Time for an upgrade
Ever since I got into doing astrophotography, I’ve been imaging with a DSLR. My very first attempts were with a Nikon D7000, and then I quickly switched to a Canon T3i. I was comfortable with it, largely because of Backyard EOS – the software I use to control my Canon. Now, as I have grown in my skills, I’m ready for a new camera with specialized features. I’m ready for the ZWO ASI174MM Monochrome planetary camera. I’m ready to kick some ass!
The need for an upgrade to my DSLR camera came about only because of my planetary astrophotography interests. Anyone who has tried to image a planet or our Moon knows about atmospheric seeing and the effects of scintillation. Perfect seeing conditions don’t exist every night, and when they do, sometimes they last only a second or two. Being able to catch these fleeting moments is known as lucky imaging. It makes sense that an imager would want to take as many images as physically possible during moments of good seeing. The Canon T3i is only able to collect an average of 20 frames per second. I really wanted to improve my ability to collect more frames in the same amount of time. I needed a high speed camera.
The ZWO ASI174MM USB 3.0 camera
I’m not going to lie, I did most of my initial research on Facebook… I started seeing several photos of Jupiter being produced with exceptional sharpness, and the photographers were doing this in ‘average’ seeing… These images were better than my best under ‘exceptional’ seeing conditions. I noticed that none of them were using a DSLR for the photos, and a few were using a camera I’ve never seen before: the ZWO ASI120MM. I started researching this particular camera online. I quickly decided that I had to own one. While saving some cash, and selling old equipment online, I checked back to the ZWO website and saw that there was a newer camera that was going to be released shortly which would be better suited for my telescope. This was the ZWO ASI174MM USB 3.0 camera. As far as planetary imaging performance is concerned, on paper the ASI120 and ASI174 have very similar specifications. However, I have a large aperture SCT, and having a bigger sensor will definitely help ensure that Jupiter or Saturn, and their moons can all exist in the frame, even if I’m using a 2X Barlow and shooting at F/20.
There are a venerable cornucopia of resources you can use to read up on the factory specifications of this camera, and therefore I’ll not pollute the interwebs with yet another rehash of what’s already been said. Instead, I’ll be covering the performance of the ZWO ASI174MM astrophotography platform in real situations as tested by me.
For starters, I placed my Orion ED80mm guide scope on my home office desk and pointed it out the window in the direction of my neighbors house. Speaking of my neighbors, I always get funny looks from them, and I can’t quite figure out why. Anyway, I connected the camera, fired up the imaging software and found focus. I played with some of the settings, and saw how my frame rate changed based on using 16bits, binning, and shutter speed. I noticed that I suffer a significant reduction in frame rate when binned. Less surprising was that the higher the gain and longer shutter speed was, the lower the frame rate became.
I set the capture duration to one second, and let the camera rip through as many frames of my neighbors roof as it could. I did this for each of my 4 filters, RGB+IR. I found that the average file size was over 100mb for each file. This is a little concerning, as my 256 GB HD is already about 60% full. At this rate, a 30 second file may be as large as 3 GB, and with 4 channels to capture, collecting enough data for a 30 second stack may eat up as much as 12 GB of disk space. Because my laptop has only a single USB 3 port, writing the files directly to a high capacity external drive via USB 2 may negatively impact my capture rate.
This fear was realized when I did first light with the camera on March 29th, 2015. I set the camera to maximum resolution, and was capturing about 120 frames per second of a waxing gibbous Moon. I took three videos, each was was 90 seconds long. The three videos produced by the ZWO ASI174MM consumed 60.8 GB of disk space! 3 minutes and 30 seconds of video ate up nearly all of my laptops free space! To make matters worse, it would take nearly 60 minutes to move this data from my laptop to an external HDD via USB 2.0. I wouldn’t know for sure, because some error occurred during the transfer, and all 60 GB of data disappeared. I’m going to need a better solution. Also, this is going to take considerable research to find the right balance of hardware if I want to use it as a high speed planetary camera!
ZWO ASI174MM external USB 3.0 HDD capture speed
It took me a while, but I was able to finally track down a 1 TB external USB 3.0 HDD that also came with a 7200 RPM drive. Best of all, it was on sale at the local computer super store for under $150. I paired this up with a 4-port USB 3.0 hub. I do have one caveat to this test: Everything will be running through a single USB 3.0 port on my laptop. I will be reading from the camera at the same time and through the same USB port that I write back out to the HDD. There will be a lot of traffic coming out of the ZWO ASI174MM, but the hope is that the port and HDD can keep up. Fortunately, I don’t require clear skies to actually perform my testing, I just need to have some kind of subject so that data can be read from the sensor of the ZWO ASI174MM Camera and fed to the external HDD.
