Dual reflecting telescope design for ultraviolet spectroheliograph

Milligan, J.

Underwood, J.

White, W. A.

English

NASA Technical Reports Server

X-614-66-132 ' 
A DUAL TELESCOPE 
FOR SPECTROHELIOGRAPHY 
IN THE EXTREME ULTRA-VIOLET 
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BY ff 653 July65 i 
r i  J. MILLIGAN 
J. UNDERWOOD 
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W. A. - w i i i i E  
MARCH 28,1966 
GODDARD SPACE FLIGHT CENTER 
GREENBELT, MARYLAND 
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A DUAL TELESCOPE 
FOR 
SPECTROHELIOGRAPHY 
IN THE 
EXTREME ULTRA-VIOLET 
J.  Milligan 
J. Underwood 
W .  A .  White 
Goddarci Space Flight Center 
Greenbelt , Maryland 
March 28, 1966 
1. INTRODUCTION 
. .  A high resolution x-ray telescope experiment was proposed and 
scheduled for flight on AOSO.(l) As stated in the original proposal 
"The measurements of intensity, spatial and temporal resolution will 
be compared with ground based visible and radio observations with the 
objective of better understanding the dynamics (temperature, density, 
ion composition, etc.) of the solar atmosphere.I1 The x-ray telescope 
was to have a resolution of 5 arc seconds, use filters to cover the 
-.  
wavelength region from 3 to 60 A, and obtain solar images every 5 
minutes. 
The objectives of the experiment proposed for ATM are the same. 
The same type telescopes, filters, resolutions and general spectral 
range are proposed. 
obtain more detailed time histories. To meet the severe schedule 
constraints, two telescopewith fixed filters will be used instead 
of a single telescope with a changeable filter. 
The added capability of ATM will be used to 
2. OBJECTIVES 
By using the capabilities of the ATM pointing control planned for 
the Apollo Applications Frogram, we propose to trace the development 
Lindsay, J. C. and Giacconi, R., "High Resolution ( 5  arc sec) X-ray 
Telescope for Advanced Orbiting Solar Observatoryg', GSFC Report 
(1) 
X-614-63-112 
in time and space of typical solar centers of activity by observing 
selected coronal radiations from Fe XVII in the 15-17 A region and 
by observations of lower excitation phenomena in the 44-60 A region. . .  
. .  Imaging will be done simultaneously in these two band passes; 
the general structure of the lower corona will be mapped in the 44-60 
A region and the development of centers of high activity will be 
traced in Fe XVII; both images will have a resolution of 5 seconds of 
arc on axis and will be obtained photographically from two X-ray/EW 
telescopes. 
will be observed in integrated fashion over the entire solar hemisphere. 
Concurrent with the imaging, both of these radiations 
From this data it should be possible to trace the development 
of various active regions in the solar atmosphere during the 14 day 
ATM mission. While 14 days is not enough time to trace the complete 
life history of a center of activity, it can be expected that enough 
centers will be observable on the sun at the time of the flight to 
allow us to measure the coronal conditions for centers at various 
stages of development. In addition this experiment will have the 
capability of obtaining pictorial histories of the development of 
representative highly transient solar phenomena, e.g. flares, in the 
x-ray region for correlation with ground based data. 
3 .  SCIENTIFIC BACKGROUND 
A considerable amount of information regarding the physical state 
of the solar corona can be obtained from Observations in the extreme 
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ultraviolet and X-ray regions (350 A to 1 A ) .  
intensities of spectral lines E. highly ionized atoms and the 
6istribution of brightness in these lines across the solar disc, one 
may determine for example, the electron density and temperature of 
specific structures in the corona. This information can contribute 
to the evertual solution of the problems of the mechanisms for energy 
and mass transport into the corona, and the relationships of these 
mechanisms to observed photospherjc and chromospheric structures. 
For this, one requires precise observations of the lower regions 
of the corona. 
limb) using coronagraphs, and for a few moments during total 
eclipses using spectrograph-camera combinations. 
continuum radiation is so intense in the visible wavelength region, 
it has not been possible to make observations of the corona in front 
