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\title{Infrared Astronomy with the}
\title{Hubble Space Telescope and}
\title{the Next Generation Space Telescope
%\thanks{this is a place for a title footnote}
}
\author{M.~Robberto\/\altaffilmark{1} %\inst{}
%\fnmsep
}

%   \offprints{M. Robberto}
   %\mail{3700 San Martin Dr., Baltimore MD, 21218, USA}

\affil{Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore MD, 21218, USA\\
email: robberto@stsci.edu}
\vspace{1in}
\centerline{\titlelarge To be published in the proceedings of the \emph{Third National Conference on IR Astronomy}}
\medskip
\centerline{\titlelarge (held 4--7 December 2001 in Perugia, Italy),}
\medskip
\centerline{\titlelarge in \emph{Memorie della Societ\`a Astronomica Italiana}}

\vfill


\altaffiltext{1}{On assignment from the Space Telescope Operation Division of the European Space Agency (ESA).}

%\authorrunning{M. Robberto}
%\titlerunning{IR astronomy with HST and NGST}

%  \maketitle
\begin{abstract}I review the characteristics of NICMOS before and after the installation of the new cooling system. Recent results on high-redshift supernovae and star formation in the Large Magellanic Cloud are used to illustrate the NICMOS performance. I present the IR channel of WFC3, a fourth generation instrument to be installed on the Hubble Space Telescope in 2004. Finally, I briefly report on the current status of the NGST project.
\end{abstract}

   \keywords{infrared: general -- cosmology: observations -- infrared: galaxies  --
   Telescopes: Hubble Space Telescope -- instrumentation: spectrographs: NICMOS --
   instrumentation: detectors: WFC3 -- Telescopes: Next generation
   Space Telescope}


%________________________________________________________________
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Introduction}
%%%%%%%%%%%%%%%%%%%%%%%%%%%
When the Hubble Space Telescope was launched in April 1990, it was equipped with five focal plane instruments all able to observe in the ultraviolet (UV) down to $\lambda \le 1200$~\AA. Only one
instrument, the Wide-Field/Planetary Camera, could cover the visible range up to the I~band. The original goal of the HST program was in fact to put in orbit a 2.4~m telescope optimized for the UV and the visible wavelengths, and the initial complement of instruments coherently exploited the design of the telescope.
Little attention, if any, was paid to the behavior of the HST optical assembly at infrared (IR) wavelengths \citep{robb00}. 
\par 
Twelve years and four Shuttle servicing missions later, the wavelength coverage of HST appears now greatly extended in the visible and near-infrared. The scientific ground of this extension is clear:  the expanding Universe shifts to the IR the light emitted in the rest frame at shorter wavelengths. Also, interstellar absorption by dust grains becomes less important at longer wavelengths, making the IR regime the ideal choice for studies of embedded fields, like star-forming regions. The advantages offered in the near-IR by a telescope above the atmosphere are also obvious: diffraction limited imaging with PSF constant across the entire field of view, full wavelength coverage, and reduced background emission due to the absence of atmospheric airglow lines.
\par
HST has had  a major impact on IR astronomy. The current near-IR instrument onboard HST, NICMOS, has provided data of unparalleled quality in a number of important fields. NICMOS is based on focal plane arrays originally designed for the HST program. One may remember that in the early '90s the development of the NICMOS detectors made IR observing routinely available to the general astronomical community, triggering a revolution in IR astronomy.
\par
In this paper, I will summarize the capabilities of the HST in the IR with both the existing (NICMOS) and future (WFC3). The performance of NICMOS will be illustrated by a couple of scientific results, whereas for WFC3, still in the development phase, only estimates can be given. In the last section I will report on
the current status of the Next Generation Space Telescope, the IR-optimized successor of the HST.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   \begin{table*}[t]
      \caption[]{Imaging parameters}
         \label{Impar}
      $$
         \begin{array}{p{0.5\linewidth}ccc}
            \hline
            \noalign{\smallskip}
            \textrm{Parameter}     &  \textrm{Camera~1} & \textrm{Camera~2} & \textrm{Camera~3} \\
            \noalign{\smallskip}
            \hline
            \noalign{\smallskip}
            {\textrm{Pixel~Size~(arcsec)}} & 0.043 & 0.075 & 0.2 \\
            %Field of View $\textrm{(arcsec \times arcsec)}$ & $ 11 \times 11 $ & $ 19.2 \times 19.2$ & $51.2 \times 51.2 $ \\
            \textrm{Field~of~View~arcsec} $\times $ \textrm{arcsec} &  11 \times 11  &  19.2 \times 19.2 & 51.2 \times 51.2  \\
            \textrm{Diffraction~limited~wavelength~($\mu$ m)} & 1.0 & 1.75 & - \\
            \noalign{\smallskip}
            \hline
         \end{array}
      $$
   \end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section {NICMOS}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) was installed on the HST during the Second Servicing Mission in February 1997. Principal Investigator has been Rodger Thompson at the Steward Observatory. The instrument was built by Ball Aerospace. NICMOS contains 3~near-IR cameras, each one equipped with its own NICMOS $ 256 \times 256 $ HgCdTe detector. The cameras simultaneously image three fields, almost adjacent, with three different scales (Table~\ref{Impar}). Each camera is equipped with 
19~optical elements (filters, grism, polarizers) covering the wavelength range from 0.8~$\mu$m to 
2.5~$\mu$m. Besides imaging, NICMOS allows for slitless grism spectroscopy with $R\sim200$ in Camera~3, imaging linear polarimetry in Cameras~1 and 2, and coronography with 0.3\arcsec occulting spot in Camera~2. A detailed description of NICMOS goes beyond the scope of this review. The reader may check the NICMOS home page at the URL \verb"http://www.stsci.edu/hst/nicmos", and in particular the latest version of the NICMOS handbook. Together with a more detailed overview of the instrument, the handbook describes in detail the observing modes currently supported.


