Considerations about telescopes for infra red astronomy

IL NUOVO CIMENTO VOL. 8 B, N. 1 11 Marzo 1972

Considerations About Telescopes for Infra-red Astronomy (*).

F. MELCH:[ORRI

Te.S.B.E. Laboratory o/ Consiglio s delle Ricerche - Bologna-Firenze Space Physics Group, Uuiversit 9 o/ .Florence - Firenze

(rieevuto fl 30 Giugno 1971; manoscritto revisionato ricevuto fl 27 Settembre 1971)

S u m m a r y . - - The choice of a telescope which may be used for ground- based infra-red observations is discussed. A number of facts suggests that two types of telescopes may be used: a small P number, low-optical- tolerance telescope in the near infra-red (up to 3.5 ~m) and in far infra- red (beyond 300 ~m); a large F number, high-optical-requirement telescope in the intermediate infra-red. The efficiency of these instruments for the study of localized sources and for sky surveys is discussed.

1 . - I n t r o d u c t i o n .

The recent d iscovery t h a t some celestial objects emi t mos t of the i r rad ia t ion in the inf ra- red region of the spec t rum has s t imula ted an increasing growth of in teres t in infra-red a s t r o n o m y dur ing the pas t few years . Airborn, bal-

loon and rocke t exper iments have been pe r fo rmed to observe these sources of radia t ion outside the a tmosphere , or a t an a l t i tude where the a tmospher ic absorp t ion and emission m a y be neglected.

Extens ive ground-based observat ions have also been carr ied out th rough the a tmospher ic windows in the nea r (up to abou t 3 ~tm), in te rmedia te

((3--30) y~n) and ve ry far (submill imetrie and mil l imetrie) infra-red. I n spite of the constra ints in t roduced b y the presence of a residual, nonnegligible

a tmospher ica l emission, inf ra- red a s t r o n o m y f rom ground observator ies appears to be a valuable research tool, because the flexibility of ground-based observat ions exceeds, genera l ly speaking, t h a t of high-al t i tude observat ions.

(*) To speed up publication, the author of this paper has agreed to not receive the proofs for correction.

167

168 F. )IELCHIORRI

This s i tua t ion led var ious au thors to discuss abou t the best telescope to be used for ground-based inf ra - red observat ions (1.3).

A realistic discussion abou t the best infra-red telescope should t ake into account several pa r ame te r s and features, such as the opera t ing wavelength range, the match ing p rob lem between the detec tor and the telescope, the a tmos-

pheric contr ibut ion, the main a im of the observat ions ( tha t is a m a p of the sky,

or a s tudy of some well localized fa int sources, or solar as t ronomy, etc.). F i r s t of all, we m a y compare two view-points : the first is t h a t convent ional

as t ronomical telescopes m a y be used also in IR-as t ronon ly with the advan tages of pract ical benefits which come f rom the facilities of a large observatory . The second, and perhaps p reva len t view today, is t ha t IR-de tec to r s cannot be matched with a convent ional telescope keeping their u l t imate performance. Fur thermore , the qual i ty of the image in a IR- te lescope can be significantly

poorer t han t ha t needed for visible as t ronomical work, so new telescopes m a y be designed, fas ter and easier to use a t an overall cost considerably lower t han

t h a t ol the in s t rumen t s t hey replace. The a im of the present work is to discuss the above problems in some

detail . F r o m the discussion it will be clear tha t i t is impossible to find the best telescope for all kinds of infra- red observat ions, bu t r a the r i t is suggested t ha t a cer tain k ind of telescope m a y be used bo th in ve ry near (less t h a n 3 ~m) and very far (beyond 300 ~m) infra-red, while a quite different k ind of telescope m a y be used in the in te rmedia te infra-red ((3--30) ~zm). These conclusions are the resul t of an analysis of the match ing p rob lem between tele-

scopes and detectors . Final ly, we discuss the location of observat ion sites wi th respect to the

absorpt ion and emission of the a tmosphere .

2. - The matching between telescope and detector.

2"1. - A telescope is said to be (~ opt ical ly ma tched ~, or in opt ical contac t wi th a detector , if

(i) Ad(I) d = At(I)t ,

where Ad----detector area,

~ba = solid angle of field of v iew of the detector ,

and At, qSt have the same mean ing for the telescope.

