Updated: May 17, 2020
My experience with getting into deep space cameras and my latest endeavor with the ZWO ASI2600mc-pro.
I started deep space imaging in March of 2013 with a Canon 450D DSLR. I purchased the camera off eBay and it was extremely inexpensive compared to the camera options on the market. After taking several images through it, and seeing other people’s images, I realized I could do better. I started researching about modifying the camera. I read about the difference between a full spectrum modification or performing the Baader modification. There were services out there, that would do the modification for me. I went with the self Baader modification route which simply swapped the stock IR filter out for a Baader IR filter. The Baader IR filter let in a wider spectrum of photons than the stock IR filter. The camera and this modification was a great stepping stone for me to learn with. It was the easiest camera for me to use and I got decent results with it. I started with an intervalometer to control the camera exposure but then I started using the DSLR with Backyard EOS. Although this worked well, each time I would image, I would have to capture a new set of dark and bias frames to be consistent with the ambient temperature of my light frames.
In April of 2018, I purchased the ZWO ASI1600mm-pro, which was a lot of fun to do narrowband imaging with and gave excellent results. It was extremely sensitive and pulled spectacular detail, especially while using the ZWO 31 mm, 7nm narrowband filter set. I also used it with an Astronomik LRGB filter set to image galaxies. This involved capturing flat frames for each filter, swapping between filters with the ZWO Electronic Filter Wheel, refocusing after each filter change and extra steps to calibrate and stack each channel. I found this to be very time consuming and demanding, so I started looking at One Shot Color (OSC) cameras. After looking at the camera specifications, I went with the ASI294mc-pro. It had a slightly smaller sensor of what I used on the Canon 450D (Canon 450D = 22.2x14.8mm and the ASI294mc-pro = 19.1x13mm), but a very similar size sensor to the ASI1600mm-pro (17.7x13.4mm). I also went with the ASI294mc-pro for the great full well depth (63700 electrons), the same back focus distance as the ASI1600mm-pro (6.5mm), which made it easy to swap the cameras between the
ASI1600mm-pro and the ASI294mc-pro, and the 14-bit ADC. Between the monochrome camera and the OSC camera, I thought I had it covered. Over time, I used various scopes to image with. I went from an 8” Newtonian f3.9, an 80mm f5.6 refractor, to a Celestron 9.25” EdgeHD SCT. I imaged at the native 2350mm (f10) focal length and purchased the 0.7x reducer by Celestron to bring the focal length down to 1645mm (f7). I added a Starizona Hyperstar to the front which brought the focal length down to 516mm (f2.2). This has become my favorite scope and setup to use. It is a lot of fun for me to image at f2.2. I can image several targets in one night at shorter exposures, or I can spend an entire night on one object. After using the fast optics, I did not like to changing out the Hyperstar with the Secondary mirror. Only those who have used fast optics would understand this. I am not patient and want great, quick results. The Hyperstar with the ASI294mc-pro was a fabulous setup and did just this. A look closer at the specifications of the ASI294mc-pro
show the pixel size is 4.63 microns. I have always used the formula as I was taught to figure out my sampling. (Pixel Size / Telescope Focal Length) x 206.265. This applies to average typical seeing of 2” to 4”. With this formula, any value over 2” is considered under-sampled and any value under 0.67 is over-sampled. With the Hyperstar and the ASI294mc-pro the value I get is 1.85”. I am on the upper end of the values which are closer to becoming under-sampled. When looking close at the image, the stars appeared square shaped and pixelated due to too few of pixels to capture at the resolution of the telescope.
