Call for Observing Proposals on the 30m Telescope

Summary

Proposals for three types of receivers will be considered for the coming winter semester:

1. the observatory's set of four dual polarization heterodyne receivers centered at wavelengths of 3, 2, 1.3, and 1.1 mm,
2. the 9 pixel dual-polarization heterodyne receiver array, HERA, operating at 1.3 mm wavelength,
3. a 1.2 mm bolometer array with at least 37 pixels.

Emphasis will be put on observations at the shorter wavelengths. In total, about 2800 hours of observing time will be available, which should allow scheduling of a few longer programmes (up to $\sim 150$ hours).

The main news, proposal formalities, details of the various receivers, and observing modes are described below.

What is new ?

The tuning range of the 3mm receivers, nominally 80 - 115.5 GHz, has been slightly extended toward lower frequencies. Both 3mm receivers, A100 and B100, have now been tested. Both receivers work correctly in SSB and high USB rejection down to $\sim77$ GHz. A test report together with a collection of spectra is made available on our web pages

Applications for frequencies down to $\geq72$ GHz will be considered. Requests for frequencies below 77 GHz should describe the precision of the calibration needed. Due to the special LO hardware requirements observations below 80 GHz presently exclude the simultaneous use of A230 and B230. Proposers should enter ``$<80$ GHz'' as special requirements on the proposal cover sheet.

Proposers should use the time estimator which will include the correct receiver temperature at the low frequencies and an extra overhead for calibration.

The second polarization module of HERA has now been installed. With the exception of one pixel where one polarization is somewhat less sensitive, the upgrade was fully satisfactory. As many observational parameters are not yet precisely known for the full dual polarization array, we request HERA proposers to consult the time estimator when estimating integration times.

The new broadband autocorrelator WILMA (Wideband Line Multiple Autocorrelator) is now operational at Pico Veleta. Hardware for synchronization with the wobbler is also expected to be in place.

Zero spacing observations with the 30m telescope needed for a proposed PdB Interferometer observation do not need a parallel proposal for the 30m telescope. It is sufficient to mark ``zero spacing'' on the interferometer proposal form. All necessary scientific and technical information about the 30m observation must be given in an additional paragraph of the Bure proposal. We strongly recommend to use the 30m time estimator.

When an interferometer proposal had not originally requested zero spacing observations, but the need for such data becomes apparent after reduction of the interferometer data, we still request a separate and self-contained proposal for the 30m telescope.

The implementation of the New Control System is advancing. A beta test is planned for November this year. The test is designed to provide complete telescope control at least for bolometer observations. More comprehensive tests are envisaged toward the end of the winter semester. The bulk of the observations will however still be observed with the present VAX-based control system which will then be 20 years old.

Applications

On the official cover page, please fill in the line `special requirements' if you request either polarimetric observations, service or remote observing. If the observations need or have to avoid specific dates, enter them here. If there are periods when you cannot observe for personal reasons, please specify them here.

We insist that the source positions and observing frequencies are correctly and completely entered on the proposal cover page. Proposals for heterodyne receivers are requested to give a complete list of frequencies corrected for source redshift (to 0.1 GHz). If in very special cases the proposers do not feel to be in a position to give this information, they should take up contact with the scheduler (thum@iram.fr). The proposers should also specify on the cover sheet which receivers they plan to use.

In order to avoid useless duplication of observations and to protect already accepted proposals, we keep up a computerized list of targets. We ask you to fill out carefully the source list in J2000 coordinates. This list must contain all the sources (and only those sources) for which you request observing time. To allow electronic scanning of your source parameters, your list must adhere to the format indicated on the proposal form (no hand writing, please). If your source list is longer (e.g. more than 15 sources) than what fits onto the cover page, please use the L^ATEXmacro \extendedsourcelist.

A scientific project should not be artificially cut into several small projects, but should rather be submitted as one bigger project, even if this means 100-150 hours.

If time has already been given to a project but turned out to be insufficient, explain the reasons, e.g. indicate the amount of time lost due to bad weather or equipment failure; if the fraction of time lost is close to 100%, don't rewrite the proposal, except for an introductory paragraph. Information given in the Observer's Comment Sheet which we request to be filled in after each observation, is linked to the proposal data base and helps to reconstruct a proposal's history.

