SPECTRA-L 200April 2010
Introduction
The cost of entry to intermediate and advance spectroscopy is high. Once the Star Analyser/ Rainbow optics gratings have been mastered the next plateau on the learning curve is a slit spectroscope with higher resolution. This would mean paying over $3000 to purchase a Baader Dados unit or a Shelyak Lhires III. Don’t get me wrong, these are excellent spectroscopes and well worth the money, but still a significant expense to the amateur.
The usual alternative is to search through Buil’s site (and books like Practical Astronomical Spectroscopy, or the latest (!!) Astronomical Spectroscopy for Amateurs) and build your own. This has successfully been done using both transmission grating and reflection grating by many DIY amateurs. But what if you have no DIY skills, what are the options? Not many…. Maybe a second hand unit or waiting on a “mate” getting round to doing the work he promised…
Now there is an alternative; the Spectra-L 200.
This is a cost effective “IKEA” kit, which contains all the critical components needed to assemble a 210mm Littrow slit spectroscope. If you or the kids can assemble an IKEA cupboard, then you certainly will be able to assemble the Spectra-L 200. Allow a couple of afternoons to lay out the components, paint, assemble and test. Other than some paint and basic handtools (Philips Screwdriver!) everything is supplied.
To fit the Spectra-L 200 to your scope you’ll need a suitable adaptor. It comes with a standard female T thread, so any of the 1.25”/2” T thread nosepieces or the SCT/ T thread will fit.
For guiding you can use a piggyback guide scope, or an Off-axis guider, or even modify a flip mirror into a beamsplitter (recommended!). Your choice.
Likewise, to mount the imaging camera; the Spectra-L 200 is fitted with a female T thread, but comes with a male T thread insert (to allow some focus adjustment) so you can use T thread adaptors/ spacers to mount your CCD or DSLR camera. The generous back-focus of 70mm, is designed to allow the use of Baader quick release couplings and accommodate the 55mm needed by a DSLR camera.
Design Overview
Similar in looks to the LhiresIII, the Spectra-L 200 is a triangular shape, 210mm x 200mm x 63mm wide made from laser cut and folded 1.5mm aluminium sheet. Target weight approx 1.2 Kg. The inside of the housing “box” is 60mm x 60mm.
The optical layout has been designed to accommodate an f7 entrance beam for 300/ 600/ 1200 l/mm gratings and an f10 beam for 1800/ 2400 l/mm gratings.
The collimator lens is a 38mm, 210 focal length achromatic doublet.
It comes standard with a 600 l/mm,30mm x 30mm, Optometric blazed grating.
With the 600 l/mm grating a resolution of R= 3000 should be obtained with a camera having 9 micron pixels. Other gratings, 300/ 1200/ 1800/ 2400 l/mm can also be fitted.
The movement of the grating is controlled by a micrometer head which gives full travel for all the gratings from Zero image position through to NIR.
The SimSpec-SpectraL V3 spreadsheet should be used to calculate limiting magnitudes and resolution for various seeing conditions/ slit/ grating/ camera combinations.
Mechanics
The main housing is laser cut 1.5mm aluminium with pressed in M4 fasteners and comes supplied as side plates, and box housing components.
The telescope mounting plate is 6mm aluminium with a central M42 x0.75 T thread.
The entrance slit, a Surplus Shed adjustable slit mechanism (restricted to a height of 3mm and a width of 0.0mm-0.5mm (500 micron), is positioned 60mm behind the telescope mounting plate. A 50mm knob and graduated scale allow the slit gap to be set.
Between the front plate and the entrance slit there is space for a Neon reference lamp in a slide enclosure. This can be positioned in front of the slit to obtain a reference spectrum.
The tilt/ position of the entrance slit can be slightly adjusted (if necessary during testing) and locked in place with a M4 screw.
A 25mm x25mm x3mm front surface mirror is mounted on a support arm to direct the incoming beam to the collimator.
The 38mm collimator lens is mounted in a fixed housing at the design distance from the slit.
The grating is mounted on a grating support plate. This plate is located onto the mounting shaft with M3 grub screws and the shaft itself runs in a 6mm precision bearing.
The tilt of the bearing housing can be finely adjusted using a push/ pull screw arrangement to achieve proper collimation with the optical axis.
The grating’s rotation is adjusted using a 0-25mm micrometer head fitted with a spherical 6mm nose. Back pressure is applied to the grating support plate with a small adjustable compression spring. This ensures the holder is always held positively in contact with the micrometer. Full travel from Zero Image through to NIR is achievable.
The imaging camera is mounted via a female T thread (a male insert is provided for fine focus adjustment). The available back focus of 70mm accommodates all DSLR and CCD cameras.
The kit of parts
The following components comprise the kit:
2 off side plates, 210mm x 200mm
1 off box section, 150mm x 140mm (inside surface)
1 off box section, 210mm x 200mm (outside surface)
1 off Telescope mounting plate, 60mm x 60mm x 6mm, T-threaded
1 off Camera mounting plate, 60mm x 60mm x 6mm, T-threaded
1 off Male T thread adaptor
1 off Mounting frame for entrance slit, 60mm x 60mm (18mm hole)
1 off Mounting frame for collimating lens, 60mm x 60mm (38mm hole)
1 off Cover plates for entrance slit, 60mm x 60mm (3mm hole)
2 off Cover plates for collimating lens housing, 60mm x60mm (36mm hole)
1 off Grating bearing housing, 40mm x 40mm x 10mm (6mm bearing, 6mm shaft)
1 off Micrometer head housing, 15mm diameter x 15mm long
1 off Grating housing pressure spring
1 off Adjustable slit mechanism, fitted with 6mm shaft.
1 off 50mm dial knob (slit adjustment)
1 off 25mm x 25mm front surface mirror mounted on support bracket (pre-painted)
1 off 38mm achromatic collimating lens
2 off 36mm O rings
1 off Micrometer head, 0-25mm
1 off Optometrics 600 l/mm grating mounted in holder (pre-painted)
Various stainless steel fasteners/ screws
Sealing tape for joints
Assembly instructions!
Optional Extras:
Neon reference lamp, with shutter slide
Neon driver PCB (12V), switch and 3m 12V cigar connector
Red LED back light illuminator and reflector for slit
Baader quick fit connection and T thread adaptors
Additional gratings in holders (300/ 600/ 1200/ 1800/ 2400 l/mm)
Additional items (supplied by user)
Spray paint: Etching primer, Matt Black (interior), your choice of exterior paint colour.
Masking tape
Tools required
Philips head screw drivers (small/ large)
Exacto knife
3mm Allen key
Small adjustable wrench
Laser telescope collimator, (not mandatory but VERY useful!)