This is a project of a modular spectrograph, for telescope stellar use, medium resolution and fairly fast. It has been conceived with interchangeable modules, aimed to future developments and enrichments, for high flexibility in applications and experimentation.
Its construction is quite simple, however it needs to be guaranteed dimensional accuracy and especially the optical alignements, in particular those of the folding mirror and the vertical offset of the grating.
To minimize weight and size, the optical solution was chosen according to the Littrow configuration.
It consists of some modules, as partially shown in Figure 2 (starting from the left side of photo):
Central Folding module: based on the popular Indian slit released by SurplusShed, slightly modified. The barrel is inserted into the focuser 1 "1/4 of the telescope.
Optics: Initially a lens reflex F50mm f:1,7 and mounting M42 (pictured). However, I also realized some alternatives, with simple achromatic lenses having different focal lengths.
Intermediate module: with a ~ 45° folding mirror and the mounting hardware for any CCD, webcam or camera, the visual module, etc..
Grating Modules (reflective): currently 2 units, respectively with 600 lines / mm, and 1200 lines / mm (both blazed).
Barrel 1"1/4 module: for webcam and visual use for educational purposes, etc.
Devices for alignment: to facilitate the rays path adjustment.
Neon calibration unit : A compact device powrerd by 1,5 volt. battery AA o AAA size (NeonLamp).
Photos are sufficiently explanatory. The compactness and short focal length optics require a good constructive care, in compliance with the critical determination of the optical paths. However, there are atjustements indispensable to trim the correct alignment of the beams, incident and diffracted. Avoiding the details here, too complicated, just to mention:
- Angular orientation of the folding mirror, a little more than 45°, as shown on the attached diagram.
- Very slight upward inclination of the grating, as indicatively represented in the same diagram. Note in picture 5, on the rear cover, one of the two screws of balancing agents on the frame of the grating.
- Tilting the grating, which in the example in question allows not only the first order spectrum, but also of the second.
To accomplish the tilting of the grating I have realized an unconventional system, which has proved to be very simple and effective: The picture 5 shows the knob keyed on a threaded rod dimeter 6 MA, which with its rotation causes the sliding of the threaded pawl, which in turn causes the rotation of the grating, through the connecting rod of harmonic steel wire hinged at both ends. The upper threaded rod (diameter 5MA) is used solely as an anti rotation of the pawl and holds nuts for the adjustable limits .
Other modules currently under construction & test:
Module with fixed reflective slit and its relative optical observation unit, for centering and auto-guide on the star;
Module flip mirror, to match the current SurplusShed slit;
Module Long Focal Lens: For solar use, with acrhomatic lens diam. 30 / focal 140 mm or more;
Module Fast Focal Lens: based on achromatic triplett diam. 25 / focal 80 mm.