%%%%%%%%%%%%%%%%%%%%
\subsection{NICMOS cooling: problems and recovery}
%%%%%%%%%%%%%%%%%%%
In NICMOS, the detectors and part of the optical train are installed inside a dewar originally using sublimation of solid Nitrogen. The design lifetime of the coolant was approximately $4.5\pm0.5$~years. During the tests before launch (Figure~\ref{nicmosfinaltest}), solid Nitrogen was periodically cooled to $\approx40$~K by flushing Helium through a coil on the back side of the dewar. Unfortunately, this  operation caused an unexpected failure. During the Helium flushing, the back side of the solid Nitrogen block was colder than the front side. The resulting cryo-pumping effect transfered the gaseous Nitrogen evaporated on the warmer front side to the back side, where extra ice formed. At the end of the test session, when the dewar was finally allowed to warm up, the extra ice increased its volume and pushed on the inner walls of
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vspace{\baselineskip}
\vspace{1pt}
%\begin{figure}[b]
 \centerline{\epsfxsize=\linewidth\epsfbox{NICMOS.eps}}
%   \resizebox{\hsize}{!}{\includegraphics{NICMOS.eps}}
   %\includegraphics{empty.eps}
   \figcaption{\footnotesize{NICMOS during the final tests at Ball Aerospace.}
 \label{nicmosfinaltest}}
%\end{figure}
\vspace{\baselineskip}
%%%%%%%%%%%%%%%%%%%%%%%
\noindent the dewar. This mechanical distortion, plastic and permanent, has produced a number of undesirable effects:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{itemize}
\item
The three cameras are no longer confocal. In particular, NIC3 camera is out of the focus range correctable with the NICMOS mechanisms, and in order to be on focus, the entire telescope needs to be refocused. Since the Wide-Field Camera~3 does not exploit the resolving power of the HST, the defocusing may be acceptable, depending on the science program. The focus shift between NIC1 and NIC2 is sufficiently small that an intermediate focus yields excellent image quality in both cameras. These are the camera normally used.
\item
The detectors have moved laterally, and there is a $\approx 20$\% vignetting in all cameras. The pupil stops are also misaligned and background removal is less than optimal.
\item
The thermal insulation of the dewar has been compromised. The mechanically induced thermal short has reduced the lifetime of the coolant from 4.5~years to about 2~years.
\end{itemize}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
On January 3, 1999, the cryogen was completely exhausted. This day marked the end of the instrument lifetime under the original cooling regime. After more than three years of inactivity, on March~8, 2002, the astronaut crew of Serving Mission 3B installed the NICMOS cooling system (NCS), a closed cycle system
intended to bring down the temperature of the instrument to $\approx 77$~K \citep{MC98}. The cryocooler, produced by Creare, Inc., is a reverse-Brayton cycle turbine design. Neon gas now flows through the cooling coil originally used to flush liquid Helium. The compression-expansion cycle of Neon removes heat from the NICMOS dewar. Heat is transferred to the cryocooler via a heat exchanger, and then transported to an
external radiator by the Capillary Pumped Loop. Among the advantages of the NCS, one must mention the high operation speed---about 7000 revolutions per second---which is ideal to minimize vibrations.

