As I have mentioned before, the instrument we used at IRTF is called SPEX. It is a medium resolution spectrograph. Specifically, it's a cross-dispersing spectrograph. What does this mean? Well, instead of dispersing light like a prism where the colors are all in one line, SPEX breaks the spectrally dispersed light into several orders that are displayed along side each other. I've included a copy of a figure I made for a previous presentation that includes SPEX data in LXD1.9 mode to give you an idea about the dispersion:
Please note that the camera takes only black and white images. I have tweaked the intensities and applied a rainbow map to the intensities using SAO DS9. During our observations we actually use two different dispersive modes. One is called SXD and the other is LXD2.1 and we merge the two together to form the final spectra. The array is a 1024x1024 Aladdin 3 InSb array. The camera is a very excellent device, but it is not without small defects that are common in all instruments. For example, check out this image below:
There you see what appears to be a pit. More interesting is the line that goes from top to bottom in the image that even show up the flat-field images:
Those lines, if I recall correctly, are actually defects in the structure of the camera's detector (I want to say that these are crystalline defects, but I'm not for certain). The above image is actually a flat from a light source inside of the camera. In the lower-left side, you can see that there appear to be overlapping orders. This effect is called persistence and is literally one of the previous exposures. In the lower-right, you can see some leftover spectra (the vertical bands) from a previous observation. To calibrate our data, we need to take spectra of a known object that has well-defined spectral lines. To do so, we expose the camera to an argon arc lamp and obtain calibration frames (called "arcs") like what follows:
In this image the most intense regions (shown in red-orange). These are the arc lines from the arc lamp. You can also see that the background isn't as clean as it was in previous images, this is simply background noise that will need to be removed during the processing steps.
Spectra from stars looks different. In the image below you can see the spectral bands from the atmosphere (the most obvious is the vertical red-orange line in the third order from the bottom), as well as the star's spectrum (red-orange bands that follow the orders). You can also see the spectra from a previous exposure (the nearly horizontal line that crosses the second order from the bottom).
In my next post I'll discuss how the spectra is reduce from it's image form to the spectral form which we are accustomed to seeing.