HIGH ALTITUDE SKY LUMINANCE MEASUREMENT

WALTER M. CLARK and HAROLD A, MULDOON

~o~hro~ Nortronics, Resewch Park, Rtfos Verdes Estates, California

iReceived 15 June 1963)

INTRODUCTION

IN THE design of a star tracking system, the radiance and spectral distribution of the daytime sky radiation at bigh altitudes are of extreme importance, These factors determine the back- ground noise level in the star tracking subsystem and, consequently, the sip-to”noise ratio and the number of navigational stars available for star sightings. Of particular importance are the characteristics of sky background radiation near the horizon and at angles close to the sun, where the sky is of relatively high intensity.

Daytime star systems normally utilize photosensors with responsivity in the blue and near-visual range of the ~lec~oma~etj~ spectrum. In this range the quantum efficiency of detectors is high, resulting in high sign&to-noise ratio and rapid acquisition and tracking capability.

The U~versity of Wi~nsi~ is currently under Contract No. NASw-66 from NASA to provide equipment for ultraviolet star photography in the X-15 flight research vehicle. The purpose of this experiment is to determine the configuration of equipment to be used for a later ex~~rn~t in the Orbiting Astrono~cal Observatory.

Prior to the incorporation of the ultraviolet cameras in the X-15, a two-channel photo- electric photometer is being flown to measure sky radiance in the ultraviolet and near- visual range of the spectrum. The NASA Flight Research Center, Edwards, California is supplying Nortronics, A Division of Northrop Corporation, with data from the near-visual channel of the photometer. Nortronics is funded by the USAF Ballistic Systems Division under Contract No. ~~694~291 for reduction of this data, and this paper is a report on the measurements completed to date by this joint effort of the University of Wisconsin, The National Aeronautics and Space Administration, and No~ronics,

THE PHOTOMETER AND SUPPORTING EQUIPMENT

The two-channel photoelectric photometer, designed and built by Dr. Arthur D. Code and William Reining of the Space Astronomy De~~ment, University of Wisconsin, has a five degree field of view and employs two photomultiplier tubes and a 200 A passband mirror which transmits energy in the 2300 A region to an ASCOP photomultiplier with CsTe photocathode and LiF window. The remaining spectral energy is reflected by ,the low pass splitter mirror to an RCA lP28 photomultiplier. This splitter mirror consists of five layers of aluminum and MgF in alternate metallic and die~~t~~ layers. The window in the X-15 camera port is Super& ~En~lehardt~ quartz with transmission to 2ooO A. This, material has low susceptibility to radiation darkening.

Located between the splitter mirror and the lP28 photom~tip~er is a near-visual filter 127

128 WALTER M. CLARK and HAROLD A. MULDOON

and a small aperture. See Fig. 1. For the first flight on October 9, 1962, a O-013 in. diameter aperture and a narrow band, aluminum coated Corning 5-60 filter having a peak response at 4200 A was used. For the next two flights, November 9, 1962 and Jannary 17, 1963, the filter was changed to a wide band Wratten-86 and was combined with a 0.025 in. diameter aperture. The integrated response of the system with these filters is shown in Fig. 2, where the W-86 filter was chosen to peak near 5200 is. Future flights are planned to include a filter to shift the response to photopic.

irk window

IP28 photomultiplier

5000 i poss (lost flight’)

LJ

1 &COP photomultiplier

FIG. 1. University of Wisconsin two-channel photoelectric photometer

Wovelength, tL

FrG. 2. Photometer response relative to photopic (Flight No. 3-14-24 same as No. 2-31-52)

Facing Page 128 I.P.

_._._.I- . . “.

FIG. 4. The Nortronics calibration source attached to the mirror in its inconel-X housing

FIG. 5. The external folding mirror mounted in the starboard camera port on the X-15

High altitude sky luminance measurement 129

The photometer gain can be changed in decades by switching the load resistor through a series of values from 100 K to 1000 MP. Overall instrumentation accuracy is approximately 5 per cent, and the instrument was tested under anticipated variations of aircraft primary power, and under vacuum from 10~ to one atmosphere of pressure.

The photometer was analyzed for polarization content and linearity by Nortronics and was spectrally calibrated by Nortronics and by the University of Wisconsin. Figure 3 shows the laboratory instrumentation arrangement used to obtain a relative responsivity

FIG. 6. The line of sight of the photometer relative to the X-15 vehicle

measurement in the near-visual channel. A Reeder thermopile was first used to calibrate the correction curve for the Bausch and Lomb High Intensity Monochromator. The photometer then sensed the output of this monochromator and the correation curve of the monochromator was divided out to yield the responsivity of the photometer.