of the solar disc at visible wavelenghts. 
By measuring the 
Such observations have been made (but only at the 
A s  the photospheric 
Even in limb observations, 
scattered phot~spheri~ radiation $2 2 S C ~ ~ C I I S  p r ~ b l e ~ .  At E ~ j l  
ana X-ray wavelengths, however, this problem Goes not occur. 
By observing the sun in these short wavelengths, one may also 
hope to obtain a better understanding of solar flares. 
one requires an instrument of sufficient spatial resolution to enable 
the X-ray flare to be closely compared with the Ha flare and the 
unaerlying sunspots and plages. 
For this 
A major auvance in short wavelength studies of the sun was made 
on March 17, 1965, with the first successful use of grazing-incidence 
- 3 -  
X-ray optics to obtain X-ray pictures of the corona. This experiment, 
flown on an Aerobee rocket from White Sands, was conducted jointly 
by the Solar Physics Branch of Goddard space Flight Center and the 
American Science and Engineering, Inc., of Cambridge, Massachusetts. 
A number of X-ray pictures were obtajned through various filters which 
served to define the wavelength region observed; the experiment 
convincingly demonstrated the potential of this new technique. 
Although these first pictures achieved a resolution of only one or 
two minutes of arc, improved techniques of mirror fabrication under 
investigation by the Solar Physics Branch at GSFC are now enabling 
resolutions of the order of a few seconds of arc to be obtained. 
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e .  
. .  
4 .  DESIGN OF INSTRUMENTS 
4.1 Imaging Elements 
We propose to use two telescopes, each of which will be 
capable of resolving solar details down to a size of 5 arc seconds. 
The proposed imaging mirrors will each have an internal diameter of 
approximately 711 and a length of 1211 and will be separated by l o l l ,  
center-to-center. 
Considerations of film grain (discussed later) dictate that 
the solar image should be at least 16mm in diameter, if we are to 
,-chieve 5 arc-seconds resolution. For each of the two telescopes 
we propose to use an imaging element having an effective focal length 
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I 
I 
of-75 inches, giving a solar image of 17.7mm diameter. The proposed 
. element will be a grazing-incidence paraboloid combination of the 
2 2 
I first kind with a collecting area of 38.5 cm . A s  the aperture is 
annular, the focal ratio as defined in the usual manner has no 
meaning for exposure purposes; but we may define an "equivalent 
f-number" as being the ratio of the diameter of the equivalent 
circular area to the focal length of the telescope. 
way, the proposed element operates at f/27. This compares very 
favorably with the f/lOOO of a typical pinhole camera or the f/300 
of a typical zone plate telescope. 
* .  
. .  
Defined in this 
During the past several years the Solar Physics Branch in 
conjunction with various contractors and research group has been 
irwestigating techniques of fabricating grazjng-incidence X-ray 
telescopes. 
by electro forming. The early results from this technique were 
In the past most of the telescopes have been fabricated 
quite encouraging, hc:J-,T.rer, dcrifig the p s s t  year it has  herome nhvio1_1s 
that further improvement of the optical performance by electro forming 
is going to be very difficult if not impossible. It has not been 
possible to obtain the final surface finishes which are required with 
the result that too much light has been scattered over the whole field. 
Wolter, H . ,  Ann. 2 der Phys. - 10, 94 (1952) 
- 5 -  
Approximately one year ago a program was initiated to attempt to 
fabricate X-ray optics using classical optical techniques. The 
results to date have been very encouraging. 
X-ray telescope fabricated in stainless steel for GSFC by the 
Speedring Corporation is shown in Figure 1. This telecope operates 
at f/44. 
The resolution indicated by this picture is approximately 7 seconds 
of arc over a field of view of 45 minutes of arc. 
final surface finish is still not up to the optical quality normally 
required for astronomical purposes its performance is better than 
that achieved by the earlier techniques. It should be noted, however, 
that this telescope is still only a first approximation to what is 
possible, since the two optical surfaces are cones instead of being 
