The second generation of Hubble Space Telescope (HST) instruments will include a near-infrared instrument. This choice has driven the development of near-infrared arrays to larger sizes and lower read noises. Rockwell International has delivered an array for use in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument; this array has been dubbed NICMOS2. NICMOS2 is a 128x128 array of HgCdTe diodes In-bonded to a switched MOSFET readout. The readout was specifically designed for astronomical use with the HST requirement of low read noise a prime goal. These arrays use detector material which is similar to that used by Rockwell in previous arrays (e.g., HgCdTe produced on a sapphire substrate), but the NICMOS2 devices differ substantially from other 128x128 arrays produced by Rockwell in having a read noise of only 30 electrons when read out using appropriate correlated sampling. NICMOS2 has now been characterized in the laboratory, and it has been used on groundbased telescopes

Blessinger, Michael

Kleinhans, William

Montgomery, E. F.

Rieke, G. H.

Rieke, Marcia J.

Vural, Kadri

English

NASA Technical Reports Server

N90-21338
Laboratory and Telescope Use of the NICMOS2 128x128 HgCdTe Array
M.J.Rieke, E.F.Montgomery, G.H.Rieke
Steward Observatory
University of Arizona
K. Vural and M. Blessinger
Rockwell Intemational Science Center
W. Kleinhans
Valley Oaks Semiconductor
I. Overview
The second generation of Hubble Space Telescope instruments will include a near-
infrared instrument. This choice has driven the development of near-infrared arrays to larger
sizes and lower read noises. Rockwell International has delivered an array for use in the
NICMOS instrument; this array has been dubbed "NICMOS2". NICMOS2 is 128x 128 array
of HgCdTe diodes In-bonded to a switched MOSFET readout (see Blessinger, et al.,this vol-
ume). The readout was specifically designed for astronomical use with the HST requirement
of low read noise a prime goal. These arrays use detector material which is similar to that
used by Rockwell in previous arrays (eg. HgCdTe produced on a sapphire substrate), but the
NICMOS2 devices differ substantially from other 128x 128 arrays produced by Rockwell in
having a read noise of only 30 electrons when read out using appropriate correlated sam-
pling. NICMOS2 has now been characterized in the laboratory, and it has been used on
groundbased telescopes.
Table 1 summarizes the properties of the NICMOS2 array.
Table 1:NICMOS2 Characteristics
Detector type
Cutoff wavelength
Readout type
Unit cell size
Nominal operating temperature
Read noise
Dark current @ 60°K
PV HgCdTe
2.5lxrn
switched MOSFET
601ax601x
60-77°K
30 electrons
1.5 e/sec
II. Laboratory Tests
Arizona has measured the quantum efficiency, linearity, read noise, uniformity, and
response to gamma radiation. In tests which duplicated Rockwell data, similar results were
obtained. Dark current data were not acquired using a NICMOS2 array but rather were taken
321
https://ntrs.nasa.gov/search.jsp?R=19900012022 2020-03-19T23:16:56+00:00Z
usingJFETintegrators(seeG. Rieke,et al. in this volume for a description of the technique);
Table 2 summarizes results for a variety of individual diodes with different cutoff wave-
lengths. In the case of the 2.5-_rn cutoff material, Arizona measurements of individual diodes
are comparable to the rates measured at Rockwell of 4.5 electron/sec at 77°K and 1.5
electrons/sec at 60°K for NICMOS2 with the output amplifier tumed off. Note that the indi-
vidual diodes have a larger area than the diodes in the array so the lack of a significant drop
between the individual diode and array data suggests that a regime is being entered where the
dark current is not proportional to diode area. The relatively small drop in dark current
between 77°K and 60°K is inconsistent with the dark current being due to generation-
recombination and suggests that tunneling may be responsible.
Table 2: Dark Currents for a selection of HgCdTe Diodes
Type Size T°K Iok(e/sec)
_,c=1.4_m 160lax 160IX 77 6.5
_,_=2.5 _lxn 130gx1301x 77 9.0
_,_=2.5_,rn 1301ax1301.t 63 2.0
_c=3.0Jxm 50btx50bt 60 6.0
_,_---4.2gm 60gx601.t 63 200
Of particular interest for NICMOS is the relatively small increase in dark current seen
for the 3.0-_m cutoff material as compared to the 2.5-1am cutoff material. This result implies
that the NICMOS spectrometer extension to 3.01xm will not exact any large perfomaance
penalties.
All of the data reported here were taken using a modified form of correlated sampling.
Figure 1 shows this technique where two samples per pixel are taken per readout. One sam-
ple is taken at the end of the integration and the second is taken immediately after with the
reset enabled. This method also requires that some images (bias frames) with extremely short
integration times be taken to calibrate the change in level seen when the reset is released.
Bias frames have proven to be very stable over periods of days. The read noise achieved with
this sampling technique is 91 electrons; Rockwell saw 80 electrons using a similar method.
The 80 electron noise level is what is expected for this technique which is subject to the full
