Solar Spectrograph


Design Team:

Jordan Creveling (EE)
Blaine Ferris (EE)
Ahmet Kartal (ME)

Team Adviser:

Dr. David Dickensheets

Project Sponsors:

Montana State University ECE Dept.
Montana Space Grant Consortium
Richardson Gratings

Project Statement:

    With growing concern due to climate variation, it is important to create measurement systems to monitor the presence and concentration of elements, molecules and particulates in the atmosphere. This team proposes the design of a low-power solar spectrograph that will measure the relative intensity of H2O Fraunhofer lines (absorption lines) in the atmospheric solar spectrum. Through the design of a durable weather-resistant system, long-term measurements of Fraunhofer lines will be collected at accessible or remote locations to monitor the changes in atmospheric gas concentrations.   


Water vapor Absorption lines (Frounhofer Lines):

    The sun which is often considered a black body radiator emits a broad spectrum of radiation that travels through the earths atmosphere. The plot below shows the total outer atmospheric sunlight (yellow) fits a Blackbody spectrum curve. When the radiation has reached earths surface certain wavelength ranges will have been absorbed by atmospheric gases (red). Note that there is a large section missing around 800nm labeled H20.  

                                    File:Solar Spectrum.png                                    

Below is a Hitran Model of the wavelength dependent absorption by atmospheric gases. Note that at 0.8um there is a defined band corosponding with water vapor absorption.

Spectrograph Calculations:

    A Solar spectrograph is an instrument that captures radiation from the sun and separates this radiation into its individual wavelengths. With a known spatial separation of individual wavelengths spectrographs can be engineered to accomplish a scientific goal. There are different variations of spectrographs however most share some specific characteristics to accomplish their tasks, these characteristics include capturing light from the sun, spatially filtering it through an aperture, collimation of the captured light, separating the wavelengths with a diffraction grating or prism, focusing of light onto a detector with a focusing lens or mirror. In this design a reflective diffraction grating is used to separate the wavelengths and a focusing lens is used to focus the separated wavelengths onto a sensor. A CMOS sensor was used in this design instead of a photodetector which would require a motor to scan the individual wavelengths across the sensor.

    The novel part of this spectrograph design is how the light is collected from the sun at various angles across and above the horizon. Generally, solar spectrographs light input slits or apertures are pointed directly at the sun. With this spectrograph design a light diffuser will be used to diffuse light incident at a maximum of 23o, such that enough of the scattered light is captured by the system. Holographic diffusers shape the diffusive light profile for higher sensitivity applications where higher transmission power is a necessity. A holographic diffuser set was obtained and tested for transmission and angular independence for this spectrograph system.

    To test the holographic diffuser an open faced optical power meter was used. A 2mm pinhole aperture was screwed onto the face of the detector such that the aperture remained approximately 1cm from the face of the detector. The holographic diffuser was then taped over the pinhole aperture where it remained approximately 5 mm away from the aperture.  A protractor was taped to the center of the apparatus and by using a ruler a 40 Watt light bulb was stationed 12 inches from the center of the apparatus. A two-dimensional diagram of the setup can be seen below. The light was initially held at 0o and moved up in 5o increments to 40o while remaining 12 inches from the apparatus. This test was done with two different holographic diffusers, a 20o and 60o diffuser. The rating of these diffusers is in regards to the maximum angle at which they will transmit optical power when light has normal incidence on its surface. The power relative to a control group incident on the detector was measured and plotted in Matlab. The control group was defined as the light bulb normally incident on the system with no diffuser blocking any light. These values can be seen over the range of input angles in the plot below.

 Holographic Diffuser test

    The diffraction grating could be considered the most important component of the spectrograph system. This importance is reinforced by the fact that it has been mentioned so much in earlier sections. Without some type of grating or prism there would be no way for light to be separated into its individual wavelengths. The three gratings that were considered for this design were the Richardson 53-*-260R, 060R, and 290R reflective diffraction gratings. The grating that was selected from the three options was the 260R grating with 600 grooves/mm and 8.6o blaze angle. The efficiency for this grating can be seen below.

    The minimum optical power required for the CMOS sensor chosen for the system is specified to be 2nW/mm2. In the holographic diffuser test the diffuser efficiency at the most extreme angle seen in this system (23o) is approximately 20%. This empirically observed parameter is shown below in the Parameters section below. Additionally, sun approximate specifications, slit and camera area, and grating and camera efficiencies were taken into account at the center wavelength (800nm) for the purposes of the power budget calculations. The calculation flow and calculations below show that the theoretical intensity incident on the camera is 31.8nW/mm2. This value is greater than the minimum camera specification, and it was therefore shown that the system should function correctly with a holographic diffuser at the input. These calculations do not take into account additional power collected from a cylindrical lens, or power lost inside the spectrograph system.

