ASI2600 Testing Continues

Test of new OAG spacing

You may recall in my last post I had discussed the plan to reset the spacing of the ZWO OAG on the scope to help with the strong vignette of the guide scope camera image. I moved the 15mm spacer in front of the OAG, mounting the OAG flush with the filter wheel. A thin 0.5 mm spacer was used between the nose piece and the 15mm spacer to act as a stop when the assembly was attached to the Moonlite 2" adapter. As expected, the pick-off prism is now rotated a bit from the center of the sensor's long axis.

Although this did improve the guide image, there is still an obvious vignetting present. The ZWO prism and light guide are just very small compared to the Celestron OAG.

Gain Settings

Early Friday morning I imaged the Sunflower galaxy, M63, so that I could compare the results of the ASI2600 with the image I took with the ASI1600 back in March. I ran a similar set of LRGB subs, 60sec exposure, but I used Gain 0. It turns out that these subs are essentially useless as I needed either a longer exposure time, or higher gain setting. The images did look rather nice, but they couldn't be stretched in postprocessing since they lacked sufficient dynamic range.

The darks at Gain 0 on the ASI2600 produce frames of 500ADU average at -10C. The 60sec lights of M63 came in at 520-560ADU. This means that once the dark frame data is subtracted from the light frames the net result will be only 20-60ADU. This is extremely low signal content, especially with a camera that has a 16bit ADC (0-65535). Any amount of stretching to bring out the faint details will produce a final image that is extremely 'posterized'.

To illustrate the point, I took a few subs of the Blackeye galaxy, M64, last evening during a very short period of clear skies in that direction. Clouds interfered with the 60sec gain100 sub so I couldn't do a comparison with the 60sec subs.

Three subs, 60secGain0, 120secGain0 and 120secGain100, are shown in the image below. A quick stretch of each was made to bring out the details. As you can see, the images themselves seem pretty nice. But this is misleading as each stretch is a different relative strength, normalized so that they all appear with the same relative level of brightness and contrast. When a background model (ABE) is created for each sub the underlying problem is made apparent. Notice the model of the 60 sec gain0 sub. There are only 11 levels of dynamic range in this image. Once stretched and processed the result will be very blotchy and posterized - there simply isn't enough data in the sub to work with. Note that one of the reasons I purchased this camera was to get higher dynamic range, 16bit vs the 12bit of the ASI1600. At gain0, 60sec, the result is worse than that of the 1600.

However, when setting the gain to 100 and increasing the exposure time to 120sec the dynamic range is greatly improved. At Gain100_120sec the mean ADU of the image is 1071, twice that of the comparison dark frame.

Reviewing the specs of the ASI2600 suggests that Gain100 should be the optimum setting as there is lower read noise at gain 100 than at gain 0, and the dynamic range is almost the same. The down side is that the full well capacity is greatly reduced (16000 down from 49000). So, for long exposures, the lower gain provides some safety in protecting from blown out star images as the camera can hold far more electrons that at gain 100. Since my general process is to take more, shorter exposure subs, the higher gain is the obvious first choice. 

Since my earlier M63 subs suffer from the low dynamic range, I will need to retake that series once the weather clears and the moon is out of the sky - probably next month - and then do the comparison. Stay tuned!