Reminders

A handbook (``The 30m Manual'') collects most of the information necessary to plan 30m telescope observations[6]. The report entitled ``Calibration of spectral line data at the IRAM 30m telescope'' explains in detail the applied calibration procedure. Both documents can be retrieved from the URL /IRAMES/otherDocuments/manuals/index.html. A catalog of well calibrated spectra for a range of sources and transitions (Mauersberger et al. [9]) is very useful for monitoring spectral line calibration. A copy of the 30m file with the calibrated spectra can be downloaded from the Spanish web site.

The astronomer on duty (whose schedule can be found at URL /IRAMES/groups/astronomy/aodsched.html) should be contacted well in advance of an observing run for any special questions concerning the preparation of an observing run (e.g. setup of on-the-fly maps etc).

Frequency switching is available for both HERA and the observatory's standard SIS receivers. This observing mode is interesting for observations of narrow lines where flat baselines are not essential, although the HERA provides the flattest frequency switched baselines known at the 30m. Certain limitations exist with respect to maximum frequency throw ($\le 45$ km/s), backends, phase times etc.; for a detailed report see [4].

Observing time estimates

This matter needs special attention as a serious time underestimate may be considered as an indication of sloppy proposal preparation. We strongly recommend to use the web-based Time Estimator (URL: /IRAMES/obstime/time_estimator.html), whenever applicable. It handles heterodyne (single pixel and HERA) as well as bolometer observations with updated instrumental parameters. Suggestions and questions can be addressed to Axel Weiß (aweiss$@$iram.fr).

If very special observing modes are proposed which are not covered by the Time Estimator, proposers must give sufficient technical details so that their time estimate can be reproduced. In particular, the proposal must give values for $T_{\rm sys}$, the spectral resolution, the expected antenna temperature of the signal, the signal/noise ratio which is aimed for, all overheads and dead times, and the resulting observing time. A technical report explaining how to estimate the telescope time needed to reach a given sensitivity level in various observing modes was published in the January 1995 issue²of the IRAM Newsletter [5]. It has been included in the 30m telescope Manual [6].

Proposers should base their time request on normal winter conditions, corresponding to 4mm of precipitable water vapor. Conditions during afternoons can be degraded due to anomalous refraction. The observing efficiency is then reduced and the temperature calibration is more uncertain than the typical 10 percent. If exceptionally good transmission or stability of the atmosphere is requested which may be reachable only in best winter conditions, the proposers must clearly say so in their time estimate paragraph. Such proposals will however be particularly scrutinized.

Pooled observing

As in the previous semesters, we plan to reserve a large fraction of the winter semester for pooled observing. The proposals participating in the pool are observed by Granada staff and cooperating external astronomers, coordinated by Axel Weiss. The participating proposals are grouped according to their demand on weather quality and will be observed following the priorities assigned by the program committee. The organization of the observing pool is described at /IRAMES/observing/flexible/flexible.html. Typically, the bolometer proposals are included in the pool, along with suitable heterodyne proposals. Participation in the pool is voluntary, and the respective box on the proposal form should be checked.

As a backup for poor 1mm observing conditions, simple spectroscopic proposals at lower frequencies are often ideal. We therefore encourage submission of such proposals, essentially at frequencies below $\sim105$ GHz and including the newly opened range below 80 GHz. The ``pool'' box should be checked on the proposal cover sheet.

Service observing

To facilitate the execution of short ($\leq$8 h) programmes, we propose ``service observing'' for some easy to observe programmes with only one set of tunings. Observations are made by the local staff using precisely laid-out instructions by the principal investigator. For this type of observation, we request an acknowledgement of the IRAM staff member's help in the forthcoming publication. If you are interested by this mode of observing, specify it as a ``special requirement'' in the proposal form. IRAM will then decide which proposals can actually be accepted for this mode.

