more than 100%, leading to clipping) to get the diffraction spikes around bright stars, thanks to the spider of the secondary mirror of the telescope. A more rigorous approach should have led to fill up the histogram to 90% to get pinpoint stars with no diffusion, to not waste the generous potential well.


Above: adjusting the exposure with the help of the histogram. The four curves are shifted to the right, meaning that the exposure is slightly too long: I was simultaneously wasting almost 10% of the potential well and uselessly recording the uniform background of light pollution and natural glowing of the sky. The right part of the histogram is truncated: some bright stars are clipped, resulting in their enlargement. The proper balance is achieved when the signal grows from 0%, keeping maximal room for high lights with little clipping. Note that the cooler is set to -15°C. In addition the software offers access to anti-dew feature (with the help of the vertical slider). The background software is PHD2 for autoguiding with the ASI120MM equipping a guide scope.

30-s, 60-s then 240-s exposures in 14 bits showed no trace of Fixed Pattern Noise even with extreme histogram stretch, nor amp glow or hot pixels at the exception of sparse and moderate color dots from slight offset of individual photosites. The color dots were automatically fixed by subtracting dark frames. Dark frames can be subtracted during acquisition with software such as SharpCap or FireCapture, or later with software such as DeepSkyStacker. I choosed the latter solution to keep control of the process. Flat frames were not necessary in this case, neither offset frames, but they can be subtracted with stacking software too. In all cases, the cooled camera naturally showed very little noise relative to DSLRs such as the venerable EOS350D and the EOS600D, not to speak of the entry-level Nikon D3200. Live focusing was extremely easy and accurate with the help of 400x, digital zooming on the screen of the laptop with the acquisition software. A DSLR is not capable of live focusing on relatively dim stars because Live View operates only with daytime illumination, bright stars or planets, or the Moon. In addition, the ASI071Mc-cool has an electronic shutter, totally suppressing vibrations at the beginning and at the end of each exposure, like a mirrorless DSLR.



Above: enlarged parts of images of the NGC2158 open cluster, gain 350, exposure 30s, RAW16, binning 1x. Left: raw, undebayered FITS frame. Center: debayered frame. Right: 149 frames, dark frames subtracted. The entire frames do not show any amp glow nor Fixed Pattern Noise, and dark frames easily fix the inevitable though very sparse and unsaturated color dots. In this early test, the camera shows the same quality as a semi-professional APS-C DSLR equipped with a third-party cooling box. The orange background in central image is due to light pollution. In these enlarged images, the accuracy reveals itself to be limited by both atmospheric turbulence (30-s exposures) and the diffraction by the optics.


Above: the Double Cluster in Perseus, shot from a light-polluted site, less than 10km north of Nantes (FRA) with no filter. 6-in/150-mm, F/4 Newton with coma corrector. 174 x 60s. Darks frames (not strictly necessary) and flat frames subtracted. Sensor was cooled down to -15°C.

The frame rate and the transfer speed are negligible when I take long exposures. The problem is the size of the images: 31 Mb for an undebayered, FITS image, 47 Mb for a BMP image (8 bits), 93 Mb for a TIFF (16 bits) image. Stacking one hundred frames is extremely long, resembling stacking large SER files with the ASI178MM: in such cases I start stacking in batch mode, the laptop runs all night long, then I retrieve the stacked images on next day. AutoStakkert has a batch mode, along with AviStack. DeepSkyStacker also accepts lists of frames to be stacked; another solution is simply running several instances of DeepSkyStacker at the same time. Post-processing to adjust levels, colors and possibly some unsharp mask or wavelets also requires a large amount of memory and a powerfull processor. This is the price to pay for playing with 16-megapixel, 14-bit, color images. Professional astronomers have the same concern and they need by far more powerful computers, e.g. to process the 268-megapixel images from the OmegaCAM which equips ESO’s VLT.


Above: Inverted image of Leo's Triplet in same conditions as the previous image at the exception of exposure: 74 x 60 seconds, and observation site: my yard in suburbs of Nantes with light pollution (ZMV=3.5). More than 40 small, faint elliptical and spiral galaxies (labelled) clearly appear up to magnitude 18.5. This is equivalent to what can be observed in the eyepiece of a 1.7-meter telescope. I even did not complete the inventory. The image includes Messier, NGC, IC, USNOA-2, and PGC catalogs. Only the 14-bit mode rescued the faint galaxies from strong light pollution because the camera can record a little more than distinct 16 000 shades.

The 071 also can image vast, diffuse nebulae, thanks to the generous transmission in deep red (Hydrogen-alpha 656nm, Sulfur-II 672nm) and blue (Oxygen-III, Hydrogen-beta). Since the Bayer matrix voluntarily becomes transparent in large wavelengths, the camera is sensitive in near infrared too: this may help to image carbon stars or stars concealed in dust/neutral hydrogen clouds. I choosed to image the Rasalgethi luminous red giant star through a StarAnalyzer 100 because the star’s flux extends to near infrared.


Above is the spectrum of Rasalgethi star with a StarAnalyzer 100 and a 5-inch reflector. It shows how far the ASI071MC-cool’s sensitivity extends to near infrared – hence it can make science too, including imaging Jupiter and Uranus in methane band, and carbon stars during their minima.


Above: eventually, I shot a region near Sadr, including the discreet NGC6910 open cluster, and a part of the IC1318 Butterfly Nebula. The observation site was my backyard with strong light pollution. Top: the image was quickly saturated in orange due to city lights (high-pressure sodium) even in exposures as short as 8 seconds ! Center: I added an Optolong UHC filter, which passes red and blue while blocking yellow and green, in order to lenghthen exposures up to 120 seconds to gather the faint light of the nebula despite the light pollution. Bottom: stacking 45 x 120s with a small 6-inch F/4 reflector and a coma corrector was sufficient not only to detect, but to decently image the nebula in color with unprecedented ease-of-use relative to a monochrome camera, with no need for narrow-band filters, filter wheel and multiple-layer alignment. This is NOT the brightest part of the nebula, which is listed in the IC catalog of photographic, faint objects.


Above: the uncropped image, 45 x 120-second exposures from my yard in suburbs with UHC filter. The bright star is Sadr in the Cygnus constellation.

COPYRIGHTED MATERIAL. Do not use unless written permission - Just contact me! N.Dupont-Bloch June 2017, updated May 2020.


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