ASI6200MM Pro - First Impressions
Recently the observatory was upgraded with a new camera, the ZWO ASI6200MM Pro. This is a 61MP full-frame monochrome camera, based on the Sony IMX455 sensor. Because this is a full-frame camera, the 1.25” filters had to be upgraded as well. The choice was made for a set of 50mm filters from Chroma. The ZWO EFW2 filter wheel with 7x50mm filter holder fits a complete set of broadband and narrowband filters. Below are some first impressions of this camera with the filters. After enough clear skies, a later blog will have more detailed impressions of image quality.
Getting the camera
The ASI6200MM Pro is still in short supply. Regular retailers did not have the camera in stock and it was difficult to find the EOS adapter. It was decided to buy the camera directly from ZWO from their website. This worked quite well and the camera with accessories was delivered from China to The Netherlands in less than 1.5 weeks.
Also the Chroma filters were ordered directly from the company’s website. After a short email exchange with their sales support on the ordering process, the order was placed. It took less than 4 weeks for the filters to arrive, well within the projected timelines.
In the box
The camera comes with several adapters to accommodate different setups. Most are M48, not ideal for a full-frame camera. M54, or even M68 would probably be a better choice. For the observatory the choice was made to work with the standard EOS-mount, so none of the adapters were required. In fact, the tilt-adjustment ring that comes standard with the camera had to be removed. The camera is screwed directly to the filter wheel with four screws. In this configuration, the nicely padded case that comes in the box does not have much use unfortunately.
The filter wheel comes with a set of rings + screws to fix the unmounted filters. These screws are tiny, and the screwdriver that came with the filter wheel was way too big. A miniature screwdriver is a must have.
The unmounted filters came nicely packed in little envelopes per filter. Printed on the side of each filter are the specifications. The narrow-band filters have a coating on one side, and while Chroma claims the orientation should not matter, they do advise that the coating layer faces the telescope. The filters came with a transmission curve for each filter. A nice QC-check before shipping that makes for a reassuring attention to detail.
Physical dimensions
The first thing noted when unpacking the camera is that it is quite a chunky beast. The camera, filter wheel and filters are all significantly larger than an ASI1600 setup. Because the design is so similar to other cameras, it is very difficult to appreciate the difference in size when looking an image on a computer screen. The size difference is also reflected in the weight. The complete set of camera with filter wheel and filters comes in at 1474g. That is almost half a kilo more than the 1002g of the comparable ASI1600 setup. A solid focuser that can handle the additional weight is important.
Settings
In the INDI driver, under ‘Controls’ there are various settings worthwhile testing and you want to select the best options for your particular setup. Below some of the more critical settings with recommendations.
Typical settings for the ASI6200. Gain 100 is a good ‘set and forget’ setting.
Gain. With the ASI6200 the question on which gain to use is mostly gone. The most universal setting is Gain 100. Noise levels are the lowest the camera can produce and dynamic range is close to the maximum of 14 stops. Higher gains don’t reduce noise levels and limit dynamic range as well as full well depth, so have no benefit. Lower gain increases dynamic range and full well depth a little bit, but at the cost of some extra noise. So Gain 100 is a perfect ‘set and forget’ setting. Alternatively one could use Gain 0 for broadband imaging and Gain 100 for narrowband imaging.
Offset. ZWO does not advise offset values in their manual, so this is up for experimentation. After trying out some different settings, it turned out that each unit in offset appears to add 10ADU in signal. So an offset of 10 adds around 100 ADU to the signal. An offset of 50 adds 500 ADU to the signal. Given the very low noise of the sensor, an offset of 10 for all gain settings is probably sufficient.
BandWidth. This is the amount of bandwidth used on the USB port to transfer the data from the camera to the computer. Default here is 40, but given the large files it was increased to 80. To be honest it did not seem to affect the speed a lot. If you’re having difficulties with the USB connection, it is worthwhile checking out different numbers here. Different computers, USB cables, hubs etc may affect what is the best setting here.
HardwareBin. Under the assumption that hardware binning is better than software binning, this was turned on. That was a bad choice. When turned on, the camera switches to 12 bit mode, regardless the binning setting in the capture software. So best is to leave Hardware Bin off, and if binning is needed, set this in the capture software (software binning).
HighSpeedMode. The documentation is not very clear what the effect of this mode is. Possibly this is related to streaming. But be aware that turning this on will put the camera in 12 bit mode. So leaving this setting off is the best choice unless you have a specific reason to turn it on.
