Regulusastro Blog

Fiddling around with the spectrograph again today… the goal: to install the software, drivers and hardware for the guiding camera. The unit is a little, tiny, compact (did I say small?) video camera made by Watek and a USB video grabber. The AudeLA software has a built in module which can see the video from the camera as a webcam device. It then can connect to and control the telescope as any autoguider can. I like the idea: live video with autoguiding built in! This reminds me a lot of the webcam used in the Coude Feed Spectrograph at Kitt Peak, though I think their guide camera is an older TV-style imager. The systems pretty much works the same way: the video camera is focused on a mirror in the guide/imaging head attached to the telescope using a small lens system. The mirror has a 50 micron hole it it. The idea is to place your target (to be spectra’ed –> I know, it’s not a real verb) on that hole. The mirror then feeds an image of that star/planet/whatever, the hole and the surrounding star field for the guide camera to see. It’s pretty slick. This allows real time focusing, real time pointing, and a secure knowledge that you are grabbing spectra of the target of interest. No more trying to get that star on the invisible slit!

While I was playing with all that, I wanted also to try attaching a camera to the spectrograph to see the spectra in color. Why not? The lens used to bring the spectra to a focus on the CCD is made by Canon, and is an EOS standard lens. Sure enough, the Canon camera was able to bring the images to a focus. Here is a colorful sample.

Above is an image of the flat field lamp used to generate flats for later calibration. Nice looking rainbow there. You now get the idea of an echelle spectrograph: the orders at the top are the long wavelengths while the shorter ones are at the bottom.

Still playing a bit, the above is a spectra of the basement lights where I have this all setup. You can see the tell tale emission lines from the fluorescent lights used down there.

The above is my favorite: this is the calibration lamp spectra of thorium-argon. All those little lines have very well known wavelengths and allow each order to be precisely calibrated. Such fun!

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!

The eShel spectrograph from Shelyak Instruments in France has arrived, and with it a lot of excitement about all the possibilities of projects for the future! The unit came into the country and stopped for a brief stint at an airport customs office before making its way here to Exeter. The box was about 45 pounds in weight and had a ton of individually wrapped goodies inside, all in excellent condition. Packaging was perfect and survived the trip from France without any issues.

Unpacking the unit is best done with a checklist and table nearby in a clean area. I have a very clean basement, dry too. The job was carried out there. Some serious time was spent learning the proper id of all the connecting pieces, some of which are not all that obvious at first glance. The majority were easy to identify: CCD camera and power supply, the optical head which attaches to the telescope, the fiber optic and power cables, the spectrograph and its lens… all easy enough. How to connect them? That is the fun part!

Here is the system completely hooked up and ready to go… with everything but a telescope for the little optical head. If you get one of these: note the use of the correct fiber cables for the starlight going into the spectrograph and the feed for the calibration lamps. The calibration lamp cable is larger than the 50 micrometer diameter cable for the starlight. Also note that the AC power goes into the calibration light box first, then is jumpered to the power supply for the thorium lamp which feeds a 10mA supply back to the calibration box. It is a little confusing, but the insides of the calibration box need power to handle logical switching directions fed into it via a com port. This brings up some interesting points: you need a PC with com ports.

The supplied software is an extension of the AudeLA package. The whole systems runs on top of that. It will manage your CCD imager (power and temperature), a guiding video camera, telescope guide control, the automatic on/off of the calibration lamps (yes, two: one is thorium for wavelength calibration, the other a white LED for flat fields! Cool!), the flip mirror in the guide head which allows either object spectra or imaging the calibration lamps, and the processing of all the final images automatically. It is pretty amazing. The setup is a bit tricky, but not bad. Make note: the software initially expects you to have already taken and formed an instrumental response FITS file… but you can’t do that until you have taken spectra of some standard star or blackbody radiator with a known temperature like a tungsten lamp. There is a checkbox to turn that feature off until you are ready. None of your object spectra will be calibrated automatically until you take care of this.

The spectrograph uses a Canon EOS lens to focus unto a QSI CCD imager (blue above). Focus is achieved by focusing the lens as you would any camera. No, it does not autofocus! The beauty of this design is that the spectrograph can reside 20m away from the telescope in a vibration free temperature controlled environment which makes for very sharp spectra and excellent calibration. Finding exoplanets using radial velocity measurements will be pretty easy with this!

The system is an Echelle spectrograph using an Echelle grating to make a merged spectrum first then is spread out using a secondary prism to break the orders of spectra apart for imaging. The result is an interesting and complicated field of many curved spectral orders. All this must be sorted out! Here is the spectrum of the sun (blue clouds actually) for example.

How to make sense of that!? Well, the software can take the calibration lamp spectrum of thorium and use it to identify the field curvature, the angle of the curves, and the wavelengths across the field: automatically. It then provides both individual spectra for each order and for the whole spectra combined.

Above are the spectra of the sun near H-alpha and then the combined spectra of the orders from about 500nm to 670nm. Note that the true blackbody curve of the sun is not evident here, because the unit has not been calibrated (YET) for instrumental response. We’re almost there! Once that is done, the wavelengths AND the flux values will be calibrated and the curves shall be accurate.