Spectroscopy: Preparing a Programme

Spectroscopy: 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 deal only with spectroscopy).

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).

Alternatively, 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…

TIP: Prepare ONE observation (one MSB), save this to the database, and ask your support scientist to check it over. You can then simply use copies of this MSB for your other targets. Changing one MSB is a lot easier than having to change a dozen or more!

The UIST Template Library

Start with a “Template MSB” from the template library (available template observations are described on a separate page; a table of DR recipes is also available). DON’T try and write your MSBs from scratch, and DON’T make huge changes to the template MSB without discussing these with your support scientist (obviously changing slits, grisms and using different offsets on sky are ok). Major structural changes are probably not necessary!

With this important point 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 UIST “folder” icon and open the folder labeled Spectroscopy. There you’ll find the available sequences for spectroscopy (plus some useful notes!). Examine those that may be of use to you by clicking on the button to the left of the icon (blue/pink square); the observations (calibration, standard and source) should be displayed (e.g. Fig.2). Click on any observation to unfold this as a “flow chart”; the elements within this observation are described below.

A Typical Spectroscopy Observation

In a nut-shell, a spectroscopy observation should comprise a flat, an arc and a sequence of “object” and “sky” exposures on 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 may be “nodded” up and down the slit so that the source is observed even in the “sky” frames. Subtraction of the sky frames from the object frames will remove OH line and thermal background emissions, giving positive and negative spectra which may be extracted separately and combined to give a spectrum of the source. An arc spectrum may be used 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 spectrum.

For an extended source, nodding to blank sky will probably be necessary. In this case only half of the data contain spectra from the target, though subtraction of the “sky” spectral images from the “source” spectral images will again yield an image largely free from OH sky lines. Sections of this spectral image may then be extracted and calibrated to give reduced data at different locations along the slit/across the extended target.

Moderately extended sources may be slid up and down the slit; here’s an example of +30arcsec and -30arcsec offsets relative to the centre of the array.

Flexible Scheduling and Minimum Schedulable Blocks:

All UKIRT observing are 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. 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 spectroscopy, an MSB usually includes a flat, arc, standard star observation and the target observation; in Fig.2a below the “opened” MSB is represented by the blue+pink square.

Flats, arcs, the UIST component and inheritance

In Fig.2a we show an example MSB for a point source. The programme currently only contains one MSB; many may be needed if multiple targets are to be observed. The “Point Source” MSB, now labeled “HK Spectrum of HH1”, 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 slit width, grism, etc.). This is achieved by placing the UIST component (the broken blue square) 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 (see Fig.2b).

The three observations in Fig.2a also inherit the standard star coordinates (from the Target component), although these coordinates are subsequently “overwritten” by the coordinates of the science star, HH1, in the third observation. (Note that, in Fig.2d, the science target observation contains a second Target component.) With this arrangement, the flat and arc will be observed (first) at the location of the standard star.

Fig.2a – the UIST component

Fig.2b – Flats and Arcs

Fig.2c – Imaging acquisition

Fig.2d – Flushing the array

Fig.2e – a longer expos time on target

Fig.2f – Repeats and Offsets

The Components of a Spectroscopy Observation

To understand the layout of a typical spectroscopy observation, consider the MSB in Figs.2a-2f. Each observation 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 (DR) Recipe. All three observations will inherit the UIST configuration from the components above them; the flat/arc and standard star observations will inherit the standard coordinates from above; the data reduction recipe is specified inside each observation.

  • The Target information component is used to enter the source coordinates. It may also be used to display a Digitised Sky Survey image of the target field, the instrument aperture size, offset positions on the sky 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, slit width, exposure time, position angle, etc. In Fig.2a 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 spectroscopy with an east-west, 4-pixel-wide slit. One 3sec exposure will be taken (with the default NDSTARE 1024×1024 readout) at each object and sky position (defined by repeats and offsets; see below).
  • The DRRecipe component allows you to select the recipe appropriate to your mode of observation, so that the DR can reduce the data on-line. An observation copied from the template library should already have the DR recipe set correctly. All object files obtained as part of this observation will be flagged with this recipe. In our example, the recipe STANDARD_STAR is used to reduce the standard star observations (Fig.2c), and FAINT_POINT_SOURCE is used to reduce the spectra of the science target, HH1 (Fig.2d).

Fig.2c shows the observation of the standard star. Recall that this inherits the UIST component and the standard star coordinates from above, so it only contains the DR recipe component. Below this DRRecipe component there is a “running man” icon or “iterator” labeled Sequence. Embedded “within” this Sequence iterator is the Spec/IFU Target Acquisition observation (an “eyeball”) and the actual spectroscopy Exposures (the Observe eye symbol), as well as a note and two nested iterators (more running-man symbols), labeled Repeat and Offset. These iterators are stacked much like “embedded do-loops” in a computer programme. With the setup in Figs.2c an object-sky-sky-object “quad”, defined by the offset iterator, will be repeated two times (specified by the repeat iterator). The offsets up and down the slit are set in the offset iterator by “p” and “q” parameters, q being along the slit and p being perpendicular to it, regardless of the slit position angle (Fig.2f). For point sources we recommend a 12 arcsecond slide up and down the slit, and an east-west slit position angle. The offsets can be changed (if a larger nod is required, say) by clicking on the offset iterator symbol. And of course, if only one quad is needed on the star, the repeat iterator can also be set to one.

