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Table of Content

This document has the following sections:

• Introduction
• Read the Manuals!
• Additional Requirements for ISAAC Phase II
• ISAAC OB Recommendations
  • Moon Constraints
  • Overheads and Short Integrations
  • Telescope Offsets
  • Maximum number of filters in one OB
  • Bright Sources (Wavelengths below 2.5 microns)
  • Bright Source (Wavelength above 2.5 microns)
  • Spectroscopy: Target acquisition
  • Spectroscopy: Nighttime calibration
  • Burst mode observations

Introduction

This page contains information specific to the creation of ISAAC Observations Blocks (OBs) for Service Mode programs. For more general OB creation information and rules, see the P2PP page or the Service Mode Guidelines for Phase 2 preparation.

Requirement Compliance Policy: Observing runs which do not adhere to procedures and policies presented in this document will not be scheduled for execution. If you feel you must violate one of these procedures or policies, you must submit a Phase 2 Waiver Request before submitting any Phase 2 material.

Read the Manuals!

• ISAAC User's Manual

• P2PP User's Manual

It is also useful to check the ISAAC Web pages for recent updates and the ISAAC Data Reduction Guide and the ISAAC Cook Book for reduction tips.

A tutorial on the use of P2PP to prepare ISAAC observations is also available.

Additional Requirements for ISAAC README Files

In addition to the general README file requirements, the following recommendations should also be followed:

• Justification for short integrations (DITs) - for the Hawaii arm only.
• A justification must be provided in the README, if the brightness of the brightest object in a field, including standard stars, exceeds the limits given below.
• Justification for obtaining night time flat fields and / or arcs for spectroscopic observations - for both the Hawaii and Aladdin arms.

ISAAC OB Recommendations

Moon constraints

The Moon does not affect IR observations in broad band imaging, most of narrow band imaging, Low Resolution spectroscopy, and in Medium Resolution Spectroscopy above 2.0 microns. Only with some Narrow Band filters (e.g. NB_1.06, NB_1.19) and with Medium Resolution Spectroscopy of faint features between the OH lines below 2.0 microns can the Moon degrade the performance. In these cases, reducing the FLI constraint to approximately 0.7 and increasing the distance to the Moon to approximately 50 degrees is generally adequate. Even here, it is important not to over-specify the constraints, as this reduces the chances of the Observing Block being executed.

Overheads and Short Integrations (Hawaii SW only)

In addition to telescope presets, field acquisition and telescope offsets, operational overheads within an observation template are directly related to DIT, the Detector Integration Time (see the ISAAC User Manual). The smaller the DIT, the higher the overheads. Resorting to the minimum DIT of 3.55s (Hawaii array) results in overheads exceeding 100%, which is difficult to accept. Users are requested to use the DITs recommended in the User Manual, and to avoid using the minimum DIT. Use of the minimum DIT must be requested by submitting a Phase 2 Waiver Request. ESO reserves the right to modify the DIT/NDIT values should this justification be missing or weak, or to lower the overall priority of the OB in question.

Minimum time between telescope offsets (Hawaii and Aladdin arm)

Maximum number of filters in one OB

Bright sources (Wavelengths below 2.5 microns)

Brightness limits: Imaging

Observations involving fields with objects brighter than 11th magnitude (BB imaging) or 8th magnitude (NB imaging) cannot be guaranteed in Service Mode and in shared visitor nights. ESO reserves the right to lower the overall priority of the OB in question in service, and not to execute the observations in shared visitor nights.

Requests for imaging observations not compliant with these limits must be submitted as a Phase 2 Waiver Request. If judged acceptable, ESO will try to devise operational strategies (e.g. observations at the end of the night, scheduling other imaging OBs after the observations in question).

Brightness limits: Spectroscopy

For spectroscopic acquisition of bright targets, the following filter settings must be used. Note that this applies not only to standard stars, but also to other bright objects in the field of view.

Important note: when the bright object in the field of view is not the science target, the target may become too faint to centre on slit due to the use of the Narrow Band filter(s). In this case (see section 'Target acquisition' below), offsets from a reference star should be used.

Bright Sources (Wavelengths above 2.5 microns)

Although the Aladdin detector is less sensitive to the effects of bright targets, there are limits to what is acceptable at wavelengths above 2.5 microns. For broad band filters, the limit is 4th magnitude in the IR. For narrow band filters, the limit is 1st magnitude in the IR. Targets brighter than these limits will not be observed. The magnitude of the brightest object in the field, including standard stars, must be indicated in the "Instrument comments" field in each OB.

Spectroscopy: Target acquisition (Hawaii arm)

OBs for which target acquisition cannot be completed within a few minutes of time will not be executed. Acquisition has to rely on either science targets or reference objects brighter than approximately 17-18th magnitude in the IR, when the acquisition is done with Broad Band filters. Exceptions will be tolerated for moving targets, and special situations to be evaluated on a case by case basis.

When the science target is fainter than the above quoted magnitude, the procedure for acquisition should rely on reference objects which are brighter than this limit. These reference objects can either be positioned in the slit together with the target by defining the appropriate position angle on sky (the preferred procedure), or be used for initial centering on slit, followed by a blind offset to move the target into the slit.