In Fig. 1 there also appears an external mirror mounted behind the camera port fairing on the X-15. This mirror and mount, designed and built by Nortronics, serves to direct the line of sight of the photometer upward at an angle of 63’ to the centerline of the X-l 5 vehicle, The aircraft pitch attitude varies from -20” to +40” while in flight, and the line of sight of the photometer varies from 80” to 20” above the northern horizon.

The mirror, made of borosilicate crown glass coated with aluminum and a hard MgF overcoat, is mounted in a cylindrical Inconel-X housing to withstand possible temperatures of 800 degrees Fahrenheit in flight due to laminar flow at low angles of attack. See Fig. 4. The mirror was tested for both thermal and vibration resistance prior to flight. The spectral reflectance was measured and taken into consideration in the data reduction after each fight.

Figure 5 shows the mirror mounted in the starboard camera port on the X-I 5 instrumen- tation bay hatch. Figure 6 indicates the line of sight relative to the X-15,

A calibration light source of known spectral distribution and luminance was clamped to the external mirror prior to, and after, each flight to calibrate the sensitivity of the photometer-recorder system. This calibration source consists of a 6 V lamp mounted in a

130 WALTER M. CLARK and HAROLD A. MULW~N

diffusing well and powered by a regulated a.c. transformer. An opal glass diffuser and Chance Brothers OB-8 filter completes the blue diffuse calibration source as shown in Fig. 4.

DATA REDUCTION

In addition to a copy of the on-board oscillographic recording of the output of the photometer, NASA furnished Nortronics with a plot of altitude versus time, a radar plot of the locus of nadir points of the X-l 5 during its flight, and a record of yaw, pitch, and roll in tabular form.

From the information provided, a comprehensive plot of attitude, altitude, and photo- meter response versus time was made for each fight. These appear as Figs. 7, 8, and 9. The attitude of the vehicle is plotted at the top of the chart as a function of the elapsed

60

FIG. 7. Sky luminance measurements relative to aircraft position and attitude using the University of Wisconsin photometer in the X-15-2 flight research vehicle; Flight No. 2-30-51 on October 9, 1962. Real

time at launch: 1059 hr, 43.7 set (Pacific Daylight Saving Time) The large fluctuations of the photometer output curve at launch is believed to be caused by the B-52

intercepting the line of sight. Motion picture films taken by a camera in the left camera port reveal that the spikes occurring between 460 and 500 set, and the full scale deflections between 500 a-rd 550 set are probably due to wisps of vapor drifting back over the X-15. The spikes at 215 set and at 370 set are due to electronic

noise in the amplifiers

High altitude sky luminance measurement 131

Time, set

FIG. 8. Sky luminance measurements relative to aircraft position and attitude using the University of Wisconsin photometer in the X-15-2 flight research vehicle. Flight No. 2-31-52 on November 9, 1962.

Real time at launch: 1023 hr, 52 set (Pacific Standard Time)

time from start of recording, which was about two minutes prior to launch of the X-15. The altitude of the vehicle is plotted across the center of the chart, and the photometer output is plotted near the bottom.

The output of the photometer is plotted relative to four ordinates; the first is the voltage output; the other three are luminance values corresponding to three different assumptions of sky spectral distributions, so one may assume the sky spectral distribution most appro- priate for the particular line of sight selected for analysis from the chart. More than one luminance ordinate was necessary because the filter in the near-visual channel was not a photopic filter, and to convert from integrated radiance units to luminance units, the particular spectral distribution of the sky must be known, or assumed. Provision has been made for three such assumptions based upon the greybody distributions: 1OO,OOO”K, 20,OOO”K, and 6,OWK.

Other high altitude sky radiance measurement programs* indicate that in the region of the sky away from the sun and at zenith angles less than approximately 45”, a sky spectral

* W. M. Clark and D. G. Swanson, Northrop Aircraft Memorandum 3813-57-5003, 24 September 1957- “Flight Altitude Sky Characteristics”.

R. Davis and K. S. Gibson, U.S. Department of Commerce Miscellaneous Publication, Bureau of Standards, No. 114, January, 1931-“Filters for the Reproduction of Sunlight and Daylight and the Determination of Color Temperature”.

132 WALTER M. CLARK and H~OLD A. MTJLDCION

iZO@ P

18OQ ‘$j 24

280 240’ !!

