Class Project: A Solar Spectroscope for the Visitor Information Station

The students of the University of Hawaii at Hilo, Astronomy 450 course (Advanced Instrumentation) have built a small solar spectroscope as a class project.

The goal of this project was, primarily, to introduce the students to all major steps in

the design and construction of optical instruments. However, it was intended to produce

a useful instrument for the Onizuka Center Visitor Information Station. The VIS

already operates a solar telescope for daytime visitors, and a spectroscope, showing

the absorption lines in the solar spectrum, was a nice addition to this telescope.

The project proceeded along these steps:

1. Establish the scientific goal of the project. In our case the goal was to provide a visual spectroscope to the solar instrumentation package of the Visitor Information Station that has sufficient resolution to show a large number of absorption lines in the solar spectrum.

2. Derive technical requirements. In the case of our project, the requirements included a sufficiently large visual image of the spectrum, a clearly defined exit pupil of the instrument with sufficient eye relieve for people wearing eyeglasses, the ability to focus the instrument. Further, we estimated the limits of sufficient brightness of the spectrum, comfort limits to the brightness, and safety limits, and discussed safety consequences of failure modes of the instrument. Finally, a design requirement for ruggedness of the instrument and elimination of most modes of mishandling was identified.

3. The basic instrument layout was established based on a paraxial model of the required optical components.

4. Commercially available components were identified from the Edmund Scientific catalog. A detailed Zemax model using the optical data of the selected components was established.

5. The mechanical dimensions established with the Zemax model were transferred to an AutoCad layout of the instrument.

6. Two components, the slit holder and the grating mount, that could not be purchased commercially, were design in detail for in-house fabrication.

7. The slit holder was fabricated on a manual lathe, with each student doing at least one fabrication step. The grating mount was prepared on a manual mill and finished using a CNC with the help of the IfA machinist.

8. The components were inspected, cleaned, painted, and test assembled.

9. The collimator was focused using an alignment telescope, the slit position was aligned using fluorescent light bulb as a source of line radiation.

10. Finally, all adjustable features were fixed in position with epoxy, the the threaded components were secured with epoxy.

11. We achieved “first light” observations of the solar spectrum in the last week of the semester. By comparison with the atlas of the solar spectrum, people with average eyesight can see a resolution of about 0.1 nm.

Optical Layout of the Solar Spectroscope

Mechanical layout of the solar spectroscope, drafted in AutoCad. All optical components

and most mechanical elements were purchased from Edmund Scientific.

Fabrication of the slitholder on the lathe.

The spectroscope is completed. “First Light” in the IfA courtyard.

The solar spectroscope in use at the Onizuka Center Visitor Information Station at Hale Pohaku. The other instruments are a 11” Schmidt-Cassegrain used for white light observations of the solar disk, and a small Hα telescope for the observation of flares and solar prominences.

An image of the solar spectrum, taken with a handheld digital camera through the eyepiece of the solar spectroscope.

Most people can see more detail than is recorded in this image. The sodium doublet in the yellow (orange here) part of

the spectrum can usually be seen as clearly resolved, for example.