In early spring this year, 2016, innovative astrophotography camera maker ZWO teased about a very intriguing new camera they were bringing to the market: The ASI1600. This camera has caught the amateur astronomy and astrophotography worlds by storm, as it is offers some highly competitive and very compelling features at a low price point not seen before for a new fully featured camera. The nearest competing CCD cameras with similarly larger sensors, in this case a 4/3″ sensor (quite large in the astrophotography world, where tiny sensors tend to rule), are almost double the price.
The ASI1600 offers some unique versatility in the marketplace, capable of operating as a normal astrophotography camera for long exposures, as a high speed planetary camera for high resolution solar system imaging, supports live viewing of planets and DSOs for EAA (Electronically Assisted Astronomy) as well as live stacking, and can even operate as a guide camera with a large field of view if necessary in a pinch. That makes it one of the most versatile cameras available for astrophotography, and a real winner for both ZWO and budget minded or beginner astrophotographers.
To add to it’s versatility, the camera comes in four key variants. There are the cooled and uncooled models, in which both the color and monochrome models come. The uncooled color version, the ASI1600MC, would compare almost directly to an astro-modded DSLR. It contains a color sensor with a bayer color filter array, but without the infrared cutoff filter normally found with DSLRs. The color cooled version, the ASI1600MC-Cool, would compare almost directly to an astro- and cold-finger modded DSLR. The prices of either of these color options is not much greater than you would have to pay in order to purchase a DSLR and cover the cost of the modifications. With the added benefit of no risks and customer support (which so far has been exemplary!).
The uncooled mono version, the ASI1600MM, is the first model that allows more CCD-like imaging. Being monochrome, it can be used with standard astrophotography imaging filters such as LRGB for color, or even narrow band filters like Ha, SII and OIII. Being uncooled, there may not be many applications for such a camera in astrophotography outside of guiding or possibly double star imaging, however ZWO also offers the ASI1600MM-Cool as well. The cool version supports competitive cooling to other low priced CCD cameras, better in some cases, at -40° to -45° C below ambient temperature. This allows dark current to remain quite low, limiting how much noise it adds to the images, and important factor for narrow band imaging in particular.
The ASI1600 is priced quite competitively. For the uncooled color model, the brand new price from ZWO direct is $799, while the cooled color model is $1080. The cooled monochrome model is $1280, which puts it about $715 cheaper than comparable CCD cameras such as the SBIG STF-8300.
Preorder and Delivery
After some contemplation and considerations of the persistent weather, I decided to skip the $10,000 camera package I had originally been considering this year, and became one of the first ASI1600 customers to pre-order the camera in mid April. By early May, I had the wonderful new ASI1600MM-Cool camera delivered and in my hands. And what a beautiful little camera it was!
It was a compact cylindrical camera, anodized a rich, bright red with a black heatsink and adapter thread. The colors fit right in with my existing equipment, which was also black and red. The camera felt quite solid, yet still light weight. It is well designed, and capable of being used at a very short backfocus, which is a boon for those using refractors and camera lenses where available backfocus is often limited.
Inside the Box
Inside the box were a number of useful adapters, as well as an interesting conversion option. The camera itself comes with a simple 11mm thick M42 threaded adapter ring already attached to the camera under a protective cap. With this ring attached, backfocus is 17.5mm, standard for ZWO cameras, and the camera can be threaded onto any standard M42 (T- or T2- threaded) telescope.
The camera also comes with a couple of alternative adapters. The first is an 18.5mm thick M42 to M48 adapter. Attaching this to the camera makes the backfocus 36mm, and allows the camera to be attached to any standard M48 (2″ threaded) telescope, or even slipped inside of any 2″ focuser with a compression fitting. The entire M48 adapter can be slipped inside the focuser, allowing backfocus to be reduced to 11mm again, or even slipped all the way inside, reducing backfocus to the minimum of 6.5mm.
The final adapter is a 30mm thick M42 to 1.25″ eyepiece holder adapter. This adapter has a 1.5mm thick flange, which increases the camera backfocus to 19mm. This adapter allows the camera to be converted for use with any standard 1.25″ telescope eyepiece holder. This can be useful for using the ASI1600 in a highly magnified planetary imaging rig, or as a guide camera (particularly with OAG). This option reduces the clear aperture, and can result in fairly significant vignetting, so it is not recommended for normal DSO imaging, however it can be very useful for planetary imaging of very bright objects.