Quant i t ies in (i) are f requent ly called (( opt ical th roughou t ~) (OT) of the detec tor and the telescope, respect ively . I f the OT of the telescope is larger

(1) W. ~k. STEIN and N. J. WOOLF: Appl. Opt., IO, 655 (1971). (2) R.B. LEIGHTON, E. E. BECKLIN and G. N]ZUGEBAU]~R: Astrophys. Journ., 1,399 (1965). (3) p. E. CL]~GG, R. A. NEWSTEAD an4 J. A. BASTIN: Phil. Trans. Roy. Soc., 264, 203 (1969).

CONSIDERATIONS A B O U T T E L E S C O P E S F O R I N F R A - R E D ASTRONOMY 1 6 9

t han the OT of the detector , the la t te r is excited by a f ract ion of the energy collected by the telescope; in the opposite case, other rays, in addit ion to those coming f rom the telescope, reach the detector . In bo th cases a degradat ion is expected in the system performance, so tha t condit ion (i) is t ha t of an ideally operat ing telescope-detector system. Moreover, as the OT of the telescope increases, we may observe more sources in a shorter t ime. More precisely, with increasing As fa inter sources can be detected, while with increasing ~b, the t ime required to per form a survey of the sky decreases (assuming a finite in tegrat ing t ime for each measurement) . So in principle the OT must be kep t as large as possible.

Le t us neglect here the optical and mechanical problems re la ted to the con- s t ruct ion of a large, small E-number telescope. In the following we will discuss the problems related to the matching of this kind of telescope with an infra-red detector, which arise f rom the fact tha t , generally speaking, the noise of IR- detectors depends on the OT of the detector .

2"2. - In the afore-ment ioned work of STEIN et al. (1) i t is assumed tha t the signal-to-noise ra t io of a detect ion system depends inversely on the linear size of the detector ; this fact is t rue only for a photoconductor or in the presence of background fluctuations. In fact, there are two sources of noise in infra-red:

intr insic noise,

background noise.

The intrinsic noise is l inear ly dependent upon the square root of the area for a photoconductor and is pract ical ly independent of size for a bolometer (actually, for ve ry large bolometric elements the difficulty met in cooling may be in te rpre ted as a source of noise, because the detec tor cannot work at the same t empera tu re as t h a t of a smaller element).

The extrinsic noise (or background noise) is due to the random fluctuations in the surrounding background, which, in the case of IR-as t ronomy ground observations, is essential ly the atmosphere.

In the last Section of this work we shall discuss the atmospheric emissivity at various alt i tudes. Le t us now assume tha t the a tmosphere emits as a black- body at 300 ~ The detec tor will be sensitive to the quantis t ic fluctuations of this t ype of emission. Assuming t ha t Wb is the root-mean-square value of the power of these fluctuations, per un i t of area and solid angle, we will expect a root-mean-square power noise on the detec tor of (*)

N E P = A �89 r 1 8 9 W d d b"

(*) The NEP is defined as the root-mean-square (r.m.s.) power of radiation which produces a r.m.s, signal equal to the r.m.s, value of the electric noise of the detector with an amplifier of 1 Hz bandwidth.

170 F. ~r

O b v i o u s l y th i s noise e q u i v a l e n t p o w e r m a y be l a r g e r or s m a l l e r t h a n t h e

i n t r i n s i c noise , d e p e n d i n g on t h e a b s o l u t e v a l u e of t h e a t m o s p h e r i c emiss ion ,

which is w a v e l e n g t h d e p e n d e n t . The u l t i m a t e p e r f o r m a n c e of a t e l e scope-

d e t e c t o r s y s t e m in B L I P ( b a c k g r o u n d l i m i t e d p h o t o c o n d u e t i o n ) cond i t i ons

has been d i scussed in t h e R e p o r t of t h e I n f r a - r e d A s t r o n o m y P a n e l of IqASA

(~Nasa, 1969) (~). H o w e v e r , t h e d e t e e t i v i t y (*) of t h e s y s t e m was e x t r a p o l a t e d

up to 1000 F m w i t h t h e a i d of a f o r m u l a v a l i d o n l y up to 50 Fm. More r e c e n t l y a