260 304 t-

240

220

2CKl zi L

180

160

140

Touchdown Time, set

FIG. 9. Sky luminance measurements relative to aircraft position and attitude using the University of Wisconsin photometer in the X-15-3 flight research vehicle. Flight No. 3-14-24 on January 17, 1963. Real

time at launch: 1059 hr, 37 set (Pacific Standard Time)

distribution approximating a IOO,OOO”K greybody would be in order. In the northern sky at angles approaching 90” from the sun and 50” to 70” above the horizon, the sky spectral distribution may exceed ~~,~~K. A~rdj~gly, the lOO$OO”K ordinate should be used as a guide in estimating luminance in these regions of the sky.

For high altitude orientation with the line of sight directed away from the sun and at an elevation angle from the horizon of less than approximately 35”, a 20,OOO”K sky spectral distribution is in order, For lower altitudes and higher elevation angles, but away from the sun, 20,OOO”K is still a fair approximation.

As the line of sight approaches the horizon, or approaches the sun, the spectral distri- bution shifts toward an even lower color temperature, reaching about 6,OOO”K at the horizon and at the sun, This is approximately true for all altitudes, with due allowance being made for greater scattering of sunlight at the lower altitudes. In these regions of the sky where the spectral distribution is in the 6,000°K to 20,OOO”K range, a ftL value may be inter- polated from two of the three ordinates provided.

High altitude sky luminance measurement 133

In future flights with a photopic filter in the near-visual chamtel, it will not be necessary to define a spectral ~s~bution, and only one 1 uminance ordinate will be necessary.

Equations were derived which give the position of the sun, the position of the Iine of sight in the sky, and the angle between them for the entire flight of the X-15. These equations and the necessary data were processed by the IBM 7090 computer and resulted in tabular data, with samples taken every 2$ set from launch to touchdown. This tabular data is far too extensive to include in this paper, but may be found in the Nortronics’ Report, NORT 63-165, “Final Report, High Altitude Sky Brightness Measurement Program” which will be published in late summer, 1963.

A polar plot of the intersections of the line of sight with the celestial sphere for flight No, 3-14-24 on January 17,1963 appears as Fig. 10. This is provided as a typical sky sampling profile,

East

North

FIG. IO. A plot of the intersections of the line of sight of the photometer with the celestial s here for flight No. 3-14-24 on January 17, 1963. Sky sampling profiles on other flights are s is&r.

CONCLUSIONS

Figure 11 is a plot of a cross section of the hemisphere of the sky taken approximately north and south. The 5,000 ft and 20,000 ft curves, measured on both sides of the sun, are taken from the S. Q. JDuntley, Scripps Institute of O~ano~aphy Final Report NS71~1~

INF.-K

134 WALTER M. CLARK and HAROLD A. MULDOON

Sun I

A 40000ft x- 15 Data .120000 ft x-15 Data 0 6000011 x-15 Data 0 Newkirk

v 6OOOOft x-15 Data 8 Comput

LIP\\ I I I 5000ft A II I

?\\I /I

,

U d

, I

h Srrinns I/ I 811

FIG.

I.0 -80 -60 -40 -20 0 20 40 60 80

Zenith angle

11. A north-south cross section of the sky showing the brightness of the sky at several altitudes, compiled from various sources

as

Contract No. bs-72039, Task 2, 10 January 1958 entitled “Aerial Measurements of Atmospheric Clarity and Sky Luminance Near Alamogordo, New Mexico”. The higher altitude curves were obtained from three sources; the series of X-15 flights being reported in this paper, the Newkirk and Eddy article, “A Coronagraph Above the Atmosphere” from the Sky and Telescope magazine, August, 1962, and finally from theoretical computa- tions based on the assumed linear relationship between pressure and sky luminance at high altitudes. The computed values are somewhat lower than the measured values below 40,000 ft because of the effect of large particle scattering at these lower altitudes.

Although the line of sight of the photometer was entirely subject to the attitude constraints of the X-l 5 vehicle, and, consequently, the section of sky sampled is very limited, note that the Nortronics’ data agrees exactly with the Scripps’ data at 20,000 ft and 20”-40” zenith angle away from the sun.

High altitude sky luminance measurement 135

As an example of the usefulness of the tabular data, we extractitwo rows from these tables for flight No. 2-30-51 ou October 9,1962. These are presented as Table 1 and indicate that brightness values approx~ating 200 ftL are logical for low view angles at these altitudes.

It is more important to assume the correct sky spectral dist~bution from the data which uses the 4200 A peak waveleugthresponse than it are from the data which uses the 5200 A peak filter. This is true because the former response is relatively far from photopic.

