Brody Fuchs
St. Cloud State University

REU program-Summer 2008
Univ. of Wisconsin - Madison
Madison, WI 53706

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Research Picture



Schematic diagram of the WHAM instrument (fig. 1)

Above is a diagram of the WHAM instrument, it is a large Fabry-Perot spectrometer coupled to a telescope (all-sky siderostat). The dual-etalon system allows for very high resolving powers. After the etalons is where the filter is inserted into the system. The filter lives in a filter wheel which can hold up to a total of 16 filters, allowing for observations of multiple lines during one night. The aforementioned dual etalon system uses two Fabry-Perot etalons in succession to select one order to transmit while suppressing neighboring orders, this results in greater resolving power. In the figure below, (a) is the transmission function of the first etalon, (b) is the transmission function of the second etalon, (c) is the transmission function of the system and the dotted line is the transmission function of the filter. It is easy to see that the dual etalon system does a good job of locally transmitting only one order, but orders are going to line up again somewhere far away which is where the filter comes in, to block out any unwanted orders.

Transmission functions of etalons and interference filter (fig. 2)

Because interference filters consist of glass with dielectric layers deposited on the surface, this creates multiple beam interference within the glass, causing the filters to act like etalons. With multiple beam interfernce, peaks are found where the condition for constructive interference is met: , where m is the order, lambda is the wavelength, n is the index of refraction between surfaces, l is the gap width, and theta is the incident angle.


The purpose of this project was to design, construct and test an instrument that could determine key parameters of 3 inch WHAM filters:

We need to know these parameters for a few reasons such as correct WHAM data interpretation, testing new filters to ensure that they meet manufacturers specifications, testing older filters to make sure that they are still in working quality because these filters can deteriorate over time for various reasons such as humidity, dust, fingerprints, etc.

Side 1 of tested, deteriorated filter (fig. 3)

Side 2 of filter (fig. 4)

Experimental Procedure

We started by taking an existing Ebert monochromator as shown below

Diagram of Ebert monochromator (fig. 5)

To use this monochromator in our instrument we took out the exit slit which allowed for a much higher bandpass for an "Ebert Spectrometer". We then used a combination of camera lenses (chromatic aberrations) to image the exit slit onto a CCD camera.

Diagram of Ebert spectrometer (fig. 6)

A dispersion test was performed to ensure that the Ebert spectrometer was in working condition. The following figures show the results of the dispersion test.

ThAr spectrum with peaks highlighted (fig. 7)

Dispersion plot of peak wavelength vs. peak pixel number (fig. 8)

Comparing this spectrum with a Kitt Peak ThAr spectral atlas, the peaks have similar separations and relative intensities. The dispersion plot shows that the dispersion is linear and it is also consistent throughout the visible range.

The actual spectral data needed sits on a platform of noise. This noise is in the form of dark current and CCD bias. There are also intensity differences among pixels which is a multiplicative factor. The way to get rid of all noise is by subtracting an average dark/bias image from both the data image and the flat field image, then divide the corrected data image by the corrected flat field image. This noise needs to be removed before the data interpretation stage.

Crude ThAr Spectrum With Average Dark (fig. 9)

Dark Corrected Spectrum (fig. 10)

White light images were produced by illuminating the entrance slit with a diffused white light source. White light images are needed for flat field images as well as to calculate the bandpass of the system. The bandpass (and exit slit) edge is where the intensity starts to fall from maximum intensity due to diffraction. The bandpass is useful to know because it needs to be large enough for the filters that are being tested. In this case it is large enough because the typical fiter width is ~20A and the system's bandpass is 92A. I have marked the edges of the exit slit on the figure below.

White Light Intensity Spectrum (fig. 11)

One more test was run before the profiling could begin, the FWHM dependence of a ThAr line on the width of the entrance slit. Since the ThAr lamp was not very bright a trade-off had to be made. When the entrance slit was smaller, the line resolution was better but the lines were more intense and broad when the slit was opened up. A happy medium was found somewhere in between.

Once the key parameters were determined to be satisfactory, the profiling could be started