1013

T

d

~1012 E u

1011

:la

lo ,O~_ 1 o ~

/ stale of art for I R-defectors

I t I ;

101 10 2 10 3 10 4. 10 5 ~. (p.m)

Fig. 1. - Detect ivi ty of an infra-red detector under a 300 ~ black-body background with a cold filter which cuts off all wavelengths shorter than ~. Curve 1) is from (4) and curve 2) (better approximation) is from (5). The intrinsic noise of the available detectors divides the operating wavelength region in two zones where the detector is, respectively, l imited by the intrinsic noise (class I) and by the background noise (class II) .

(4) NASA, 1969: Report o] the In/ra-Red Astronomy Panel o] NAS A (NASA SP-213, Chairman E.P. NEY, 1969). (*) The detect ivi ty is defined as the inverse of the NEP divided by the square root of the area of the sensitive element; the detect ivi ty in absence of background noise is called << intrinsic detect ivi ty >). (5) S. F. J i coBs and M. SARGENT I I I : Infrared Phus., 10, 233 (1970).

CONSIDERATIONS ABOUT TELESCOPES FOR INFRA-RED ASTRONOMY 171

compute r calculat ion has been per fo rmed (~) and the resul ts are shown in Fig. 1. We m a y note t h a t the intrinsic de tec t iv i ty below 3 ~m and above 600 ~m is smaller t h a n the m a x i m u m de tec t iv i ty allowed by the B L I P conditions. So,

in these infra-red regions, only pract ica l considerat ions l imit the OT of the detector , bu t in principle the match ing condit ion (i) m a y be satisfied a t any value of OT wi thout an increase in the noise of the system. Pract ical ly, in the

p resen t s t a te of the ar t , the m a x i m u m size of a bo lometer is abou t (1 • 1) cm 2 wi th a field of view of 1 sr. So we m a y use as a ma tch ing condit ion in prac- t ica l cases

(ii) A~r = 1 cm ~ s r .

On this basis, the values of the l inear field and of the t ime required for a comple te m a p of the sky are p lo t ted in t e rms of the d iamete r of the pr incipal mi r ror {Fig. 2).

1.5

o

1.0

~ 0 . 5

I 0 6 0

I I I I I

1 2 3 4 5 D(m)

6

4 ~

i2

Fig. 2. - Performances of a class I telescope: a is the linear field of view, D the diameter of the principal mirror, t/v the time required ~o make a survey (in units of integrating time v) with a signal-to-noise ratio of 10 (practical values of integrating time range from 0.1 up to 103 s).

I n the in te rmedia te infra-red, one expects all te lescope-detector systems to be l imited b y background radia t ion, so t h a t wi th increasing OT the per- fo rmance of the sys tem will be degraded. This fact leads us to conclude t h a t the pr incipal a im for a ground-based i n t e r m e d i a t e - I R telescope is the s tudy of small localized sources (point sources) ins tead of a t t e m p t i n g to make a m a p of the sky. A large F - n u m b e r telescope (10 or more) together wi th a small dimension detec tor m a y be used to reduce the cont r ibut ion of background. Assuming a prac t ica l l imi t of 20 for the F - n u m b e r and (0.1 • 0.1) m m 2 for the

172 r. MELCHIORRI

area of the detector , we have the match ing condit ion

(iii) A tC t = 2 .5 .10- : em 2 s r .

Obviously, the t ime required to make a map of the sky in these conditions

is long and the telescope is usable to s tudy point sources only. In conclusion, the types of telescopes required in infra-red as t ronomy

m a y be broadly divided in to two classes:

I) a large, small F -number , telescope, to work in the near (up to 3 ~m)

and far infra-red (beyond 600 ~m);

I I ) a large, large F -number , telescope to work in the in te rmedia te infra- red (between 3 and 30 ~m).

In the following we will discuss the efficiency of these telescopes and find tha t the dist inction we made in two different classes of telescopes is con- firmed by the different requi rements in the qual i ty of the image in the near, in te rmedia te and far infra-red.

3. - Optimum telescope size.

