stellar diameters.' 2 "13. J. B. Breckinridge is with California Institute of Technology, Jet. Propulsion Laboratory, Pasadena, California 91103; H. A. McAlister.
Kitt Peak speckle camera J. B. Breckinridge, H. A. McAlister, and W. G. Robinson
The speckle camera in regular use at Kitt Peak National Observatory since 1974 is described in detail. The design of the atmospheric dispersion compensation prisms, the use of film as a recording medium, the accuracy of double star measurements, and the next generation speckle camera are discussed. Photographs of double star speckle patterns with separations from 1.4 sec of arc to 4.7 sec of arc are shown to illustrate the
quality of imageformation with this camera, the effects of seeing on the patterns, and to illustrate the isoplanatic patch of the atmosphere.
Introduction
Astronomical speckle interferometry is a technique which utilizes a spatial interference effect produced by
natural index of refraction fluctuations in the earth's atmosphere caused by small temperature variations. The small refractive index variations form a random phase screen across the pupil of a large telescope.
The
image of an unresolved star is observed broken up into many small speckles or fringes. These fringes or spe-
ckles are in continuous motion and require exposures on the order of 10 ms or less to freeze them.
A camera is positioned after the focal plane of the large telescope. The camera optics magnify the primary image plane so that the output image plane scale is typically as large as 1 sec of arc/4 mm. At this mag-
nification, and for short exposures, the object must be relatively bright, and an image intensifier is required to record the speckles or fringes. Review discussions of astronomical speckle interferometry are given by Dainty' and Worden.2 We describe the Kitt Peak photographic speckle camera which has been in use since 1974 recording
speckle interferograms for the image reconstruction of a Ori,3 ' 4 the measurement of many double stars's position angle and separation,5 - 9 the measurement of the diameter of the asteroid Vesta,'0 a study of the physics of atmospheric speckle formation," and a measure of stellar diameters.'
2 3
"1
We show photographic prints of typical speckle patterns obtained with the camera and include a description of the Kitt Peak coherent image processor used to reduce ensembles of double star speckle patterns.
We
illustrate the spatial correlation for double stars and show, for the first time, high quality pictures to demonstrate the isoplanatic patch. Speckle Camera
The speckle camera is one of at least four in use today
at several large telescopes around the world. Each instrument is different. Schneiderman and Karo'4 discuss the camera built for the 1.6-m Air Force telescope on Maui, Hawaii; Beddoes et al. 15 discuss the camera used at the 2.5-m Isaac Newton telescope; and Labeyrie
et al. 16,17 (the inventor of this technique) describe his camera which has been used on the 5-m Mt. Palomar telescope. Figure 1 shows a sketch of the photographic speckle camera which is used at the 4-m Mayall telescope and the 2.1-m telescope at Kitt Peak National Observatory (KPNO). The camera, originally conceived by Lynds, 3
4
was constructed by the Image Tube Devel-
opment Laboratory at KPNO, in large part, with equipment already available. Referring to Fig. 1, light from the telescope passes through an electromechanical shutter (1) at the front of the speckle camera system and reaches focus at the telescope image plane (2). The telescope image is re-
layed and magnified by a microscope objective (3) J. B. Breckinridge is with California Institute of Technology, Jet Propulsion Laboratory, Pasadena, California 91103; H. A. McAlister
is with Georgia State University, Physics Department, Atlanta, Georgia 30303; and W. G. Robinson is with Kitt Peak National Observatory, Tucson, Arizona 85721. Received 9 August 1978. 0003-6935/79/071034-08$00.50/0. ©Optical Society of America. 1034
APPLIEDOPTICS/ Vol. 18, No. 7 / 1 April 1979
through an atmospheric dispersion compensation prism
(4), narrow-pass band filter (5), and a mechanical shutter (6) onto the photocathode (7) of the image intensifier. The magnified image is amplified by the image intensifier and presented on the output phosphor screen (8). A transfer lens (9) then relays the image from the phosphor screen through the film camera shutter (10) onto the film (11).