When I used the ASI294mc-pro on the Celestron 9.25” SCT at F7, 1645mm, the value I calculated is 0.58” which indicates I am over-sampled, which increases the imaging time as light is spread across more pixels than needed for full resolution. Through my experience using deep space astrophotography cameras with various scopes, I have learned it is much better to be over-sampled than under-sampled. If you are planning to use a camera with various scopes, I would look at either the scope you plan to use the most to base the pixel size on or run the sampling formula with all the scopes and look for that happy medium. You can also try drizzle. Not dither but drizzle. If you are planning on drizzling, you will want to dither. That is a tongue twister. Drizzle is a technique that improves the resolution of an under-sampled image while dither slightly moves the scope in random directions during image capture in between exposures. Regardless of the sampling, my experience with the ASI294mc-pro has been excellent. I live in a colder climate in Upstate, New York, in the Northern Adirondacks (44 degrees latitude), where the temperature in the winter averages -12 degrees Celsius and the summer temperature averages 15 degrees Celsius. With these low temperatures and using the TEC cooler built in the ASI294mc-pro, I am able to cool the sensor to -20 degrees Celsius all year round which reduces the noise in the images. The ASI294mc-pro easily cools the sensor to 35 degrees Celsius below the ambient temperature. With the ASI294mc-pro, I found the noise level of an uncalibrated light frame to be moderate, but when calibrated with darks, the image was smooth and the noise was very minimal. A look at the stretched dark frames from the ASI294mc-pro, revealed amp glow, noise and hot pixels. I also dithered to eliminate walking noise which appeared in my images. The walking noise was visible in the image as a rain pattern running down the image. I was always able to process this out during processing, but when I take care during capture and use proper calibration frames, it makes processing a lot easier and the images appear clean and smooth at the end. I want to stress that, even though the ASI294mc-pro has distinct amp glow, I never had an issue with calibrating it out of my light frames by using dark frames.
Now that galaxies are starting to come up, I have been wanting to maintain my fast speed of f2.2 with the Hyperstar and at the same time, image the small galaxies. My options would be to take the Hyperstar off and go f7 or get a camera which has higher resolution and smaller pixels to crop much of the image out. I started looking at the cameras offered by ZWO and found the ASI2600mc-pro to be the camera which matched my goals. It first caught my attention when ZWO advertised it as a camera with a new Sony IMX571 back-illuminated sensor which was claimed to improve the camera sensitivity and reduce noise. ZWO also claimed there was absolutely no amp glow from this camera which I found interesting since every camera I have used has had amp glow. From looking further at the specifications, it had a larger APS-C sized sensor at 23.5x15.7mm, still maintained a deep full well at 50000 electrons, smaller pixels at 3.76 microns, greater resolution at 26 megapixels, and a 16-bit ADC which provided greater dynamic range. On paper, this looked like a great camera for my Hyperstar setup. When I plugged the pixel size in with the focal length, my sampling looked much better at 1.50”. It is better sampled than the ASI294mc-pro on my Hyperstar.
Upon receiving the ASI2600mc-pro and unboxing the camera, I thought, “Wow, this is a massive camera.” The camera was much heavier than the ASI294mc-pro and had a wider diameter. The ASI294mc-pro weighed in at 14.8 ounces and the ASI2600mc-pro weighed in at 24.7 ounces. The ASI294mc-pro has a diameter of 78mm and the ASI2600mc-pro has a diameter of 90mm. I was immediately worried about the secondary obstruction. I measured the secondary obstruction on the Hyperstar and it was 104mm, so I was safe. I also worried about tilt due to the weight, but the ASI2600mc-pro had a built-in tilt adjuster. The back-focus distance on the ASI2600mc-pro was 17.5mm, which is the same distance as using the ASI294mc-pro (6.5mm back focus) with the 11mm collar spacer. It was cloudy for the first several nights after I received the camera, so I spent time building dark frames and dark flat frames for my calibration frame library. ZWO recommended using two gain settings; zero and 100. I built two libraries at each of these recommended gain levels and at a sensor temperature of -20 degrees Celsius. To achieve this temperature while indoors, I put the camera in the refrigerator. This kept the camera cool, so there was less stress on the TEC cooler and the refrigerator acts as a dark box, with no light leakage. The dark frames were smooth and showed no signs of any amp glow when I stretched them. They were much cleaner than the ASI294mc-pro. I read more about the camera and learned more about it. I should have done this before getting the camera, but it was all good news I stumbled across. I never had an issue with a ZWO camera frosting up, but I have read about it being a