Following (1) we shall in t roduce an efficiency e to describe the per formance of IR-telescopes, however, we mus t distinguish two different efficiencies:

e~: telescope efficiency in tended to s tudy localized sources; i t is the inverse of the t ime required to obta in a signM-to-noise ra t io = 10 during the observat ion of a fa in t source;

es: telescope efficiency in tended to realize a m a p of the sky; it is the inverse of the t ime required to make a m a p of the sky with a signal-to-noise ra t io = 10.

e L m a y be computed for telescopes of both classes defined in the previous Section, while sz is meaningless for class I I telescopes.

3"1. E]ficincy o] class I telescopes.

i) Telescope-bolometer system. The bolometer N E P m a y be assumed independent of the OT; the value of the signal appears to be l inear ly de- penden t upon the square of the d iameter D of the pr inc ipa l mir ror and the square root of the in tegra t ing t ime 3, i.e.

noise R = constant ,

signal S oc D ~ ~�89


TO eva lua te s~ we p u t the condi t ion 8 / R = 10 and derive v. We obta in

eLoc D 4 .

To eva lua te % we note t h a t the t ime required to observe 2z sr is 2~v/~2~

f rom which

d~D 2

where d is the l inear dimension of the bo lometer and F is the E - n u m b e r z ]/D (]----focal length of the telescope).

ii) Telescope-photoconductor sys tem. We r e m e m b e r t h a t the noise of a photoconductor increases wi th the l inear dimensions of the detector :

Roc d ,

S oc D 23 �89

Following the same considerat ions of case (i) we obta in

D 4 D ~ s~ oc d- ~ es ~: F~ �9

Perhaps the mos t i m p o r t a n t fea ture is t h a t in bo th cases es is p ropor t iona l

to the inverse of F ' .

3"2. E//ieiency o/ class I I telescopes. - I n this case the de tec tor is a lways in B L I P condit ions, so t h a t the ~TEP m a y be wr i t t en as p ropor t iona l to dD//. Following the same procedure as above we find

D 4

I t is i m p o r t a n t to note t h a t in this case e~ increases as F s.

3"3. Conclusions. - The resul ts are p lo t ted in Fig. 3 and 4, respectively. The efficiency increases always wi th the d iamete r of the pr incipal mirror ,

however class I telescopes reach the m a x i m u m efficiency a t ve ry small F -numbers~ while class I I telescopes require large E-numbers . We note t ha t our resul ts are not in contradic t ion with those obta ined by (1) in the case of inter- media te infra-red, bu t they prove t h a t i t is no t correct to ex t rapo la te those par t icu la r resul ts to the whole inf ra- red spect rum.

174 r. MELCttIORRI

t0 z,

10 3

10 2

1 10

o lO i i i l i i i

2 4 6 8 D(m)

10 J ' ~ L ( D = 2 m ) ~

;lass I I J [L( ~ ~ m] / - 10 ] ~ ~ c l a s s l l

c c

10 -1 ~ s s I

10 -2 I * I I I I I 2 4 6 8

F- number"

Fig. 3. Fig. 4.

Fig. 3. - Efficiency of class I and class I I telescopes vs. the diameter of the principal mirror: eSB is the efficiency for survey using a bolometer as detector (class I), eLe is the efficiency of localized sources using a photoconductor as detector (class I), ~ is the efficiency for localized sources (class I I telescopes), ~b t is the field of view.

Fig. 4. - Efficiency of class I and class I I telescopes vs. T'-number. We may note tha t e s decreases and e L increases with 2", generating two classes of telescopes: class I and class I I .

4. - On the quality of the image.

SCOTT (6) f irst p o i a t e d ou t t h a t t h e p r i n c i p a l d i f fe rences b e t w e e n the op t ics

of i n f r a - r e d a n d v i s ib le l i g h t s y s t e m s come f r o m t h e (( i m a g e >) size r e q u i r e m e n t s .

This f ac t is p a r t i c u l a r l y e v i d e n t in fa r i n f r a - r e d , whe re t h e d i f f r ac t ion l i m i t s

a re m o r e a n d m o r e cogen t . S t a r t i n g f rom th i s i d e a J o ~ s o N et al. (7) p r o p o s e d

a n d b u i l t some la rge q u a s i - o p t i c t e l e scopes , us ing m e t a l l i c m i r r o r s up to 60

inches in d i a m e t e r .