MASK
(6) (7)
(8)
TELESCOPE PRISMS
FILTER
IMAGETUBE RCA70021 3 STAGE
CAMERA
Fig. 1. Schematic cross-section view of the Kitt Peak National Observatory speckle camera. Light from the telescope passes through an electromechanical shutter (1) at the front of the speckle camera system and reaches focus at the telescope image plane (2). The telescope image plane is relayed and magnified by a microscope objective (3) through an atmospheric dispersion compensation prism (4), narrow-pass band filter (5), and mechanical shutter (6) onto the photocathode (7) of the image intensifier. The magnified image is amplified by the image intensifier and presented on the output phosphor screen (8). A transfer lens (9) then relays the image from the phosphor screen through the film camera shutter (10) onto the film (11).
Table I.
(i)(i)
Five MicroscopeObjectives and the Effective Focal Lengthand Plate Scale for Each at the 4-m and 2.1-m Telescopes.
Objective magnifica-
tion 10 15 20 25 30
Effective
Plate
Effective
Plate
focal length at 4m
scale at 4 m (Sec of
focal length at 2.1 m
scale at 2.1 m (Sec of
arc/mm)
320 480 640 800 960
0.67 0.45 0.34 0.27 0.22
arc/mm)
160 240 320 400 480
1.27 0.85 0.64 0.51 0.42
The microscopeobjectives used are standard Bausch & Lomb' 9 achromats model 31-10-22-01 and 31-1027-01, having a magnification of 1oX and 21X, respec-
tively, when used with a standard 160-mm microscope tube. The speckle camera system was designed so that the effective magnification of the objective can be changed slightly by varying the distance from the objective to the photocathode of the image intensifier. The best image quality is obtained when the microscope objective is used at its design focal conjugates. The back focal distance of the objective (3) is set during use
to typically 160 mm. Table I gives the magnification of these objectives and the approximate effective focal length of the telescope and plate scale for the speckle camera system for Cassegrain focus of the 2.1-m and the MATCHINGOIL
Ritchey-Chr6tien focus of the 4-m telescopes. The atmospheric dispersion compensation prism assembly is shown in greater detail in Fig. 2. Prisms are numbered 1, 2, 3, and 4. Detail design discussion of the
optical prisms appears later in this paper under the section title: Atmospheric Dispersion Compensation Prisms. Here, we describe their fabrication, assembly, and adjustment. In Fig. 2, prisms 1 and 4 are crown glass of equal wedgeangle, and prisms 2 and 3 are flint COLLAR THREADED CYL IND ER
glass of equal wedge angle. The crown/flint prism 1 and 2 are cemented together and cemented to a ring or collar
which is free to rotate within the threaded cylinder assembly. The dispersive power of the assembly is adjusted by The front shutter is Unibilitz'8 model 225LOA055X5 rotating prisms 1 and 2 (attached to the collar) with respect to the threaded cylinder holding prisms 3 and having a 25-mm aperture. The original shutters were unreliable and would self-destruct after approximately 4. The azimuth, or orientation of the dispersive power, 500 cycles. The factory made a slight design modifi- is adjusted by rotating the entire threaded cylinder assembly containing all 4 prisms inside its holder. cation, and the shutters are now reliable. A Unibilitz The first step in constructing the prism was to fabshutter timer controller, model 300C, is used for exporicate the two glass elements (BK-7, F-2) and cement sure timing and synchronization. Fig. 2.
Atmospheric dispersion compensation prism assembly.
1 April 1979 / Vol. 18, No. 7 / APPLIEDOPTICS
1035
them together as shown in Fig. 3. Four prisms were
then biscuit-cut from the large slab as indicated by the dashed lines in Fig. 3. The glass-air surfaces of prisms 1 and 4 were AR-coated and assembled as shown in Fig. 2. The two prism assemblies 1/2 and 3/4 have Cargille20
immersion oil Type B between them which allows the prism pairs to rotate independently about the optical axis. The filter shown at plane 5 in Fig. 1 may also be a
combination narrow-pass band filter and neutral den-
The speckle camera system also has a removable calibration mask that can be placed in front of the microscope objective in the focal plane of the telescope. This mask is used to determine magnification of the microscope objective and spatial changes in magnification due to distortions caused by the system optics and magnetically focused image intensifier and is used as an alignment aid. Figure 4 shows a photograph of the KPNO speckle camera.
