|
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.
|
