Design feasibility and performance of semiportable field instrument for monitoring changes in earth albedo via earthshine measurement

Franklin, F. A.

Maestre, L. A.

English

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d 
STUDY OF LUNAR EARTHSHINE BY 
PHOTOELECTRIC PHOTOMETRY 
Grant NGR 09-015-025 
Fina l  Report 
F o r  the Period 1 November 1965 to 30 September 1967 
February  19 
Smithsonian Institution 
Astrophysical Observatory 
Cambridge, Massachusetts 021 38 
https://ntrs.nasa.gov/search.jsp?R=19680011503 2020-03-12T08:36:47+00:00Z
STUDY OF LUNAR EARTHSHINE BY 
PHOTOELECTRIC PHOTOMETRY 
Grant NGR 09 -0 15 -0 25 
Fina l  Report 
F o r  the Period 1 November 1965 to  30 September 1967 
Pro jec t  Director 
F r e d  L. Whipple 
Pr incipal  Investigator 
F r e d  H. Franklin 
By F r e d  A. Frankl in  and L. A. Maes t re  
Smithsonian Institution 
Astrophysical Observatory 
Cambridge, Massachusetts 02138 
802-176 
TABLE OF CONTENTS 
Section Page 
1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
2 CONSTRUCTION OF THE INSTRUMENT . . . . . . . . . . . . . . .  3 
3 METHOD OF OBSERVATION . . . . . . . . . . . . . . . . . . . . . .  9 
4 RESULTS WITH THE 5-INCH INSTRUMENT . . . . . . . . . . . .  1 2  
5 CONCLUSIONS O F  THE SMALL-INSTRUMENT STUDY . . . . .  14 
6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15 
7 ATTACHMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16 
ii 
STUDY OF LUNAR EARTHSHINE BY 
PHOTOELECTRIC PHOTOMETRY 
Grant  NGR 09 -0 1 5 -0 25 
1. INTRODUCTION 
The project for  which this final r epor t  m a r k s  a culmination had a two- 
fold purpose: 1) to provide accurate  photoelectric photometry of the earthli t  
portion of the moon, pr imar i ly  to establish an albedo f o r  the ear th ;  and 2) to 
investigate the various instrumental  and observational constraints that such 
a type of measurement  imposes.  In particular,  we wished to know whether 
small, semiportable instruments could be used a t  a number of field stations 
to monitor via earthshine measurements  possible changes in the ea r th ' s  
albedo. In the Attachment to this report ,  we present  an account, a l ready 
published (Franklin, 1967), centering on the f i r s t  purpose. The observations 
reported a r e  two-color photoelectric measu res  obtained with a 16-inch 
instrument on 14 nights at the Boyden Observatory in South Africa. 
are in close accord with the visual observations of Danjon (1954) and present  
new photometric parameters  for  the ear th  in the blue. In Sections 2 and 3, 
we discuss  the resu l t  of the second purpose: the design, construction, and 
operation of a 5-inch field instrument.  
resul ts  obtained by that instrument  at the Smithsonian astrophysical observing 
station a t  Organ Pass, New Mexico. 
They 
Section 4 gives the photometric 
Certain aspects  of this study grew f r o m  a project initiated in 1957 by 
F. L. Whipple. This ea r l i e r  work (Bakos, 1964) emphasized a technique 
pioneered by Danjon (1954), that is, use  of the so-called cat's eye photometer, 
to obtain visual measurements  of the earthshine f r o m  six stations at widely 
different latitudes and longitudes. 
somewhat disappointing. 
The results of that earlier study were  
Although it was indeed possible to der ive visual 
1 
albedos for  the ear th ,  averaged both over the period of a year  and over all 
latitudes and longitudes involved, the large sca t te r  in these observations 
prevented a m o r e  refined analysis.  We re fe r  the r eade r  to Bakos (1964) for  
details ,  and we echo here  his conclusion: that data  much m o r e  homogeneous 
and accurate  were  required before fluctuations in the ea r th ' s  albedo due to 
seasonal o r  meteorological changes could be  reliably determined. 
s t r e s s  he re  that we did not address  ourselves  to this problem in this study. 
