Preparing for CGS4 observations with the ORAC-OT

Preparing for CGS4 observations with the ORAC-OT

This document overviews the preperation phase of observing with CGS4 under ORAC. This mainly involves the use of the ORAC-OT – the Observing tool. Whilst you will no doubt be running the OT whilst you are observing to make any last minute changes, you should have the bulk of your preperation done before arriving at the telescope.

General ORAC-OT introduction

The general ORAC-OT documentation overviews the various concepts of the ORAC system, and makes a good introduction to this more specific document on how to use the ORAC-OT to prepare for CGS4 observing.

Getting started with the ORAC-OT

First up, launch the oracot and open up the “UKIRT Template Library” from the file menu. Also in the File menu, select “new program” to create yourself an empty program.

Now a trivial example of editing soemthing: In the new program window that just appeared, double click on where it says “Science Program”. An edit window appears immediately to the right of your science program window. Type in a title for this science program into the box labeled “Title”. Ignore the other fields.

Basically, you prepare your science program by copying items from the template library, and editing them to your needs. In the template library, open the CGS4 folder by clicking on the little handle icon to it’s left – the handle twists down and you can see the contents of the section. You should see a list of template cgs4 observations, looking somewhat like this:

Now, you’re going to want to do the array tests at the start of your observing, so copy the “array tests” observation into your science program. To do this, put the mouse over the array tests, press and hold the left mouse button and move the mouse into your science program window. Now the OT is a bit picky about exactly where you drop stuff. Move the mouse (still dragging the little blue square observation icon”) over the title or icon of your science program. As you go over this, you’ll notice a little red and white diagonal arrow sign appear over your science program icon. This indicates that if you drop it there, the array tests observation will go into your science program, which is what you want, so go ahead and release the mouse button. You should now have something like this:

Adding a Flat and Arc

Next, you will want to take a flat field and arc lamp observation. Drag over the “flat and an arc” observation from the template library. This time, when you drop it, you’ll want to have the pointer over your array tests, and you should see a white and red arrow sign, with the arrow pointing straight down.

A note on the little arrow signs: The arrow pointing straight down indicates that when dropped, the new item will be placed below the one where the arrow appears. A diagnoally down/right arrow indicates that the idem will appear inside the one where the arrow appears. We’ll see more of this later.

At this point, you might like to change the title of the flat and arc, for example to add a comment about which band this is. Do this in the same way that you edited the title for your science progam.

Now, we’ll look inside the flat and arc at the components that make it up. Click in the little handle next to “flat and an arc” to expand the view. Notice how things inside other things get indented to the right (inside the flat and arc observation are four components, one of which is a “Sequence” component, itself containing more items). Whenever something contains things inside it, it has a little handle sign to it’s left, demoting whether it’s being displayed expanded (handle vertical) or condensed (handle horizontal). Simply click the handle to toggle this. Things should now look like this:

The “CGS4” component contains the instrument configuration for CGS4. Double click on it to bring up the CGS4 configuration editor into the right hand panel of the window:

Edit any of the values as necessary. Don’t worry about the exposure time and coadds that you set here, as these ones aren’t actually used – the exposure times for the flat and arc are set specifically, as we’ll get to shortly. When you’re happy with the values, click on the “Flat” component inside the sequence. You should get a flat component editor looking like this:

Note carefully: The parameters for the flat field observation don’t automatically update if you edit the fields in the CGS4 configuration component. To have the software estimate the flat field parameters based on what’s set in the CGS4 configuration component, click the “Default” button in the flat field editor. If you then go and change the settings in the CGS4 component, you’ll need to go back into the flat field and hit default again. If after taking a flat, you find that the default values aren’t quite right, then you can simply edit the exposure time etc directly in the flat field component.

Now go into the Arc component, and hit the default button to get the default arc parameters for the configuration you’ve selected.

Adding a Standard Star

We have a library of bright G stars built availiable in the OT to speed things up here. Don’t worry if you want to use a star that’s not in there, we’ll get back to that shortly.

Open up the CGS4_bright_G_library from the file menu of the main OT window. Simply pick a star, then drag it into your science program, taking care when you drop it that you’ve got a vertically downward pointing red and white arrow sign on the flat and arc observation. You might want to collapse the display of the flat and arc first, by clicking the little minus sign to the left of the “flat and an arc” label.