%%%%%%%%%%%%%%%
\subsection{NICMOS today}
%%%%%%%%%%%%%%%
The first test images taken with the refurbished NICMOS demonstrate that the instrument is again fully operational. Performance is actually improved. Since the NCS allows the detector to operate at a temperature slightly higher than with the original cooling system, the quantum efficiency turns out to be
higher without paying the price of a significant increase of dark current. Early tests show that faint objects with $m_{AB}=22$ over 1~arcsec$^2$ at 1.6~$\mu$~m in NIC2, F110W, now provide S/N~=~36, against a S/N~= 27 before the installation of the NCS. Scientific observations resumed in mid-June 2002, and there is general agreement between the calibration data and the performance predicted with the latest version of the Exposure Time Calculator, available on the URL: 
\verb"http://stsdas.stsci.edu/ETC/NIC/"\\\verb"nic_img_etc.html".

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Science with NICMOS}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
In is beyond the scope of this paper to account for the scientific discoveries made by NICMOS. I will therefore highlight only a few results, selected to provide an overview of the instrument's capabilities.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure*}%[t]
   \centerline{\epsfxsize=8cm\epsfbox{Riess.ps}\epsfxsize=6cm\epsfbox{fg1.ps}}
%   \resizebox{\hsize}{!}{\includegraphics[clip=true]{Riess.ps}
%   \includegraphics[width=14cm]{fg1.ps}
%   \includegraphics[clip=true]{fg1.ps}
   \caption{\footnotesize Left panel: WFPC2 images of 1)~portion of the HDF-North where SN~1997ff has been located (top); 2)~close up of the white box area, with the white arrow pointing to the host galaxy. Its redness is due to the billions of old stars residing there (bottom left); 3)~difference of two frames taken two years apart showing the SN (bottom right). Right panel: color-composite images of the same region taken from NICMOS images. Note that the field here is slightly rotated with respect to the WFPC2 images. }
\label{fit1}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
\begin{figure*}%[t]
   \centerline{\epsfxsize=5in\epsfbox{9933bw.ps}}
%   \resizebox{\hsize}{!}{\includegraphics[clip=true]{9933bw.ps}}
   %\includegraphics{empty.eps}
   %\includegraphics{empty.eps}
\caption{\footnotesize Upper Panel: WFPC2 F336W, F555W, F814W (blue, green, red, respectively) composite of a field in the 30~Doradus region. Note the location of the triple stellar system (Knot~1) at the apex of a large dust pillar, whose summit has been blown off; the pillar is oriented toward R136, outside the field to the SW of Knot~1. Note also the "inverted~Y" configuration of four prominent dark globules north of Knot~1. Lower Panel: NICMOS~J, H, K (blue, green, red, respectively) composite of the same field. Note the faint, very red object within the easternmost of the four dark globules north of Knot~1.} 
\label{Walborn}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%
\subsection{Accelerating universe}
%%%%%%%%%%%%%%
Type Ia supernovae (SNe~Ia) are standard candles, i.e.\ their apparent brightness allows us to obtain an accurate measure of the distance of the host galaxies. By combining distance with redshift, one can verify if the rate of expansion of the Universe is constant over time. The fact that at redshift $z\simeq 0.5$
SNe~Ia are fainter than expected provides a strong indication that the Universe is now accelerating. Models predict that acceleration follows a previous deceleration phase, and therefore supernovae at largest redshifts are needed to confirm our understanding and constrain the basic cosmological parameters.
Supernovae at larger redshifts are expected to peak in the near-IR. They are extremely faint (at $z=1.6$ they have  $m\approx 24$ in~J and~H) and require extremely stable photometric conditions in order to measure with high accuracy their brightness on top of the host galaxy. The absence of atmospheric seeing, together with the stability of the PSF over long time-scales, makes this an ideal field of application for NICMOS.