Remote observing

This observing mode where the remote observer actually controls the telescope very much like on Pico Veleta, is available from the downtown Granada office, from the MPIfR in Bonn, from the ENS in Paris, from the OAN in Madrid (near Parque de Retiro), and from IRAM in Grenoble. This observing mode is available to projects without any particular technical demands and to experienced 30m users. The prospective remote observer should note ``remote observing'' as a special requirement in the proposal cover sheet.

After time has been awarded to a proposal, the P.I. is requested to give sufficient detail to the secretary, Cathy Berjaud (berjaud$@$iram.fr) on how the remote observer can be contacted. Please note that IRAM is not responsible for the remote stations in Paris, Madrid, or Bonn.

Remote observers affiliated with the MPIfR or other institutes near Bonn should contact F. Bertoldi (bertoldi$@$mpifr-bonn.mpg.de) or D. Muders (dmuders$@$mpifr-bonn.mpg.de) at MPIfR for a short introduction to the remote observing station. Remote observers in the Paris area may contact M. Gerin (gerin$@$lra.ens.fr) for arrangements. Astronomers who want to use the Madrid station are requested to contact J. Alcolea (j.alcolea$@$oan.es). Remote observers in or near Grenoble please contact H. Wiesemeyer (wiesemey$@$iram.fr) at IRAM. Observers visiting the 30m might opt to do some of their observing from Granada if it eases their travel constraints. In this case, a Granada astronomer should be contacted as soon as possible, arrangements on very short notice may not always be possible.

Technical Information about the 30m Telescope

This section gives all the technical details of observations with the 30m telescope that the typical user will have to know. A concise summary of telescope characteristics is published on the IRAM web pages.

HERA

The HEterodyne Receiver Array is expected to be available for most of next winter. The 9 pixels are arranged in the form of a center-filled square and are separated by $24''$. Each beam is split into two linear polarizations (after a successful upgrade in March) which couple to separate SIS mixers. The 18 mixers feed 18 independent IF chains. Each set of 9 mixers is pumped by a separate local oscillator system. The same positions can thus be observed simultaneously at any two frequencies inside the HERA tuning range (210-276 GHz).

A derotator optical assembly can be set to keep the 9 pixel pattern stationary in the equatorial or horizontal coordinates. Receiver characteristics (of the single polarization system) are listed in Tab. 1, and an updated user manual (version 1.7) is available on our web page.

Frequency tuning of HERA, although fully under remote control and automatic, is substantially more complicated than for the observatory's other SIS receivers. Although the tuning is still known for only a few frequencies, (the 3 CO isotopes at 230.5, 220.4, and 219.6 GHz; CS at 244.9 GHz; HCN at 265.9 GHz; HCO$^+$ at 267.6 GHz; DCN and HC$^{15}$N at 217.2 and 259 GHz; H$_2$CO at 225.7 GHz; H30$\alpha$ at 231.9 GHz), HERA proposals for any frequency within the nominal tuning range of 210 - 276 GHz will be considered, but we cannot guarantee at this moment that these proposals can actually be done. In any case, HERA observers should send the list of their frequencies to Granada as early as possible.

HERA can be connected to three sets of backends: VESPA with the following combinations of nominal resolution (KHz) and maximum bandwidth (MHz): 20/40, 40/80, 80/160, 320/320, 1250/640. The maximum bandwidth can actually be split into two individual bands for each of the 18 detectors at most resolutions. These individual bands can be shifted separately up to $\pm200$ MHz offsets from the sky frequency (see also the sections on backends below).

$\rhd$

a low spectral resolution (4 MHz channel spacing) filter spectrometer covering the full IF bandwidth of 1 GHz. Nine units (one per HERA pixel) are available. Note that only one polarization of the full array is thus connectable to these filter banks.

$\rhd$

WILMA with a 1 GHz wide band for each of the 18 detectors. The bands have 512 spectral channels spaced out by 2 MHz. WILMA will be available after successful completion of the current tests.

