wpo
- equatorial & galactic plane camera tracker
astro-optics
page digital
still cam - images of the sky
Clockwork
drive in a digital age....
and
nova hunting made simpler by Maurice Gavin
This 1 rev-per-day surplus clockwork 'motor' was bought many decades ago and converted to a simple camera tracker as shown here. It has survived into the instant digital age and 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.
It accurately tracks a 135mm fl lens on my DSLR for two minutes - adequate for my light polluted skies. My favourite Jupiter 85mm fl f/2 Zenith lens reaches mag 11 in 30sec - sufficient for basic astrometry and photometry of brighter comets, asteroids and variable stars. The background starfield will contain many variable and red stars and brighter galaxies, planetaries and diffuse nebulae.
Coverage via my Canon 300D/Rebel APS = half of Orion. The full-frame Canon 5D would cover most of Orion with this same 85mm fl lens. I have no Canon lenses [except the supplied zoom] but many Pentax screw/M42 thread lenses which couple perfectly via a Kood M42-EOS adapter. Working manually is preferable for astrophotography but zoom lenses are best avoided. A cable [or delayed-action] release is essential to avoid shake spoiling the exposure. It's the largest clear aperture, for a given exposure, that goes fainter - not lens f/ratio. Fast f/ratios attract skyfog and limit exposure times. Some typical images are here. I also use my Minolta D7 digital camera with fixed zoom 7.2 - 51mm lens on the drive to good effect.
Due to my proximity to London, light pollution is onerous and although LP filters help they are not totally effective. For decades I was unaware of the true colour of my local sky but digital colour left me in no doubt - it's a hideous ginger. My solution was simple. The selected raw image is copied [via PaintShop Pro; Photoshop etc] as a separate image and gaussian blurred to 'remove' all the stars and then subtracted from the original image. The ginger sky turns a pleasant neutral grey and ready for modest contrast stretching and sharpening to a presentation image as below. Click here - Psyche+Parthenope.
Nova hunting made simpler
The tradition route to novae discovery [and instant fame!] is short exposures via a wide-angle camera aimed at the Milky Way [galactic plane] where virtually all discoveries are made. The camera is usually equatorially mounted to track the stars so they don't blur during the exposure. However the galactic plane is tilted at 63 degrees to the earth's equatorial plane and the resultant Milky Way montage forms a ragged mosaic wasting frame space and time to secure it
A more productive approach is to align the galactic plane through the centre of each successive frame to form a complete galactic panorama with the minimum number of exposures. The figures adjacent show two options using either a 'wedge' to tilt the galactic camera or the 'U' channel with one arm inclined to support the galactic camera.
A
50mm fl lens full frame coverage = 40o x 27o ie >1000
square degrees per photo! Allowing 30o net coverage on the long
axis of the frame [5o overlap each end], 4 photos will cover
a third of the Milky Way - 5 photos = 160o or virtually the
Milky Way from horizon to horizon when well placed in summer and autumn.
Rotating the camera through 90o to cover a 40o wide
swath [20o either side of Galactic Plane] x 22.5o
net per photo - 7 photos = 160o coverage - perhaps 30 minutes
for a complete survey.
The
equatorial mount is aligned on Polaris in the usual way and the 'U' channel
and camera rotated on their axes so the camera axis points at one of the
galactic poles as illustrated. These have no bright defining stars but
the alignment can be checked by rotating the camera on its [galactic pole]
axis to scan along the Milky Way that contains bright stars like Altair,
Vega, Deneb etc before the photo survey begins.
The
camera is rotated on its axis at regular angular intervals between exposures
to give an overlap between adjacent frames until the survey is complete.
Virtually any suitable camera, both film and digital, can be used. Such
a survey is ideally suited to full frame Canon 5D when coupled to a 50mm
fl f/1.8 lens which gives the best angular coverage, stellar penetration
and instant results for comparison against 'master' images.
The
clockwork camera mount has not been tested in 'galactic' mode but a 'galactic
wedge' has been piggybacked on my Meade LX200 as below.
*North Galactic Pole at RA12h 50m; Dec +27o in Coma Ber - use Arcturus & Alkaid at end of the Plough to locate NGP.
*South Galactic Pole at RA00h 50m; Dec -27o in Sculptor - use Fomalhaut & Difda [ b Ceti] to locate SGP.
*equals Galactic Plane offset from the Celestial Equator. * each frame duplicated to eliminate artefacts.
images/text copyright - Maurice Gavin - Aug 2001/Jan 2005 rev1