More r e c e n t l y STEIN et al . (1) p o i n t e d ou t t h a t t h e r e q u i r e d (<image >> size

wi l l be a c o m p r o m i s e b e t w e e n d e t e c t o r size, d i f f r ac t i on p a t t e r n a n d see ing

spo t , howeve r t h e i r c o n s i d e r a t i o n s a r e l i m i t e d to w a v e l e n g t h s s h o r t e r

t h a n 20 ~m.

(6) ]~. ~I. SCOTT: ~/~1"00. I R E , 1530 (Sept. 1959). (7) H . L . JOHNSON, R. I. ~r B. ]RIARTE and W. Z. WIS~-I]~WSKI: Comm. L u n a r

and Plane tary Lab. , 4, 99 (1966).

CONSIDERATIONS ABOUT TELESCOPES FOR I N F R A - R E D ASTRONOMY 1 7 5

Although there is no doubt t ha t in infra-red (especially in far infra-red) the qual i ty of the image (and also the polishing of the mirror) m ay be poorer t han the qual i ty usually required for visual observations, there are no quanti- ta t ive considerations in l i t a ra ture on this point . There are two main dist inct problems:

1) Can we use spherical mirrors ins tead of parabolic mirrors?

2) How good must the surface of the mirrors be?

10 / 2

10

E E

~b

~I0

u)

10 -I

.N e3

10 -2 J

10 -1 10 0 101 10 2 10 3 10 4

F-number

Fig. 5. - Radius of the spots due to the 3rd order aberrations and the diffraction for various /~-numbers. The zones of practical interest for class I and class II telescopes are indicated.

The first problem can be solved by comparing the diffraction disc with the possible aberra t ion contr ibut ion. Assuming tha t the aper ture stop of the telescope coincides with the pr incipal mirror, we may neglect, among 3rd-order aberrations, dis tor t ion and field curvature . Spherical aberrat ion, coma and ast igmatism are p lo t ted in Fig. 5 for a spherical mirror v s . F-number , together

1 7 6 F. MELCHIORRI

with the diffraction disc for var ious wavelengths. The formulae used are

r: - - ( 1 / 6 ~ ) D / F 2 : radius of the m i n i m u m confusion disc for spherical aberra t ion ;

r~ = (1 /8 )d /F ' - : m a x i m u m width of the p a t t e r n of a comat ic image;

r~ (d '- /D)I/F"-: length of the sagi t ta l line of an as t igmat ic image;

r,~--1.22).F: radius of the first m i n i m u m of the diffraction disc.

F r o m Fig. 5 we m a y note tha t spherical aber ra t ion produces a disc with a d iameter always greater t han the d iameter of diffraction in the range

of pract ical dimensions of detector . Thus one m a y conclude t h a t the parabolic

figure is required. We r e m e m b e r t ha t the equat ion of a parabola is y = x: /4] ,

thus to t r ans fo rm a spherical mir ror into a parabol ic one with the same focal

length the amoun t of ma te r i a l to be removed is

~1 R - - ( R 2 - - x") ~ - - x : / 2 R .

where R is the radius of the spherical mirror . I t is well known tha t in the case of short focal length (i.e. small F -number ) ,

there is a definite gain in using a parabol ic ins tead of a spherical mir ror be-

cause the length of the sagi t ta is much shorter . The second problem is re la ted to the optical tolert~nce of the surface re-

quired to obtain a spot with a d iameter smaller t han the d iamete r of the detector. The m a x i m u m permissible angular d isplacement between the real sur-

face and the ideal one m a y be expressed as

]tgfi <d, cos [arctg D]

where fl is the angular accuracy of the optical surface. This relat ion is p lo t ted in Fig. 6 vs. the _F-number of the telescope. We

m a y note tha t class I and class I I telescopes are separa ted by a factor greater t han :10 in the qual i ty required for the f ront surface of the pr incipal mirror. In fact~ the tolerance required for class I telescopes is satisfied by plast ic mirrors or some search-l ight mirrors , which m a y be found as surplus. On the contrary , the requi rements for class I I telescopes are more str ingent, and quasi- optic mirrors mus t be realized; the cost of this t ype of mirrors of the same

d iamete r as above would be about ten t imes grea ter (7).