The Filter Holder
In addition to the adapters, the camera also comes with a replacement sensor compartment cover. The camera sensor is by default covered by anti-reflective glass (AR glass), which helps to seal the camera compartment. The alternative sensor compartment cover has not glass, and instead has a thread compatible with 1.25″ filters. This allows a filter to be screwed into the camera as close as possible to the sensor (which avoids additional vignetting from the filter), and also allows filters to be used with camera lenses that do not allow a lot of backfocus (such as some M4/3 or other mirrorless camera lenses).
The camera comes with two cables. One is a USB 3.0 data cable for connecting the camera to a computer for control. The camera primarily runs off of the USB 5v power supply, and as such does not absolutely require an additional power source. To use the integrated TEC for regulated cooling, you will need to purchase a 2.5×5.5mm center-positive 12V power supply capable of supplying up to 3 amps of current. The cooler itself will draw up to 2 amps, but to make sure the cooler is never starved for power it’s a good idea to get a supply capable of delivering higher current if necessary.
In addition to the USB cable, the camera also comes with an ST-4 port and cable that allows it to be connected to any mount with a companion ST-4 port. This allows guide commands to be sent to the mount through the camera, in the event that you choose to use the camera as a guide camera. For some specialized cases, the ASI1600 can indeed serve as a high resolution, large field guide camera, which may be ideal for OAG ports or ONAG on large reflecting telescopes, where finding a guide star in a small field can often be difficult.
Since it’s original release, ZWO has upgraded the ASI1600 twice. The first upgrade, dubbed v2, made two significant changes. The first was the removal of the ST-4 port, replacing it with an integrated USB 2 hub. This hub allows the new companion ZWO EFW that was released not long ago to be connected directly to the camera, rather than to a separate USB hub. Additionally, the second port allows a guide camera to be connected directly to the integrated USB 2 hub as well. The v2 model also includes updated design for holding the desiccant tablets in the sensor chamber, making it much safer when removing the sensor compartment cover. For those who own an ASI1600 v1 model, I don’t see the changes as a fundamental need to upgrade, nor any reason why the resale value of your model would be significantly hurt. This camera is still in high demand, and if you do have a need for the integrated USB hub, selling your v1 and upgrading to a v2 should be pretty easy.
ZWO made further improvements in the v3 model of the camera. The first major improvement is the use of an optional external desiccant plug, sealed with o-rings, making it possible to swap out desiccant plugs very quickly and without needing to open the sensor chamber. For those with high humidity and the need to periodically swap out tablets for dry versions, this will be a very useful update. The second major improvement is a redesigned heatsink for the cooler. The heat from the heatsink is now physically redirected to the AR window in the front of the sensor cover. The heat from the cooler will now heat the window, which should keep it above the dew point and prevent dewing issues on the AR window. Since the sensor chamber is sealed, this should resolve dew issues for those in high humidity areas.
The ASI1600 packs some interesting features. Aside from one of the most important, being very cost effective, it also has some of the lowest read noise on the market, a very high readout rate and very high framerate, configurable hardware ROI, and one of the shortest backfocus on the market. These features make the camera extremely versatile. It is capable of imaging bright objects as well as extremely faint objects, planets as well as DSOs, can be adapted to pretty much any accessory hardware on the market, and can even serve as a large field guide camera.
Before we dive into these in detail, it also needs to be noted that the ASI1600 uses a Panasonic sensor, rather than a Sony sensor. While most of the top CMOS astro cameras that have hit the market in recent years use Sony sensors, the ASI1600 is a bit unique in that it uses the Panasonic sensor. More on the sensor in a bit.
Low Read Noise
The ASI1600 has some of the lowest read noise of any astro camera on the market right now. With minimum read noise around 1.13e-, it is rivaled only by a couple of Sony IMX sensors that may bust the 1e- barrier to read noise (the IMX178 in particular gets down to around 0.7e- read noise at it’s high gain setting). Even at the unity gain setting, which is the most commonly recommended setting for beginners, read noise is around 1.6e-. The minimum gain setting has read noise of 3.5e-, which still puts it ahead of most other cameras on the market, including the vast majority of CCD cameras.