Once you’ve got a star there, you need to edit that observation to match the configuration you’re actually using. There’s an easy way to do this. First expand the flat and arc observation and the standard star observation so that you can see all their components. You don’t need the details of the sequence at this point, so you can collapse the sequences in each to save screen space if you like. Now double click the cgs4 component in the standard star, and note the magnitude of the star. Now, single-click the CGS4 component in the flat and arc observation, and click the copy button (the one on the right of the scissors (cut) button). Now single click the CGS4 component in the standard star, and hit the paste button (the one to the right of the copy button – looks like a clipboard). When it asks if you want to replace the component, say yes. Now double click the CGS4 component in the standard star to get the editor window back, and set the magnitude back to what it was. The software will then autmatically set the exposure time and co-adds for that magnitude. you can over-ride these manually by simply changing the values. We’re still working on the defaults so you might have to do this. The best thing to do is to ask your TO for the count rate after they complete the peakup, then set the time to give you around 3000 counts.

Adding a Target observation

First of all, you have to decide which of the observations in the template library is most appropriate for your target. For point sources, this is probably point source, nod along slit. For extended sources, you’ll want extended source, nod to blank sky. If you are in doubt as to which sequence to use, ask your support scientist or contact the CGS4 instrument scientist for advice.Drag the one you want from the template library into your science program. As an example here, we’ll use the point source, nod along slit one. Drop it so as to appear after your standard star.

Now, in your science program, double click on where it says “point source, not along slit” and in the editor panel that pops up, type in a title for this observation. This isn’t used as the target name or anything, it’s for your own reference, and should be reasonaby descriptive as you will choose this name from a list of your observations to specify that this is the one you want to observe.

Setting the target co-ordinates etc

Now click the little handle next to your target observation to expand the view, and click on the “Target List” component. In the panel that appears, fill in your target name (this is what will go into the data headers etc) and the RA and Dec of your target. If you wish, you can enter a target name and click the “Resolve” button to have the OT query the target name on SIMBAD and get the co-ordinates from there. If you want the name resolver to query an different database, simply select it in the pull down to the left of the resolve button.

Next, we’ll go into the plot window to check things and to set a guide star etc. Click on the “Plot” button in the lower left of the target editor window. In the window that appears, go into the “Calalog” menu and under “Image Servers”, select “Digitised Sky at JAC”. After a pause and some flashing lights, a digitised sky survey image of the sky area you’re looking at should fill in. To the lower left of the image are a set of togle buttons, collectively labeled “View”. Turn on the ones for “Sci Area” – shows the slit, “Dichroic” and “X-head” – the latter two between them show you the area of sky from which you can select a guide star. Now go to the “Catalog” menu again and under “Catalogs”, select “Guide star catalog at CADC”. When the progres box disapears, a list of HST guide stars will have been loaded. Turn on the “Catalog” view button, and these will be highlighted on the sky survey image. To manually inspect the list of stars, select “Browse” from the “Catalog” menu.

To the left of the window, there’s a selection of buttons labeled “Mode”. Click on the one labeled “GUIDE”, now point and click on the star that you wish to use as a guide star. If you select one of the ones from the catalog, the positions from the calalog will be used. If you prefer, you can just click on a star at any position on the sky survey image. If you’re doing this, then you might want to use the zoom facility to give you more accuracy. You must select a star that is within the purple X-head circles (and avoid ones close to the edge as you’re going to be nodding) and not hidden by the green dichroic edges (again, and not too close).The position editor window should now look something like this:

Returning to the target editor panel in your science program window, you’ll notice that it has filled in the guide star co-ordinates you chose, and if you edited your position using the “drag” facility, the co-ordinates have updated.

Setting Exposure times etc

Next, go back to the flat and arc observation and copy the CGS4 component again. Replace the one in your target observation with this, then go into it and set the magnitude or exposure time that you wish to use.

Note: The BL option in the magnitude pull down selects the minimum time necessary to become background limited. You might want to observe for longer than this, but bear in mind that the time between nods equalls this time multiplied by the coadds and the sampling. See the CGS4 handbook for more details.

The sequence

The only thing you’ll usually wish to change in the sequence is the number of repeats. The “Offset” itterator executes a 4 position jitter (Object – Sky – Sky – Object). The repeat iterator sets how many times to repeat this sequence. Simply double click the repeat iterator and set the number.

Data Reduction Recipies for CGS4

In general, if you based your observations on the template library, the DR-RECIPE component will be all set up for you, and you won’t need to chnage anything.

The ORAC-DR section of the CGS4 manual provides more info on what the individual recipies do and the observing sequences they are appropriate for. If you are unsure or if you think you will need a recipe that is not provided in our standard selection, you should contact your support scientist or the CGS4 instrument scientist well in advance of your observing run.