WITH A DIGITAL SLR CAMERA AND
By Maurice Gavin - http://www.astroman.fsnet.co.uk/
Many decades ago at an early Winchester Weekend, I and a number of colleagues, bought for a few pounds 1 rev-per-day surplus clockwork ‘motors’. Most got converted to simple, if unique, star-trackers typically for Zenith cameras with 58mm fl f/2 Helios lenses. Mine, shown below with camera attached, has survived with a new lease of life into the digital age. It sits on my patio under wraps [except for a bi-annual spray of WD40] and can be rapidly brought into action in a matter of moments.
With care it accurately tracks a 135mm fl lens on my DSLR for about two minutes – adequate for my light polluted skies. Typically my favourite Jupiter 85mm fl f/2 Zenith lens reaches mag 11 in 30sec - sufficient for basic astrometry and photometry of brighter asteroids. Coverage via my APS sized detector equals half of Orion. The full-frame Canon 5D, for example, would cover most of Orion with this same lens. It’s the largest clear aperture, for a given exposure, that goes fainter - not lens f/ratio. Fast f/ratios attracts skyfog!
I don’t have any Canon lenses [except for the standard zoom that came with my Canon 300D camera] but many-fixed focal length Pentax screw/M42 thread lenses. A Kood M42 to EOS adapter ensures a perfect fit and infinity focus at the lens mark. Working in manual mode is no disadvantage for astrophotography – in fact it’s preferable and more positive.
Some typical images are at the end of this article and at http://home.freeuk.com/m.gavin/digsky.htm . I also use my Minolta D7 digital camera with fixed zoom 7.2 – 51mm lens on the drive to good effect. A cable [or delayed-action] release is essential to avoid shake spoiling the exposure.
Due to my proximity to London, sky pollution can be onerous and although special filters can reduce the problem they have downsides and are not totally effective. For a decade, using monochrome cooled CCDs, I was unaware of the true colour of my local sky – digital colour leaves one in no doubt - it’s a hideous yellow-brown. My solution was simple. The selected raw image is copied [via PaintShop Pro; Photoshop etc] as a separate image and heavily blurred [Gaussian Blur] to ‘remove’ all the stars and then subtracted from the original image. The yellow-brown sky turns a pleasant neutral grey and ready for modest contrast stretching and sharpening to a presentation image.
Does the title imply a sprung wound ‘motor’ is unnecessary? I found that if the camera and counterweight were imbalanced very slightly in favour of the camera [if ‘west’ of the meridian] then gravity alone was sufficient to cause the clock to tick giving a controlled ‘fall’ to track the stars! I’m sure some of these motors are still around and such a project is pleasantly free from mains/ battery power or leads to trip over.