With the ultra low read noise, the ASI1600 is a powerhouse for capturing faint details on just about any object, including those with very low surface brightness. The low noise allows for much shorter exposures to be used, even with narrow band imaging, which can have distinct benefits over long exposure imaging that the CCD community is so far familiar with. With shorter exposures, imaging gets easier, challenging objects once reserved for the $50,000 dark site observatory imager become accessible to beginners or lower budget imagers using average equipment in their city back yards. This kind of sensor is just a herald of the future shift to come, from CCD to CMOS. Much better CMOS sensors exist, and it is only a matter of time before bigger players enter the realm of low noise CMOS cameras.
High Frame Rate
Thanks to it’s CMOS technology, the Panasonic sensor in the ASI1600 is capable of very high readout rates in the sub-second range. In contrast, CCD cameras often require several seconds to download each frame, possibly tens of seconds. DSLRs also often require several seconds to download. Depending on the software used, such as SharpCap or SGP, the real-world download rate for the full frame may be as little as 0.06 seconds to maybe as much as 1.5 seconds, based on my own experience. This fast readout rate is a bonus when it comes to DSO imaging with shorter exposures.
The readout rate can be increased well beyond 0.06 seconds when using the hardware ROI (region of interest) feature of the camera. Like most of it’s companions in the CMOS astro camera world, the ASI1600 supports custom ROI. This is a very useful feature for solar system object (SSO) imaging. The full frame or half the frame may be used for the moon and sun, while very small ROI down to as little as 100×100 pixels can be used for planetary imaging, double star imaging, etc. As the ROI area is reduced, the maximum frame rate the camera can achieve with a program like SharpCap increases. With ROI below 400×400, a maximum frame rate of nearly 300fps can generally be achieved. This can be a great help in getting past seeing effects with planetary imaging.
One of the more intriguing features of this camera is it’s short native backfocus requirement. The standard backfocus for most astro cameras of this class is 17.5-18mm. ZWO’s standard is 17.5mm. This particular camera, however, is fitted with a removable 11mm thick M42 female adapter that actually threads onto an M42 male thread machined directly into the camera’s face plate. Removing the 11mm adapter reduces the camera’s backfocus requirement to a mere 6.5mm. This ultra short backfocus gives this camera a very wide range of compatibility with existing M42 accessories on the market. Anything with a female camera-side thread can be directly attached to the camera. This includes many filter wheels, for the mono version of the camera.
The compatibility of this camera can be expanded to a much greater number of accessories via custom Precise Parts adapters. This can bring in M48 accessories into the fold, as well as filter wheels such as the Atik EFW2 (the filter wheel I chose myself) which uses a dovetail flange design for the camera side adaptation which support rotating the camera without needing to rotate the scope (a handy feature, to say the least…but one which in practice I have found is a bit more difficult to deal with than originally anticipated; I ultimately just left my camera oritentation to the FW fixed, and rotated the scope). There are few accessories on the market that the ASI1600 cannot be adapted to when using Precise Parts adapters thanks to it’s short backfocus.
Panasonic 16MP 4/3 Sensor
The heart of the ASI1600 is it’s sensor. This is a fairly unique sensor in the market, as it has not only a color version, but also a monochrome version that was sourced by ZWO for this camera. This allowed ZWO to bring the first relatively large field (over 1″ format), monochrome CMOS camera for DSO imaging to market. The Panasonic sensor is 16 megapixels, and has a 4/3 format. That makes it identical in size and shape to the KAF-8300 CCD sensor, one of the most popular astro camera sensors on the market and a long term staple of the astrophotography community.
The Panasonic sensor delivers some of the lowest read noise around, at 3.5e- RMS at Gain setting 0, which is ~4.88e-/ADU. At the unit gain setting of 139, the camera has around 1.55e- RMS read noise, which is sufficient for the vast majority of users, delivering competitive DR. At the high gain setting of 300, the camera drops to 1.13e- RMS read noise, which is lower than all but a couple of smaller ZWO and QHY CMOS planetary cameras. This versatility in gain and read noise allows the camera to be highly adaptable to a very wide range of telescopes, camera lenses, atmospheric environments and imaging goals. The camera can be used at low gain for maximum dynamic range (12.04 stops), at unity for general purpose imaging, or high gain for chasing faint details.
Low Dark Current
In addition to the low read noise, this sensor also has very low dark current when cooled. Below 0C, the camera performs exceptionally well on the dark current front, with 0.02e- or less dark current. Ideal performance is reached somewhere between -15C and -20C for commonly used exposure lengths, with dark current dropping to a low of 0.006e-/s. Below -20C, I have never measured any further improvement in dark current, with the exception of extremely long exposures. This camera has a maximum exposure of 2000 seconds (~33 minutes), however to date I have never found a need to use such an exposure, at any gain setting, even with very narrow 3nm filters.
