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 37 or 117 pixels

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 telecope is now being operated under a new control system (NCS). Although allmost all features available under the old control system are already operational in the NCS, a small number of restrictions still hold. Most visibly, the only coordinate system fully supported by the NCS now is equatorial J2000.0, and we request observers to use this system for their source coordinates and offsets. Source offsets can be in the Equatorial J2000.0 system, projection``radio''; in the``true angle'' horizontal system; and in the Nasmyth system (receiver offsets). SET ANGLE UNIT other than [arcsec] is no yet supported. The ``Raster'' command is still not available. OTF, ONOFF or TRACKING (with frequency switching) commands together with SIC loops should be used instead. Observations with the rotated wobbler which are of interest for bolometer mapping, are in preparation. Remote observing is not yet implemented, so astronomers are asked to travel to the observatory, or in case of short and straightforward observations, consider service observing. The NCS team makes a strong effort to maintain a web page (../IRAMES/ncs30m) where the current status is described. The dual polarization HERA is operational together with its backends for high (VESPA) and low spectral resolution (WILMA, 4 MHz filters). Although tuning parameters are now available for a large range of frequencies, it is still recommended to send us HERA frequencies in advance. HERA observations will be organized as an observing pool similar to the bolometer observations. Like last semester, a bolometer array, preferentially the 117-channel MAMBO II, which should be used for observing time estimates, will be available.

Applications

On the official cover page, please fill in the line `special requirements' if you request either polarimetric observations or service 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 upon receiving, with proposals for heterodyne receivers, a complete list of frequencies corrected for source redshift (to 0.1 GHz) and precise positions. 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 equatorial 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^ATEX macro \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. For continuation of proposals having led to publications, please give references to the latter.

Reminders

A handbook (``The 30m Manual'') collects much of the information necessary to plan 30m telescope observations[6]. It is however outdated in many sections, and we recommend also to consult the NCS web pages (../IRAMES/ncs30m) 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/mainWiki/AstronomerOnDutySchedule) should be contacted well in advance 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 single pixel SIS receivers. This observing mode is interesting for observations of narrow lines where flat baselines are not essential, although the spectral baselines with HERA are among the best known in frequency switching. Certain limitations exist with respect to maximum frequency throw ($\le 45$ km/s), backends, phase times etc.; for a detailed report see [4]. This report also explains how to identify mesospheric lines which may easily be confused in some cases with genuine astronomical lines from cold clouds.

Observing time estimates

This matter needs special attention as a serious time underestimate may be considered as a sure sign of sloppy proposal preparation. We strongly recommend to use the web-based Time Estimator (URL: /IRAMES/obstime/time_estimator.html), whenever applicable. Versions 2.6 and higher handle heterodyne (single pixel and HERA) as well as bolometer observations with updated instrumental parameters. 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. The details of the procedures on which our time estimator is based are explained in a technical report published in the January 1995 issue² of the IRAM Newsletter [5]. 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 flux/temperature calibration is more uncertain than the typical 10 percent (possibly slightly more for bolometer observations). 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 previous semesters, we plan to pool the bolometer with other suitable proposals into a bolometer pool. HERA projects will be pooled with other less demanding project into a HERA pool. Both pools will be organized in several sessions, occupying a significant fraction of the totally available observing time. The proposals participating in the pools will be observed by IRAM staff, the PIs and Co-PIs of participating projects and other cooperating external astronomers. The pool observations will be organized by the pool coordinators, Stéphane Léon (bolometers) and Helmut Wiesemeyer (HERA). The participating proposals are grouped according to their demand on weather quality, and they get observed following the priorities assigned by the program committee. The organization of the bolometer observing pool is described at ../../../IRAMES/observing/flexible/flexible.html. A description of the HERA pool will follow soon. Bolometer and heterodyne proposals which are particularly weather tolerant qualify as backup for the pool. Participation in the pool is voluntary, and the respective box on the proposal form should be checked. Questions concerning the pool organization can be directed to the scheduler (thum@iram.fr) or the Pool Coordinators, Stéphane Léon (leon@iram.es) and Helmut Wiesemeyer (wiesemeyer@iram.es).

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 in 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, used to be 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. However, due to the transition to the telescope's new control system, remote observing will not be operational during several months. Observers are strongly encouraged either to consider service observing for short ($\leq8$ h) and simple proposals, to participate in a pool, or to come to the telescope.

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. Whenever possible, HERA observations will be pooled. The 9 dual-polarization 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 2005) 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 are listed in Tab. 1, and an updated user manual (version 1.9) 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. HERA observers are therefore advised to send a list of their frequencies to Granada at least 2 weeks ahead of their run. Recent observations have shown that the noise temperature of the pixels of the second polarization array may vary across the 1 GHz IF band. The highest noise occurs towards the band edges which are, unfortunately, picked up when HERA is connected with VESPA whose narrow observing band is located close to the lower edge of the 1 GHz band. Therefore, while not as important for wide band observations with centered IF band, the system noise in narrow mode is higher (factor 1.5 - 2) as compared to the first polarization array. We do not recommend to use the second polarization for frequencies $>241$GHz. HERA can be connected to three sets of backends:

$\rhd$

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.

HERA will be operational in two basic spectroscopic observing modes: (i) raster maps³ 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 HERA 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.