AntiDewHeater. The ASI6200MM is equipped with a dew heater for the coverglass of the sensor. Since it is always difficult to see if there is any dew problem on the sensor, and there is no real downside of using it, it is probably best to leave this on at all times. Energy consumption is very modest at only 3W.
First connection…
Version 1.8 of the INDI driver for ZWO cameras (indi_asi_ccd) supports both ASI6200 and EFW2 filter wheel. When starting the INDI server and connecting in Ekos, the camera was immediately recognised and settings could be applied as described above. But the first major showstopper was revealed when capturing an actual image. Ekos kept downloading…. Even after minutes no image had reached the computer so something must have been wrong. To problem-solve, the camera was connected directly to the laptop, rather than through the separate on-scope minicomputer. This worked perfectly and images downloaded within a second. So the issue must be somewhere in the minicomputer/server.
The solution came from a post on the INDI forum. Apparently on Linux there are default memory allocation settings for the driver that are too limiting for the ASI6200. Increasing the default of 100 to 160 did the trick. This can be done by specifying the -m option and manually starting the indiserver with the following command:
Most people will use INDI web manager, an easy to use tool to remotely control selected INDI drivers. In order to change this memory parameter in conjunction with INDI web manager, you need to modify the file \\usr\local\lib\python2.7\dist-packages\indiweb\indi_server.py. In the line ‘def __run(self, port):’ change the code to:
File-size and speed
A 61MP camera produces large files. In the case of the ASI6200 this is 122.4 MB for a 1x binning FITS-file. At 2x binning this reduces to 30.6 MB. This has downstream consequences for storage and processing. It also has consequences for downloading the images from the telescope to the laptop. If connected directly to the laptop with thunderbolt/USB3 connection, this download is near instantaneous. But the observatory is setup as remote, using a WiFi connection between telescope and laptop. Getting 122MB files over a WiFi connection is not instantaneous. Below is a table of measured download speeds. This is in a test environment. In real life these numbers vary a bit, depending on other network traffic taking place, exact placement of router/telescope, etc.
Overall the 20s is a bit on the long end, but not too bad. For purposes of polar alignment, focusing, plate solving etc, 2x binning can be used, bringing the time back to a very acceptable 5s. Some images may be taken in a cropped (e.g. square) format. As can be seen, cropping to the format of the ASI1600 gives comparable download speeds as the ASI1600 itself. If 60s images are taken, 20s downloads in between takes away 30% of observation time. That would certainly be too much. In such case images can be stored locally on the minicomputer and later downloaded to the laptop. If 300s images are taken, only 6% of observation time is lost due to downloading, relative to 2% with the ASI1600. Then the issue becomes irrelevant.
Bias and Dark files
The bias and dark files that come out of this camera are in a league of their own when compared to the ASI1600. First, there is absolutely zero amp-glow. The dark frames are very even across the frame, as can be seen in the following images.
AMP-glow is pretty much absent in the ASI6200
Bias files are equally clean. It is remarkable how little variation there is between pixels. The following images show some statistics of superbias images, each being the integration of 100 individual bias frames. The variation between pixels, expressed as the avgDev for the ASI6200 is 0.06, compared to 0.74 for the ASI1600. That is more than 10x improvement.
Bias frames of both cameras show a much narrower spread in pixel values for the ASI6200
Upon closer inspection of the longer exposure dark frames, something strange appeared. Scattered around the frame were what could best be described as ‘hot pixel areas’. Only visible at extreme stretching, and then only when zooming in, but still, they were certainly there. There does not seem to be a fixed pattern, but a random distribution across the frame. Shorter dark frames did not show these areas, but clearly visible at anything 3 min and above. Location was identical between frames, so it seems related to the sensor/camera. Like hot pixels, they will probably average out during calibration, but certainly something to be aware of and even more of a reason to apply dithering.
A couple handful of ‘Hot pixel areas’ on fixed position, but randomly distributed over sensor visible from 3min exposure and longer.
Flats
During the first night, the focus was on narrow-band images, so flats were only made with these filters. A better test would be to do flats with a luminance filter. Vignetting appeared to be minimal. The fall-off in the extreme corners is around 4%. Calibrating images with a proper set of flats should easily correct. And even uncorrected it would probably not even show in actual images.