HERA is operational in two basic spectroscopic observing modes: (i) raster maps of areas typically not smaller than $1'$, in position, wobbler, or frequency switching modes, and (ii) on-the-fly maps of moderate size (typically $2' - 10'$). Extragalactic proposals should take into account the current limitations of OTF line maps, as described in the User Manual, due to baseline instabilities induced by residual calibration errors. HERA proposers should use the web-based Time Estimator. For details about observing with HERA, consult the User manual. The HERA project scientist Karl Schuster (schuster$@$iram.fr), or Albrecht Sievers (sievers$@$iram.es),the astronomer in charge of HERA, may also be contacted.

Given the risk involved in the upgrade and considering that some relevant observational parameters are not yet well known for the full array, we request HERA proposers to use the parameters of the present single polarization array when estimating integration times. As mentioned above, the times scheduled for the successful proposals may then be adjusted, once the parameters of the full array are known.

The single pixel heterodyne receivers

Four dual polarization SIS receivers are available at the telescope for the upcoming observing season. They are designated according to the dewar in which they are housed (A, B, C, or D), followed by the center frequency (in GHz) of their tuning range. Their main characteristics are summarised in Tab. 1. All receivers are linearly polarized with the E-vectors, before rotation in the Martin-Puplett interferometers, either horizontal or vertical in the Nasmyth cabin. Up to four of these eight receivers can be combined for simultaneous observations in the four ways designated as AB, CD, AD, and BC. These are the only valid combinations and they are depicted in Tab. 1. Note that single pixel receivers cannot be combined with HERA nor with the bolometers. Also listed are typical system temperatures which apply to normal winter weather (4mm) at the center of the tuning range and at 45 elevation. All receivers are tuned by the operators from the control room. Experience shows that it normally takes not more than 15 minutes to tune four such receivers.

Table 1: Heterodyne receivers available for the next winter observing season. Performance figures are based on recent measurements at the telescope. $T^{\ast }_{sys}$ is the SSB system temperature in the $T^\ast _A$ scale at the nominal center of the tuning range, assuming average winter conditions (4mm precipitable water vapor) and 45 elevation. $g_i$ is the rejection factor of the image side band. $\nu _{IF}$ and $\Delta \nu _{IF}$ are the IF center frequency and width. Note that the 8 standard receivers can be combined in 4 different ways.

receiver		polar-		combinations				tuning range		$T_{Rx}$(SSB)		$g_i$	$\nu _{IF}$	$\Delta \nu _{IF}$		$T^{\ast }_{sys}$	remark
		ization		AB		CD		AD		BC		GHz	K	dB		GHz	GHz	K
A100		V		1				3				80 - 115.5	45 - 65	$>20$		1.5	0.5	120	1
B100		H		1						4		81 - 115.5	60 - 85	$>20$		1.5	0.5	130	1
C150		V				2				4		129 - 183	70 - 115	15 - 25		4.0	1.0	200
D150		H				2		3				129 - 183	60 - 150	8 - 17		4.0	1.0	200
A230		V		1				3				197 - 266	85 - 185	12 - 17		4.0	1.0	420
B230		H		1						4		197 - 266	95 - 160	12 - 17		4.0	1.0	420
C270		V				2				4		241 - 281	125 - 290	10 - 20		4.0	1.0	900	2
D270		H				2		3				241 - 281	130 - 300	9 - 13		4.0	1.0	900	2
HERA		H,V										210 - 276	110 - 380	$\sim 10$		4.0	1.0	400	3

1: tuning range extended to $\geq72$ GHz under special conditions (see text)
2: noise increasing with frequency
3: tuning parameters are not yet complete

Tuning range extended below 80 GHz

Several molecules of high astrophysical importance have transitions in the frequency band 66 - 80 GHz, i.e. between the atmospheric O$_2$ absorption band and the low frequency edge of the nominal 3mm tuning range (see Tab.1). Both 3mm receivers, A100 and B100, have now been tested below 80 GHz with partially encouraging results. Both receivers work correctly in SSB and high USB rejection down to 77 GHz when fed with a suitable foreign local oscillator. Below this frequency, the USB rejection progressively weakens, until the mixer becomes fully DSB near 72 GHz. At still lower frequencies, however, the tuning behavior is very irregular, and high USB gain ($g_i \gg 1$) makes the receivers very noisy and calibration difficult. A detailed test report is available on the IRAM web site (at /IRAMFR/PV/veleta.html) together with a sample of spectra.