Our pr imary  aim was to develop techniques to obtain data of sufficient obse r -  
vational accuracy so  that a sea rch  fo r  fluctuations would be a meaningful 
problem. We feel  that we do have a small number of such observations, but 
they a r e  too few to represent  m o r e  than a reconnaissance. 
W e  shal l  
P 
2 
2. CONSTRUCTION OF THE INSTRUMENT 
The instrument used in this study was intended to measu re  photoelectri- 
cally the intensity of the earthlight reflected f r o m  areas of the lunar surface 
directly in t e r m s  of adjacent standard stars of known brightness.  Since our 
interest  was ultimately to make these measurements  f r o m  stations widely 
distributed in latitude and longitude, we wished to develop an instrument that 
would involve only a small initial investment for  construction and that, while 
incorporating ruggedness with low operating cost, would be capable of 
measuring the earthshine with the highest precision possible. 
we suggested a 5-inch instrument.  
To this end, 
The optical arrangement  of the resulting instrument  is shown in F igure  1. 
Light f rom the object under investigation entered the 5-inch diameter  F / 5  
Achromat (1) and formed an image on a front-surfaced aper ture  plate (3). 
The position of the image on the aper ture  plate could be controlled by an  
observer  looking through a guiding eyepiece (9) a t  the aper ture  plate by means 
of a beam-split ter  (2) fixed a t  a 45-degree angle in the path of the light f r o m  
the 5-inch lens. 
8 a rcmin  in diameter  in the aper ture  plate. The light next passed through 
interchangeable B (Corning 5030 t 2mm Schott GG13) and V (Corning 3384) 
f i l t e r s  (4), interchangeable c l ea r  and opaque neutral-density f i l t e rs  (5), a 
f ield lens ( 6 ) ,  windows on the cold box (7), and onto the photomultiplier tube 
The image could then be centered s o  it fell  through a hole 
( 8 ) .  
An interesting feature  of the instrument  was the cold box used to 
re f r igera te  the detector for  reduction of da rk  current .  The box was a 
Research  Model TE-101 TS thermoelectr ic  unit, which was capable of cooling 
the detector face f r o m  ambient temperature  to f O .  25°C of any selected 
temperature  between -10°C and 20°C. 
the tube chamber and the detector was afforded for  uniform cooling of the 
tube. In addition, the box employed a double-pane insulating window that 
Excellent contact between the walls of 
3 
reduced the effects of blackbody radiational heating of the detector cathode 
and that prevented dewing of the detector face and the window surfaces .  
1 
I 
1. 
2. 
3.  
4. 
5. 
6. 
7. 
8. 
9.  
5-inch diameter  achromatic  objective lens 
10 / 9 0  beam split ter  
Field stop 
Neutral  density f i l t e rs  #1, 2, and 3 
B, V f i l t e rs  
Collimating lens 
Condensation inhibiting windows 
EM1 9558 photomultiplier 
Guide eyepiece 
F igure  1,  Optical components of albedo photometer. 
The detector was an EM1 9558 11 -stage photomultiplier with an S-20 
photocathode. 
efficiency . 
The tube was selected for  its high sensitivity and high quantum 
Our electronic equipment, block diagrammed in F igure  2, included a 
Northeast Scientific DC power supply to dr ive the detector.  
e lectrometer  Model 1250 was used as the DC amplifier. This was followed by 
a divider c i rcui t  designed to provide signal attenuations in one-half magnitude 
s teps  and a Honeywell Model 15;: s t r i p  char t  recorder .  
A General Radio 
F igures  3,  4, and 5 a r e  photographs of the albedo equipment. 
4 
P 
k 
c, 
(I] 
d 
.r( 
0 a 
Q) 
cd 
w 
0 
2 
E 
cd 
k 
M 
cd 
a 
2d u 
0 
.d 
4 a 
e, 
k 
2J 
.r( 
cr 
5 
Figure  3 .  The albedo photometer and telescope. 
6 
Figure  4. The telescope and electronic equipment. 
7 
Figure  5. The electronic equipment. 
