Instrumental Response: Another Journey

To calibrate a spectrograph in terms of wavelength is one thing: get a lamp with known emission line wavelengths and use it to calibrate on a pixel-by-pixel basis the wavelengths of light coming out of the instrument. Ok – done. Thorium has been a good help here.  How to get the instrument’s response in terms of sensitivity to different wavelengths is a whole different ballgame, one that has proven interesting for some not-so-expected reasons.

Generally, the process is to take spectra of some radiator (a lamp) that has a well known spectral output in terms of intensities across all the wavelengths the instrument is designed to handle. This usually means taking some incandescent lamp and getting spectra of it. This lamp must have a continuous spectrum and be as close to an ideal blackbody radiator as possible. Tungsten lamps work well for this, as do some halogen bulbs. The key to blackbody radiators is that one can tell you their theoretical spectral curve if you know their temperature…. Planck and Wien were helpful here.

Trip to the hardware and home supply store: Lightbulbs. Incan-what? Descent? Incan – descent? Wha…?? All these places have gone UBER-green making it darned near impossible to find a well documented incandescent light bulb! I bought several types from several makers in the hope that at least one major company would have an available spec sheet for me to get the color temperature information that I needed. Sure enough: SYLVANIA soft white halogen bulbs have a convenient spec sheet available if you call their tech support. 2850K color temperature for their 28 Watt model. Awesome. I was out to win the race after all.

I took the spectrum of this little glowing heat-maker and got the instrumental response to what should have been a perfect blackbody curve. Now, CCDs are not the nicest of instruments. They are quirky. Mine like blue and far red. Green… not so much.

  1. Take spectrum of lamp with the CCD at a known temp (-10C in my case for now).
  2. Take a bunch more.
  3. Average them.
  4. Divide that spectral curve across the whole wavelength domain by the expected Planck curve for a 2850K blackbody radiator.
  5. Spline this result to smooth out the rough features of the spectrum. The result is the instrumental response for the spectrograph at that temp across those wavelengths.
  6. Use this newly created response curve to correct the recorded spectra of astronomical objects.
  7. DONE!

And you thought that using biases, darks, and flat fields was bad!  The file maintenance here is a real game. So here are a couple of spectra for you to enjoy…. and yes, that have been calibrated for wavelength and spectral response of the instrument. Joy! Mission complete.


The above is the Hydrogen-Alpha region. Below is the Sodium Doublet. Look! Lines between the doublet of sodium. Cool resolution!  I am liking this instrument a lot. Note the rough area below around 5760: instrumental noise caused by a not-so-perfect-fit in the modeling of the field curvature for that particular spectral order. Tuning time!


About johnb

- Director of Grainger Observatory, Phillips Exeter Academy. - Variable-star-crazed astronomer, but have done research in other areas. - Drummer, archer, pilot, chef, friend, pet owner, husband, father, Train-nut.
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