As stated in previous posts, I was chosen along with Carolyn Bushman of Wendover Jr/Sr. High School to fly on SOFIA as Airborne Astronomy Ambassadors. On the evening of June 25-26, 2013 our dreams and preparation finally paid off as we flew on a science observation flight aboard SOFIA. This post will describe the first half of our voyage. Previous posts described our trip to Palmdale, CA and the Dryden Aircraft Operations Facility (DAOF) where SOFIA is based. We were joined there by Matt Oates and Dan Ruby, educators from the Reno, NV area. Matt teaches gifted and talented programs in schools in Sparks, NV and Dan is the director of the Fleischman Planetarium at the University of Nevada at Reno.
In the three weeks since our flight, I have cleaned up my photos and captured the video clips. I’ve also been working on a revised version of my script for the final video I’m making as part of my outreach plan.
On the day of a flight, preparations begin early in the morning. The science instruments, including FORCAST, must be kept cold using liquid nitrogen and helium. Every 72 hours the cryostats are filled and the instrument monitored. The airplane is towed out of the hangar and onto the tarmac where it is fueled up and inspected.
Meanwhile, mission planners finalize the flight plan and crew manifest. Prior to boarding, a briefing is held in a conference room on the second floor of the hangar building at the Dryden Aircraft Operations Facility. The Mission Director for our flight was Randy Grashuis, assisted by Sybil Adams. Karina Leppik was our Science Flight Planner and Jim De Buizer was the Test Director, who acts as the lead Staff Scientist. In addition, there were the flight crew, telescope operators, computer programmers, additional scientists, reporters from the L.A. Times and Aviation Week, NASA publicity people, safety and equipment technicians, a graduate student representing one of the principal investigators, two interns for the STAR program, and us, the Airborne Astronomy Ambassadors. Altogether there are seats for 32 people on board, and we filled up most of them.
Randy led the briefing, going over the manifest to make sure it was accurate. It is uploaded into the emergency beacons on board the airplane so that a crew list is available to rescuers. The flight path was discussed. Tonight, our flight would take us over much of the western United States, from Palmdale northeast along the Colorado River to Moab, Utah, then southeast across northern Texas to Louisiana, then north to Missouri and west to Colorado, then northwest across the northern arm of the Great Salt Lake to the Oregon-Washington border, and finally south and southeast again to Palmdale. We discussed the weather report along the way – mostly smooth skies at 40,000 feet but some possible high-altitude turbulence across northern Texas. We also went over the science objectives and targets for each leg. We received a handout showing the exact headings and course adjustments during each leg.
It was finally time to board SOFIA for our flight. We walked from the main hangar around to where SOFIA was parked and took some photographs before climbing the stairs into the airplane. We got our geared stowed away, including midnight snacks and drinks, camera equipment, and flight plans. The safety technicians provided additional training.
I was able to join the flight crew in the cockpit, and strapped myself into the jump seat behind the pilot as they ran through their pre-flight checklists. During most of 2012, SOFIA was undergoing additional modifications. Custom-built digital avionics were installed in the cockpit, which allows the plane to better monitor the telescope cavity and weight. Once the telescope is locked on a target, the scope flies the plane as the plane turns around it.
We taxied out to the runway, passing one of the 747s used to carry the space shuttles. At the end of the runway, the captain briefed his crew.
Then we turned onto the runway, got clearance from the tower, and accelerated. The giant jet lumbered down the runway toward the sunset and lifted off. We turned back to the east and began our climb into the stratosphere as the sun set behind us.
The Telescope and Counterweight
For such a large telescope to point accurately at a target while flying on an airplane, it must be carefully isolated from the normal vibration and turbulence of the airplane itself. It also has to be perfectly balanced in order to turn its 17 ton mass. The telescope’s mirror is 2.7 meters in diameter and is made from Zerodur, a special type of glass that has almost zero thermal expansion and resists cracking in the extreme cold of the stratosphere. To lighten the weight, much of the mirror’s underside was removed by drilling holes in a honeycomb pattern. The mirror components were assembled in Augsberg, Germany in 2002 and a special rig designed to hold it as it was transported to Waco, Texas for installation in the telescope cavity.
