User:Alots/ALOTS
In April I did an article stub about http://en.wikipedia.org/wiki/Airborne_Tracking_Imageing with a focus on the less known ALOTS system and the description of unusual images aquired by this system during the Apollo programm. The stub was intended for others I informed about it to contribute their knowledge. After some build up it was to link to other articles of Apollo, Rocketry or Telescopes. But it was soon renamed to http://en.wikipedia.org/wiki/Airborne_Lightweight_Optical_Tracking_System and deleted within a week. Not even spelling and grammer were corrected. Regarding the reasons for deletion I will point out that I`m not "a five year old", neither was it "a hoax" nor was my intention to hurt the "reputation of Wikipedia." I thought only of Wikipedia as a place for collaboration. Now I will try it again. Not as an article but at a user side. I put in new data I got on ALOTS and broaden the scope beyond ALOTS. Everyone is free to edit it.
Airborne Telescope Observations[edit]
Besides balloons, airplanes were used as airborne telescope platforms. The Airborne Lightweight Optical Tracking System (ALOTS) was for tracking and observing rocket launches and reentries. The Kuiper Airborne Observatory (KOA) was for astronomical objects. Its successor is Stratospheric Observatory for Infrared Astronomy (SOFIA), currently under construction.
To overcome limitations of ground based optical tracking and imaging of rockets, a airborne camera system was developed by the USAF for the Apollo program. This first Airborne Lightweight Optical Tracking System (ALOTS) pod went operational in 1967. Compared to an airborne system, ground based optical tracking and imaging is limited by clouds and in resolution and contrast by atmospheric conditions. Fixed stations are further limited by the direction of sunlight towards the rocket.
The pod was "22 feet long and six feet in diameter", mounted on the cargo door of an EC-135N. The EC-135N was a modified version of the Boeing 707. It was an ARIA (Apollo Range Instrumented Aircraft) and its main purpose was as airborne tracking stations. The nose section was modified to house an S-band tracking antenna dish of 7-foot diameter. Of the 9 EC-135N aircraft the USAF had, 4 were modified to carry ALOTS pods.
Here the strange looking EC-135N with pod in flight. As the ALOTS was a forward looking system, only the artist was free to revert the flight direction.
At present, the paper by D. C. Marquis, "Optical tracking; a brief survey of the field," Appl. Opt. 5, 481- (1966) is the only one I know with a brief mention of ALOTS:
"Present efforts on the Air Force Eastern Test Range, which are somewhat representative of the optical tracking field, include a Mobile Optical Tracking System (MOTS) and the Airborne Lightweight Optical Tracking System (ALOTS). MOTS consists of a tracking pedestal, mounted on a trailer, which is capable of holding many lens and camera systems up to 457 cm in focal length. It uses gearless electric torquers, automatic ir tracking, and closed-circuit television to produce high-accuracy and high-dynamic-rate tracking. ALOTS has a 508-cm focal length, 50.8-cm aperture optical telescope mounted in an Air Force KC-135 (Fig. 5). It uses two coaxially mounted television systems (wide and narrow fields) to produce high-accuracy automatic tracking for the main telescope." (APPLIED OPTICS / April 1966 / Vol. 5, No. 4 / page 483)
The Fig. 5 shows a pod very like the model above with a side window. This side window was probably for tests during development as the later operational pods missed it. The 2 m focal system with a 16 mm camera mentioned in another web source was probably only during development too.
The data of the telescope ringed a bell. Its a 20 inch f10 system. The Celestron Company was famous for f10 Schmidt-Cassagrain-Telescops (SCT). I remmember I saw in the early 1980s an offer for a 20 inch f10 SCT by Celestron wich was presented as a large but still man portable unit. On a photograph one man had it in his hands. I know of rumours that Celestron first produced for the military and such a SCT was already for sell in the 1960s. As the SCT is the most compact way for a large field / large focal lenght telescope, I assume ALOTS was a 20 inch Celestron SCT.