The cooler on the ASI1600 is also very competitive, rated to cool below the ambient temperature by 40-45 degrees Celsius. This makes it competitive with some of the fairly high end CCD cameras out there from manufacturers like SBIG and QSI, who generally offer -45C cooling on most of their mainstream cameras. The camera can definitely be pushed to a delta-T of -45C, however you will need to make sure that you have sufficient airflow around the camera to move the excess heat away from the heat sink.
Indoors, where the air is often relatively still, I am unable to achieve a full -45C delta-T, and I stick to -40C. Outside, where there is usually some amount of airflow moving exhaust heat away from the camera, achieving -45C delta-T is possible, although it doesn’t leave as much headroom to regulate the temperature. This makes the camera viable for use in warmer climates, while still achieving the -15C cooling required to push dark current levels to minimums.
High Dynamic Range
The sensor also delivers competitive dynamic range. Most CCD cameras have somewhere in the realm of 11 to 12.7 stops of dynamic range, with the rare gem having as much as 13 stops (i.e. the KAF-16803). The popular KAF-8300 has ~11.5 stops, while newer Sony ICX sensors have as much as 12.5 stops or so. The ASI1600 has a native analog dynamic range of 12.52 stops. Due to the use of a 12-bit ADC (analog to digital conversion) unit, the “extra” .52 stops is clamped to 12 stops, a limitation imposed by the ADC unit. The ADC is actually integrated into the sensor, in fact one ADC unit is integrated for each column. This advanced design is what allows the ASI1600 to achieve such low read noise, as well as it’s very high readout rate. At the unit gain setting, the camera has around 11.41 stops of dynamic range, on par with the KAF-8300. At the high gain setting, the camera has around 9.2 stops of dynamic range.
As this camera supports a variable gain setting from 0-600 through it’s drivers, using only the three default presets is not a requirement. The camera can be configured for any gain setting, which lead me to investigate some alternative settings that I felt were more optimal. Based on my testing, the most optimal setting for this camera, one which delivers the highest dynamic range possible while concurrently minimizing read noise as much as possible, is gain setting 76. This brings you to exactly 12 stops of dynamic range with only 2e- read noise, vs. the 3.5e- you have at the low gain setting. This is beneficial for using moderately long exposures and chasing faint details, which makes it a good setting for narrow band. It also supports a decent full well capacity of 8192e-, making it a great setting to use for LRGB imaging at a dark site as well.
Additional testing also lead me to gain 200, which delivers about 10.6 stops of dynamic range, and read noise of only 1.3e-. This is a great setting for darker skies, very narrow band filters, and longer focal lengths where dynamic range is not as much of a concern. Chasing very faint details at high f-ratios is much easier at gain 200. Great for distant galaxies or narrow band.
Like many of it’s planetary camera counterparts, the Panasonic sensor in the ASI1600 has relatively small pixels. At only 3.8 microns, they are smaller than most DSLR and CCD pixels. This allows for high resolution imaging with smaller scopes, which is again a real bonus for beginners or lower budget imagers who cannot afford a $10,000 high end mount to handle a big high f-ratio scope. With such small pixels, image scale at 600mm is a mere 1.3″/px. In contrast, the popular KAF-8300 has an image scale of “/px. On a night of decent seeing, at 600mm focal length, stars have an FWHM of around 2”, making them very tiny, sharp, and non-dominating in the field:
With such small pixels and a large field, this camera is an excellent companion to camera lenses. Historically radically undersampled with most CCD cameras, lenses in the range of 135mm to 300mm can become great options for beginners looking to get into the astrophotography game with gear they may already have, but with the opportunity to do narrow band imaging (which delivers higher quality data that is easier to process from anywhere, even a light polluted city.)
As with any piece of equipment, there are usually some drawbacks to counterbalance the beneficial traits. The ASI1600 is no exception to that rule, and has a couple traits that should be noted. The camera, being CMOS and having integrated technology like many other CMOS sensors out there, can suffer from amp glow. Additionally, with certain scopes and very bright stars, a microlens diffraction artifact may present that can be rather distracting. Neither issue is really going to be a problem for most imagers, particularly not the amp glow as it is easily correctable. The microlens diffraction may be an issue for some imagers who don’t want to deal with it, especially if they tend to image objects with very bright stars.