The pressure bulkhead between the cavity and the cabin is the balancing point of the telescope assembly. A perfectly spherical ball bearing is sealed inside the bulkhead, floating in a thin layer of oil. On the other side, a counterweight balances the mass of the telescope. The electronics for each detector is placed inside racks and forms part of the mass of the counterweight.
To prevent vibration from reaching the telescope, an isolation system consisting of air-filled rubber donuts prevents movement in all three directions. Active and passive mass dampeners have also been added. With all of these systems, the telescope is able to point at a target with an accuracy of under 0.2 arcseconds, or about one fifteen thousandth of a degree
Pointing the Telescope
Inside the cavity, the telescope is able to rotate up and down about 30 degrees without vignetting, or cutting off the view with the edges of the door. It cannot rotate or slew sideways. To do that, the plane itself must change its yaw, or turn around its vertical axis. This means that each leg of our flight must be carefully calculated so that the telescope, which is on the left side of the airplane, can be pointed at the right target at the right time of the night as the earth rotates beneath us. A pool of candidate targets proposed by the principal investigators is drawn from to complete each leg. Since SOFIA returns to its home base each morning, targets must be sequenced so that something can be observed on each leg as we loop around. To complicate the navigation, some observations take longer amounts of time to gather enough signal from the background noise. They usually take from one to four hours. We also have to stay out of no-fly zones such as the air force test range in western Utah and avoid approaches to busy airports. Some flights go over the Pacific Ocean, others over the western United States. We are not allowed to fly over Mexico.
Once we reached our initial observing altitude of 38,000 feet, the telescope door was opened up.
The first science leg of our flight was to observe the star Tania Australis, one of the stars forming the hind claws of the great bear, Ursa Majoris. It is a red giant in its death throes, gradually brightening as its helium core contracts to where it will soon fuse carbon. It is blowing dust and gas into interstellar space with a strong stellar wind. SOFIA’s telescope looked at this dust, which is more visible in the mid infrared. Its declination of 41 degrees, 30 minutes made it visible to the SOFIA telescope as we flew on an azimuth heading of 28 degrees from true north.
The Light Path
As light from a target such as Tania Australis enters the telescope, it is reflected by the parabolic primary mirror. A secondary hyperbolic mirror reflects the light back down to a flat tertiary mirror tilted at 45 degrees. This mirror is dichroic, meaning that it reflects only part of the light, in this case infrared. Visible light passes through the first surface and is reflected off the bottom of the mirror. Both beams of light travel along a Nasmyth tube through the central bearing and pressure bulkhead. The visible light is reflected off a final flat mirror and bounces to the side to a focal point camera. The infrared light comes to a focus inside the detector instruments mounted on the instrument flange under the counterweight. In our case, the instrument was FORCAST.
The Guide Scopes
To point the telescope accurately at the desired targets, two smaller telescopes are mounted on the main telescope housing. The smaller one is used for general pointing, and has a field of view of 40 arcminutes, or about 2/3 of a degree. The larger of the guide scopes points the main telescope more accurately, with a field of view of ten arcminutes. Finally, the camera located at the visible focal point is used for precise pointing, since it gathers the actual light from the primary mirror. It has a field of view of only 1.0 arcminute, or 1/60th of a degree. At our station, we can see the three guide scope fields of view displayed on a monitor, with digital overlays labeling the stars in each view. The left view shows the general guide scope. The small blue rectangle in the center is the field of view of the mid-range guide scope, shown in the middle view. The small blue rectangle in this view represents the field of view of the focal point camera, shown in the right field.
NGC 7129 and Cepheus A
Our Tania Australis leg had taken us along the Colorado River until we reached Moab, Utah. We then turned to a heading of 118 degrees toward the southeast to observe our next two targets, NGC 7129 and Cepheus A. Both are active star-forming nebulas in the constellation Cepheus. NGC 7129 has the appearance of a rosebud in this false color infrared image from the Spitzer Space Telescope. Cepheus A contains a huge protostar sending out collimated pulses of radio waves along its rotational axis. Light emitted from water and methane molecules is polarized into collimated maser beams by strong magnetic fields spiraling out from the protostar.