TABLE 1

Nadir Nadir Altit LOS LOS B’ B’ B’ Univ Sun Sun Scattering Long Latit Azim EIev l~,~‘K2O,~*K 6,OOO”K Time Azim Elev Angle

w

117-Y 3.5*1” 70,000 350.7” 21.1” 94 II& 261 18.083 147.7” 43*3” 112.2” 117.5” 35.1” 66,500 3555” 16.3” 140 174 386 18.085 147.7O 43~3~ 115.3’

Although this program has resulted in a significant contribution to the knowledge of high altitude sky brightness, data taken close to the sun may be questionable because of as yet undetermined glare characteristics inherent in the photometer. Also, in order to place greater weight upon the accuracy of the data from the flights, a better knowledge of the spectral distribution of the sky at high altitudes would have been desirable, at least across the spectral response range of the photometer.

Several irregularities may be noticed on the data plot of flight No. 2-31-52 (X-15 No. 2, 31st successful launch, 52nd attempt) on November 9, 1962. The first irregularity is of course that this was a short endurance, low altitude flight. Unlike the other flights, which were launched at a point approximately 80 miles north of Las Vegas, Nevada, flight No. 2-31-52 was launched a few miles north of Tonopah, Nevada, about 90 miles north of Death Valley, California.

Unable to obtain full thrust from his rocket engine on this flight, the pilot, Jack McKay, shut down his engine and began the emergency fuel jettison procedure as he commenced a left turn towards the east and headed down for his first emergency landing site, Mud Lake. Al~ough he came in heavy and fast due to a malfunction of his landing flaps and the fact that he still had fuel on board, McKay made a good landing on the parched mud of the dry lake bed, After a landing roll of a few hundred ft, however, the left tail skid collapsed, dropping the X-15 onto its left wing tip and causing it to veer to the left and start an oblique skid which caused the nose wheel to collapse. After considerable momentum had been lost, McKay jettisoned his canopy. A few set later, friction overcame momentum and rolled the vehicle to the right. The starboard wing tip dug into the ground and flipped the X-15 over onto its back in which position it skidded a few ft further befare coming to rest. The waiting rescue crew reached the battered rocket vehicle in a matter of seconds and soon had extricated the pilot, who, although the top of his helmet had been ground flat, suffered only relatively minor injuries.

Although the top of the inst~entation bay hatch cover was extensively damaged and the mirror and camera port were filled with Nevada soil, neither the mirror, its housing, nor the photometer were damaged, other than the mirror being scratched. The spare mirror, which had been sent to Wisconsin for reflectance tests in the ultraviolet region of the

136 WALTER M. CLARK and HAROLD A. MUL~N

spectrum, was returned to the Nortronics Research Park, Palos Verdes Estates, California, and the photometer was shipped back to Wisconsin for testing after the crash. The decision had been made by the USAFjBSD to transfer the equipment to another X-15 vehicle and continue the High Altitude Sky L ice Measurement Program.

Also on the data chart of this f-light, there appears a drop in sky brightness at the time of engine shutdown. This suggests the possibility that there had been a contribution to the apparent sky brightness from the rocket exhaust, possibly due to a scatter phenomenon. The other flights, however, do not exhibit this phenomenon at the time of engine burnout. If there had been a contribution of brightness due to rocket exhaust scatter light during this flight, then there should have been a si~cant rise of brightness which would have been sustained when the engine was ignited two seconds after launch rather than being delayed until 20 or 30 set after launch. There is not enough data available to completely substantiate whether or not there is a scatter phenomenon functioning. In either case, however, the post- burnout data should not be involved with this problem, if it exists. The possibility of this phenomenon will continue to be under surveillance in future flights scheduled to altitudes in excess of 360,000 ft.

Acknowledgenrents-The authors wish to bring recognition to the following persons whose cooperation was essential to the High Altitude Sky Luminance Program: Dr. Arthur D. Code and William Reining of the Space Astronomy Department, University of Wisconsin, Capt. Charles A. Davidson, Headquarters Ballistic Systems Division, Air Force Systems Command, USAF, Norton Aii Force Base, San Benardino, California, M. AIan Covington, Louvic P. Thomas, William W. Albrecht and Robert E. Brush of the NASA Flight Research Center, Edwards, California, Frank F. Crandell of Northrop Nortronics, who acted as technical consultant, and Norman P. Laverty of Northrop Nortronics, who directed the Nortronics’ participation in the program.