SN~1997ff is the furthest SN~Ia observed to date, at a redshift $z\approx 1.7$. It was discovered during the repeated observations of the Hubble Deep Field-North, and serendipitously monitored with NICMOS during the GTO campaign \citep{Thompson+99}. The NICMOS data (Figure~\ref{fit1}) \citep{Riess} have constrained the redshift at $z=1.7\pm0.1$, and the brightness is consistent with an earlier decelerating universe
with cosmological parameters $\Omega_M\approx 0.3$ and $\Omega_\Lambda\approx2/3$. Various astrophysical phenomena have been invoked to provide sort of conventional explanation to the apparent acceleration of the universe, but the NICMOS results seem to rule them out, confirming that the acceleration is real and due to some form of ``dark energy.''

%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Star Formation in the LMC}
%%%%%%%%%%%%%%%%%%%%%%%
The Large Magellanic Cloud, with an average metallicity $Z\simeq0.3~Z_\odot$, hosts a rich variety of 
star-forming regions of different age and stellar composition. The most spectacular field is certainly the giant complex of HII regions, 30~Dor. Interpreted for a long time as a rather uninteresting evolved HII
region, 30~Dor is now considered a milestone in our understanding of the formation and very early evolution of massive stars. The region is the most active site of massive star formation within the Local Group, and the nearest example of the more distant and unresolved starbursts. \par The 30~Dor region is powered by R136, a 2~Myr old massive cluster. The entire area is extremely crowded and the superb image quality of NICMOS, with PSF almost constant over an extended field of view, is required to obtain IR data with quality comparable to that obtained in the visible also by the HST. The NICMOS data \citep{Walborn} show that a ``second generation'' of star formation is triggered by R136. Many new IR sources, including multiple systems, clusters, and nebular structures, are found in the periphery of R136 (Figure~\ref{Walborn}).
Dust pillars with O~stars at the top are visible, oriented directly toward R136. This provides strong support to the hypothesis that triggered star formation may occur at the heads of dust pillars. The higher mass density associated with the local star formation episode on top of the pillars shields the neutral material lying behind from the advancing main ionization front, actually creating the pillars. The detection of jets from massive stars is also important, as it indicates that disk accretion still occurs around newly formed, visible massive stars. There are only a few known cases of massive young stars powering bipolar outflows within the Galaxy.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{The Wide Field Camera 3 (WFC3)}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Wide Field Camera~3 will be installed on the HST in 2004 during Servicing Mission~4. Unlike the preceeding instruments, WFC3 is ``internally'' built by a team of astronomers and engineers from NASA, STScI, JPL, and industrial contractors, mainly Ball (BASD),  with an independent oversight committee appointed by NASA representing the worldwide astronomical community. WFC3 (Figure~\ref{fit1}) has two independent channels, one for the UV and visible wavelengths (UVIS, $\approx2000$~\AA ~to $1~\mu$m) and the other for the near-IR
wavelengths (IR, $\approx0.8$ to 1.7~$\mu$m). The UVIS channel is based on a pair of butted 4K$\times$2K thinned, back-illuminated CCDs produced by Marconi (UK), whereas the IR channel is based on a new HgCdTe detector 1K$\times$1K made by Rockwell Science Center. The IR channel has a scale of approximately $0\farcs121 \times 0\farcs135$/pixel, with a field $\approx127\arcsec \times 137\arcsec$. The rectangular shape is due to the detector tilt with respect to the chief ray caused by the off-axis optical design.