Given the partially encouraging test results, applications for frequencies down to $72$ GHz will be considered. Please enter a remark like ``$<80$ GHz'' under Special Requirements on the proposal cover sheet. Requests for frequencies below 77 GHz should additionally describe the precision of the calibration needed.

Some of these low frequency proposals, particularly if they are not particularly sensitive to calibration errors and if they do not simultaneously request frequencies in the 1.3mm window, may be well suited as a mediocre weather backup in our pooled observing sessions. The ``pool'' box on the proposal cover sheet should then be checked.

General point about receiver operations

Tuning of the single pixel/dual polarization receivers is now considerably faster and more reproducible than before. Particular frequencies, like those below 80 GHz or near a limit of the tuning range, may still be problematic, and we recommend in such cases to check with a Granada receiver engineer at least two weeks before the observations. HERA observers, however, are requested to send their frequencies as soon as their project gets scheduled.

Polarimeter XPOL

An upgrade of the IF polarimeter [16] is now available, where the cross correlation between the IF signals from a pair of orthogonally polarized receivers is made digitally in VESPA. The new observing procedure XPOL generates simultaneous spectra of all 4 Stokes parameters. The following combinations of spectral resolution (KHz) and bandwidth (MHz) are available: 40/120, 80/240, 320/480, and 1250/640.

Although successful XPOL observations were made at several frequencies, experience is still limited, particularly with respect to long integrations and observations of extended sources. Data reduction software using CLASS enhanced with a graphical user interface is available (H. Wiesemeyer). A short guide (at /IRAMFR/PV/veleta.html) describes XPOL observations. Polarimetry proposals will be considered with the restriction that the target sources be not larger than the main beam.

MPIfR Bolometer arrays

The bolometer arrays, MAMBO-1 (37 pixels) and MAMBO-2 (117 pixels), are provided by the Max-Planck-Institut für Radioastronomie, Bonn. They consist of concentric hexagonal rings of horns centered on the central horn. Spacing between horns is $\simeq 23''$. Each pixel has a HPBW of 11$''$. We expect that MAMBO-2 will be normally used, but MAMBO-1 is kept as a backup.

The effective sensitivity of MAMBO-1 for onoff and mapping observations is $\sim35$ mJys$^\frac{1}{2}$. For MAMBO-2 effective sensitivities of $\sim\!40$ mJys$^\frac{1}{2}$ (ON/OFF mode) and $\sim\!45$ mJys$^\frac{1}{2}$ (mapping mode) were measured. Since in the mapping mode all beams cover the inner region of the map area, MAMBO-2 turns out to be more sensitive if areas of $2'$ and larger are to be mapped (see the Time Estimator). The sensitivities apply to bolometric winter conditions ( $\tau(\small {250{\rm GHz}})\sim$ 0.25, elevation 45 deg, and application of skynoise filtering algorithms). In cases where skynoise filtering algorithms are not or not fully effective (e.g. extended source structure, atmosphere not sufficiently stable), the effective sensitivity is typically about a factor of 2 worse. Such projects should only be observed under atmospheric conditions with low skynoise (i.e. stable atmosphere, no clouds, little turbulence), unless the expected signal is about 1 Jy/beam or stronger.

The bolometer arrays are mostly used in two basic observing modes, ON/OFF and mapping. Previous experience with MAMBO-2 shows that the ON/OFF reaches typically an rms noise of $\sim2.3$ mJy in 10 min of total observing time (about 200 sec of ON source, or about 400 sec on sky integration time) under stable conditions. Up to 30 percent lower noise may be obtained in perfect weather. In this observing mode, the noise integrates down with time $t$ as $\sqrt{t}$ to rms noise levels at least 0.5 mJy.