sity filter in nominal 50-mm by 50-mm size. Most of the spectral filters used have been 10-25-nm full width
at half maximum. The mechanical shutter at plane 6 (c.f. Fig. 1) is between the filter and the image intensifier photocathode, is manually operated, and protects the image intensifier during a change of prism orientation of filter. The image intensifier assembly consists of a 40-mm RCA2 ' three-stage magnetically focused intensifier, model C70021-SP2. The input photocathode has a S-20 spectral response, and the output phosphor has a P-11 spectral response. The photon gain of the inten-
CROWN
sifier is approximately 1.6 X 105at the input wavelength
of maximum photocathode response. Magnetic focusing is accomplished with a KPNO-built permanent magnet producing single-loop focus within each stage in the intensifier. High voltage is supplied to the intensifier tube by a KPNO-built voltage divider and a CPS2 2 high voltage power supply model 100P.
Fig. 3.
FLINT Crown and flint prisms are shown cemented together prior
to coring to make the elements for the atmospheric dispersion compensation prism assembly.
The
image intensifier assembly, the most costly part of the speckle camera system, was originally made by the KPNO Image Tube Development Laboratory as a backup system for a spectroscopic instrument. The transfer lens used to relay the image intensifier output from the phosphor screen to the film was originally an F/2 (at infinity) Zeiss, operated at a magnification of unity. The F/2 Zeiss lens was later replaced with an 85-mm F/1 (at infinity) Repro-Nikon
23
lens for
added transfer efficiency. The Nikon lens has an adjustable iris. The original film camera was a motorized Nikon capable of holding 250 frames of film (approx. 10 m). The
motorized Nikon proved to be very difficult to load and had an unreliable film advance mechanism. Because of the operation of the film advance mechanism, the frame spacing was very erratic, and automatic transforming or processing of the developed film became difficult. The motorized Nikon film camera was replaced with a motorized Leica2 4 film camera, model Ducoflex 35. The Leica film camera has a vacuum platen, extremely accurate positive film advance mechanism, holds approximately 750 frames (30 m) of 35-mm film, has a focal plane shutter, has throughthe-lens viewing, and is very reliable for this application. The front and rear shutters shown at positions 1 and 10 in Fig. 1 are synchronized electronically.
The front
shutter is used to determine the exposure time of the image (typically 8-20 msec), and the rear shutter is used
to decrease the effect of thermally induced noise from the intensifier between exposures. 1036
APPLIEDOPTICS/ Vol. 18, No. 7 / 1 April 1979
Fig. 4.
The Kitt Peak speckle camera apparatus on a laboratory
bench. The large circular plate at the top bolts onto the Cassegrain focus of either the 4-m or 2.1-m telescopes.
The large diameter cyl-
inder is the permanent magnet for the image intensifier. Shutters, microscope objective, and prism assembly are in the vicinity between the cylinder and the large circular plate. 'l'he recording camera is
shown in the lower part of the photograph.
Table II. Wavelengthand Refractive Index for Air at Kitt Peak Mountain under Average Conditionsof Pressureat 600 Torr and Temperature at
16'C Wavelength (nm)
values in Table II we write AFC
219.9 X 10-6 tanz and
or the dispersion is
Index (n - 1) X 106
486.1 587.6 656.3
6F =
b = 217.6 X 10-6 tanz. The difference in this deviation AFC = 2.3 X 10-6 tanz.
(1)
The prism layout shown in Fig. 2 will be used to describe the design. Elements 1 and 4 and elements 2 and 3 are the same glass. The deviation 6 of a prism Qf small angle a, composed of material of index n, is given by 6 = (n - 1)a. The index of refraction of the prisms at three wavelength points is of concern: (1) that wave-
219.9 218.1 217.6
length for which the light is to be deviated toward the Atmospheric Dispersion Compensation Prisms
blue (486.13 ni or F); (2) that wavelength for which the
The technique of speckle interferometry has been successfully used without corrections for atmospheric
light is to be deviated toward the red (643.85nm or C); (3) that wavelength which is undeviated (589.
nm or
applications to stellar objects have used atmospheric dispersion compensation. Glass prism compensators
d). Let the wedge angle of prism 1 be a and that for prism2 be /, forprism 1,6 da = (Nda - 1) a; bFa= (NFd - 1) a; bcd = (Nd - 1)a. For prism 2, add = (Nd3 1) /3; bFF= (NFf - 1)j; 6,# = (N - 1)0. Figure 2 shows that the angle for prism 1, that is, a, is in the
Schneiderman and Karo14 and by Labeyrie1 6 in their cameras. The diffraction grating application is dis-
negative sense to the sign for prism 2, that is, F. Each prism pair is to have zero deviation for the 587.6
dispersion for solar physics research.