The two channels view the same on-axis field, but not simultaneously; a channel selection mechanism diverts the beam to one or the other channel. The optical design of the IR channel is all-reflective, with the exception of a lens performing the spherical aberration correction at the cold pupil. The cooling system is based on thermoelectric coolers (TECs). While TECs provide virtually unlimited cooling capability, the price is reduced cooling power. The operating temperature of the IR detector will be, in fact, close to 
150~K, much higher than $T\approx77$~K of NICMOS. At 150~K, conventional HgCdTe devices with cut-off at $\approx2.5~\mu$m generate an intolerably high dark current. Since the dark current is related to the bandgap, by shortening the cutoff wavelengths to $\approx 1.7~\mu$m it is possible to keep the dark current below the natural limit imposed by the average zodiacal background, estimated in approximately 
0.2~electrons/second/pixel. This constraint defines the long wavelength cutoff of WFC3. 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vspace{\baselineskip}
%\begin{figure}
   \centerline{\epsfxsize=2in\epsfbox{WFC3.eps}}
%   \resizebox{\hsize}{!}{\includegraphics{WFC3.eps}}
%   \includegraphics{figura5b_articolo.eps}}
   \figcaption{Schematic of the WFC3 optical bench. The light from the secondary mirror is intercepted by the pick-off mirror protruding at the bottom of the figure, then enters the optical bench where it
immediately encounters the channel selection mechanism. A mirror can be placed to reflect the beam to the IR channel. In the figure the UVIS channel is on the left, the IR channel is on the right. \label{fit1}}
\vspace{\baselineskip}
%\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
The IR filter wheel accommodates 15 passband filters, two grisms and a blank position, including
broad-band filters, medium-band filters centered on molecular features and nearby continuum, and 
narrow-band filters probing interstellar diagnostic lines.
\par 
Up to 1.7~$\mu$m, WFC3 offers a 6.4 larger field of view than NICMOS/Camera~3, with substantially better spatial sampling and sensitivity, depending on the final detector performance. According to the most recent
predictions, WFC3 will reach S/N~=~5 in 10~hr for a point source with H~=~26.3.  An exposure time calculator is maintained at the STScI with the most recent values for the instrument throughput,
filter transmission and detector performances. It is found at the URL: 
\verb"http://www.stsci.edu/instruments.wfc3."\\
\verb"etc-list.html".

