Ursa Major - probably the best known constellation in the northern sky

The Plough - Part of Ursa Major

(The image above only shows the part of the constellation known as the Plough, or Big Dipper.
The full constellation extends quite a long way to the right and below the area shown)

The original image was taken with Canon AE1 with a 50mm f1.8 lens [1]. The camera was stationary during the exposure. With a longer exposure time the stars would have started to show trails. The solution to this is to guide the camera, but this is easier said than done !

The slide was then copied using slide duplicator onto 25 ASA print film, the main reason being to reduce the orange 'skyglow' (from streetlights) and make the stars stand out better against a dark background.

The 6 inch x 4 inch print was scanned [2] at 400dpi 24bpp colour, resulting in a sprite file [3] of 8.9Mb.

It has been retouched [4] to remove obvious defects which did not appear to be stars - single pixels or small groups of irregular shaped pixels. It's possible that the latter could have been feint stars or galaxies but the resolution, brightness or shape was not high enough to be confident of this.

Converting the image [5] to the GIF file seen above reduces the file size to an incredible 8,663 bytes, for an image 1919 x 1165 pixels. It has been reduced to 1/3 of this (640 x 388 pixels) to fit your browser screen. Save the GIF image as a file you will see much more detail than in the image above. (RISC OS users can drop the file into !ChangeFSI)

Obviously the processing loses some of the definition of the original, but a large number of stars and their colours can be seen, amongst them the seven stars of the Plough, which include the double star Mizar and Alcor (¼ way in from left, ¼ way down).

The moral is - you too can take pleasing astronomical photos with modest equipment.

To avoid star trails here's a rough guide that I calculated. With a 135mm lens (on a 35mm camera) an exposure time of 5 seconds is about the maximum you can rely on. Halve that focal length and you can double the exposure time, double the focal length and you must halve the exposure time, but unfortunately the stars (and planets) are not very bright, so a high speed film and a lens with a large aperture are almost essential, and being well away from street lights would help!

More to come later - including a photo with deliberate star trails.

If you would like a more detailed guide to taking pictures of the night sky I can thoroughly recommend "Astrophotography for the Amateur" by Michael Covington, published by Cambridge University Press.

[1] 12 seconds at f1.8 with 800 ASA film
[2] Software - !DPlngScan 1.14, Hardware - Plustek OpticPro 9636T
[3] Acorn's bitmap format
[4] Software - !Paint 1.95
[5] Software - !InterGif 6.12

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