In conclusion:

a) Class I telescopes: the best solution appears to be a large mirror with short focal length matched with a large detector ; the optical requirements are not severe, so t ha t parabol ic plast ic mirrors or search-light mirrors m a y be


10

~)

10 2 b) -

E E

10 ~

10 - I 10 0 101 10 2 10 3 10 a

F- number

Fig. 6. - Radius of the spot of a parabolic mirror with a surface finished with an accuracy ft. The zones of interest for class I and class I I telescopes are indicated: a) f l = 1 0 -3rad, b) f l = 1 0 - 3rad, c) f l = 1 0 -4rad, d) fl----10 -Srad.

used. The weight of the i n s t rumen t should not be much more t h a n 500 kg for a mirror of 1.5 m in diameter .

b) Class I I telescopes: the best solution appears to be a large mirror wi th a long focal length, ma tched with a small de tec tor ; a well finished mirror (glass or metal l ic) m u s t be used. The weight of the i n s t r u m e n t cannot be lower t h a n 4000 kg for a mi r ror of 1.5 m in d iameter .

5 . - S i t e s f o r o b s e r v a t i o n .

The mos t i m p o r t a n t cont r ibut ion to the a tmospher ic absorp t ion is due to H20 and CO2 in the near and in t e rmed ia te IR , and to H~O in the fa r infra-red. Le t us assume t h a t the absorp t ion coefficient a t a given wavelength is g for H~O; as the content of H20 in the a tmosphere increases, the absorpt ion also increases. We m a y in t roduce a layer Z of precipi table water , so t h a t the absorp-

12 - I I NUOVO Cimento n .

100

101

i

0 ~3 i~2

I

10 -~ L(cm)

i0 ~ i01 102

1 7 8 F, MELCHIORRI

Fig. 7. - Transmi t t iv i ty of the atmosphcre (in units of the absorption coefficient S) for various equivalcnt lengths L of precipitable water ( taken from (s)). See text for the use of the curvc at various wavelengths.

I0

b

~ I O

lO

3

10 class I class II

I0

v ~3

0

class i

r-,. co o~ cr r

residuclL water

content : I mm H20

10 " i i

10 o 10 ~ 10 3 10 ~ 10 5

Fig. 8 . - Atmospheric t ransparency around 3000m of al t i tude (1 mm of precipitable H20 ) vs. ).. The zones of operation of class I and class I I telescopes are indicated.

(8) C . W . ALLEN: A s t r o p h y s i c a l Q u a n t i t i e s (New York, 1966).


t ion will depend upon SL. However , t h e L a m b e r t law. of exponent ia l absorpt ion

is no longer val id for bands made of discrete lines. The effect of the layer on

the t r a n s m i t t i v i t y is shown in Fig. 7. I f we known the t r ansmi t t i v i t y a t a ce r ta in wave length wi th a given value of L, we normal ize the curve of Fig. 7 a t the same value of / i and then we m a y obta in the t r anspa rency for other

values of Z. Pract ica l ly , L ranges f rom 10 m m (sea level, h igh-humidi ty site) to 0.7 m m (high a l t i tude over 3000 m). The dependence of ~ upon the wavelength has been eva lua ted b y var ious authors (8) and par t icu la r ly b y T~raO~- LACXRRIEV et al. (8) for the far infra-red. F r o m the above da ta we p lo t ted in

Fig. 8 the expected t r anspa rency a t an a l t i tude of abou t 3500 m with a resid-

90

80

70

60

5O

4O

30

20

o 10 ~2 10 2 10 3

ttm

\ 3'

E

10 -I 10 o 101 crn of H20

Fig. 9. - Gain in transmittivity from ground to mountain sites in intermediate infra-red ((8--14) ~tm) and in a far infra-red (460 ~m) window.

ual water content of 1 m m precipi table water . One m a y see t h a t some measure- men t s m a y be carr ied out in the far infra-red, s ta r t ing f rom about 400 ~m. Al though the use of h igh-mounta in observator ies appears to be in general recommendable , we m a y n o t e t h a t i t is more i m p o r t a n t for fa r inf ra- red observat ions. I n Fig. 9 we p lo t t ed the t r anspa rency of the a tmosphere for var ious

(9) P. TURON-LACARRI~U and J, P. V]ZRDET: Ann. d'Astrophys., 31, 237 (1968),

1$0 F. MELCHIORRI

contents of H~O for the 8 - - 1 4 band and a far infra-red band of about 460 ~m.