The amp glow is a common issue with CMOS cameras. It can be quite bad with some cameras, particularly Sony IMX sensors with integrated image processing logic. They often have bright starburst diffraction. The ASI1600’s Panasonic sensor, thankfully, does not have severe glows. It has a pair of bubbles along the right edge, and a very faint upper left corner glow. These glows, as with any camera that has amp glow, are easily correctable with well matched darks and unscaled dark calibration. An example of the ASI1600 glows…worst case scenario using a 2000 second exposure:
Seems horrible, however when calibrated with a proper master dark, hardly an issue as can be seen here:
The key with calibrating these subs is to make sure the temperature, gain, offset and exposure of the darks exactly match that of the lights. With regulated cooling, that should not be an issue. Another key is when calibrating, make sure that you disable any kind of dark optimization or dark scaling features of your calibration program. Scaling the darks will result in them losing their match with the lights, and they will improperly calibrate, leaving remnants of glow behind.
A diffraction artifact, microlens diffraction occurs when light diffracts off the microlenses of the sensor pixels and that light is somehow reflected back to the sensor. In the case of the ASI1600, it appears this can happen, and the light will reflect back from the sensor cover glass. This is the only issue with the ASI1600 that may actually be a problem for some, as it is a bit annoying when it happens. It only occurs with very bright stars, such as Gamma Cas, Sirius, possibly the Pleiades, etc. It does not occur with most stars, and many have reported that it has never occurred for them at all, indicating it may also be dependent on the kind of scope used. In my case, I use a rather large aperture fast refractor (150mm aperture at f/4, Canon 600mm f/4 telephoto lens), which can pack a lot of starlight into each resolved star, possibly exacerbating the issue in my case.
Worst case scenario, with a very bright star, you might encounter this, which was done with an Ha filter:
This is the worst I’ve ever seen the issue, by a long shot, and only occurred on this particular image. I’ve encountered the issue in other cases, although not nearly as severely. An example of Pleiades with an L filter:
This is a more common presentation. The problem seems to be more pronounced with narrow band filters. On stars smaller than these, I have not generally seen any issues. In most cases, if a diffraction artifact presents, it appears more like a vertical and horizontal diffraction spike, without any visible microlens reflections. For the most part, this issue can be avoided simply by avoiding very bright stars when imaging, and it can also probably be avoided by using a smaller aperture scope (an 80mm wouldn’t pack nearly as much light into each star, meaning less light to diffract off the microlenses.)
Having used the ASI1600 for a while now, I have to say I think it is probably one of the best purchases I have made for my astrophotography. I am very pleased with the camera overall, it has performed extremely well, is very easy and fun to use, and has allowed me to maximize the potential of my 600mm lens that I use as a telescope. Considering I’d been on the verge of dropping about ten thousand dollars on the much larger, much heavier and much, much more expensive Moravian G3-16200 camera with the large KAF-16200 sensor, a camera which I now know would not have worked properly with my Canon EF lens mount, for the money spent on the ASI1600 it’s been an entirely regret-free purchase. Wonderful camera for those looking to move beyond DSLR. Wonderful camera for those looking to get into astrophotography without having to struggle against LP. Wonderful camera for those on a more limited budget and lower end companion equipment (i.e. mounts). Wonderful camera in general, one that performs extremely well in the vast majority of situations, with an extremely broad range of compatible equipment.
One final word on the ASI1600. Thanks to it’s low camera noise, it is possible to use very short exposures to maximize resolution, minimize seeing effects, and still get good SNR on the object you are imaging. With short exposures comes some unique opportunities to record things that would otherwise be impossible to record with long exposures and higher noise cameras. One such case is my exploration of Pleiades last year, during what appeared to be a meteor shower. Caught on film!
This image sequence is a series of 10 second frames, exposed one after the other with no delay except a momentary jump when dithering kicked in (every 10 frames in this case). What would have normally just showed up as a thin meteor streak in a single sub turned out to be an intriguing smoking meteor, a somewhat rare phenomena when a meteor vaporizes as it enters the atmosphere. The spinning meteor burned off as it raced through the upper ionosphere, leaving behind a corkscrew trail that faded into the night. Such a capture would never have been possible with a higher noise camera and classic long exposures. 😉