kTC noise of the integrating node. A better method which avoids kTC noise and compensates
for low frequency drifts is also shown in Figure 1 and is labelled "triple-correlated" sam-
piing. Rockwell has used this method to achieve a readnoise of 30 electrons which is inde-
pendent of integration time.
Because the read noise as expressed in electrons depends on knowing the integrating
node capacitance, this must be measured, and is one of the keys to the good performance of
the N'ICMOS2 array. Rockwell measured the capacitance by measuring the total current flow
322
,J
_f./i ""12]
c_ I Ei
.->-- l S
Q ,'_>{i('
£
<I
!: F
_'_>_i >
.,>,<"!or_l. A 13
A A
/- //- .....
i _m(:"
3
i] /" ""
,)
C"J :'_
[ 3 ,¢_"7.>-,
L
<]
])
t-
T _r',l 6.?
.j
Figure 1: Sampling methods used in reading NICMOS2 arrays.
through the array and obtained a value of 0.10pF with over 80% of the capacitance estimated
to be attributable to the HgCdTe diodes. The Arizona measurement took advantage of the
fact that signal strength and noise have different dependencies on the capacitance:
V, ig,_t = ei /C
V.o,,, = e_lC
2V_oi,, = e2ilC _
--= Cle
vL.
C = capacitance, e = electron charge, i = photoelectrons/sec
The capacitance of 0.127-1-.035pF measured at Arizona agrees well with the Rockwell data.
For the read noises reported in this paper, a value of 0.10pF has been used.
The quantum efficiency was measured for a central region on the array by imaging a
spot onto the array. This spot was illuminated by a blackbody source and a combination of
cold and warm filters were used to define the bandpass. Figure 2 shows results from a NIC-
MOS2 array at 77°K and an older 64x64 array (Rieke et al. 1987). The newer array does not
exhibit as sharp a drop in QE towards Iwn as did the older array. The quantum efficiency of
the NICMOS2 array was also measured at 63°K and did not change significantly at the lower
temperature.
ORIGINAL PAGE I_;
OF POOR OUALIlrY
323
ORIGINAL PAGE IS
OF POOR _ALITY
0,7
U
(b
U
t'(*(D [)4 /_Z/
k_ /
+J / / "> _'( / / /0
(D 0 ;> 1 /
01 -[
[) -- • [- F
\\
\.
1 1P
l
/
_;tdc>u" f'-_4xC4 r_r'rr_,y
+ O! For" NICMI!_i! (:_rrm, y
I 1 • ] -" 1 T [ T - T T T
14 16 18 2 _E P4
Wo, vet-on.c.j th (_m)
Figure 2: The quantum efficiencies of a NICMOS2 army and an older (_lx(_l HgCdTe array
measured at 77°K.
One problem with the NICMOS2 array which has been used most extensively is its
gradient in quantum efficiency from one comer to the diagonally opposite comer. The image
in Figure 3 was obtained looking at sky at 2.2_Ti; no correction procedures of any sort
including bias correction have been applied to this image. The gradient in quantum efficiency
and "striping" due to column to column variations are apparent. The quantum efficiency
ranges from over 70% to about 25% in the worst areas. This gradient is much smaller at
1.6_aa and essentially gone at 1.251ma. The "striping" is seen in bare NICMOS2 multiplex-
ers, and is very stable. Flatfielding removes all traces of this column to column structure in
the image.
The linearity of the NICMOS2 array has been assessed in two regimes shown in Fig-
tires 4 and 5. The array should be subject to an inherent non-linearity because the photocur-
rent is collected on the combined capacitance of the diode and MOSFET, and the diode
capacitance is a function of the bias across the detector and hence depends on how much
charge has already been collected (see Hoffrnan 1987). The degree to which this non-
linearity poses a problem depends on how rapidly the diode capacitance varies with bias and
on how much effort can be expended on calibrating it and removing it from the data in the
image processing steps. HgCdTe diodes have the excellent characteristic that they change in
capacitance relatively slowly; from a bias voltage of -0.2 to -0.5 volts, they exhibit a change
of only 4% in their capacitance. This voltage range is a typical operating regime for NIC-
MOS2 arrays. Data taken by varying the integration time while viewing a constant source of
324
Figure 3: An unprocessed image taken of the sky at 2.2pro.
illumination have been used to study the linearity. In the low signal case plotted in Figure 4,
NICMOS2 appears very linear with fro small signal collection problems seen. In Figure 5, the
large signal linearity is examined with no problems apparent until the bias level is
approached. The bias used in taking these data was 0.5 volts which yields an effective well-
depth of about 300,000 electrons. No linearity corrections are needed if NICMOS2 is oper-
ated at less than 85% capacity, and no corrections have been applied to the telescope data
reported below.
325
ORIGINAL PAGE IS
OF POOR QUALITY
t,q
t-
O
U
NICHFIS2 SmaLL Signal Lineari±_
g
f.,
! ! i
Time (see)
Figure 4: Small signal linearity for a NICMOS2 array.
t_
t-
O
U
NICHO:_2 LinearltE
i ..........
100 200
Time (see)
Figure 5: Large signal linearity for a NICMOS2 array. The bias level corresponds to 0.5
volts.
Another important attribute of an array intended for use in space is the ability to with-
stand charged particle hits. NICMOS2 arrays have not themselves been subjected to radiation
testing. A similar readout has been, and limited testing of HgCdTe diodes has been carried
326