8 
3. METHOD OF OBSERVATION 
Our intention was to look for  a net change in the intensity of the ear th-  
shine a f t e r  the observations had been corrected for  the effects of aureole,  
atmospheric extinction, and average phase variation of the earth.  Then, any 
change in brightness,  assuming these contributions could be accurately dealt  
with, would represent  changes in the albedo of the ea r th  due to changes on the 
ea r th ' s  surface o r  in the atmosphere.  
extinction and the aureole effects with sufficient precision to permi t  residual 
brightness effects to be detected. 
The problem was to measu re  the 
To eliminate atmospheric extinction, we used a simple technique that 
involved making B - V  observations of a standard s t a r  near  the moon in eleva- 
tion and azimuth each t ime lunar readings were  taken. Thus, assuming that 
the reading for  the lunar aureole was properly made, a comparison between 
the measured brightness for the s t a r  corrected for  the effects of sky back- 
ground and the measured brightness of the earthshine would eliminate the 
effects of extinction. 
Correcting for  the aureole in the earthshine readings was a m o r e  difficult 
proposition, since any measurement  of the earthlight a l so  measures light 
scat tered f r o m  the bright c rescent  onto the da rke r  portion of the moon. 
decided that a method for  measuring the aureole  suggested by D r .  Frankl in  
and employed in some ea r l i e r  work would be fruitful (see Attachment). 
involved determining isophotes, o r  contours of equal brightness,  for  the 
scat tered light f r o m  the c rescent  moon. 
provide the shape of ra ther  than quantitative values for  the aureole.  First, 
we measured the intensity of the scat tered light around the moon at various 
phases by permitting the moon to dr i f t  a c r o s s  the diaphragm of the photometer. 
The intensity curves  result ing f r o m  the dr i f ts  were  then plotted around a 
m a p  of the moon, and contours were  drawn through the points of equal bright-  
ness  on the dr i f t  curves.  
We 
It 
The contours were intended to 
F o r  phases to 1 s t  quar te r  and af ter  3rd quar te r ,  
9 
the contours near  the dark  l imb showed a tendency to l ie paral le l  to the 
terminator a t  points slightly to either side of the line passing through the 
center of symmetry  of the crescent .  
for  points just  off the da rk  limb a s  well. 
This paral le l ism to the terminator  held 
A typical observing sess ion  with the ins trurnent proceeded approximately 
as follows: we took earthshine readings in B and V by setting the photometer 
near  the dark  limb, slightly below the line of symmetry  of the c rescent  
(point A on Figure  6). 
lunar l imb on the sky (point B on Figure  6), along the contour passing through 
point A, and took measurements  of the sky background brightness in two 
Next, we s e t  the photometer at a point just  off the 
colors .  Setting of the photometer at points A and B 
the use  of a special  graduated ret ic le  in the guiding 
was made possible through 
eyepiece. 
B 
Figure  6. Observed isophotes around lunar crescent .  
After  a series of earthshine-sky background measurements  had been 
made, we took B and V readings on a standard s t a r  that was of spec t ra l  type 
s imi la r  to that of the sun and that was a t  near ly  the s a m e  zenith angle as 
10 
the moon. 
too low in the evening sky f o r  reliable measurements  to be obtained o r  until, 
in the morning, the sky  became as bright as the comparison s t a r .  
We repeated the lunar and star readings either until the moon was 
11 
, 
4. RESULTS WITH THE 5-INCH INSTRUMENT 
Both the observing and the data-handling techniques outlined in the 
Attachment and used fo r  the data supplied by the 15-inch instrument, have 
also served,  with the one exception noted below, to gather data  with the 
5-inch instrument and reduce them. 
recently been executed should be regarded a s  a feasibility study ra ther  than 
a s  an established r e sea rch  effort, we have used one alternate procedure. 
In order  to avoid determing the B, V transformation coefficients for  this 
instrument,  we have used stars of nearly so la r  spec t ra l  c lass  as comparison 
objects for  lunar photometry. 
Because the 5-inch program a s  it has 
Measurements were  obtained a t  Organ Pass ,  New Mexico, on eight nights 
during the summer  of 1967. 
five best  nights only. 
