Quantifying the ASI174MM sensor noise
One of the most demanding challenges any astrophotographer will face with every project is generating a sufficiently high Signal to Noise ratio (SNR). There are several kinds of noise in every sub-exposure and a detailed description of each kind of noise goes beyond this posting. My focus here is the method I used to quantify the ASI174MM sensor noise as it relates to long exposures. The simplest way to do this test is to accumulate a series of dark frames in a controlled setting. I should mention at this time, that the ‘noise’ I am actually measuring is really signal. Remember that when I am referring to the ASI174MM sensor noise in this article, I’m really referring to the thermal signal generated by the imaging sensor itself.
Taking the dark frames
This is a single 180 second frame from a series of tests run on my ASI174MM Camera. This shows the challenge of trying to deal with the ASI174MM sensor noise.
I had to determine the ASI174MM sensor noise for various exposure times. Because the sensor is not cooled, and is also a CMOS chip, I was not able to accumulate a dark frame library the same way cooled CCD cameras can. I could still create a dark frame library for observing the ASI174MM sensor noise, however.
With two filters in the path of my optical train, I am almost certain I’ll be running very long exposures for light frames. I’ll need dark frames with a corresponding time component to offset any of the thermal noise accumulated by the sensor as it warms up. Some resources online have shown that the the ASI120MM camera is noisier than desired for deep sky photography, however, the ASI174MM has a different chip, and was also shown to have lower sensor noise.
I know that if I am able to successfully use the ASI174MM for DSO imaging, I’ll be taking photos with a variety of exposure times. Therefore, this test will also need to include several exposure times for the dark frames. I chose to build a out a small dark frame library with the below settings. You can click the links to see the full resolution master frames. (opens in new window/tab) Because they’re full resolution, and there are so many I decided not to embed them. This page would take forever to load!
- 5 dark frames at 30 seconds.
- 5 dark frames at 60 seconds.
- 5 dark frames at 120 seconds.
- 5 dark frames at 180 seconds.
- 5 dark frames at 240 seconds.
- 5 dark frames at 300 seconds.
With this test library I should be able to identify a sweet spot of exposure times using the ASI174MM for imaging deep sky objects. A huge challenge that I’ve been dealing with over the last few months is learning a new camera control software platform.
FireCapture was my original platform, however it doesn’t offer features I consider essential, such as a delay between exposures to allow the sensor to ‘cool’ or a dithering feature to be used with PHD2. Also, I’ve grown to think that the interface is too flexible as to be clunky and cumbersome. I want something a little more like BackyardEOS which has a more logical layout, and more intuitive control over the camera. I’ve tried SharpCap and APT, and have since settled on APT as the platform of choice.
My last few silent months have been spent tinkering with FireCapture, SharpCap and APT to learn which I like best. Now that it’s settled, I’ve dedicated some time to learning the necessary features and functions of APT. The Astrophotography Tool is missing one minor feature which is not necessarily a deal breaker by not outputting the sensor temperature. For simple visual inspection, knowing the temperature is not necessary. However, if I were to do a detailed analysis of the heat, and how it contributes to the thermal signal in the ASI174MM sensor noise I’d have to use FireCapture.
One final observation I had made when conducting the tests was the considerable amp glow present in long exposures. Large amounts of amp glow are all but impossible to remove during post processing for me. Fortunately, my tests were published online immediately after they were completed, and Sam from ZWO came across one such test that highlighted the problem. Sam reached out to me to beta test a new driver aimed specifically at reducing the amp glow part of the ASI174MM sensor noise. I had to redo my tests to help him validate the driver, so this article needed to be postponed, and then updated to reflect the new driver.
ASI174MM driver 126.96.36.199 beta testing
I spent 5 days taking hundreds of dark frames in my home office, testing the driver. I had access to three versions of the driver and I wanted to test the newest beta against previous versions to see the improvements. Depending on the driver installed the ASI174MM sensor noise and amp glow could be completely different.
It took a day for me to figure this out, but apparently, there are default gain and offest settings coded into the driver. Older versions used a much lower gain for the ASI174MM so the sensor noise appeared lower. This cost me a day or two of testing, but was a simple enough adjustment to make in APT.
My driver testing strategy was straight forward enough: Install a driver. Launch APT. Capture dark frames of varying time lengths. Create master dark frames. Upgrade ASI174MM driver to next version and start over.
My dark frames were captured in the following time lengths. Once again, you can click the links to see the full resolution master frames:
The master dark frames were made using DSS and the recommended settings for having a single light frame (which was a copied dark frame). The dark frames were stacked in their original FITS format. This would ensure minimal loss of data for analyzing the ASI174MM sensor noise in the master dark frames. Since this was not an in depth analysis of the the performance of the chip and driver, I didn’t need to do a pixel level comparison in a camera inspection program. Simple side-by-side images easily showed the amount of amp glow present in the camera, and the improvement the new 188.8.131.52 driver had on the ASI174MM sensor noise.
The new ASI174MM driver (which was still a ‘beta’ release during the writing of this post) decreased the overall sensor noise for the ASI174MM. A major component to the signal was the amp glow. It’s beyond my technical expertise at this time to say how much of the removed sensor noise was related to the amp glow. It’s obvious to me in the high quality, full resolution images, that the amp glow has physically moved, and is now sourced in the lower right corner of the dark frames. This corresponds to a location in the PCB where there are two 220 ohm resistors. as seen in the image below.
I also believe that the remaining improvement was due to better overall noise reduction in the software. One theory I proposed online is that the ASI174MM sensor doesn’t warm up as much using the new software. It’s clear that the ASI174MM sensor noise has been reduced to a level where long exposure deep sky astrophotography with this camera is now possible.
ASI174MM sensor noise test conclusions
When I began testing the dark frames I was immediately frustrated at the amount of amp glow and overall sensor noise of the ASI174MM camera. Based on my early tests, doing any meaningful deep sky photography with this camera is all but impossible. The ZWO ASI174MM camera was very ‘hot’ – in some tests getting as warm as 107 degrees F. All that heat simply accumulates as signal the longer the exposure is allowed to be. Until the new 184.108.40.206 driver from ZWO becomes available for the camera I don’t expect to see any multi-minute exposures produced by the camera. That being said, hopefully the new driver is released to the rest of the world soon. When that day comes, ZWO will have a nice entry level DSO camera on their hands.
In an upcoming post – you will see how well the camera can do on deep sky objects. I’m still working the kinks out of a few other bugs in my kit. Once that’s been squared away, I should be able to test the filters in the field. I will also see how the camera performs with narrow band imaging as well!
I’d like to hear from you about how you think the ASI174MM sensor noise has affected your imaging. Post a comment below and let me know your successes and failures!
Thanks for reading, and remember, keep looking up!