out.In bothcases,thetestingconsistedof exposingthedeviceto a 10mCurieAm TM gamma-
radiation source mounted within a few centimeters of the device being tested. In other situa-
tions the deposition of energy by gamma rays induces effects similar to those induced by
other types of radiation; more extensive testing of NICMOS2 arrays and HgCdTe diodes
should be done to verify this assumption. A TCM-1000B switched FET readout with 2.5-1am
HgCdTe diodes showed no sensitivity changes when exposed to the Am TM source. This test
was conducted with the array viewing a relatively high background. The other test consisted
of exposing a 3.0-Hm diode being read out with a JFET integrator to the radiation source.
This diode showed complete recovery to its normal dark current 5 minutes after being irra-
diated at 20% of an HST South Atlantic Anomaly passage. Recovery may have been faster,
but a few minutes are required for an accurate dark current measurement at these levels.
Other testing at Rockwell has not revealed any problems, but more radiation testing is needed
before certifying the NICMOS2 array for space use.
Ill. Telescope Use
The NICMOS instrument will operate at wavelengths where observations can be done
from groundbased sites. A camera for such use has been built, and the performance of the
NICMOS2 in an actual astronomical situation has been characterized. The camera is shown
schematically in Figure 6. Note the use of a cold pupil and cold filters to limit the radiation
reaching the detector array. The background levels at 2larn from a groundbased telescope are
not very different from those expected at this wavelength on HST because the larger pixel
scale used on the ground is compensated by the fact that HST is warmed to temperature of
76°F, much wanner than is typical for a groundbased site. At shorter wavelengths the
groundbased backgrounds are much higher than will be seen from HST because of the OH-
airglow.
\\
\
\
[i
i ";
/I } _ • J
2'_ II I:' I
'\/ I'+_
/
Figure 6: Camera optics schematic
\
01
F>Arl;L:_r/!:4 (,! t_'!"_
327
ORIGINAL PAGE I_
OF POOR QUALITY
The equipment used to read out the array at the telescope is very compact and is based
on clock waveforms being generated using EPROMs. The hardware does not do any of the
differencing required by the readout sampling; all differencing is done in the computer con-
trolling the array readout (a 80386-based computer in this case). The readout electronics
themselves are very compact and occupy a volume of only .009 cubic meters. These
electronics digitize to 16-bits and when used with the NICMOS2 array provide a gain such
that one unit of the digitizer corresponds to 19 electrons. The system noise without an array
is 40 electrons, and with the array, yielded a read noise of 91 electrons. The electronics also
include protection circuits for all lines controlling the array and a real-time video display
which is very useful for focussing and alignment.
P
O
Figure 7: The Crab Nebula at 2.2gm.
Figures 7 and 8 show fully reduced data taken with the NICMOS2 camera using the
Steward Cbservatory 1.5-meter telescope on Mt. Bigelow, AZ. The telescope has an f/45
infrared secondary which was used for these observations. The plate scale of 1.8"/pixel is
optimized fc "studying extended sources. This plate scale yields a total field-of-view of 3.8
328
arc minutes. These images were produced by beam-switching between the object and adja-
cent sky every 60 seconds. The 60-second exposures which were recorded for analysis con-
sisted of 6 co-added 10-second exposures which were necessary to avoid overfilling the
array. The final pictures were produced by subtracting a bias frame from each of the object
and sky frames, ratioing the resulting frames on a pairwise basis, and then averaging all of
the ratio frames. This is the same technique that has been used with our previous arrays
(Rieke et al. 1987), and results in flatfielding to 1 part in 20000. The sensitivity achieved
using the NICMOS2 array at K(2.2lam) on the 1.5-meter is 21.5 magnitudes per square arc
second (one sigma in one hour of integration composed of co-added frames).
120
tt0
t00
90
8O
70
60
50
4O
30
2O
t0
0
0
0
t0
10 20 30 40 50__60 70 80 90 t00 110 t20
om 0 t.o t.t2 t • va - . OB
Figure 8: Part of the edge-on galaxy NGC3044 at 2.21am.
At this point, the full capabilities of the NICMOS2 array have yet to be
exploited in groundbased use, but it has already shown that it is capable per-
former.
329
/ 41
IV. Summary
The NICMOS2 128x128 HgCdTe array developed for use on HST is a
sigqificant step forward in the technology available to astronomers. Its readout
has been designed with astronomical use in mind, and it delivers a level of
performance at 30 electrons read noise that will permit full use of HST as well
as very interesting investigations from the ground.
V. References
Hoffman, A. W. 1987, in Infrared Astronomy with Arr___, ed.
C. G. Wynn-Williams and E. E. Becklin, p. 29.
Rieke, M. J., Rieke. G. H., anf Montgomery, E. F. 1987, in
Infrared Astronomy with Arrays, ed. C. G.
Wynn-Williams and E. E. Becklin, p. 213.
330


Laboratory and telescope use of the NICMOS2 128 x 128 HgCdTe array

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server