These reference objects should be stars or point-like objects. Blind offsets from a reference object should be limited to approximately 1 arcminute. Offsets that are too large could result in the TCS selecting a new guide star, and poorer acquisition accuracy. Experience has shown that the offsets from the reference star are often non-accurately defined. For instance, if the offsets are computed from a previously taken ISAAC image, the distortion at the edge of the field can affect the reliability of the offsets if a constant plate scale was assumed. Also, users tend to choose the brightest object in the field, which can then be far from the target, although very often there are fainter reference targets, still bright enough to satisfy the limits mentioned above, that would have been much better choices. Users should try to use reference objects as close as possible to the target, preferably as aligned as possible with the slit, to minimize offset measurement errors, rather than trying to use the brightest reference target.

To the extent this is possible, it is recommended to position a reference object in the slit together with the target, as this allows to control the position of the target on the spectral image and to monitor the flux through the slit, etc.

If you elect to use blind offsets, you must define them in the appropriate MoveToSlit template. The finding chart must clearly indicate the reference and science targets and the position of the slit .

Spectroscopy: Night time flat fields and arcs

Due to non-reproducibility effects involving the grating and the slit, there are usually slight differences in the position of the grating and the slit when flats and arcs are taken, which is usually the following day. This can limit the accuracy at which spectroscopic data can be flatfielded and wavelength calibrated. To circumvent this, special templates have been created to allow flat fields or arcs to be taken at the end of the spectroscopic templates (ISAACLW_spec_cal_NightCalib for the Aladdin arm, ISAACSW_spec_cal_NightCalib for the Hawaii arm). If used, these templates must be attached at the end of spectroscopic OBs.

It is believed that the flat field non-reproducibility problem only affects observations at high signal-to-noise ratio (S/N >100). It is therefore recommended that users use the night time calibration templates to take flats immediately after their observations if they want high signal-to-noise data. One should also take night time flats for the telluric standards. As this is not part of the ISAAC calibration plan users will have to provide the appropriate OBs. To get the highest signal-to-noise ratio one should set the nod throws of the telluric standard and the science target to be the same.

Observers who do not wish to obtain such high signal-to-noise data (this applies to most observations done with ISAAC) can safely ignore the night time flat field calibrations.

Requesting night time arcs is usually not necessary for observations below 2.2 microns, since the OH lines provide an in situ wavelength calibration. Above 2.2 microns there are few OH lines, so we recommend that users requiring accurate wavelength calibration in the SWS1-MR mode above 2.2 microns use the night time calibration templates. Alternatively one can use the many telluric features as an in situ wavelength calibration, or as a means of determining the wavelength offset for observations calibrated with the daytime arcs.

Little experience has been obtained with similar problems in the Aladdin arm. However, the same template has been created for the Aladdin arm, in case users feel they need accurate flat fields or arcs. Note that the arcs should usually not be necessary, since the sky leaves plenty of telluric features for wavelength calibration. In the M band, it is not possible to do accurate wavelength calibration with the arc spectra. Although we will provide an arc that is taken with the grating in third order, the telluric features should prove to be more accurate.

In all cases, the use of the ISAACLW_spec_cal_NightCalib or ISAACSW_spec_cal_NightCalib templates should be justified in the README file. ESO reserves the right not to execute these night time calibrations if they are not properly justified.

Burst mode observations

The Burst and FastJitter modes are offered both in VM and in SM. However, in the case of occultations, only disappearances are offered in SM. VM must be requested in the case of appearances.

Limitations:
-- The data cube can contain a maximum of 32000 planes (frames), i.e. the maximum NDIT is 16000 in Burst mode and 32000 in Fast Jitter mode.
-- The maximum data cube size is 262Mb. Once the window size has been selected, this limits the number of frames/reads and vice versa. The data cube size in b is given by the relation: Xpix*Ypix*4*NDIT. Table 26 of the ISAAC User's Manual reports some conservative upper limits for the data cube size. Data cubes too close in size to the limit of 262Mb can results in the loss of few frames (NDIT).

Be aware of the naming conventions for the BURST/FastJitter mode OBs:

FastJitter OBs (BURST=F) should start with the prefix "FAST" in their name

Burst OBs (BURST=T) which do not make use of the EVENT keywords (EVENT.DATE=0 and EVENT.TIME=0) should start with the prefix "BURST" in their name

Burst OBs (BURST=T) which make use of the EVENT keywords (EVENT.DATE=YYMMDD and EVENT.TIME=HHMMSS) need to include the time at which the science template (not the acquisition!) should start, i.e. the UT time of the EVENT time minus half the total exposure time. For example, let's assume that you are exposing for 30 sec in total and let's assume that your event occurs at UT date YYMMDD and UT time HHMMSS, then, your OB name should include the following prefix: BURSTUTYYMMDDHHMMss, where ss=SS-30/2=SS-15.

Send comments to <usd-help@eso.org> Last update: 20 Dec 2007