One may see tha t the t ransparency in far infra-red, comparing the results of

~ sea level~> observatories and mounta in observatories, increases by about 5, while the gain is lower than 2 in the intermediate infra-red, in the case of high

mounta in observatories. So a telescope intended for intermediate infra-red

studies operating beyond 3500 m should be of about 3.6 m in diameter to com-

pete with the 5-meter Hale telescope; on the contrary, a telescope intended

for far infra-red observations should be of about 2 m in diameter.

10 I~

1013

g"

f l012 E

5

10 m

,al j

/~1) z2) /

ClCZss II class I / /

~/ / / / / s t a t e o f ac t

f o r I R -de tec to rs

I I I

1010[ I I I i

100 101 102 103 104 105 ,l (~m)

Fig. 10. - Detectivity of a BLIP detector looking to the zenith: curve 1) deteetivity computed using the emissivity data of the atmosphere listed in the literature (s,,); curve 2) deteetivity under 300 ~ black-body background (compare with Fig. 1).

I n conclusion, there is a definite gain in using infra-red telescopes at high

altitudes. This gain is larger for class I telescopes operating in the far infra- red; in this ease a plastic mirror of about 1.5 m in diameter will be equivalent to a 3.5-meter telescope operat ing at sea level.

Finally, in Fig. 10 we evaluated the deteet ivi ty of a system operating at

high altitude, using for the atmospheric emission the data listed in l i terature (s.9).

CONSIDERATIONS ABOUT TELESCOYES FOR I N F R A - R E D ASTRONOMY 1 8 1

6 . - C o n c l u s i o n s .

A number of facts suggest t ha t two types of telescopes may be used in infra-red as t ronomy.

Class I telescopes: small E-number ratio, low optical tolerance telescopes. They should work in near infra-red (up to 3.5 y~n) and in far infra-red (beyond 300 ~m); they should be matched with large detectors and m ay be used both for survey and localized-source observat ion; a definite gain is ex- pec ted ib the ins t rument is located in a high-al t i tude observatory; a 2-meter telescope would be compet i t ive with the 5-meter Hale telescope. The weight of this ins t rument should not be more than 1000 kg and the cost should be less than $ 20000.

Class I I telescopes: large F -number high optical requi rement telescopes. They should work in the in termedia te infra-red ( (5- -30)~m) and should be matched with ve ry small detectors and used only to s tudy localized sources. No pract ical gain is expected f rom working in mounta in observatories. The weight of these ins t ruments (2 m in diameter) should be of about 4000 kg and the cost of the order of $100 000. There are two reasons to recommend there al- ization of this type of ins t rument ; the lower cost in comparison with a convent ional telescope of the same diameter (7) and the requirements of t ime on exist ing large telescopes (1).

~:kr

I acknowledge grateful ly the assistance and advice of Dr. S. STILLER during a recent informat ive voyage through the Uni ted States. Of par t icular help have been the discussions with Dr. G. NEUGEBAUER and his colleagues and co-workers. I would like to t h a n k Prof . F. SCANDO~E of Officine Galileo, for the interest in this work. I acknowledge also the constant encouragement and criticism of Prof . A. BONETTI and of the colleagues of the Group.

�9 R I A S S U N T O

Si svfluppano alcune considerazioni suUe caratteristiche dei telescopi adatti ad osser. vazioni infrarosse in osservatori di alta montagna. Si individuano due classi eli telescopi: quelli adatti ad operare tra 5 e 30 micron (finitura ottica, lunga focale, eosto elevato) e quelli adatti alle regioni (1--5) ~ e (300--1000) ~ (finitura quasi ottiea, corta locale, costo limitato).

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Considerations about telescopes for infra red astronomy