wpo - Ebert style spectrograph from SCT
....astro-optics
page....Questar
QMAX spectrometerwpo - Ebert style spectrograph from SCT
An
Ebert spectrograph can be built around a Newtonian or SCT [corrector removed]
where the single mirror is essentially spherical in figure against the
two small areas [equal to grating size] allocate to collimator and
viewing
telescope respectively. The slit and eyepiece/detector
are located on the common focal plane but offset equally from the central
grating. This offset introduces astigmatism into the system
but is used beneficially to reinforce spectral lines at right angles to
dispersion as samples below.
These solar spectra, via a 15cm Bausch & Lomb Criterion 6000 SCT converted into a stigmatic Ebert spectrograph, show promise. The lines of calcium II in blue, magnesium triplet in green, sodium duo in yellow [6A separation] and Ha in red recorded with a digital camera handheld behind the eyepiece. A temporary box lid with three apertures for slit, grating and eyepiece replaces the glass corrector plate. The centrally located 1200 lines/mm blazed grating with housing is from my WPO stellar spectrograph. With a focal length of 30cm the focal plane scale = 20A/mm.
The Questar QMAX spectrometer [US$4250 = UK£2700] is the current leader in visual solar spectroscopy - a rescaled QMAX spectrum from their website is added to the D line spectrogram below for comparison. QMAX claim a spectral resolution of 0.17A [17 trillionths of a metre] with a dispersion 11.6 Å/mm in the focal plane of Questar's 8mm Brandon ocular at 5500 Å eg approximately double the current 1st order spectrum dispersion of my Ebert spectrograph. A comparable dispersion is possible by doubling the focal length of the imaging telescope to 60cm, swapping to a higher dispersion grating or simply using the 1200g/mm grating 2nd order spectrum as below.
Hi-res magnesium triplet
below taken via a narrower slit and tripod mounted digital camera to remove
shake.
Inset same region on Ilford
FP4 film via massive SLR
spectrograph [90cm fl+600 l/mm grating] -1979.
RH magnesium spectrogram
via 2nd order spectrum eg scale = 10A/mm - comparable to QMAX.
...
More spectra below via digital
camera on tripod behind eyepiece. Visually the spectrum is stunning
with thousands of lines clearly resolving [perhaps more than can be recorded
with a camera] including FeI @ 5168.908A from FeII @ 5169.050A in green
light - a difference of only 0.142A.
Note:
One-dimensional spectrograms below without text can be uploaded to ISP
server 3 pixels high and expanded via HTML code to any 'height' eg here
to 48 pixels.
As an experiment the 1200g/mm grating swapped for a small Littrow prism for ultra low dispersion with focal plane scale of 400A/mm eg 1/20th of the grating. The whole visual spectrum from violet to red [range ~3000A] ~7.5mm long and contained in a low power eyepiece. Sodium duo D1/2 and magnesium triplet visible but barely resolved.
The
above Ebert spectrograph assumes the SCT corrector plate is set aside.
Thus any cord on the primary mirror [clear of the central perforation]
will serve to focus the slit and eyepiece - see adjacent schematic.
It
may
be possible to leave the corrector + secondary in place if the corrector
plate optics cause minimal aberrations to the spectrograph. Subsequent
tests indicate the above proposal doesn't work - the corrector plate causes
severe distortion of the spectrum.