Spectroscopy With Filter-Wheel Gratings

A fantastic new tool is available to help non-professional (and professional) astronomers do low-resolution spectroscopy. There are now gratings that you can drop in your filter wheel ( the Star Analyzer SA-200 for example) and use to take spectra of objects without the imaging setup for your telescope. This page is setup to provide a few free resources for people that want to start using this interesting innovation.

Our Test Bed

The advice we give here has been developed through experience with using a Star Analyzer SA-200 in the filter wheel for the imaging setup on the Barber Observatory 20-inch Telescope. We have no financial or other arrangement with the builders of this equipment so the advice we give here is from that perspective.

I have been pretty impressed with the demos of the latest versions of the RSPEC software they sell with the grating. But sorry Mac and Linux users.... it is Microsoft only. For this reason and also for the purpose of educating people about how RSPEC works, I have set up the following tutorial.

Units for Wavelengths and Graphing Spectra

In this tutorial I determine the wavelength scale in nanometers (nm). This is purely a personal preference. Angstroms are also a popular unit. Multiply nanometers by 10 to obtain Angstroms.

Another personal preference expressed in this tutorial is to graph spectra so that longer wavelengths appear on the right of the X-axis. This is known as the "red is right" approach because longer wavelengths are red and they are on the right. The double meaning of the word "right" should not be taken literally here. Just as many professional astronomers plot long wavelengths to the left on the X-axis (because of the relation between frequency and wavelength for light). So "red is right" is just one way to do it, NOT the only correct way to do it.

Grating Tutorial

This tutorial is in two parts:

  1. How to extract spectra from images taken with your filter wheel grating.
  2. How to wavelength calibrate those spectra.

How to extract spectra from the images with your grating

Before you start this tutorial you will need the following:

Watch the video and/or follow the outline below.

Steps to Extracting Your Spectrum

  1. Load the dark corrected FITS image into DS9.
  2. Activate the projection tool through the "Region" menu in the toolbar on the top of the DS9 window (Region -> Shape -> Projection).
  3. Use the arrow to draw a projection for a bright star with an obvious spectrum in the image.
  4. Grab (at a point other than the tabs) the projection and move it to the intended target.
  5. In the graph window for the project, select "Save" from the "File" menu to save the data from your projection

You now have a text file of ordered X,Y pairs that you can graph. The X coordinate is pixels from the start of the projection and the Y coordinate is the brightness of the projection at that pixel.

How to wavelength calibrate your spectra

For the second part of this you need the following to begin:

Steps to wavelength calibration

  1. Use the DS9 projection tool to extract a spectrum of the planetary nebula.
  2. Find the pixel in the projection where the zero order image is centered. This is your zero_point.
  3. Identify the bright features in the spectrum and record the pixels they are centered on.
  4. Divide the known wavelength of the feature by the distance in pixels between the zero order image (zero_point) and the feature in the projection (pixel).
  5. That ratio (nm/pixel) can now be used to solve any spectrum you extract from any image with the projection tool for the wavelength using the formula: (wavelength) = (nm/pixel) * [(pixel) - (zero_point)].

Every time you extract a new spectrum you will re-determine the position of the zero order image (zero_point) but the ratio (nm/pixel) will remain the same as long as you are using the same grating and do not alter the distance between your filter wheel and your CCD camera.

Follow this link for examples of what we have done with our SA-200 grating.

Return to the Spectroscopy Workshop page

Return to the Barber Observatory web page.

Return to John C. Martin's web page.

Last Modified: July 25, 2014