So, I received my new ASI071 one-shot color camera over a month ago. But the weather here is typical for our winters – one big, continuous cloud bank punctuated with very rare nights of clear skies. As a result, I did some general indoors testing and dark testing, got my USB configuration upgraded, and waited for a clear night. One finally arrived on Jan 15. It was very cold though so I didn’t want to spend much time outdoors, and the moon was about 85% illuminated so I just went for the brightest target, Orion. Wasn’t expecting too much so didn’t set up for a full night of imaging. This was just a test run of the new camera.
But to back up a bit, I actually did get a short clear break about a week before that, just for a couple hours, and was able to do some test exposures in an effort to see where I was in relation to Jon Rista’s 20xRN rule (20 times the Read Noise). I was just aiming at a random star field.
I first needed to figure out how to set gain and offset. There are 3 data points provided in the driver settings as potential starting points:
• Maximum Dynamic Range: 0 Gain, 8 Offset
• Unity Gain: 90 Gain, 20 Offset
• Minimum Read Noise: 240 Gain, 65 Read Noise
Using the chart on the ZWO site, I could determine the real gain (Electrons per ADU) and Read Noise for various gain settings. The offset I used was just interpolated between the three settings provided by ZWO in the driver. I then calculated the 16 bit value I would shoot for in SGP for the 20xRN. I tried to get an exposure time long enough to produce a mean value in SGP that was greater than or equal to 20xRN, and short enough to saturate only a few stars of the brightest stars at most. Here’s the results:
I calculated the 20xRN value for a variety of gain/offset settings, but did not test them all. The last two columns are the test data. Note that I was able to achieve the 20xRN exposure for all but the 240-gain setting test. The dynamic range had been reduced enough in that test that many stars were saturating unless I kept the exposure short. In the zero gain test, no stars saturated. As the gain increased, more stars began to saturate at the 20xRN exposure, with the 150-gain test being barely tolerable.
Of course, this test was on a single night with its specific level of skyglow, and nothing in the image except some random stars. I would have to do some test exposures on the night of actual imaging, but at least this gave me some idea of what I was dealing with. Normal exposures would likely be in the 3 to 5 minute range, which is about the same as I was experiencing previously with my DSLR.
So ... back to Orion on the night of January 15th. After doing all the upfront testing, I’d know what to shoot for as a mean value in SGP – except I forgot to bring that chart out to the observatory! Oh well, too lazy to go back in and get it, it’s just a night for a quick test, and I know the Orion core is bright so I’ll just go for a small amount of saturation at unity gain and be done with it. Took a few test shots and it looked like 2 minutes would be a good exposure. Shorter than what I was thinking so I figured I probably wouldn’t hit the 20xRN target. So I set up SGP for 40 lights at 2 minutes. Went back out an hour and a half later to run a bunch of flats, bias and dark frames.
Next couple of weeks I finally got into PI and started processing this image. Went through it several times learning the ropes, and this is what I came up with on my last time through:
Here's the full image in Astrobin if you want a closer look:
There’s a couple of things to point out here. First is the dust on the left. Remember this was a night with 84% moon illumination? There’s no way I would have pulled that out of the moon’s LP with my cooled DSLR. The noise would not have let me stretch it that far. I’m frankly amazed at what this new camera can do! The cooling is much more efficient than the DSLR, providing a MUCH lower noise floor. That lets me use most of the camera’s dynamic range, rather than the lower part of the range being unusable due to noise.
In addition, while both my T3i and the ASI071 have 14 bit data, I used the T3i at ISO 800 to 1600, which limited the dynamic range to 9-10 stops. The ASI071 at unity gain is giving me 12.5 stops – that’s a huge improvement! Which brings up the second thing to notice – the core is not blown out. The Trapezium stars are indeed saturated, but prior to being stretched, the core was easily visible. Took me a while to learn how to brighten up the dim parts while keeping the core intact, but that’s all from the same image. So it’s a testament to the dynamic range of this camera that you can cover everything from the core of Orion to the dim outer dust areas in a single exposure. I could have even done better if I had reduced the gain to say 50 and picked up another half stop of range, at the cost of more exposure time.
As for the 20xRN rule, I looked at my subs while processing and found that they had a mean value in SGP of right about 500. That’s well over the calculated 20xRN target of 296, so I could have reduced my exposures more, still gotten good results, and had the brightest stars a bit less saturated.
So I’d have to say I’m a happy camper with this new camera. It blows away my T3i, despite the fact that I had done a full spectrum modification and added cold-finger TEC cooling.
Edited by Midnight Dan, 05 February 2017 - 03:39 PM.