The purpose of these legs was to look at dense star-forming clusters. Several competing models exist that attempt to explain how stars form in these clusters, and these observations will help define and constrain the theoretical models. The principal investigator is Professor Lee Mundy of the University of Maryland.
We encountered a hiccup in the SOFIA science software as we traveled these legs, but computer programmers onboard were able to fix the problem on the fly. Literally. Because of this timely problem solving, the observations were successful.
The Airborne Astronomy Ambassadors Station
While SOFIA was grounded in 2012 for the avionics upgrade, several other systems were improved. A new bank of monitors was installed especially for the educators who would fly aboard. The four of us took turns sitting at the station, which had four monitors and three seats. Dana Backman showed us how to interpret the monitors, which show the observing status, the current leg, the heading and altitude, outside temperature and air pressure, telescope and door positions, vibration, loads for the turbulence dampening systems, and many other data streams.
We also shared the station with Carey Baxter and Rebecca Salvemini, two interns who are part of the STAR program, which stands for STEM Teacher and Researcher. It is a nine-week summer internship for aspiring science and mathematics teachers in the California State University system. Interns work in NASA, NOAA, and NSF facilities throughout California. During our stay in Palmdale, we had the chance to meet several of these future educators who are working at DAOF and at Dryden Flight Research Center.
We also had several members of the press. Beth Hagenauer and Jim Round represented NASA public relations. They interviewed each of us as we flew across northern Texas. We also had Amina Kahn, a science reporter from the Los Angeles Times, and Guy Norris from Aviation Weekly aboard.
Once we reached the Texas-Louisiana border, we turned north on a heading of 1.5 degrees. Our purpose on this leg was to collect light from a well-known source in order to calibrate the grism spectrometers inside the FORCAST instrument. A grism is a cross between a diffraction grating and a prism, and its purpose is to split infrared light into a spectrum so that the strength of each wavelength can be recorded, which gives us a fingerprint of the types of molecules and elements in the instrument’s field of view. The grisms in FORCAST are made of silicon or of thallium indium bromide. These materials are opaque to visible light, but transparent in infrared.
To calibrate the grism, the telescope was pointed at Arcturus, or Alpha Bootes. It is an orange giant star much like what our sun will become, and its spectrum is well studied. As Luke Kelly put it, having the grism look at a known star is like having a new thermometer and deciding where to put the marks by testing how it reads at known temperatures such as the freezing and boiling points of water.
FORCAST is the Faint Object infraRed CAmera for the SOFIA Telescope. It is a mid-infrared camera that can image from two channels simultaneously. Each channel has a wheel of filters that are rotated into place for narrowband and broadband imaging in the 5-8, 17-25, or 25-40 micron regions of infrared. These are the regions where the SOFIA telescope is most sensitive. A grism can also be rotated into place on each channel, turning it into a spectrometer.
FORCAST uses silicon arsenide and silicon antimony detector arrays with 256 x 256 pixels and a resolution of 0.75 arcseconds per pixel. It has already provided highly detailed images of the supermassive black hole at the center of our galaxy and the accretion disk of heated material orbiting around it. It has imaged protostellar environments, superluminous blue variable stars, the winds from dying stars, and the infrared energy emitted by Jupiter and its moons. As one of the first completed instruments, FORCAST has been in use aboard SOFIA for over two years. The Principal Investigator is Terry Herter of Cornell University. He is assisted by Ryan Lau, a graduate student whom we met while we were in Palmdale, and by Luke Kelly, who is working to calibrate the new grisms.
Using the Grisms
Once the grisms for each channel were calibrated, we were ready to use them to make observations on our next leg. Just north of Branson, Missouri we turned due west to observe G35.2N IRS 1-1. We were at the midpoint of our flight.