The observation of the science target, HH1 (Figs. 2d-2f) is much the same as the observation of the standard described above EXCEPT for the optional “flush-array” Darks and the UIST Spec/IFU iterator used to change the exposure time for the science target. The HH1 observation also contains a target component which provides the coords of HH1 and a guide star.

The use of the darks to flush the array is described below (these are less important now that coadds have been implemented with imaging-acquisition). The UIST spec/ifu iterator is used to override the short exposure time used on the standard star (and defined in the “blue” UIST component inherited by this observation of HH1). In this case, 240secs will be used per exposure on HH1 (Fig.2e). Finally, the offsets and repeats (Fig.2f) are again used to move between the target and sky positions, and to repeat this “quad” of exposures to build up signal-to-noise. For HH1, 5 repeats with 240sec exposures would give a total of 20mins on the target. All of these data will be grouped together into a reduced spectral image by the DR, so that the observer knows just how great his or her final data will be …

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 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 put down the slit, in imaging acquisition mode. The TSS will do this at the telescope. However, the standard and science target observations must already include the “Target Acquisition” eyeball (e.g. Fig.2c and 2e). By clicking on this icon in the OT you can enter either 9-10th mag for the standard star, or an appropriate magnitude for your fainter science target. 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 20secs or more may be needed; either enter this exposure time directly or select a fainter source magnitude. Source acquisition is discussed further on a separate page. Beware of latency, however (see below) – for acquisition of 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 less than 1%. Because imaging acquisition involves taking images, often with long-ish 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 to some extent “subtract off” when skies are subtracted from object frames.

The problem can 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 available 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.

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. If you already have a project ID (e.g. u/03a/99) and password you may then store it to the telescope site (Database – store to telescope site). The programme can then 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 available here. A UKIRT Support Scientist is assigned to each project (Visitor or Queue-scheduled) to assist with the preparation of OT observations.

Choosing the best exposure time

In order to be background limited in the non-thermal regime (<2.3 microns), the sky noise must be greater than the array read noise (discussed earlier). Times needed to reach background-limited performance are listed in the section on sky counts. Narrower slits will obviously require longer, though beyond about 2.2 microns, the background increases markedly as the thermal background from the sky and telescope begin to increase, and the background-limited exposure time drops rapidly. However, for non-thermal spectroscopy of faint sources, long exposure times will be needed: 240sec exposure times are recommended for all sources fainter than about 9th magnitude, or 120sec if the OH sky lines are not being subtracted off too well (because of cirrus, say). At the end of the day, the longest “possible” exposure time should be used, given the caveats outlined below.

As an example, during commissioning the same star, Roque 25, was observed 4 times with the HK grism. On each occasion, the same total time (16 minutes) was spent on the target. However, this sixteen minute period was made up of individual exposures with different exposure times. The white spectrum in the figure below represents the co-addition of 48 10sec exposures; the individual exposures were not background limited and the source is barely detected. The blue spectrum, on the other hand, comprises just 2 240sec exposures. This time, the H and K-bands are easily discernible and a decent detection was attained (the red and green plots represent 16 30sec exposures and 4 120sec exposures).

Fig.3 The same 16-minute-total HK-grism spectrum of Roque 25, but using coadd x exposure times of 48x10sec (white), 16x30sec (red), 4x120sec (green) and 2x240sec (blue).

As already noted, in photometric conditions even longer exposures may be preferable, particularly in the J-band. The plot below is the (flat-fielded) difference of two 600 second exposures on a standard star. Note how well the sky lines subtract off, even after 20 minutes.

Fig.4 A spectral image using the old/bad IJ grism (since removed from UIST); the image shows the difference of two 600 second exposures, with the star slide along the slit.

The drawbacks to using long exposures include variations in the sky background and OH line intensities, OH line saturation (at H and K), and the increasing possibility of spikes on individual or small groups of detector pixels. If the critical wavelengths are well clear of OH lines (assuming sufficiently high spectral resolution is being used to allow observations between the OH lines), then the ~5-10 minute variation timescales of the OH lines may not cause concern (i.e. imperfect sky subtraction may not be a problem). But if a desired spectral feature is close to an OH line, and particularly if conditions are not photometric, then these variations can become problematic. In addition, long exposure times can lead to a lot of wasted telescope time if a fault should occur or a bad observation be loaded and executed.

For spectra obtained while nodding along the slit, subtraction of the negative spectrum from the positive spectrum will remove most of the sky and OH fluctuations because both vary slowly across the rows of the array. When observing faint and compact sources it is always advisable to nod a small number of rows along the slit, so that the cancellation of sky and OH residuals is as accurate as possible (you are also more likely to keep the source on the slit!). Remaining residuals can be removed with polyfitting techniques, using blank sky rows adjacent to the rows of interest (although doing this will increase the noise in the final spectrum a little).

Finally, note that when observing in between the OH lines at moderate spectral resolution (with the short-/long- grisms) it may be impossible to reach background limited performance. The lower-resolution HK grism, on the other hand, is background limited at “reasonable” exposure times at all wavelengths because an OH line is present in almost every resolution element. Background limited exposure times are listed here.

To check whether a particular emission line falls on or near a strong OH sky line, consult the Tables of OH Spectra compiled by Tom Geballe and Tom Kerr from CGS4 observations.