possibility and this crossed my mind with the larger sensor. I discovered the ASI2600mc-pro had a built-in polyimide dew heater built in to the front of the camera. So not only does this camera cool the sensor, but it also has a dew heater to keep the front window of the camera free of frost and dew. While talking about the TEC cooler, the camera has a two stage TEC cooler which can lower the temperature of the sensor to 35 degrees Celsius below the ambient temperature just like the ASI294mc-pro. Not only does it keep the sensor cool for lower noise, it also allows me to use consistently cooled calibration frames. When I attached the ASI2600mc-pro to the Hyperstar, I checked the collimation of the scope’s optics. I noticed the field of view was much wider, I had to re-collimate the optics as the stars were not pinpoint across the field. I adjusted the collimation and I adjusted for minor tilt. Once the camera was set on the Hyperstar, I started star hopping across the sky to see how the camera would perform on targets. The moon was in a waning crescent phase and I was imaging under Bortle 3 skies. I initially used a 2” Astronomik UV/IR L2 filter on the scope. After the first session and reading more on the ASI2600mc-pro, I realized it has a built in D60-2 IR filter, so the UV/IR filter is not needed. The ASI294mc-pro just has an anti-reflectivity window. Without the UV/IR filter on the ASI294mc-pro, I found the stars would have star bloat and the UV/IR filter really tightened up my stars. I started with the Horsehead nebula, at gain 100 and 60 second exposures. There was a gusty wind which caused my
guiding to be off a bit. The colors pulled from the ASI2600mc-pro were beautiful. I could notice a difference in the dynamic colors, especially in the Flame Nebula, compared to the ASI294mc-pro. Again, like I noticed in the dark frames, the noise was very minimal and the image appeared very smooth. I zoomed in to the stars and confirmed the stars were round and less pixelated like I figured it would be. After shooting a hour of Horsehead, I wanted to go after a smaller target to check what the image would look like when cropped. I shot 20 x 60 second frames of Comet C/2017 T2 PANSTARRS near the Double Cluster in Andromeda. I was amazed at how smooth and clean each individual light frame looked. I zoomed into the comet and again, noticed the difference in the higher resolution of the ASI2600mc-pro. In comparison of colors produced by the ASI294mc-pro and the ASI2600mc-pro, I felt the colors in the ASI2600mc-pro were more natural. In the past, I had imaged several smaller sized comets with the ASI294mc-pro. With the ASI2600mc-pro, the detail of the comet and stars were much more clear. I then moved on to M101 and took 3 hours of 60 second
exposures. I then moved on to NGC891 and took 3 hours of 60 second exposures. I used the newly released ZWO ASIair Pro for image acquisition and control. When finished capturing, I used my Spike-a-flat panel on the front of the scope to capture flat frames. During the imaging session, the outside ambient temperature was -18 degrees Celsius. The camera worked flawlessly. I had no issues cooling the camera and I kept the camera’s dew heater on. I took the camera off the Hyperstar to verify no frost was present.
I transferred the camera files over to my laptop. This is where I immediately found an issue that I did not consider. Each image file size from the ASI2600mc-pro was 50,966 kilobytes. The ASI294mc-pro was only 22,846 kilobytes. This really lengthened the time needed to calibrate, stack and process the images and it bogged down my computer. I used Astro Pixel Processor to calibrate and stack the images and PixInSight to process the images. Once the files were transferred, calibrated and stacked, I found the images produced by the ASI2600mc-pro were much easier to process. A simple bump in the saturation really brought out a lot of colors in the nebulas, stars and the galaxies. With the ASI294mc-pro, I used various masks to get the colors to pop out. This was not as necessary with the ASI2600mc-pro. I was even able to crop NGC891 into an acceptable field of view without losing too much resolution and becoming too pixelated.
For those who are looking to do narrowband imaging with an OSC camera, the ZWO duo band filter and the Optolong L-Enhance filter work well. I even used the OPT Quad Band filter with the ASI294mc-Pro and got amazing results. Or you can try your hand with narrowband filters and construct a Hubble Pallett using the Sulfur II, Hydrogen Alpha, and Oxygen III filters. There are many options available to OSC users. Sure, the mono will out perform the OSC camera, but we are for the most part amateurs. I see OSC images that put mono imagers to shame and I see mono imagers that really out shine the OSC cameras. At the end it is the technique and process.