2526
However,
are used for the speckle cameras at Kitt Peak 3 and at the I. Newton 15 telescope. A diffraction grating is used by cussed in detail elsewhere 14 and will not be repeated. Here, we give the design for a compensation system composed of several glass prisms. A telescope located on the earth views the sky through the refractive medium of the earth's atmosphere. Only
light, or
when the telescope is looking straight up at the zenith is there a nearly uniform layer of refractive medium across the pupil. At angles from the zenith of other than zero, the air forms a prism wedge across the pupil. Most astronomical imaging systems are of relatively low magnification, and atmospheric dispersion is not
angles is given by the desired dispersion. Let PAFC represent the difference in the deviation for light at 486.1nm and 653.3 nm. Then
a problem.
However, the speckle camera has very high
magnification and requires atmospheric dispersion compensation to create round looking speckles. Airy in 1870 suggested that this transverse chromatic aberration introduced by a prism of air is corrected to first order by a wedge of glass.27 Air has a dispersion curve different from that found for any glass,
and dispersion correction over large wavelength intervals using glass prisms is impossible.
However, a per-
fectly adequate design is found using a zero deviation (for sodium light) direct vision prism assembly.2 8 The direct vision prism assembly shown in Fig. 2 contains
two identical crown/flint prism pair assemblies. Figure 1 shows the location of the prisms in the speckle camera. For a telescope viewing away from the
zenith at an angle z, the earth's atmosphere acts as a prism whose refractive index is that for air and whose prism angle is z.
We calculate dispersion prism power required to compensate the atmospheric dispersion at wavelengths of 486.1 nm and 656.3 nm. Table II gives wavelength
and refractive index for air at Kitt Peak Mountain under average conditions (barometer = 600 Torr, temperature = 16'C), which were calculated from Edlen. 2 9
If z is zenith distance, if the deviation of the star light at 486.1 nm is 6 F, and that for 656.3nm is 6c,using the
(Nd,, - 1)a - (Nda - 1) = 0.
(2)
Hence, with the glasses defined, the ratio of the prism angles is given by Eq. (2). A specific value for the prism
PAF, .(4'-N ,N + fl(N c
PAFC= (NF, - 1)a + (NFO- 1)
+(N - l)a +(NC- 1)f. (3)
From Eqs. (3) and (2), we find = NdONda,
-
1
N
)
(4)
Therefore, given a particular PAFCand assuming the selection of glasses, is found from Eq. (4), and a is found from Eq. (2). The dispersion PAFpis for one cemented prism pair. One pair is rotated with respect to the other to either add or subtract dispersive powers. The PAFc for one prism pair need only be one-half of the
total. Therefore, to correct for atmospheric dispersion across the 486-653-nm interval, from Eq. (1) AF, = 1.2 x 10-6 tanz.
(5)
In Fig. 1, the atmospheric dispersion compensation prisms are shown a distance
in front of the image plane
at 7. Let f be the focal length of the telescope and speckle camera assembly as seen looking out through the system from plane 7. From Eq. (3), if the linear displacement between 486-nm and 653-nm light on plane 5 from one prism pair only is equal to the linear displacement between 486-nm and 653-nm light on plane 5, caused by the atmospheric dispersion, we write (PAF,)/F = (Fp)/l, and the requireddispersivepower for a single pair is given by (PAFC)= (1.2 X 10-6 tanz) - (f/l). 1 April 1979 / Vol. 18, No. 7 / APPLIEDOPTICS
(6) 1037
The speckle camera has 1 0.15 m. The focal length of the 4-m telescope at the Cassegrain focus is 31.2 m. The microscope objective used is assumed to give a focal length f of 480 m (see Table I), which gives a plate scale
of 0.45 sec of arc/mm. It was planned that the camera operate to zenith distances as large as 71.60, or to tanz = 3.0. The required dispersive power for a single pair of prisms is, from Eq. (6), PAFC= 1.2 X 10-2 rad. This
high dispersive power is required if the white-light speckles are to be recorded. However, several investigations (1, 5, 14, 16) show that if high contrast speckles are required optical interference filters with X/AX 100
may be needed. Equation (6) is rewritten to include the optical bandwidth,
AX, PAFC = (1.2 X 10-6 tanz)(f/)[(AX)/X].
