wpo-faint object spectrograph [fos]
This
is my versatile wpo spectrograph designed
around the Littrow principle where a single lens serves as collimator and
camera lens.
text
& images (c) Maurice Gavin - 2000
2000
Sept 20 - this evening, as a follow-up to last Saturday's Pro-Am
meeting at London UO, the 1200 l/mm grating in the WPO spectrograph
swapped for a prism to produce ultra low-res spectra [R~50]. A prism
produces a single spectrum and has minimal light loss compared to a grating.
Although prismatic dispersion is non-linear this is not really a problem
with computer resampling. Such prismatic spectra contain little 'detail'
but are exceptionally 'bright' enabling faint objects to be recorded in
relatively brief exposures. The objects targeted included emission
line stars Nova Aql'99[2] = Aql V1494 [~mag 10/11],
P
Cyg [~mag 6] and Wolf-Rayet stars in Cygnus [mag
6.5 - 13.5] and some faint K/M type stars which show broad absorption features.
Note the brevity of the P Cyg exposure for a magnitude 6 star !
As was highlighted at the above meeting amateur supernovae hunters, at which the UK excels, confirmation from professional observatories as to the spectral type following discovery is becoming increasingly difficult and perhaps the discoverer or their associates may need to undertake this work. Typically SN type II show [like P Cyg] H-alpha at 656.3nm in emission and SN type Ia, the broad absorption band of Si II at 610nm. Such SNe features are not necessarily as strong as the sample spectra shown here but the system shows promise and it is possible to project exposure times necessary to reach objects down to mag 17/18 - the current level for most SNe discoveries. The spectra shown here are taken without a slit [which in itself absorbs precious starlight] thus the field around the object is also recorded which may include the parent galaxy. Exposures are limited by skyfog at the f/ratio of the telescope.
Footnote: Chanced on a 1924 book to find the renowned spectrograph at Imperial College - London is identical in optical layout to the WPO miniature fos - even to the pick-off prism for collecting light from the slit! Small world.
2000 Sept 21: Using the MX9 CCD in unbinned mode [11um pixels] the resolution is raised [R~100] - aided by stopping the 'scope aperture down to minimize aberrations via the temporary dispersing prism.
2000 Sept 23 & 24: Optics realigned to use full 30cm aperture of SCT at f/6.3 for minimum exposures in unbinned [11um mode] res ~R100 via MX9 CCD. Faint variable stars AG Peg [Wolf-Rayet type emission line spectrum] and SS Cyg [cataclysmic/ dwarf novae] near min light mv12.4. The latter included here with other stars showing Helium and Hydrogen in emission. Irregular variable TX Piscium is perhaps the brightest of the carbon stars - just visible to the naked eye mv5.5 - 6.0 and of deep red colour. Its N-type spectrum profile is quite distinct. Uranus [included here] and Neptune targeted very low down in Cap/Aqr.
Daylight colour spectrum [Starlight Xpress MX5c one-shot colour camera + dispersing prism + slit] shows the strong twin H & K calcium lines in violet [left] and correctly records the IR spectrum [right] as colourless.
2000 Oct 5: a compilation of spectra via the lo-res prism ie Nova Aql 99[2] = V1494 Aql, M13 and planetary nebula NGC 6543 which shows considerable structure to permit enlargement. NGC 6543 was just recordable in 0.5s exposure so bright is the OIII line !
More spectra via the WPO spectrograph