Some corrections to the above referenced information about ALOTS. I was one of the airmen assigned to maintain the system at Patrick AFB. The aircraft was an NKC-135A (serial number 55-3123) that did not have the famous "droop snoop" as the EC-135N. The pod was mounted to the port side of the aircraft and housed the two television cameras and the telescope which was used to film the Apollo launches. The pod had an optically flat window that was used for viewing for the tv's and telescope. It could rotate approximately 270 degrees up and down. Forward/backward movement was limited by the window. The electrical system for the television cameras was designed to auto-track the launch vehicle by locking onto the contrast of the rocket and the background. If the system would not lock, the operator would follow the rocket with a joystick at the interior console. In addition to the launches at Cape Kennedy {now Cape Canaveral), ALOTS covered several re-entries and a total solar eclipse over Florida. During the launch flights, the major networks would alternate on the aircraft and send the television signal back to the Cape to be converted to a signal for commercial use. This was done by a pool of the networks, so all had the same tv pictures.
The atmospheric seeing problem[edit]
The main factor in telescope resolution is atmospheric seeing. Its caused by wind or thermal convection. It limits the resolution of telescopes most to around 1 arcsec.
The resolution of a perfect telescope in vacuum is given by the Rayleigh Criterion to 1.25 * lamda / d (d = diameter of aperture, lamda = wavelenght). For visible light that gets to
Resolution = 13 / d (d in cm and Resolution in arcsec)
Due to atmospheric seeing the 5 m Mount Palomar telescope got no better images of Mars than a 0.15 m amateur refractor. It changed since the 1990s by the HST, short time exposure technique and addaptive optics.
The airplane telescopes were away from of the thermal convection and ground wind turbulence. But it faced other problems. To protect the ALOTS telescope against stagnation pressure by Mach 0.7 airflow it was behind a protective glas lens at the front of the pod. Such a lens could have some degrading effect on the telescope performance. Of much worse influence was probably the airflow itself. The direct impact on the pod and the boundary layer turbulence from the nearby aircraft fuselage. From the pictures below (of Apollo 7 and Apollo 11 seperation) one can estimate the ALOTS resolution to about 2 arcsec.
Thats the same like estimated for the Kuiper Airborne Observatory. The KOA was in the fuselage of a Lockheed C-141A with a 91 cm mirror vignetted to 74 cm and operated from 12.5 to 13.7 km with a speed of 0.5 (at 7.6 km) to 0.8 Mach. It was concluded after some tests that the shear layer was responsible for a 2 arcsec resolution in visible light by fluctuation in a 0.3 ms time scale. Other factors resulted in a total degradation of the KAO resolution to 5 arcsec. (Elliot, et al: Image quality on the Kuiper Airborne Observatory. I - Results of the first flight series, Astronomical Society of the Pacific, vol. 101, Aug. 1989, p. 737-764) To my knowledge the "shear layer" of the KAO was created by a special fence to protect the fuselage structure against turbulence vibration affecting the cavity structure. It was not realy laminar but more like the aircraft's boundary layer.
For SOFIA, a 2.5 m aperture IR telescope in a 747, a 2 arcsec resolution is expected. As the resolution of this big telescopes are no better than a 7 cm telescope for $300, its still the only way to get some crucial infrared data. The IR light tells much more about stellar objects than the visible light. And the possibility to update and change the detector every day is a big advantage over a much more expensive space telescope. In 1977 the KAO demonstrated the unique cabability of an airplane telescope in the southern Pacific. In this remote area the line of sight between Uranus and the star SAO 158687 intersected. By monitoring this star occulation the KAO discoverd the unexpected rings of Uranus. In an occulation years later the IR capability of the KAO discoverd the atmosphere of Pluto,
Examples of ALOTS filming and photography[edit]
Enhanced version of an image from an ALOTS film of the launch of the Saturn 1B with Apollo 7 at Oct. 10. 1968. The original by NASA is here
The NASA photo was published with the caption: "Apollo 7 seems to brush past the 525 foot high building where the Saturn V rockets were assembled. Taken from an airplane at 35,000 feet, the photograph is an illusion; the building is about five miles from the launch site." Apollo 7 started from Launch Complex 34 what is 10 km from the Vehicle Assembly Building (VAB). The VAB is 160 meters tall, 218 meters long and 158 meters wide. More info here A view with the most similar angle I found was this
The image has some geometric distortion. Assuming its mainly from the viewing perspective one may get some data on the aircraft position. It was at an elevation of about 13.6 deg over the horizon and had a position 45 km downrange in 130 deg (SE) from the VAB. From that data the rocket in the image has already an altitude of about 2.5 km.