Table 1 gives details of observations on the 
Table 1. 
Lunar photometry with a 5-inch telescope. 
Lunar V 
Date phase per a rc sec2  B (VE-VS) (BE-%) 
Aug. 28.4 88" 14.22 14.82 10.65 10.42 
29.4 75 14.33 15.02 10.76 10.62 
10.52 30.4 63 14.04 14.92 10.47 
Sept. 9 . 1  63 14.56 15.22 10.99 10.82 
30.4 47 14.47 15.06 10.90 10.66 
Each V o r  B entry resul ts  f r o m  3 to 7 separa te  settings on the moon. 
Measurements a r e  reproducible to an accuracy of N 0 . 2  mag. 
setting uncertainties proved the major  source  of e r r o r .  
Guiding and 
12 
A comparison of this table with the one that presented resu l t s  fo r  the 
15-inch instrument  (Table 1 in Attachment) a r e  in general  agreement. 
feel, however, that one point needs to be s t ressed .  Although, as Table 1 of 
this repor t  suggests,  a small instrument is capable of providing mean values 
of the brightness of the earthshine,  i t  cannot achieve sufficient accuracy to 
permi t  measurement  of smal l  fluctuations (- 0.1  mag) probably associated 
with meteorlogically induced changes in the ea r th ' s  reflectivity. 
this statement is aptly demonstrated by Table 1 for  phase angles >45". 
Weather conditions never permitted observations a t  smal le r  phase angles so 
that observations at phase angles <45" with an accuracy of 0 .1  mag remain 
a t  best  a dubious possibility. 
We do 
W e  feel  that 
13 
.. 
5. CONCLUSIONS OF THE SMALL-INSTRUMENT STUDY 
4 
Our work with the 5-inch instrument  has permitted us  to draw the 
following conclusions about its usefulness in monitoring possible changes in 
the ea r th ' s  albedo by means of earthshine measurements .  
1. The 5-inch instrument can, in the hands of a skillful observer ,  give 
individual measu res  of the earthshine to an accuracy of about 0. 2 mag. 
Thus, it provides an average value of the ea r th ' s  brightness,  but not of 
fluctuations about the mean. 
2.  The 5-inch instrument could be improved for  performance superior  
to that investigated here  by (a) a smal le r  diaphragm, perhaps 2 to 3 arcmin  
in diameter  ra ther  than 8, and (b) bet ter  guiding and setting arrangements .  
In connection with (b), our observations made c lear  that  unless i t  is possible 
to see  fea tures  in the earthshine par t  of the moon (and with our instrument  it 
was not), repeatable setting of the photometer cannot be made. This lack of 
setting repeatability made i t  impossible for  us  to judge if  the night-to-night 
fluctuations of the measured earthshine intensity were  due to setting of the 
photometer on lunar a reas ,  such as maria and mountains, having different 
reflecting character is t ics ,  o r  to other causes .  
3 .  Any project specifically designed to look fo r  changes in the ear th ' s  
reflectivity would do well to employ an instrument  substantially l a rge r  than 
a 5-inch one. 
4. A compromise is very  likely possible wherein an  instrument as small 
as a 5-inch is used at phase angles <45", thus providing a series of measu re -  
ments  during those 4 to 5 days of the lunar cycle when the earthshine is 
brightest .  
14 
6. REFERENCES 
BAKOS, G. A. 
1964. Measures  of the earthshine. Srnithsonian Astrophys. Obs. Spec. 
Rep. No. 162, 56 pp. 
DANJON, A. 
1954. Albedo, color,  and polarization of the ea r th  in The Ea r th  as a 
Planet, ed. by G. P. Kuiper, pp. 726-738, Univ. of Chicago 
P r e s s ,  Chicago. 
FRANKLIN, F. A. 
1967. Two-color photoelectric photometry of the earthshine. Journ. 
Geophys. Res . ,  vol. 72, pp. 2963-2967. 
3 
15 


Study of lunar earthshine by photoelectric photometry Final report, 1 Nov. 1965 - 30 Sep. 1967

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