Deep Sky Image Processing Example

Click on the images below to see the full sized image.

The image above is a raw photo from the CCD camera.

This is a 5 minute exposure, with the CCD chip cooled to -15 degrees C.

It was taken with a ST-7XME Camera.

Notice that it is very noisy, and the light level is uneven.

Dark frames need to be subtracted from the raw image to eliminate dark current effects

that cause a lot of speckles in the raw image. Above is a dark frame: 10 minutes at -15 degrees C.

It needs to be exposed at the same temperature as the image. To make a dark frame the shutter

of the camera is closed, or the lens cap is placed on the telescope.

Also, because the optical system does not evenly illuminate the camera frame (vignetting), and because

dust collects on the optical surfaces, flat frames need to be taken to cancel-out these problems.

Above is a flat frame for the M51 image. It shows vingnetting in the unevenness of the illumination, and

the round “donuts” are caused by dust near the CCD chip. It was taken immediately after the M51 images were taken,

with a diffuser (T-shirt) affixed over the telescope aperture, and the telescope pointed at a white board lit by a fluorescent light. The idea is to

capture an image from an very evenly illuminated source, with no intensity variations. This allows the unevenness of the

telescope, and the dust donuts to be cancelled.

The above image has had the dark frame subtracted and the flat field applied.

Notice that a lot of the noise and unevenness has been eliminated. Still, it is pretty noisy.

This is due to the small number of photons that have hit the detector. A longer exposure could

fix this, but that would require the telescope to track the object through the sky to a much higher

degree of precision, and could cause problems of over-exposure.

The above image was made from averaging 20 dark subtracted, and flat fielded images.

The image is much smoother and shows a lot of detail. The images need to be aligned with

each other. This can be done automatically with most astronomical-imaging software. The software that

runs the camera can be instructed to take a large number of exposures automatically.

The image above has been processed to enhance the low level details using a Digital Development Process (DDP) algorithm.

This adds extra “punch” to the image. There are many more processing techniques that can be applied to the

image. It’s usually best to resist the urge to over process the image.

Background image is M104, the sombrero galaxy,

processed as described above.

Telescope: Intes 6” f/6 MN-61 Maksutov-Newtonian.

SBIG ST-7XME & ST-237A CCD camera.

Losmandy GM-8German equatorial mount.

Software: CCDSoft (camera control), MaxIm DL and AIP4WIN (Processing)

Phil Schumacher 2004