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{The Next Generation Space Telescope (NGST)}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Like HST, the Next Generation Space Telescope is a collaborative effort between NASA and ESA, joined by the Canadian Space Agency. The development of the project is lead by the NASA's Goddard Space Flight Center. The Space Telescope Science Institute is responsible for the Science and Operation Center. 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vspace{\baselineskip}
%   \begin{figure}[t]
   \centerline{\epsfxsize=5cm\epsfbox{Ball.eps}}
%   \resizebox{6cm}{!}{\includegraphics{Ball.eps}}
   \figcaption{\footnotesize{TRW/Ball concept of the NGST telescope.}   \label{ball}}
%    \end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\vspace{\baselineskip}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   \begin{figure}[h!]
%   \centering
\centerline{\epsfxsize=5cm\epsfbox{lockheed.eps}}
%   \resizebox{6cm}{!}{\includegraphics{lockheed.eps}}
   \figcaption{\footnotesize{Lockheed-Martin concept of the NGST telescope.}  \label{locked}}
%    \end{figure}
\vspace{\baselineskip}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
The goal of NGST is to directly attack the most fundamental open questions of contemporary astrophysics: What is the history and the structure of the Universe? How do galaxies form and evolve? What is the history
of our Galaxy? How do stars, brown dwarfs, and planetary systems form? What is the nature of dark matter? Such an ambitious task requires extreme design parameters:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
\begin{itemize}
\item
6m class mirror, lightweight and deployable;
\item
Telescope passively cooled by large sunshade;
\item
Second Lagrange Point (L2) of Earth-Sun system orbit;
\item
Image quality diffraction limited (Strehl ratio=0.8) at 2~$\mu$m;
\item
wavelength range 0.6--10~$\mu$m, with zodiacal-light-limited performance, instrument limited up to 
20~$\mu$m;
\item
imaging and spectroscopic instrumentation over the wavelength range;
\item
5 year minimum lifetime, 10 year goal.
\end{itemize}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

As of today (June 2002), two proposals are competing to be the prime-contractor of NGST: TRW/Ball 
(Figure~\ref{ball}) and Lockheed-Martin (Figure~\ref{locked}). The selection is expected at the end of summer 2002. NGST will be equipped with three instruments to be housed in the Integrated Science Instrument Module (ISIM): the near-IR camera (NIRCam), the near-IR spectrometer (NIRSpec) and the mid-IR instrument (MIRI), plus the guide system. The consortia building the instruments have been recently selected:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
\begin{itemize}
\item NIRCam will be built by a team lead by the University of Arizona (lead investigator Marcia Rieke);
\item NIRSpec will be built by ESA with a primarily European science team;
\item MIRI will be build in a 50/50 collaboration between NASA and individual European nation states, managed by ESA. The US-lead implementing center is JPL (Instrument Scientist Gene Serabyn), whereas the US science team is lead by G.~Rieke.
\end{itemize}
%%%%%%%%%%%%%%%%%%%%%%%%%%%5
The NGST guider will be provided by Canada.

%%%%%%%%%%%%%%%%%%
\subsection{NIRCam}
%%%%%%%%%%%%%%%%%5
NIRCam will be the imaging instrument for the wavelength range $\sim0.6$--$5~\mu$m. The instrument will be composed of 4 modules: two for imaging and two for imaging spectroscopy through tunable filters. The two imaging modules will be identical, so that the field of view can be maximized within the reliability and cost constraints of the NGST project. Light entering each imaging module will be split by a dichroic into two optical arms: a short wavelength arm ($\lambda\le2.5~\mu$m) and a long wavelength arm ($\lambda\ge
2.5~\mu$m). They will observe the same field with different spatial resolutions: one Nyquist-sampled at 
2~$\mu$m, providing $0\farcs034$/pixel on a 4k$\times$4K detector, the other Nyquist-sampled at 4~$\mu$m, delivering $0\farcs068$/pixel on a $2K\times$2K detector. The resulting field of view in both cases will be $2.3\arcmin\times4.6\arcmin$,  which will double when the other imaging module is considered. The two other modules will be dedicated to imaging spectroscopy---one to the short wavelengths (1.2--2.5~$\mu$m) and the other to the long wavelengths (2.5--45~$\mu$m). The spatial scale is $0\farcs068$/pixel and the spectral resolution of $\approx1$\%. Moreover, all modules are to have a coronographic mode, with a spot on the focal plane and a coronographic mask on the pupil wheel. Used in conjunction with the tunable filters, the coronographic mode will allow us, for example, to obtain low resolution spectra of faint objects near bright stars (e.g.\ exoplanets). 
\par 
The expected point-source sensitivity, S/N~=~10 in broad band imaging (R~=~5), is of the order of a few nJy  in 100,000 seconds. Further information can be obtained at the URL:\\
\verb"http://ircamera.as.arizona.edu/nircam"

