Astronomical Society of Edinburgh
Journal 43

Postcard from Earlyburn

Image of the Orion Nebula (see text for details)

Image of the Orion nebula

Inspired by Nikki MacLeod's talk at last years' Members' night [1], I went out and got myself a webcam to use as cheap CCD astro camera. To acquire images from the camera I use the AstroVideo application [2] on Windows 98, on a Pentium II 267 MHz laptop. Individual exposures are necessarily much shorter than one second; AstroVideo can add up a number of such frames before writing the image to disc, and it can take a sequence of such images with a single button click. The output is appropriately stored as 32-bit FITS rather than an 8-bit graphics format.

The immediate adding of frames that AstroVideo does will not take care of movement of the object across the detector due to tracking errors. So part of the summation has to take place later (off line) when there is time to determine the shifts between the constituent images. For this data reduction I use the Red Hat Linux installation on my laptop and various applications from the Unix Starlink Software Collection [3].

The webcam had its original lens removed and a standard M42 photo gear glued on. It can therefore be used with any M42 photo lens, but also with my Celestron 8 in prime focus or eyepiece projection. Since the individual frames are so short, accurate polar alignment or guiding are not strictly necessary. I have even taken images with just a 50 mm lens on a stationary tripod - no tracking at all. On the other hand, more disc space and time must be spent on collecting and combining the images.

More information about using webcams for astro-imaging can be found at or through two Web sites ([4] and [5]) that are listed below. Much more specific to how I use the webcam is [6].

The image of the Moon on the front cover was taken on 2001/01/13 from Earlyburn with an f = 400 mm, f/6.3 tele photo lens. Since the field of view is only 25' by 19', this is a mosaic of 9 images. These in turn are added up from 16 frames, which should increase the dynamic range of the camera from 7 bit to 11 bit. The final mosaic was also fed through an unsharp-masking algorithm. This suppresses large-scale features and emphasises the finer detail and thereby gives a sharper appearance.

The image of the Orion nebula was taken on 2001/02/12 from Earlyburn in the prime focus of the telescope, hence f = 2000 mm, f/10. Since the field of view is only 5.0' by 3.8', this mosaic of three images shows only the innermost part of the nebula. For each of the three fields up to 256 frames from the webcam are added up, 8 frames per image on-line and 32 such images off-line.

Just above and to the right of the centre is the Trapezium, the quadruple star theta1 Orionis. There light heats the surrounding gas to 8000 or 10000° and causes it to shine. To the left of the Trapezium is the 'dark bay', where cold dark gas obstructs the bright nebula. Below the centre is the star theta2 Orionis A. Just above it runs the shock front where the bright nebula and the starlight plough from the top into the dark cloud below.

References

Horst Meyerdierks