Using my home office and a window on the other side of the house, I was able to setup the camera to point at a neighbors roof (again). I didn’t even need to focus the telescope for this test, I’m not looking for data quality, just the ability to save data. A focused image should be about the same file size (roughly) as an unfocused image. At least, that’s the theory…
Well, now it’s on to the speed test! I had everything connected to my laptop, and launched the FireCapture program. I changed the directory that the camera output saves to the root of the external HDD drive. I changed the Region of Interest (ROI) to the full sensor, which is 1936 by 1216, and let her rip! This thing is still pretty snappy, but things could be better. It’s obvious from the paltry frame rate (bounced around the high 20’s to low 4o’s) that I’m not using this frame size and method often… I really would only need to capture the full frame if I were shooting the Moon or the Sun, and normally those aren’t my preferred subjects. For planetary work, I’d certainly want to shoot a smaller ROI, such as 1024 x 768. Below I have a series of screen grabs of various capture sessions where the only variable I changed was the ROI
As you can see from this simple but effective test reducing the ROI of the ZWO ASI174MM to smaller sizes helps to increase the throughput of the USB 3.0 port and hub to allow a huge number of frames to be saved per second directly to an external HDD! Binning the largest ROI (full sensor) also helped the camera save nearly 130 frames per second at peak. There is considerable fluctuation in the frame rate still, and it’s not entirely stable.
There is no need for any expensive SSD external drives just yet – which I nearly bought. My biggest takeaway from this test is that I can read from the camera while writing to the external HDD on the same USB port, and I can do so at a very fast frame rate!
ZWO ASI174MM first light (again)
Since my original first light with the ZWO ASI174MM was a bust due to another monumental computer glitch (read: likely user error), and all of my lunar data disappeared from my laptop during the copy to my PC I had to take a mulligan. This time, my target was a much more in line with my astrophotography interests: Jupiter.
On April 25th I setup the Celestron, laptop, external HDD, USB 3.0 Hub, the ZWO ASI174MM and filter-wheel. I did an all-star polar alignment and slewed to Jupiter. Once I found focus in the green channel, I tweaked the gain, exposure and gamma settings to my liking. I cycled back to the red filter and began recording 3000 images per channel. I got quite good at moving the filter-wheel and synchronizing the FireCapture program in the dark. I was able to record all 3 channels in under two minutes. I recorded 5 sessions, with three channels per session. Some sessions recorded directly to the external HDD and others were recorded to my laptop then moved.
When capturing directly to my external hard drive with a ROI of 1024 x 768, my frame rate was somewhere around 190 frames per second. If I binned the image my ROI changed to 960 x 592 and the ZWO ASI174MM frame rate dropped to about half, averaging 90 frames per second. Recording a ROI of 1936 x 1216 (Full frame, no binning) directly to the laptop hard drive yielded 126 frames per second. These in the field tests are in line with my bench tests from the previous section, which was really encouraging!
There are some trade-offs with binning the image, obviously. My frame rate decreases to half the speed I get with 1024 x 768, however because the signal to noise ratio or SNR increases dramatically, I was able to actually expose Jupiter properly, and also capture all 4 of the Galilean satellites at the same time! Previous photos that I’ve seen required doing two stacks, one with Jupiter properly exposed, and another over exposed image to get the moons. Later the two images are layered together to get the whole system. With enough practice and honing the settings in a little better, I think I can continue to improve upon what I’ve been able to do in my first attempt, and really start showing the planet and the whole moon system in action! Try doing that with a DSLR!
Fire capture supports several output files and when I finally figured out what I was doing, I saved my files directly to AVI format. This is very convenient for me as AutoStakkert! 2 likes the AVI format. Also, they are easier to work with than a huge directory full of image files such a FITS or JPG. Of the 5 sessions I recorded, I was able to create two very nice images of Jupiter and the Galilean system. My very first session recorded was more or less useless because some channels recorded to AVI while others saved all the frames as SER format. Because AVI is a series of compressed JPG images contained in a single movie file, my concern was that the SER file saved by the ZWO ASI174MM camera would be compressed differently and the files quality would different enough to affect the outcome of post-processing.
I need a few more outings with the ZWO ASI174MM and FireCapture to really pin down the optimum settings for gain, exposure and gamma. I think I’m close with the settings I used Saturday night. I hope that my next posting about the ZWO ASI174MM camera should be more of a How-To for planetary astrophotography. I believe that I know enough now that I can show a progression of frame-rate and image quality when modifying the settings for planetary astrophotography in FireCapture.
The bottom line
As far as cameras go, this thing is a beast! Blazing fast capture rates at large resolution and the ability to write these files to a USB 3.0 External HDD on the same USB port is mind blowing for me! Before testing, I had a feeling that writing the the external drive with any ROI setting would impact my frame rates dramatically. It may not be as fast as the reported 390 frames per second on the website (640 x 480 ROI), but averaging between 90 and 190 frames per second is still a massive improvement over the Canon T3i DSLR’s average of 20 frames per second. I can collect the same amount of frames in about 10-20% of the time. This means that by using this camera I can record the same amount of data in less time. More frames in less time increases my chances of getting many high quality frames. More frames in less time means I can get a large stack of data with less planetary rotation blurring my results.
Recording each channel to a monochrome camera also ensures that my color depth is more accurate. Each pixel is a real combination of RGB data, not an extrapolation of neighboring pixels. My first light with the ZWO ASI174MM camera was a bit clumsy for me compared to using the familiar Backyard EOS and Canon T3i. Even still, my results from April 25th, 2015 are among my best planetary images yet!
Clear skies, and thanks for taking the time to read!