Thursday, January 10, 2019

Wide Angle Sky Pictures

First Try

My ZWO ASI120mc color video camera has a small 150-degree all-sky lens which can be mounted directly in front of the camera. When this lens is in place the camera can be used without a telescope to take wide angle images of the sky. The next picture shows the camera with the 150-degree lens attached.
On the relatively mild, clear, moonless night of January 5th I decided to try wide angle sky imaging for the first time. It was a rare night in my back yard with fewer bright nearby house lights than usual.

The camera was attached to a fixed camera tripod and did not compensate for Earth's rotation. A ten-second exposure with camera tilted slightly southeast produced the following image.
Orion and Gemini are rising at the bottom. Taurus and Auriga are in the center. Just above Perseus at top center an airplane is passing with blinking lights. Turning the camera northeast moved the view a bit and revealed Cassiopeia at top left of center above Perseus.
A few houses away a neighbor decided to turn on an outdoor spotlight producing annoying glare. With 10-second exposures surrounding lights are overexposed.
Tilting the camera to look up at the zenith eliminated neighborhood lights around the horizon. The next picture is the best of all my trials.
The image above was produced by stacking four 10-second exposures. Although the background sky looks grey from light pollution, hints of the dim winter Milky Way can be seen slightly left of center running vertically upward through the constellations Cassiopeia, Perseus and Auriga. Pegasus, Andromeda and Ares are visible to the right of center. The bright object to the right of Pegasus' square is Mars. Even the small dim patch of the Andromeda Galaxy is barely visible. (Click on the image for a larger view.)

Unfortunately, I spent most of the time trying to take single frame "snapshots" instead of capturing several frames and then stacking the results. I should have used this video camera to take actual videos! That's what happens when I rush outside on the spur of the moment without planning the imaging session.

Long exposure times were not possible because Earth's rotation would eventually smear star images. Also, light pollution washes out background darkness as exposure time increases. The next sequence of "snapshot" images shows the effect of increasing exposure time. In order, from first to last, the images have exposure times of 10, 15, 20, and 30 seconds.
Notice the increasingly bright sky background in the above sequence as the winter Milky Way becomes more noticeable. Notice also the scattering of red pixels throughout the images, probably caused by thermal noise and "hot pixels" in the uncooled camera. (The red pixels are easier to see if you click to get an enlarged image.) In the future I'll need to use a dark frame to eliminate background thermal noise.   

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People say I'm crazy doing what I'm doing
Well they give me all kinds of warnings to save me from ruin
When I say that I'm o.k. well they look at me kind of strange
Surely you're not happy now you no longer play the game

People say I'm lazy dreaming my life away
Well they give me all kinds of advice designed to enlighten me
When I tell them that I'm doing fine watching shadows on the wall
Don't you miss the big time boy you're no longer on the ball

I'm just sitting here watching the wheels go round and round
I really love to watch them roll
No longer riding on the merry-go-round
I just had to let it go

John Lennon