%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{NIRSpec}
%%%%%%%%%%%%%%%%%%%%%%%%%
NIRSpec will cover the wavelength range $\sim0.6$--$5~\mu$m with resolving power $\sim100$ and $\sim1000$. The design will take advantage of an innovative Micro-Electromechanical System (MEMS), namely a 
micro-shutter array. This will allow the user to simultaneously observe hundreds of objects through the openings of an array of micro-shutters located at the focal plane. It was recently decided to proceed with the micro-shutter technology rather than with the similar micro-mirror approach, where the shutters operate in reflection rather than in transmission. The projected pixel size will be approximately 100~mas, and the
sensitivity, S/N in 100,000 at R~= 1,000, will be of the order of a fraction of a $\mu$Jy.\par Further information can be obtained at the URL:\\ \verb"http://astro.estec.esa.nl/NGST"

%%%%%%%%%%%%%%%%
\subsection{MIRI}
%%%%%%%%%%%%%%%%

MIRI will provide imaging and spectroscopy between 5 and 28~$\mu$m. The functional requirements made by the Mid-Infrared Steering Committee are:
\begin{itemize}
\item
Wavelength range 5--25~$\mu$m, possibly up to 28.2~$\mu$m in order to cover an important emission line of $H_2$;
\item
Diffraction limited optics over the entire wavelength range and large field size. Pixel size $\sim0\farcs12$/pixel, field $\sim 2\times2$ arcmin;
\item
Coronographic imaging capability;
\item
Spectroscopic mode with $R\sim100$ in the range 5--10~$\mu$m;
\item
Integral field spectroscopy with $R\sim1000$--2000 with pixel size $\sim0\farcs2$ and $\sim0\farcs4$ over a field of view of $\sim 2\times2$ arcmin;
\item
Lifetime 10 years.
\end{itemize}
The proposed agreement between NASA and ESA assigns to ESA the responsibility for the optical and mechanical/cryogenic parts; the US team will be responsible for the detectors and electronics, system integration, calibration, etc.

%%%%%%%%%%%%%%%%%%
\subsection{NGST Detectors}
%%%%%%%%%%%%%%%%%%
A m~=~33 source observed with the NGST will provide less than 1~photon/second on the detector. In order to detect such low flux, it is necessary to couple the extraordinarily low background conditions of the 
L2~site with a new generation of IR detectors having extremely low noise. These detectors are under development at the Rockwell Science Center (1--5~$\mu$m HgCdTe technology), in collaboration with the team of D.~Hall at the University of Hawaii; at Raytheon Infrared Operations (InSb technology) in collaboration with the team of W.~Forrest at the University of Rochester, and at the NASA Ames Research Center, where the team of C.~McCreight is developing and characterizing Si:As mid-IR detectors. A fourth laboratory, the Independent Detector Testing Laboratory (IDTL, PI D.~Figer) has been funded at STScI to characterized both HgCdTe and InSb detectors in a comparative hardware setup. 
\par 
The detector technology for the 1--5~$\mu$m channel will be selected in the summer 2003.

\section{Conclusions}

As a final remark, let me underline that through the ESA participation, both the HST and the NGST are facilities open to the European scientific community. Staff at the STScI and at the ECF can be contacted for further information about instrument performance, observing time application and data mining in the huge and publicly available archive of HST data.

%%%%%%%%%%%%%%%%%%%%%%%
\begin{acknowledgements}
I'm indebted to my colleagues of the Space Telescope Science Institute, in particular J.~MacKenty, 
M.~Stiavelli and D.~Calzetti, for our daily exchange of ideas on the topics of this paper. Thanks to the organizers of this interesting meeting and to S.~Ciprini of the LOC for his help editing this contribution. I take this opportunity to acknowledge Prof.\ Maffei for his books, my early readings on astronomy, that inspired me when I was a teenager.
\end{acknowledgements}
%%%%%%%%%%%%%%%%%%%%%

\bibliographystyle{aa}
\begin{thebibliography}{}

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