wpo
- colourful spectra via drift method
Solar system
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These shots are via my Canon
Rebel =300D DSLR with a Rainbow
Optics transmission grating in the convergent beam of an undriven 4"
Meade SCT allowing the star and its spectrum to drift across the field
in typically 20 seconds exposure. The
spectrum of zero mag star Vega has had no enhancement in colour and contrast
and fairly indicates the colour representation and spectral range for the
Canon CMOS detector - something rarely provided by manufacturers.
Other stars include white alpha Oph [A-type] and orange/red stars alpha
Boo + epsilon Peg [K-type] and alpha Her + beta Peg [M-type] below.
Digging deeper...Recently on CN Forum [under Variable star/ Deepsky / DSLR+Digital Camera] I’ve
shown colourful spectra of the brighter stars using a static 4” telescope with DSLR
and Rainbow Optics grating by allowing the star and its spectrum to drift across the
focal plane in exposure between 10s and 30s. The singular point of starlight is
stretched in two dimension and this limits how faint spectra can be obtained -
typically mag 2.5 to mag 3 [without tracking the star].
Going fainter means a larger scope aperture [8” = x4 light grasp of 4”], focal
reducer [to slow down the star’s trailing across focal plane], moving the grating
closer to the sensor [without obstructing the instant-return flip mirror!] and high
ISO ratings like 1600 ISO [instead of 400 ISO]. These in combination seem under test
to reach another 4 mags fainter to mag 6 or 7. Locating the grating closer to the
sensor makes the spectrum shorter and brighter but looses resolution or detail.
1600 ISO produces course spectra but these are easily smoothed by carefully stacking
multiple images of the same star later via software like Photoshop or PaintShop Pro.
The most pleasing results come from targeting white A-type dwarf stars like Vega,
Altair, Sirius etc which have strong hydrogen lines. A-type giants like Rigel, Deneb
etc which have very fine hydrogen lines are mostly unresolved with this setup. Other
stars to target are cooler K and M-type stars like Arcturus, Antares, Aldebaran,
Betelgeuse and the Mira variables like chi Cyg or R Aql near maximum light. G-type
like Capella should just resolve the H and K lines of calcium II in far violet.
Freeware starcharts like Cartes du Ciel colour code all the stars to mag ~12 to
their colour index [= spectral class]. Here’s some spectra taken last night -
Sept 5 showing gamma Aql and nearby red Mira variable R Aql at mag 7 near current
maximum light [=AAVSO reports] of spectral class M5e-M9e showing typical banded
structure.
images& text copyright Maurice Gavin 2006