Today, I have acquired my first atmospheric spectrum taken from my office windows. The system was looking at an autumnal cloud (so a pretty grey one!). A simple python GUI has been made recently to control the camera via the CL2USB3 framegrabber, and is shown on the image below:
The GUI displays the camera sensor image in the main window (in a gray scale colormap) with 2 associated profiles respectively on the bottom and on the right hand side of the sensor image. These profiles give a cross-section across the spatial (horizontal) and spectral (vertical) directions. The exposure and frame rate of the camera can be controlled separately, but I have used the ALC (automatic Light Control function) to optimize the InGaAs sensor dynamic.
The horizontal profile shows the intensity distribution of the cloud across the slit, and we can notice that it is composed of 2 brighter features. The vertical black lines therefore correspond to darker areas, within the cloud. The spectral profile is taken on the brightest part of the cloud. The horizontal lines are the interesting features. They represent the main atmospheric absorption bands.
The image below shows the light path from the scene (i.e. the cloud observed through my window – in fact this is not the cloud used for the spectra shown here above!!!) up to the camera sensor. The scene is imaged onto the spectrograph slit using an objective lens (shown in the previous post). The light passes through the slit and is then dispersed by the freeform grating producing the spectra (vertically) for every spatial points across the slit.
The atmospheric transmission is composed, in the Infrared, of low absorption spectral windows or bands. Each of these bands is denominated by a letter. Ground astronomical IR instrumentation, which aims to look at the light from the stars and galaxies through the atmosphere, have been optimized to deliver the best possible image and contrast in these bands. The image below shows the main bands in the spectral range of the Owl mini 640 camera:
There are 3 main bands, at around 1020nm, 1220nm and 1630nm. The strongest water absorption is between the J and H bands at around 1400nm.
So the above spectrum, which is acquired by our system, is the spectrum of the sun (black body spectrum with the radiance peaking in the visible range around 500nm) observed through the atmospheric transmission window, and finally reaching the camera sensor, which has in turn, a maximum sensitivity between 1000nm and 1500nm.
The image below shows the position of the different IR bands on a spectrum acquired with the system.
Next time, I will look at calibrating the system and at assessing its performances by measuring the smallest spatial and spectral features that it can image.