The new Ramen Airborne Spectroscopic Lidar (RASL) instrument being designed under NASA's Instrument Incubator Program at Goddard Space Flight Center will be capable of several atmospheric and cloud measurements that address high priority NASA / Earth Science Enterprise (ESE) objectives.
In order to better understand weather and climate, RASL will be capable of making measurements that include:
• water vapor mixing ratio
• aerosol and cloud backscatter coefficients
• aerosol extinction coefficient
• cloud water content
RASL will be compatible with several NASA research aircraft including the DC-8 and the P-3. This will be the first remote sensing instrument to provide this broad range of measurements from an airborne platform. In addition, the RASL is being designed so that it can be used in a ground-based laboratory between airborne missions for further technology development efforts.
At the heart of the RASL instrument will be a 24-inch aperture F/1.5-F/5.3 Cassegrain telescope Optical Tube Assembly (OTA) now under design and construction by DFM Engineering, Inc. The OTA will have the capability to look in any attitude and will use a fully passive temperature compensated structure so there will be nearly zero focus shift over the expected operating temperature range within the various aircraft or on ground based observations. The structure is similar to our astronomical telescopes and LIDAR OTAs (Optics-in-a-Box™) and uses Invar spacers as one part of the temperature compensation
The OTA must safely withstand up to 5 G loads in any direction and have all structural natural resonant frequencies above 60 Hz, while weighing only 200 pounds. The weight requirement requires that the mechanical structure be made mostly from aluminum and use a lightweighted primary mirror. While aluminum telescope components are more costly to fabricate than steel parts for a given stiffness, (Telescope Structural Article: Steel vs. Aluminum) the overall weight can be made less.
The structural and thermal analysis has been performed by Dr. Frank Melsheimer, the drawings have been completed by the DFM staff, and the primary mirror cell has been machined and tested for strength and stiffness.
The primary mirror cell started out as 200 pounds of aluminum plate and has been machined down to the final dimensions and now weighs 42 pounds. The stiffness of the cell in "piston" (along the optical axis) is 347,000 pounds per inch producing a natural resonant frequency while supporting the primary mirror of 190 Hz - three times the required value.
Stay tuned for further details as we progress on this very interesting and challenging project.