In the mapping mode, the telescope is scanned in azimuth (also the direction of the wobbler throw) in such a way that all pixels see the source once. A typical single map³ with MAMBO-2 covering a fully and homogeneously sampled area of $150''\times150''$ (scanning speed: $5''$per sec, raster step: $8''$) reaches an rms of 2.8 mJy/beam in 1.9 hours if skynoise filtering is effective. If sky noise filtering is not effective (see Time Estimator), much more time may be needed. The area actually scanned ( $7.3'\times6.5'$) must be larger than the map size by the wobbler throw and the array size ($4'$) if extended emssion is to be properly restored. Shorter scans may lead to problems in restoring extended structure. Mosaicing is also possible to map larger areas. Under many circumstances, maps may be co-added to reach lower noise levels, but this may require vey sophisticated data reduction (please contact the experts). If maps with an rms $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}1$ mJy are proposed, the proposers must indicate how they plan to reach this ambitious goal.

The bolometers are used with the wobbling secondary mirror (wobbling at a rate of 2 Hz in azimuth). The orientation of the beams on the sky changes with hour angle due to parallactic and Nasmyth rotations, as the array is fixed in Nasmyth coordinates. Bolometer proposals participating in the pool have their observations (maps and ONOFFs) pre-reduced by a data quality monitor which runs scripts in the newly developed MOPSIC. This package, complete with all necessary scripts, is also installed for off-line data analysis in Granada and Grenoble. It is also available for distribution from the IRAM Data Base for Pooled Observations or directly from R. Zylka (zylka$@$iram.fr). The older software packages (NIC [7] and MOPSI[8]) are still available.

Bolometer proposals will be pooled together like in previous semesters along with suitable heterodyne proposals as long as the respective PIs agree. The web-based time estimator handles well the usual bolometer observing modes, and its use is again strongly recommended. The time estimator uses rather precise estimates of the various overheads which will be applied to all bolometer proposals. If exceptionally low noise levels are requested which may be reachable only in a perfectly stable perfect winter atmosphere, the proposers must clearly say so in their time estimate paragraph. Such proposals will however be particularly scrutinized. On the other extreme, if only strong sources are observed and moderate weather conditions are sufficient, the proposal may be used as a backup in the observing pool. The proposal should point out this circumstance, as it may affect the chance that the proposal is accepted and observed.

The Telescope

Beam and Efficiencies

Table 2 lists the size of the telescope beam for the range of frequencies of interest. Forward and main beam efficiencies are also shown (see also the note by U. Lisenfeld and A. Sievers, IRAM Newsletter No. 47, Feb. 2001). The variation of the coupling efficiency to sources of different sizes can be estimated from plots in Greve et al. [12].

At 1.3 mm (and a fortiori at shorter wavelengths) a large fraction of the power pattern is distributed in an error beam which can be approximated by two Gaussians of FWHP $\simeq 170''$ and $800''$ (see [12] for details). Astronomers should take into account this error beam when converting antenna temperatures into brightness temperatures.

Variable and sometimes large astigmatism which deformed the telescope's main beam during many years[3] was shown to result from temperature differences between the telescope backup structure and the yoke. The recent installation of heaters in the yoke by J. Peñalver has nearly completely removed the astigmatism[15].

Table 2: Main observational parameters of 30m telescope.

frequency	$\theta_b$ ['']	$\eta_F$	$\eta_{mb}$	$S_\nu$/T$_A^{\ast}$
[GHz]	(1)	(2)	(3)	[Jy/K]
86	29	0.95	0.78	6.0
110	22	0.95	0.75	6.3
145	17	0.93	0.69	6.7
170	14.5	0.93	0.65	7.1
210	12	0.91	0.57	7.9
235	10.5	0.91	0.51	8.7
260	9.5	0.88	0.46	9.5
279	9	0.88	0.42	10.4

(1)	beam width (FWHP). A fit to all data gives:
	$\theta_b$ [''] = 2460 / frequency [GHz]
(2)	forward efficiency (coupling efficiency to sky)
(3)	main beam efficiency. Based on a fit of measured
	data to the Ruze formula:
	$\eta_{\rm mb}=1.2\epsilon \exp(-(4\pi R \sigma/\lambda)^2)$
	with $\epsilon=0.69$ and $R\sigma=0.07$

Pointing and Focusing

With the systematic use of inclinometers the telescope pointing became much more stable. Pointing sessions are now scheduled at larger intervals. The fitted pointing parameters typically yield an absolute rms pointing accuracy of better than $3''$ [10]. Receivers are closely aligned (within $\leq 2''$). Checking the pointing, focus, and receiver alignment is the responsibility of the observers (use a planet for alignment checks). Systematic (up to 0.4 mm) differences between the foci of various receivers can occasionally occur. In such a case the foci should be carefully monitored and a compromise value be chosen. Not doing so may result in broadened and distorted beams ([1]).

