IFU: Preparing a Programme

IFU: Preparing a Programme

Preparing an Observing Programme: the UKIRT-OT

FIRST TIME USERS: Please read the General Introduction to the OMP before reading the notes below (which are specific only to the IFU). If after doing this these notes are still as clear as mud, try the long-slit spectroscopy page on programme preparation.

Your complete observing programme can be prepared either in Hilo or before you arrive in Hawaii from your home institute (provided you have access to the ukirt-ot). From any Unix or Linux box in Hilo (or at the summit) type ukirtot to run-up the observing tool (the OT).

Click this to runup the ukirt-OTAlternatively, on KAUWA at UKIRT just click on this icon on the tool-bar at the bottom of the screen.

A small window will appear (containing a photo of UKIRT) in addition to the copyright notice window; you may use the former to open existing programmes, create new programmes or access the database. If you’re new to ORAC, close the copyright box and read on…

The UIST Template Library

The expectation is that most users will work from “Template observations” (available Template observations are described on a separate page – a table of DR recipes is also available). The “Template Library” contains observations that can be modified to suit your specific needs (see Fig.1). It is probably unwise to try and write an observation completely from scratch. Thus, with this in mind, open the Template Library by selecting this option from the menu under “File” (top-left corner of the small “UKIRT” window). At the same time, create a new programme by selecting this option from the same (File) menu. After a few moments, two Programme windows – like the one shown in Fig.1 – will appear.

In the template library, click on the button to the left of the “folder” icon labelled IFU templates. There you’ll find the available observations or “sequences” for IFU spectroscopy. Examine those that may be of use to you by clicking on the button to the left of the blue/pink icon; the component observations should be displayed as a flow chart, as in the example below.

Like ordinary spectroscopy, an IFU observation should comprise a flat, an arc and a sequence of “object” and “sky” exposures of a standard star, followed by a similar sequence of object/sky frames on the target itself. The example below contains all of these components.

Point sources should be “nodded” in and out of the IFU field of view, unless the seeing is very good, in which case it may be possible to nod the star between the top and the bottom half of the elongated IFU field. Subtraction of the sky frames from the object frames will remove OH line and thermal background emissions, giving a series of spectra in adjacent slices in the scrunched spectral image. An arc spectrum will be used by the DR to accurately wavelength-calibrate the data, and a similarly-reduced standard star spectrum can be used to divide out atmospheric absorption bands and flux-calibrate the source spectra. The DR will then “chop up” the scrunched spectral image and create a data cube (see the ORAC-DR pages for full details).

For an extended source, nodding to blank sky is likewise necessary. A larger nod (than the default in the template observation) may be necessary.

Flexible Scheduling and Minimum Schedulable Blocks:

From semester 03A onwards all UKIRT observing will be flexibly-scheduled. Consequently, observations must be grouped within “Minimum schedulable blocks”, or MSBs. An MSB represents the minimum amount of data that needs to be obtained for an observation to be useful. In the OT, an MSB is represented by a blue and pink cube. You or indeed any other observer will then be equipped to properly observe one or more of your targets, simply by executing everything in the MSB. For IFU spectroscopy, an MSB usually includes flat, arc, standard star and target observations.

Flats, arcs, standards…

In Fig.1 the “IFU, Nod to blank sky” MSB has been opened; it contains a Calibration (flat and arc) observation , a Bright (standard) star observation and a Science target observation. UIST must be set up in exactly the same way for the flat and arc as for the standard and target observations (i.e. same position angle, grism, etc.). This is achieved by placing the UIST component (the broken blue square labelled UIST in Fig.1) above the three observations. The observations then “inherit” the UIST component parameters; the slit width, position angle, grism, etc. Only the exposure time is changed in each observation, as described below. The flat and arc have default exposure times, set by clicking “Use defaults” in the flat and arc observations.

The UIST instrument component and Target component are described further below.

Fig.1 An IFU Observation in the template library (click for a full-sized image).

The components of an IFU observation

Each observation (the blue squares) needs three components (the “broken” blue squares), which specify the Target information (target and guide star coordinates), the UIST instrument configuration and the Data Reduction Recipe (DRRecipe). These can be contained “within” each observation, or they can be “inherited”. In Fig.1, for example, all three observations inherit the UIST configuration from the component above them. The flat/arc and standard star observations also inherit the standard star coordinates (so that the flat and arc are observed at the location of the standard). The science target observation contains its own coordinate information, which overrides the inherited standard star information (see Fig.2 below, where the target is called HH2). The data reduction recipes, to be used to reduce the calibration, standard and science target data, are then specified individually inside each observation (see e.g. Fig.2).

  • The Target information component is used to enter the standard star or science target coordinates. It may also be used to display a Digitised Sky Survey image of the target field, the instrument aperture size, and various guide-star catalogues (see this ORAC-OMP Guide for a comprehensive description of this tool).
  • The UIST instrument component is used to select grism, exposure time, position angle, etc. Here the IFU mode is also selected. In Fig.2 the UIST component is highlighted, so that the UIST configuration is displayed on the right half of the window: in this case, UIST has been set for HK IFU spectroscopy with the IFU’s major (6-arcsecond long) axis orientated east-west. One 7sec exposure – appropriate for the standard star – will be taken with the default NDSTARE 1024×1024 readout area.
  • The DRRecipe component allows you to select the recipe appropriate to the mode of observation, so that the DR can reduce your data on-line. An observation copied from the template library should already have the DR recipe set correctly, so these shouldn’t need changing. All object files obtained as part of this observation will be flagged with this recipe.
Fig.2 An IFU MSB showing the components of the science target observation.