the paraboloid-hyperboloid combination we propose to use. Perkin 
Elmer is presently in the final stage of assembly of a similar 
telescope in Quartz but with paraboloid-hyperboloid surfaces of high 
optical quality. Preliminary tests are indicating surfaces to at 
least 1/30X in the visible. They have encountered no unforseen 
fabrication problems. 
for the ATM mirror fabrication. 
An example of a small 
Figure 2 is a visible photograph taken with this telescope. 
Even though the 
We therefore propose to use this technique 
Each telescope will be fabricated by grinding and polishing 
from quartz. ‘.lltiple-beam interferometric techniques will be used 
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. 
t o  monitor the f i n a l  polishing process, i n  order  t o  achieve a surface 
f igu re  accurate  t o  a t  least  1/30 wavelength of v i s i b l e  l i g h t .  
f i rs ' t  surface w i l l  operate  a t  a grazing angle of incidence of 
approximately io 30' and the second surface a t  about lo 7 '  f o r  the 
sho r t  A telescope. 
The 
. .  
e .  
The combined r e f l ec t ion  eff ic iency f o r  telescopes operating 
a t  these angles i s  given i n  Table I.  
experience i n  obtaining X-ray images of the  sun w e  estimate t h a t  the 
t i m e  reso lu t ion  f o r  t h i s  system t o  be of the order of 10 sec.  o r  
less. 
On the bas i s  of the pas t  
TABLE I 
- A (A) REFLECTION EFFICIENCY 
13.3 
16. 
31.4 
44 
3 5% 
17% 
56% 
7 5% 
42% 
35% 
69% 
7 5% 
4.2 F i l m  and Camera 
The most useful  emulsions f o r  work i n  the  E W  and X-ray region 
are those with a high s i l v e r  content and l i t t l e  o r  no protect ive 
- 7  - 
supercoat of gelatin, such as Ilford Special Unsupercoated Industrial 
G ,  Kodak-Pathe SC7 and SWR. These are the fastest emulsions now 
available for this wavelength region; but they are also quite grainy, 
having a resolving power of perhaps 24 lines/mm. 
solar image is therefore required (16mm diameter at 5 arc-seconds 
resolution). These films are also extremely sensitive to pressure 
and abrasion, and it is not feasible to roll them tightly on a reel 
without some means of spacing the turns away from each other. We 
propose to use a film strip 70mm wide with double-thickness edges 
extending inward 23mm on each side, leaving a central strip 24mm 
wide within which the 17.7mm solar images will be centered. 
A fairly large 
It is estimated that the camera proper would extend no more 
than 15 inches behind the focal plane; the total length of the 
experiment would therefore be about 90 inches. 
4.3 Filters 
Each telescope will contain a single fixed filter; one will 
use quarter-mil aluminum (8-17 A passband) and the other mylar coated 
with aluminum. 
by lines of Fe XVII lying between 13 and 17 A ;  similarly, the major 
components in the mylar passband are lines of si IX and Six. 
Most of the energy in the A1 passband is contributed 
4.4 Ancillary Sensors 
We plan to use 3 proportional counters in addition to the 
Two of the counters will work in approximately two X-ray telescopes. 
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the same wavelength regions as the telescopes. 
Angstrom region the other in the 44-60 Angstrom region. These 
will be used to put an absolute calibration on the X - L I ~  pictures. 
The third sensor will operate in the 2-8 Angstrom region. 
primary purpose will be to monitor highly transient activity. 
of the sensors will employ electronic pulse-counting with subsequent 
registry of their integrated outputs as binary numbers on a bank 
of neon bulbs. At regular intervals, the binary numbers will be 
transferred as a string of light and dark spots onto an appropriate 
data frame on the photographic film. 
One in the 8-20 
. .  
, - .  
Its 
Each 
1 
Note that the data from any of the three ancillary sensors 
is initially in a pulse-rate form. This data will also be recorded 
01-1 tfie oi-j,-board tape -----Am- ~t k7i11 t yds  be 29 ~ = C V  nloptrnnir J ----- - ---- - L C L W L U C L  . 
task to provide the astronaut with a suitable simple display from 
these sensors; we propose to do this, and also to provide him with 
an indicator of the mode the experiment is in at any moment ("Quiet" 
or lgActive'l). The purpose of this display is explained in the next 
section of this proposal. 
5 .  OBSERVING SCHEDULE 
Our proposed observing schedule is tailored to the following 