A big difference with the ASI1600 setup was the required exposure time. With the FlatCap light panel at full power and Gain300, exposures for H-alpha flats are typically between 3 and 6 seconds, to get to around 25000 ADU. With the ASI6200 with Chroma filters, this requires exposures of at least 10 seconds, and still does not get quite to 25000 ADU. This is probably related to the 3nm filters used relative to the 5nm filters in the ASI1600 setup.
A fall-off or approximately 4% in the extreme corners means vignetting is very well under control.
Noise
What is immediately noticeable in the files from the ASI6200 is how clean the images come out. This is in part due to the 61MP files, which are typically viewed at less than 100% magnification. But also if you zoom in and inspect detail, noise is significantly less than in the ASI1600. This is probably due to the higher bit-depth as well as the more modern backside illuminated design of the sensor. Below are 96x64 pixel zooms from both cameras. For fair comparison, black- and white-point for both were set to the same values, based on the image with the widest range (ASI1600). Midpoint was then stretched to 25% grey value. The difference in noise is quite noticeable.
Noise level of ASI6200 is notably lower in the ASI6200 compared to the ASI1600
Temperature
The sensor temperature can be adjusted to 35ºC below ambient temperature. Tested at ambient temperatures between 10 and 20 ºC, this was also really the limit and required pretty much 90% of cooling power. Pushing the cooling to 100% one might get a degree or so lower, but not much. The ASI1600 is more powerful. It is advertised at 40-45ºC below ambient temperature, in practical use reaches 45ºC quite easily. This has probably to do with the much smaller sensor to cool in the ASI6200. Also it appeared that the controller seemed a bit more gradual in its adjustments. Where the ASI1600 ramps up and down power quite quickly, the ASI6200 responds a lot slower, resulting in more gradual temperature changes.
More important than temperature is the dark current associated with these temperatures. At -15ºC, the ASI6200 has a dark current of 0.0008 e/s/pix, whereas the ASI1600 comes in at 0.009 e/s/px at that same temperature. That is more than 10 times higher dark current at the same temperature. Even at the lowest possible temperatures, the ASI1600 can not reach the dark current levels of the ASI6200.
For the observatory, the choice was made to always set sensor temperature at -15ºC. This would allow imaging at ambient temperatures up to 20ºC, well within the scope of weather in The Netherlands. Keeping a constant sensor temperature across sessions reduces the number of required calibration files.
First Light
It is time to do some actual work with the camera. After all, the proof of the pudding is in the eating. So luckily enough there were some clear skies and the new camera could be tested in a real-world situation. Below an image in H-alpha of IC1805, also known as the Heart Nebula. Because of the many different tests and changes in the process, the collected calibration frames could not be used, so the image is just the pure integration of 18 frames of 5 mins each, making a total of 1.5h of exposure. No processing has been applied other than stretching.
Heart Nebula (IC1805) in H-alpha. Total 1.5h exposure, no calibration, no processing.
There is definitely a bit of noise still present, but proper calibration and more exposures could reduce that. The level of detail in the image, and the depth into the cloudy areas is pretty good. And this is still with no processing and a total exposure of ‘only’ 1.5h.
Since this is a full-frame camera, it is much more sensitive to small imperfections in the projected image. It turned out that the FL67 flattener and the whole imaging train were performing well. A little bit of softness/elongated stars in the top-right corner, but nothing too distracting. Probably not worth going in with tilt correctors to adjust. The EOS-mount connector, in combination with the 3mm filters apparently gave the right back focus distance to not get any artefacts. The whole system did seem to be a bit more sensitive to precise focusing. Probably it would be good practise to add autofocusing routines into all-night imaging sessions.
Good sharpness across the frame and into the corners, with slight star elongation in the top-right.
Conclusion
The ASI6200 is a camera that benefits from the latest in sensor technology, and that shows. Noise is reduced, dark current is low and amp-glow is pretty much absent. The first images show a great amount of detail and depth while maintaining a sharp image field across the large sensor. It is also a demanding camera. The bigger size and weight will be more demanding on the focuser. The large file sizes transfer slower and demand a lot of computer power in processing. Start-up issues as with the driver settings always happen when you change something in the setup, and can be easily addressed. So as a first impression, the ASI6200MM Pro seems like a solid modern camera with promising image quality.
It’s now time to put the camera through its paces, use it for a lot of imaging and see how it will stand up in real world astrophotography. So it’s too early to draw definite conclusions, but it is off for a good start. Let’s hope for some clear skies shortly.