Astro Imaging with a Digital Camera

In November 2001, I purchased a base model digital camera, (Kinel digital webcam/camera maximum 640x480 resolution 300,000 pixels) with the intention of using it for general photography. I had no thought of using it for astro imaging until I was looking at the Moon one day and placed it in front of the eyepiece and took a few happy snaps.

The images were quite good, so I thought I'd try it on my 8" equatorial driven telescope. This required making a mount for the camera. The first mount was okay but awkward to use, making it difficult to centralise the camera with the eyepiece at high powers. I'm now on Mark 3 mount and it works well.

The problems

There was also a lot of work to do on the 8" 'scope; backlash in the RA drive caused the planets to wander out of view before the drive would take up, especially with small field of view at high powers.

I had to make the telescope mount more rigid, and construct a declination drive, the scope also needed to be remote controlled from my observing hut because this is where the computer is housed. With these problems solved, and with every adjustment a better result.

You might think all this un-necessary for imaging the Moon and planets. However the camera is very sensitive to vibration. This produces blurred images, and since the lens on the camera is very small (3mm), it must be precisely centred with the eyepiece. Camera lens distance from the eyepiece doesn't appear critical, but it seems the best is the same as eye relief distance.

The other problem with the camera is contrast adjustment. This was overcome by stopping down the aperture and filtering. With more expensive digital cameras, you usually get a display screen, with a cheaper camera you don't. This meant I had to place a remote monitor at the telescope to achieve final alignment and focus of the image. When this is done I switch back to the main monitor in the observing hut.

Capturing the image

The camera itself is used in web cam mode - either on record or just as a continual unrecorded image. In continual, you do a frame capture when you have a moment of steady seeing appear. In record mode you capture as a movie in AVI format. You may then play it back and pick the best images from the frames, then convert the appropriate frames into a still image format -- JPG, TIFF or whatever you prefer to use. Now it's just a matter of playing around with the images in a photo editing application to achieve the desired result.

Easy guiding

The other use I've found for the camera is as a guider for astrophotography, imaging stars of magnitude 6 to 7 on a 8" scope. I defocus the star image, and stick a scale in arc seconds on the computer monitor. Using the remote controls I keep the defocused image centred against the scale, enabling precise tracking for piggy back photography.