Below the DRRecipe component in Fig.2 there is a “running man” icon or “iterator” labelled Sequence. Embedded “within” this Sequence iterator is the Imaging Acquisition observation, five optional short darks (used to flush the array after acquisition), then a UIST IFU/Spec iterator, a Repeat iterator and the actual Offset iterators (more running-man symbols) which nod the telescope between the object and sky positions. The “eye” symbols are the actual exposures at each position. IMPORTANT: the observes (the eye symbols) must be labelled as “Observe” and “Sky” (as they are in the above example, and in the template library) for the IFU DR to work properly.

After source acquisition and array “flushing” (both discussed below), the UIST Spec/IFU iterator changes the exposure time. This iterator is needed because the UIST component, inherited by the observation of HH2, is set up for 7sec exposures on the standard. Longer exposures will probably be needed on the science target. (By selecting a source magnitude, a sensible exposure time will be set.) The iterators below “UIST IFU/Spec” are then stacked much like “embedded do-loops” in a computer programme. With the setup in Figs.2 an object-sky-sky-object “quad”, defined by the offset iterators, will be repeated five times (specified by the repeat iterator) to build up signal-to-noise on HH2. The offsets are set in the offset iterators by “p” and “q” parameters, q being along the long (6″) axis of the IFU, with p being perpendicular to it, regardless of the IFU position angle.

The standard star observation will be essentially the same as that shown above, except that the “UIST IFU/Spec” iterator is not needed, because the exposure time has already been defined in the UIST component (broken square) higher up. Also, fewer repeats may be required.

Finally, to view the whole sequence of observations (telescope moves and filter changes) written as a simple text list click on the Sequence “running-man” icon and hit “show”.

IMPORTANT: If you change the wavelength (or anything for that matter) in the UIST component, you must click on “Use default” in the FLAT, ARC and the flush-array DARK. This ensures that these observations pick up the changes made in the UIST component. Remember, though, to set the flush-array dark exposure time back to 1 or a few seconds (and 1 co-add) – you don’t want to be taking lengthy darks to flush the array.

Imaging acquisition

For both the bright standard and faint science target the source will be “acquired”, or centred in the 3.3″x6.0″ IFU field-of-view, in imaging acquisition mode. The TSS will do this for you. However, the standard and science target observations must include the “Spec/IFU Target Acquisition” eyeball (Fig.2) for this to be possible.

Exposure times for acquisition: By clicking on the acquisition “eyeball” in the OT, you can enter either 9-10th mag for the standard star or a higher magnitude for a fainter science target. An appropriate exposure time and number of coadds will then be automatically set. For the bright standard the shortest possible exposure time must be used (9-10th gives the minimum 1sec full-array readout). For a faint science target a total of 10 or 20secs may be needed. Source acquisition is discussed further in the long-slit spectroscopy pages. Beware of latency, however (see below) – on faint targets use short exposure times and a few coadds (e.g. 4 x 5sec for the HK grism) rather than one long exposure (1 x 20sec).

Image Latency

UIST suffers from image latency, i.e. residual signal (like dark current) at the less than 1% level. Because imaging acquisition involves taking images, often with longish exposures through a very broad spectral blocking filter, this can leave some residual sky signal on subsequent frames. Likewise, if a bright star is observed in acquisition, there may be a residual (weak) image of the star in the next frame or two. This latent signal gets weaker with time, and it should “subtract off” when skies are subtracted from object frames.

The problem can (to some extent) be avoided by using short exposures and a few coadds for imaging acquisition, rather than one long exposure. The penalty is readout overheads, specifically 1-2 seconds per coadd. We recommend using three or four 5sec exposures for faint targets (one 1sec exposure for a bright target). However, even with these short exposures, residual signal from imaging acquisition could still introduce additional noise to the first few frames taken directly after imaging acquisition. Consequently, it may be a good idea to “flush” the array, by taking a few short darks after imaging acquisition, and before taking a first long (perhaps two or three-hundred second) spectrum of the science target. The optional flush darks are avaiulable for this purpose; they are potentially useful for very faint targets and/or long spectroscopy exposure times and/or short wavelengths, although their usefulness is limited – the residual signal fades with time, not the number of read-outs.

Exposure times

Generally, the “longest possible” exposure time will give the best performance. Background-limited performance with most of the UIST grisms requires long exposure times. With the HK grism this is a few minutes in the K-band and 5-10 minutes in the H-band; with the short-K grism 5-10 minutes is also required (depending on wavelength and whether an emission line coincides with a sky line or not). Note, however, that OH sky-line intensities will vary on these time scales, so the importance of perfect sky subtraction may limit the exposure time you can use (as well as saturation on the source, of course). For recommendations on exposure times please have a look at the section on preparing observations for UIST spectroscopy.

Saving and Storing your handy-work…

When preparing MSBs, keep saving the file to disk: click on “File – Save As” at the top-left corner of the programme window. Once the programme is complete, save it to disk one last time. You may then store it to the telescope site (Database – store to telescope site), using your project ID (e.g. u/03a/99) and password (received through email). The programme can later be retrieved from the database at the summit and your observation executed.

HOT TIP: Set up one MSB – for just one flat/arc, standard and science target, say – then send this to your Support Scientist. He or she will check it over. In most cases, you can then simply copy this MSB “n” times and just change the coordinates of the standards and science targets.

The above discussion and example is of course meant only as a brief guide. A more comprehensive guide to the OMP, the OT and flexible scheduling in general is avaliable here. A UKIRT Support Scientist is assigned to each project (Visitor or Queue-scheduled) to assist with the preparation of OT observations.

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