Cassiopeia Observatory

It was a Darks and Cloudy Night

Posted: 23 December 2011

I am testing some image processing software and had been waiting for clear skies in order to acquire some "dark frames" and image a couple of faint DSOs. However, the weather had not been cooperating, so Thursday night, 22 December, I decided to just take some "darks" even though the sky was cloudy. I opened the observatory at 1740 MST, 45°F. I mounted the Nikon D7000 DSLR at prime focus of the Meade 8" LX200-ACF and attached the camera AC power adapter. I left the aperture cover on the telescope so that no light entered the telescope. I then waited an hour for the camera to cool down to the ambient temperature.

I began taking the dark frames at 1840 MST, temperature 43°F. Over the next two and half hours, I did 36 exposures. At each ISO setting of 800, 1600, 3200, 6400, 12800, and 25600, I took exposures of 30 seconds, 1, 2, 3, 5, and 10 minutes. I did the six exposures at one ISO setting and after the set was completed, I turned off the camera for 2 minutes to let it cool back down. I then did the next ISO setting and repeated the exposures, followed by a 2 minute cool down period.

Since the sky was cloudy I left the observatory dome closed and used only red floor lighting inside. While the dark frames were being acquired, I listened to the audio from the movie "Forbidden Planet" on my iPod. At 1933 MST, I began hearing raindrops hit the dome. Fortunately, it lasted only a few minutes. At 2105 MST, I went outside of the observatory and was surprised to see stars. The sky had cleared up somewhat, although there were still clouds that would prevent any DSO imaging this night. A few minutes later, the final dark frame exposure was completed.

During post-processing of the dark frames in Aperture software, I increased the luminance levels of the images until noise appeared. These were then exported as TIFF images for use by the software I am testing.

Here are a couple of examples of the dark frames, with the luminance increased to just show the noise. The first one is a 5 minute, ISO 6400, exposure. This is my usual faint DSO imaging setting.

As you can see, very little noise affects my DSO images at this exposure setting. Increasing the luminance further would, of course, show more noise.

This is a 10 minute, ISO 25600, dark frame, and is a worse-case scenario. I rarely use ISO settings above 6400 when imaging DSOs, but at times the higher ISO settings are useful for framing test images of very faint DSOs.