. set of ground rules. 
(a) 14-day mission. 
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(b) 90 hours of actual observing time. 
(c) 1000 feet of film, preferably 7Omm wide, \ .  
(d) Astronaut will have an ItX-ray activity meter" supplied by this 
7 -  
experiment; he will be able to override the internal experiment 
logic in exceptional or unforeseen situations. The actual 
amount of in orbit logic changing j s  dependent upon the 
number of command lines available to the experimenter. 
We plan to use a format on the film such that data are 
presented in sets of three items each; viz., (1) a solar image 
in 44-60 A, (2) a solar image in Fe W I I ,  and ( 3 )  a frame of binarized 
data from the ancillary sensors, including a portion of the film 
devoted to calibration and/or a fog reference level. Each set will 
occupy a maximum of 3 inches along the film; the separate parts 
of each data set will be positioned in a fixed relation to each 
other, probably not contiguous (See Figure 3 )  owing to the planned 
simultaneous exposure of the film through three gates spaced 5 inches 
apart. A film capacity of 1000 feet will allow 4000 data sets to 
be taken. 
5.1 Exposure Program 
In order to sensibly apporti.on the film footage and at the 
same time obtain useful statistics, we plan for approximately 89% 
of the 90 observing hours to operate in a low data-rate I1quiettt mode 
wherein one data set is taken every 12 minutes; 100 feet of film is 
- 10 - 
. 
allocated to this mode, corresponding to 400 data sets or 48~" 
minutes of observing time. The remainjng 11% of the observing 
time will be devoted to a high data-rate l*activel1 mode wherein 
one data set is taken every 10 seconds; 900 feet of film will be 
Feriod 
Between 
Exposures 
12 min. 
used for 3600 data sets or 600 minutes of observing time. 
example of how this 600 minutes of "active'I mode time might be used, 
one might, for instance, register two 100-minute large events and 
twenty 20-minute moderate-to-small events. Table I1 summarizes the 
exposure program. 
As an 
Data Sets 
400 
A system of thresholding logic will be used to define when 
the experiment should switch from '!quiet'' to 'Iactivel1 mode, there 
to remain for a pre-set interval of 20 minutes unless over-ridden 
by the astronaut who may, at his option, hold the flactiveI1 mode 
for a much longer period. 
I I 
I 
90 hr. 
4000 i 
TABLE 11 
- 
1000 
Mode 
Quiet 
Active 
TOTAL 
i 10 sec. 3600 
Total Time 
80 hr. 
10 hr. 
Film Footane 
100 
900 
- 11 - 
6 .  WEIGHT AND POWER 
Power for the actual operation of the experiment will be less 
7 .  than 10 Watts average. 
be necessary to supply active heating elements to the experiments 
to solve the thermal problems. 
difficult to estimate at the present time because of the unknown 
interfaces with the ATM and the unknown requirements which will 
be placed onus by the astronauts for EVA recovery of the film. 
However, the experiment should weight approximately 150 pounds 
if no radiation shielding is required, and if no unusual requirements 
However, it should be realized that it may 
The weight of the experiment is 
are received from the ATM contractor or the astronauts. 
7 .  PROBLEM AREAS 
A s  pointed out above, timely solution of the interfaces between 
the various experimenters and the ATM mount j s  imperative. 
Contamination from the life support system for the astronauts, and 
from the jets in the Apollo guidance and control system poses 
serious problems for any optical experiment. 
are left in the service module, and if the experiment is unshielded, 
the film wili become unusable in a fraction of an hour. 
required to protect the film from this radiation environment (if it 
must be supplied by the experiment) might possibly weigh more than the 
original experiment. 
If the Co60 sources 
The shielding 
. 
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I 
I 8. DELIVERY SCHEDULE 
. .  The present schedule for experiments requires that the 
experiments must be delivered to GSFC in July 1967 f o r  testing 
and integration in order to meet the first launch date. 
I 
- .  
The 
I relatively simple experiment proposed can meet this schedule if 
timely definitions of the interfaces are made. 
- 13 - 

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A dual telescope for spectroheliography in the extreme ultra-violet

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