wpo
- my Lensless Schmidt Camera
Bernhard
Schmidt, describing his unique astrograph in the 1930s [it has no visual
application], said that if scaled down in size and of modest f/ratio then
the complex and expensive front corrector plate could be replaced by a
simple ‘hole’ to admit starlight! In 1989 I co-wrote a paper
in the
BAA Journal
about building such a Lensless Schmidt Camera [LSC] that used 60mm square
photo-film as detector and was an 1/8th scale model of the Mt Palomar Oschin
48-inch Schmidt camera.
Recently I acquired a Meade DS-2114s OTA eg modified Newtonian with 114mm f/4 spherical primary plus Barlow-like Jones-Bird corrector lens to remove spherical aberration giving a final f/ratio of f/9. However collimation proved difficult so optical performance was poor. However it seemed ideally suited for conversion to a modified LSC with the minimum effort.
1]
remove the 25mm diameter Jones-Bird corrector lens from the rackmount
2]
retain the secondary flat and rackmount for ease of focusing
3]
use a modern 1-1/4" diameter Lodestar CCD detector located deep within
the rackmount to reach prime focus
4]
add an aperture-stop at the radius of the primary on a single aluminium
arm
5]
LSC focal plane is curved at half the radius of primary or ~10 micron across
the tiny Lodestar CCD detector and can be ignored.
Various aperture-stops could be tested - shown are 75mm [swing-aside] and 90mm apertures which equate to an effective 63mm f/6 and 80mm f/5 instrument respectively allowing for the 40mm diameter central secondary obstruction. Star images proved excellent to the edge even with the ‘faster’ and larger 90mm aperture-stop. Although the CCD camera was tiny [in Schmidt camera terms] it easily encompasses the whole moon with room to spare in good resolution - see sample images.
Drive
system: The universal Meade DS single arm
alt-azimuth mount [above] under Autostar control makes for a transportable
instrument. Tracking was adequate for 5s exposures and recorded stars
to mag 13.5. With rapid refresh on the laptop screen it gives a 'live-view' experience that outperforms large Dobsonians in revealing galaxies like M51 despite its modest aperture.
Images could be auto co-added to ~100s under SX Lodestar software for deeper stellar penetration to ~mag 15/16 but an imaging sequence beyond a couple of minutes show field rotation through alt-azimuth tracking especially towards the zenith. Extended exposures can be created by combining a series of shorter exposures in software like DeepSkyTracker that account for field rotation.
As an experiment a Rainbow Optics grating taped directly over the CCD within the LSC rackmount focuser to form a slitless stellar spectrograph - resultant spectra proved very sharp!
This series with larger 90mm aperture-stop f/5 system - no darks or flats but hot-pixel removal and contrast stretch.
text & images copyright Maurice Gavin June 2008
via www.digits.com