Wobbling Secondary

• Beam-throw is $\le 240''$ depending on wobbling frequency. At 2 Hz, the maximum throw is $90''$
• Standard phase duration: 2 sec for spectral line observations, 0.25 sec for continuum observations.

Throws of $120''$ should be avoided.

Backends

The following four spectral line backends are available which can be individually connected to any single pixel receiver and, if indicated, also to HERA.

The 1 MHz filterbank consists of 4 units. Each unit has 256 channels with 1 MHz spacing and can be connected to different or the same receivers giving bandwidths between 256 MHz and 1024 MHz. The maximum bandwidth is available for only one receiver, naturally one having a 1 GHz wide IF bandwidth. Connection of the filterbank in the 1 GHz mode presently excludes the use of any other backend with the same receiver.

Other configurations of the 1 MHz filterbank include a setup in 2 units of 512 MHz connected to two different receivers, or 4 units of 256 MHz width connected to up to four (not necessarily) different receivers. Each unit can be shifted in steps of 32 MHz relative to the center frequency of the connected receiver.

The 100 KHz filterbank consists of 256 channels of 100 KHz spacing. It can be split into two halves, each movable over a range of 500 MHz, and connectable to two different receivers.

VESPA, the VErsatile Spectrometric and Polarimetric Array correlator, can be connected either to HERA or to a subset of 4 single pixel receivers, or to a pair of single pixel receivers for polarimetry. The many VESPA configurations and user modes are summarized in a Newsletter contribution [14] and in a user guide, but are best visualised on a demonstration program which can be downloaded from our web page at URL /IRAMFR/PV/veleta.html. Connected to a set of 4 single pixel receivers VESPA typically provides up to 12000 spectral channels (on average 3000 per receiver). Up to 18000 channels are possible in special configurations. Nominal spectral resolutions range from 3.3 KHz to 1.25 MHz. Nominal bandwidths are in the range 10 -- 512 MHz. When VESPA is connected to HERA, up to 18000 spectral channels can be used with the following typical combinations of nominal resolution (KHz) and maximum bandwidth (MHz): 20/40, 40/80, 80/160, 320/320, 1250/640.

The 4 MHz filterbank consists of nine units. Each unit has 256 channels (spacing of 4 MHz, spectral resolution at 3 dB is 6.2 MHz) and thus covers a total bandwidth of 1 GHz. The 9 units are designed for connection to HERA, but a subset of 4 units can also be connected to the backend distribution box which feeds the single pixel spectral line receivers. All these receivers have a 1 GHz RF bandwidth except for A100 and B100 (500 MHz only). At the present time, a 4 MHz filterbank cannot be used simultaneously with the autocorrelator or the 100 KHz filterbank on the same receiver.

An on-line calibration routine automatically writes calibrated spectrometer data, including the 4 MHz filterbanks, to the Linux machines. The routine, although still experimental, works for all observing modes. A logical link named ``data.30m'' pointing to this file of calibrated spectra is made available in all newly created project accounts.

The new autocorrelator WILMA consists of 18 units which connect to the 18 detectors of HERA. Each unit nominally provides 512 spectral channels, spaced out by 2 MHz and thus covering a total bandwidth of nominally 1 GHz of which 930 MHz are usable. Each band is sliced into two 500 MHz subbands which are digitized with 2 bit/1 GHz samplers. An informative technical overview of the architecture is available on the backend section (URL: ./IRAMFR/TA/backend/veleta/wilma/index.html) of our web pages.