    The resolution of the CMOS sensor is stated to be 1280 x 1024 pixels, however the resolution of the sensor given in the number of pixels it contains is different than the resolution of the Solar Spectrograph device. The resolution of the spectrograph device refers to the detail in which the desired spectral range can be imaged across the CMOS sensor. This resolution limitation is ultimately a function of the size of the entrance slit of the spectrograph. A balance between the optical power input and the spectral resolution needs to be made for functionality of the system. These are inversely related, and are both dependent on the input slit size. From the general resolution calculation below it can be seen that if slit size increases, resolution decreases. However, in this case power input into the system will also be increased. On the other and if the slit size is decreased, resolution would increase and power would decrease. The slit size will be determined empirically, and will be uniquely designed for this system in order to balance the power and resolution requirements.  The calculated resoultion 
given the sensor area, resolution and slit size of 50 um, the system resolution will be 0.16 nm/pixel. This resolution signifies that over the 25 nm wide water vapor absorption feature being imaged there will be 157 data points representing the feature out of the 1024 pixels.


Spectrograph Design:

    A collimating lens will be placed the focal length, 30 mm away from the slit. The focusing lens will be place about 8 inches from the face of the grating with the sensor placed at the focus, 30 mm. The lens is placed 8 inches from the diffraction grating face because it will take around 8 inches for the diffracted rays to clear the incident collimated beam. This also sets the distance between the collimating lens and diffraction grating to be at least 8 inches. The focusing lens will be angled at 10.6o such that all of the 800 nm rays are captured at a normal to the focusing lens. This will result in the 800 nm wavelength of light to be focused in the middle of the CMOS sensor. It is inherent that not all of the light reflected off of the grating will be captured by the focusing lens as the spot size incident on the grating is equal in diameter to the focusing lens. The beam then proceeds to elongate in the dimension of spectral spreading this causes vignetting of the slit imaged spectrum when a fraction of the beam is captured by the focusing lens. This results in the extreme wavelengths at 700 nm and 900 nm to have a lower relative power on the sensor face compared to the 800 nm wavelength which should be entirely captured on the sensor. When the system is being prototyped an effort to maximize power captured by the focusing lens will be made by adjusting the incident angle of the diffraction grating such that the focusing lens can be moved closer to the grating, decreasing the effects of vignetting. A diagram of the Lens optical design layout can be seen below.       Optical Layout

    The system housing will be assembled using a metal frame in order to ensure stability and a planar system. Aluminum sheets will be cut to the desired dimensions. Then, the sheets will be bent and welded together in order to create the housing box and frame. The box must be designed to be optically insulated. If any external light enters the system except through the optical slit, any optical power measurements will be compromised. To accomplish this optical insulation, the inside of the box will be painted with a low-reflection black paint in order to absorb as much undesirable light and reflected light as possible. The top and bottom box covers’ edges will be will be sealed with foam strips in order to create a system that is insulated optically and from moisture and weather. The optical components will be mounted on an aluminum mounting plate. The optics will be mounted on this aluminum plate using lens mounts, posts and post holders from Thor Labs. The slit and diffraction grating will be mounted on the edge of the box, as opposed to mounted internally as shown in the figure. This will help minimize any additional light entering the box. Currently, the slit aperture is specified to be 5.32mm tall and 50um wide. This size will accommodate the height and power specifications of the camera. The slit may be widened to increase power while sacrificing resolution, or narrowed for the opposite effect.


    This budget does not include costs that will factor into construction of the optical system housing. However, there is room in the budget to account for the creation of the housing system. Specifically, 160 dollars remain for housing construction. Additionally, some aspects of the design may be cut from the system. The optical post holders are not necessarily required. Instead, the optics could be mounted by hand to decrease price. While the post holders would make mounting simpler, they are not required. The most expensive element in the design is the CMOS Camera. It is required, however, because it is able to operate under low input power conditions, which will occur in this system. The diffraction grating is supplied by Richardson Gratings free of charge, and does not count toward the budgetary constraint. Additionally, a laptop computer may be used during the competition to obtain and interpret data. The aluminum mounting plate and other mechanical aspects of the design will be machined and altered in Montana State University’s machine shop.