The ALOTS camera pod of the EC-135N is known as 508 mm lens system. At a range of 45 km such an optic has a theoretical resolution limit of about 0.06 m. As the image shows, the optic only got around 0.5 m. We see the cars in the parking lots around the VAB, the staircase and part of the window grid of the launch controll building. Some of the black dots, specialy those on the top of left VAB closing collector, may be humans observing the launch. The darker tone below the VAB may be caused by a cloud shadow.
ALOTS (crop) of Apollo 11 during ascent with insert at lift off imaged from the same direction. The original NASA photo is here. About half of the first stage is engulfed by the exhaust of the main engines. This F-1 engines were designed with a nozzle to get maximum thrust at lift off from the ground. That means the jet out of the nozzle has the same pressure as ambient air. As the rocket ascents the ambient airpressure drops. Therefore the jet expands further after it left the nozzle. That widens the exhaust plume at altitude. But in case of the F-1 an additional effect comes in. The exhaust of the F-1 feed turbins is injected inside the F-1 at half height towards the nozzle walls. This part of the F-1 exhaust has less speed, was very sooty and expanded much more on altitude. This part of the exhaust plume is able to move upward and engulfes first the engines and at last almost the whole first stage. We already see some darkening of the white painted stage by this sooty backflow.
[Right] Enhanced ALOTS image of Saturn V first stage seperation. Separation occurred at an altitude of about 38 miles, some 55 miles downrange from Cape Kennedy. The large smoke cloud is the remain of the first stage exhaust plume. As the rocket no longer acelerated the cloud passed by and expanded upward inside the low pressure zone behind the Saturn V Mach-cone. One of the few cases where the Mach-cone of a large vehicle gets visible. At the sides the smoke cloude pushes forward until it gets stoped by the low density atmosphere.
The solid rockets engines in the fairing of the first stage (see insert) are on full thrust. The three visible orange/white color strips are the solids exhaust. The white ring at the level of the interstage is where the solid exhaust hits the inside of the Mach-cone and gets reflected downward. The white trail behind the rocket is spill of unburned fuel.
ALOTS (crop) of Apollo 11 after first stage seperation. The full NASA Photo is here. The large cloud behind the stage is unburned fuel out of nozzles of the F-1 main engines. Above the engines two white dots are the residual plumes of the solid retro rockets. Almost all of the first stage is black now from the intense sooty backflow.
The five dots on the back of the second stage are J-2 engines running. Their H2O exhaust is invisible but the inner side of the nozzle has some luminosity - much like the Space Shuttle main engines. From the end of the second stage we see three faint lines backwards. This is the diminishing exhaust of four solid rockets. They created some artificial gravity to help for start of the J-2 engines.
The original image has reference marks and the black image border visible. As the system is called a "70 mm" and had a 5 m focal lenght, one can calculate the range of the rocket. It comes to around 70 km. The image resolution is close to the grain of the film. That is typical for an optimized system were optical and grain resolution usually match. The resolution is about 1 m what comes to 3 arcsec at 70 km.
References[edit]
Advanced Range Instrumentation Aircraft 55-3123, 28 January 2010