(7)
The maximum dispersive power which might be required at Kitt Peak is found if we use f = 960 m, tanz = 2.47, AX/X = 0.3, = 0.15 n in Eq. (7), to find PAFc = 5.7 X 10-3 rad. We choose Schott BK-7 for the crown element (that prism associated with angle a in Fig. 2) and
Schott F-2 for the flint element (that prism associated with angle /3in Fig. 2). From Eq. (4) we find / and from Eq. (2) we find a = 16.00°.
=
13.34°,
The tolerance on the prism angle is found from an estimate of the size of the system resolution element. At 5000 A, the Airy disk diameter from a 4-m telescope is approximately 0.030 sec of arc. If the effective focal
length of the system is 500 m, the linear scale at the image plane is 0.4 sec of arc/mm. There are therefore 13 Airy disk diameters across 1 mm at the image plane. From Fig. 1, we see that I 150 mm. The angular tolerance on the rays is 1/(13 X 200) = 5.1 X 10-4 rad or 0.03°. The tolerance on the prism angle is the index of refraction times the ray tolerance, or 0.030. The prisms have a clear aperture of about 2 in. (5 cm).
nominal observing practice is to record 50 exposures, each of 0.02-sec duration using interference filters with bandwidths of approximately 20 nm centered at 517 nm or 552 nm. The exposures are recorded on 100-ft (30-m) rolls of Tri-X film. The film is purchased
prerolled onto spools which allow the Leitz Docuflex camera to be unloaded and reloaded in about 30 sec, or while the telescope is being pointed to the next program
star. No significant amount of telescope time is lost. The full 35-mm format is used for each frame, permitting about 750 exposures (15 stars) to be made on each roll. The back shutter, built into the Leitz camera shown at 10 in Fig. 1, remains open for 0.25 sec after
triggering the front timing shutter. The camera framing rate of approximately 0.7 frames/sec is suited to the binary star program but may be somewhat slow for programs requiring very large numbers of exposures of individual objects.
The information capacity of the KPNO speckle camera system can be expressed in either the total number of resolution cells on the developed film or as the space-bandwidth product.3 0 For this calculation, we assume that the resolution of Tri-X film is 20 lp or cycles/mm. The nominal objective magnification of 15 gives a plate scale of 0.45 sec of arc/mm at the 4-m telescope. The diffraction limit of the telescope at X = 0.5 Am is 0.031 sec of arc. There are, therefore, 14.5 Airy diffraction diameters across 1 mm of film. The film
resolution is 20 cycles/mm. The film resolution therefore matches well the system resolution. The film is 25 mm X 35 mm, which, if one resolution cell is given as a telescope Airy diffraction pattern, yields
a total number of resolution cells on each frame of the film of 1.8 X 105. Figure 5 shows the high quality speckles which are
obtained with this system. The four frames were ex-
Speckle Recording and Processing
The KPNO speckle camera has been used at the Cassegrain foci of the 4-m and 2.1-m telescopes on Kitt
Peak. Between September 1975 and December 1977 nearly 140,000 exposures of stellar speckle patterns for
2755 known or suspected binary stars were obtained. The camera is operated with high time efficiency at these telescopes, a significant factor when valuable large
telescope time is allocated for binary star astrometry. The peak achieved observing rate of 175stars per night suggests that as many as 200 observations will be obtained during a long winter night. A detailed discussion
of the binary star program and the data reduction techniques is given by McAlister. 5
Here, we briefly
summarize the data acquisition and analysis techniques currently in use. The followingprocedure may not be suitable for all problems to which speckle interferometry is applied, but is most effective for survey programs
where very large amounts of double star data must be processed.
