wpo
- comparison of current amateur spectrographs
These nomograms provides a general guide to the
relative dispersion of some current amateur spectrographs.
The
spectral dispersion of a spectrograph, and in turn its spectral resolution,
depends on three factors - 1] camera focal length**; 2] grating dispersion;
3] pixel width. Figures below show the relative dispersion [bottom
line] in Angstroms per pixel [A/pixel] of some current Littrow/ Ebert type
spectrographs including my
homebuilt WPO devises. It assumes either 11um [or 22um binned]
pixels via my Starlight Xpress MX916 CCD camera. Smaller pixels
will raise the dispersion and spectral resolution proportionately.
The proposed LHIRES
III has the highest native resolution with a 2400 l/mm grating and
auto-collimator of 200mm fl. Alternative gratings lower its resolution
but increase the swath of spectrum covered for fainter objects.
Littrow/ Ebert type spectrographs use a single lens or mirror [=SBIG SGS] respectively to serves the dual role of collimator and camera lens [eg auto-collimator] so the slit is projected full size onto the CCD. The SBIG DSS-7 has a separate camera lens of ~half the focal length of the collimator and images the slit half-size onto the CCD - thus the entrance slit can be wider to admit more light and thus record fainter objects in a given exposure.
In amateur terms a 'high' dispersion spectrograph like the LHIRES III is confined to bright objects eg solar or stars brighter than mag 6 in great detail whilst the 'lowest' dispersion spectrograph like the Rainbow Optics and Staranalyser gratings are more appropriate to faint stellar objects but only to lowly resolution. The two SBIG spectrographs cover the mid-resolution zone. The exposure duration, for a given signal strength, is dependent on 1] scope aperture; 2] spectral dispersion; 3] overall spectrograph efficiency [light throughput].
** The Rainbow
Optics/Star-analyser gratings are used in convergent beam of telescope
close to the CCD - the 'fl' = distance of grating from CCD.