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 depicted in Tab. 1. Note that they cannot be combined with HERA nor with the bolometers. Also listed are typical system temperatures which apply to normal winter weather (4mm of water) at the center of the tuning range and at 45$^\circ$ elevation. All receivers are tuned by the operators from the control room. Experience shows that it normally takes not more than 15 min 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 pwv) and 45$^\circ$ 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	2
B230	H	1			4	197 - 266	95 - 160	12 - 17	4.0	1.0	420	2
C270	V		2		4	241 - 281	125 - 290	10 - 20	4.0	1.0	900	3
D270	H		2	3		241 - 281	130 - 300	9 - 13	4.0	1.0	900	3
HERA	H/V					210 - 276	110 - 380	$\sim 10$	4.0	1.0	400	2, 4

1: tuning range extended to $\geq 72$ GHz under special conditions (see text)
2: noise increasing with frequency
3: performance at $\nu<275$ GHz; noisier above 275 GHz.
4: the V-array of HERA has slighly higher noise which may vary across the IF band.

Extended tuning range: 72 - 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). Tests have shown that both 3mm receivers, A100 and B100 have good performance (good upper sideband rejection and system temperature) in the range 80 - 77 GHz. The receivers become increasingly double sideband below 77 GHz, until their behavior becomes erratic around 72 GHz. Due to the rapid variation of the image gain, special care must be exercised with calibration. A new image gain calibration tool is provided and described in the test report available on the IRAM web site (at ../../../IRAMFR/PV/veleta.htm). The report includes a set of reference spectra. Following the considerable demand for this frequency range in the last 2 semesters, the LO hardware has been simplified. As a result, observations in the 72 - 80 GHz range do not require any special arrangements. But note that the A230 (B230) receiver is not available when the A100 (B100) receiver is used below 80 GHz.

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 in the range 72 - 80 GHz or those near a limit of the tuning range, may still be problematic. In these cases, we recommend 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, designated 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, and 320/480. Although successful XPOL observations were made at many frequencies, experience is still limited, particularly at 1.3mm wavelength and with respect to observations of extended sources. Considerable progress was made in reducing polarization sidelobes, notably for Stokes V. Interested users should contact C. Thum for details. Data reduction software using CLASS enhanced with a graphical user interface is available (H. Wiesemeyer). A short guide (at ../../../IRAMFR/PV/veleta.htm) describes XPOL observations. Polarimetry proposals for observation of extended sources should demonstrate that their observations are feasible in the presence of the known sidelobes (see [16]).

MPIfR Bolometer arrays

The bolometer arrays, MAMBO-1 (37 pixels) and MAMBO-2 (117 pixels), are provided by the Max-Planck-Institut für Radioastronomie. They consist of concentric hexagonal rings of horns centered on the central horn. Spacing between horns is $\simeq 20''$. 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 both bolometers for onoff observations is $\sim 40$ mJys$^\frac{1}{2}$ and $\sim45$ mJys$^\frac{1}{2}$ for mapping. The rms, in mJy, of a MAMBO-2 map is typically

$$rms = 0.4 f \sqrt{v_{scan} \Delta s}$$

where $v_{scan}$, in arcsec/sec, is the velocity in the scanning direction and $\Delta s$, in arcsec, is the step size in the orthogonal direction. The factor $f$ is 1 (2) for sources of size $<30''\ (>60'')$. It is assumed that the map is made large enough that all beams cover the source. The sensitivities apply to bolometric conditions (stable atmosphere), ( $\tau(\small {250{\rm GHz}})\sim$ 0.3, 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. For those projects, only atmospheric conditions with low skynoise (i.e. stable atmosphere, no clouds, little turbulence) are recommened 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 below 0.4 mJy. In the mapping mode, the telescope is scanning in the direction of the wobbler throw (default: azimuth) in such a way that all pixels see the source once. A typical single map⁴ 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. Much more time is needed (see Time Estimator) if sky noise filtering cannot be used. The area actually scanned ( $8.0'\times6.5'$) must be larger than the map size (add the wobbler throw and the array size ($4'$), the source extent, and some allowance for baseline determination) if the EHK-algorithm is used to restore properly extended emission. 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. If maps with an rms $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}1$ mJy are proposed, the proposers should contact R. Zylka (zylka@iram.fr). The bolometers are used with the wobbling secondary mirror (wobbling at a rate of 2 Hz). 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 and the wobbler direction is fixed with respect to azimuth during a scan. Bolometer proposals participating in the pool have their observations (maps and ONOFFs) pre-reduced by a data quality monitor which runs scripts in 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, but cannot process data obtained with the NCS. 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 (quasi 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 affects positively 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. A variable and sometimes large contribution to the error beam was known to come from telescope astigmatism[3]. Extensive work during the last years had shown that the astigmatism resulted 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: 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.

Unnecessarily large wobbler throws should be avoided, since they introduce a loss of gain, particularly at the higher frequencies, and imply a loss of observing efficiency (more dead time).

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 inside the 500 MHz IF bandwidth, and connectable to two different single pixel receivers (must be set up in narrow band mode). VESPA, the versatile spectrometric and polarimetric array, 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.htm. 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. The wideband autocorrelator WILMA consists of 18 units. They can be connected to the 18 detectors of HERA. Each unit provides 512 spectral channels, spaced out by 2 MHz and thus covering a total bandwidth of 1 GHz. 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 at URL ../IRAMFR/TA/backend/veleta/wilma/index.htm. Note that WILMA cannot presently be connected to any of the single pixel receivers.