The speckle camera has been primarily used at the 4-m telescope with a 15X microscope objective giving an effective focal length of 476 m (see Table I). The 1038
APPLIEDOPTICS/ Vol. 18, No. 7 / 1 April 1979
Fig. 5. Illustration of the quality of astronomical speckles, recorded with the Kitt Peak Speckle Camera. Upper left shows 57 cnc, p = 1.5 sec of arc, m = 0. Lower left shows 12 Lyn, p = 2.0 sec of arc, Am = 0.9. On the right the speckle pattern for y Leo, p = 4.6 sec of arc, Am = 1.5, recorded on nights of considerably different seeing is shown.
posed to illustrate the presence of an isoplanatic patch and to show pictorially the appearance of astronomical speckle patterns under different seeing conditions. The frame in the upper left shows the double star 57 cnc, p
Table Ill. The Present KPNO System Compared to Five Suggested ConfigurationsExpressedin Terms of Estimated Gain, Resolution Elements, and Estimated Capital Cost.
= 1.5 sec of arc, Am = 0. If the reader moves his eyes
System
between the two stars, a certain degree of spatial cor-
Present KPNO A B C D
relation is obvious. The frame in the lower left shows the double star 12 Lyn, p = 2.0 sec of arc, Am = 0.9.
The two frames on the right are the same star: y Leo;
Estimated gain
Resolution elements
Estimated capital cost
1.0 0.25 0.15 0.18 0.18
1.0 0.3 0.8 0.7 1.0
1.0 16 240 480 192
p = 4.6 sec of arc; Am = 1.5, recorded on nights of con-
siderably different seeing. Quantitative analysis of the atmospheric isoplanatic patch appears in two recent papers. 31 32 Here, in this paper, we show high quality double star speckle interferograms to illustrate the effects of the isoplanatic patch and seeing. The exposed Tri-X film is developed using the KPNO
Kodak Versamat film processor with typeA chemistry. This processing is found to be qualitatively similar to developing this film with Kodak D-19. For double star measurements, a positive copy on fine grain film is made
commercially of the original negative film record to increase the signal-to-noise in the optical power spectrum (modulus of the Fourier transform) of the speckle photographs. The spatial frequencies present in the speckle patterns are mapped out using a coherent image
processor whose construction is described in an unpublished report3 3 and is schematically similar to Fig. 5-5a of Goodman. 3 0 The composite power spectrum of the set of 50 exposures is formed on /2 a sheet of 4 X 5-in. (10 X 12.5-cm) Kodak Ektapan film. Fringes
imply the existence of a binary star whose angular separation and position angle on the sky can be inferred from the fringe spacing and orientation by providing the
appropriate calibration data. The KPNO coherent image processor incorporates an automatic film advance and exposure mechanism for
which detailed drawings can be obtained from the engineering department of KPNO. A liquid gate is used to eliminate surface reflection and film scratch effects. The liquid-used with the gate is tetrachloroethylene, which was selected on the basis of work performed at Eastman Kodak Company.3 4 To date approximately 61,000 individual frames of speckle patterns from 1200 observations of stars have been measured. Approximately 400 of these observations are for resolvable binary stars, the remaining 800 objects either being found single or unresolved at the epoch of observation to the diffraction limit of approximately 0'0.35 for the 4-m telescope. Calibration for both scale and origin of orientation required to calculate separation and position angle is performed by photographing the speckle pattern of a single star produced with a double slit mask in the converging beam from the primary telescope mirror. The telescope is thus converted into Anderson's version35 of the classic Michelson stellar interferometer. Fringe spacing in the calibration spectrum is determined only by slit/telescope geometry and mean wavelength of the observed bandpass, all of which are determined sufficiently well to permit a ±0.6% uncer-
tainty in the calibration for angular separation. Photographic measurements at the Cassegrainfocus permit the orientation of the double slit mask as projected onto
the sky to be determined to within +02. Fringe spacing and orientation for binary star composite spectrum are determined by measuring points along a set of fringes with the two-coordinate Grant measuring engine at KPNO. Least squares linear fits to the set of measurements giveparameters with formal internal errors frequently less than 0.1% for fringe spacing and 0.05 for orientation. Thus the final errors associated with measurements of angular separation and position angle for binary stars whose separations exceed approximately 0 2 are often calibration limited. In general, 4-m speckle observations have accuracies of
