Updated on June 15, 2021, 9:19 am
This is a quick-start guide to using Hydra for the new observer, or for the old observer who needs a checklist.
Hydra is a robotic fiber multi-object spectrograph: 132 fibers are positioned on a steel plate lying at the Cassegrain focus of the 4m Blanco telescope. At f/8 the plate covers an area of 40' on the sky. In addition to the normal fibers there are 12 fiber bundles that are used to `image' guide stars. The purpose of the guide bundles is to:
(i) relate the astrometric positions of the targets on the guide to the real positions on the sky and
(ii) guide during exposure. It is thus *essential* that guide stars and targets come from the same catalog so that their relative astrometry is good to at least 0.3'' (20% of the fiber diameter).
The fibers then lead to a bench spectrograph where they are reimaged onto a slit and then a CCD after reflection through a grating. Moderate resolution (~a few thousand) to echelle spectroscopy (at R > 15,000) can be performed on Hydra (single order only). Slit masks are provided (with considerable loss of light) to increase the resolution to a potential 45,000 in echelle mode.
The robotic file positioner needs a 'configuration' file that tells it what moves to execute to place fibers on to your objects. Do not forget to assign a number of fibers to blank sky positions to measure and remove the sky. The number of fibers to be placed on the sky depends on your program, but 12 is a good number for most uses.
You can assign fibers using the program `hydrassign' and use the Hydra emulator to see what the fiber configuration looks like. Both programs are available on the CTIO web site.
If you need first night run assistance (because you never used Hydra before, or you are rusty), email the Hydra scientist. It is often possible to snare an extra half-night of data taking (and instruction) during the engineering night that takes place at the start of each Hydra block. Consider also coming up a day in advance to see the previous observer using Hydra.
When you arrive at the telescope, the Hydra station shows two screens. The one on your left has a blue window and a maroon window. The blue window is the Arcon controller, and is used for data taking. The maroon window is an IRAF window and is used for quick inspection of the data. More on this below.
On the other screen, the fiber positioner is actually controlled. When you come up in the afternoon (after lunch) the fibers should be in a `large circle' position. This is the position which you need for taking calibrations. See later (`Observing') for how to position fibers.
1. You need biases. Making sure the dome is dark, go to the Arcon window. Check that the path (>pwd) is set to place data in your directory. There should be a directory called /xxxx/xxxx/yournamehere/n?. If not, cd to that directory and type cl> set path=HDR$. Go into obspars (type as IRAF command) and change what's needed (if it is not already set).
To take biases type cl> zero. Arcon will ask how many and start working. It takes about 3 minutes to read out the data. Unfortunately, Arcon produces .imh files only.
2. Once the biases are done you want dome flats. These are the main flats needed for data reduction. Ask the staff to point the telescope at the white spot and turn on the lamps. Take a short exposure. Use `implot' in the IRAF window to check the counts. Take a number of dome flats with the appropriate exposure. The command is cl > dflat in the Arcon window. Note that rather long exposures (~20-30 minutes) are needed for a good flat in echelle mode.
3. You may want to take some arcs too. The command is > comp. Usually 10s are more than enough, but this depends on resolution. You may want to have a quick look at the spectra to make sure that the wavelength region is correctly set up (particularly in echelle mode, where the free spectral range of the order exceeds the CCD size). See below on a quick way to do this.
4. Sky flats. These are not needed. However, for throughput calibration, you may wish to take a `milky flat'. This is a flat taken with the dome open and the diffuser screen in. The staff will set this up for you. Once they are ready the command is > sflat. Exposure of 1 or 2 seconds sufficies. This is usually taken in the afternoon. Only one is needed per run and wavelength setting.
5. Darks. These are unnecessary. Check that there is no light leak (usually in the left-hand corners) of the chip in your dome flats. If you see one, tell the staff, as they need to cover the LEDs in the Arcon box better if this is the case.
You are now ready to configure your field. First thing: always do this at zenith. Never attempt to configure anywhere else. Second thing: before configuring any field you need to park all the fibers. In the Hydra GUI go to File > Run Script. Select 'configparkall' and accept. The sets of scripts also include a 'configlargecircle' option to place your fibers in a large circle.
Once you are parked go to the Hydra GUI again. Select File > Setup Field. The dialog box that pops out will ask you for
(a) the name of your configuration file;
(b) the observing period and
(c) the ST at midpoint.
In order to calculate this look up the ST when you start, add 30 min. for configuring and setup and 1/2 of the exposure time. Start configuring. A dialog box may pop out claiming that some fibers cannot be assigned. The reason for this is that although your configuration file is valid, there may be slight differences in time that lead to a potential fiber collisions or exceed the maximum angle of the fiber or its extension.
When the configuration is finished 'Park' the gripper. Ask the TA to go to the field. Move to the `Arcon' window and `type' settemp. This sets the temperature for focussing operations. See below for focussing operations (the first thing that happens during the night, after the telescope pointing is checked). Focussing is automatic after the zeropoint is set at the beginning of each night.
Once you are at your field you may want to take an arc and possibly a projector flat. This produces an image of the fibers in your configuration and may be useful for data reduction (it makes extraction of the fibers easier). To get an arc type > comp. To get a projector flat type > pflat. Once you are done, go back to the Hydra GUI. Click on `Lamps' and select 'Park' to put the lamps out of the optical train. Activate the Gripper.
The TA will now ask you to look at the guide fibers. Highlight them with the mouse (left-click on the violet circles, the display can be zoomed in). Then select 'View object' in the pop-up GUI. The gripper camera should show your bright (R=10 to 15, but prefer bright ones if possible: 12 works very nicely) guide stars nicely in the center of the gripper jaws. The TA will have you look at all the guide stars. Once this is done, park the gripper and use the Positioner Tab to select 'FOP guider on' (wait for the TA to tell you to do so).
When the guiding works, go to the Arcon tab and type 'observe' or 'object'. Choose the number of observations and exposure time. The system will hang for a moment. Check that the guiding is OK (sometimes the focus changes too much and the guide stars are temporarily lost). Once the TA gives you the go-ahead, hit any key to start observing.
When the observation is finished, take the FOP guider off in the Positioner tab, take any arcs and pflats you may need at the end of the exposures, park the lamps and return to zenith for the next configuration. 'Flatten' the field (there is a tab for that), 'park' all the fibers and then start with the new configuration. Do not forget to type `settemp' before each new observation.
If you want a standard star, the quickest way is to activate the gripper, select (by left-clicking) a fiber close to the center of the field and then select 'Move to gripper position'. Make sure that the fiber you choose is not too tangled with other fibers. Then ask the TA to move the telescope to the standard and guide the star into the fiber, using the gripper camera. Whan it is there, park the gripper. Type >observe in the Arcon window. Go back to the GUI and activate the gripper. Make sure the standard is still in the fiber (if not, have the TA put it back). Hit any key to continue.
7. Quick look.
You may want to make sure the data look OK. If the targets are bright enough, you may use the IRAF task `apall'. Type >apall obj1234.imh. Say 'yes' to all the questions. The task will show you the fibers and an aperture. `q'(uit) and then say 'yes' to yet more questions. The trace 'apall' chooses is 2nd order. Use :order 3 and then type f to refit. Then 'q' again. More 'yes'es and you should see a nice extracted spectrum. Hopefully it looks OK...
At the start of each night the TA will ask you to view a guide star bundle placed on a suitably bright star. If bright enough one of your guide stars should be fine for this. You need to type `settemp' to get the temperature of the telescope trusses. When the focus is decided upon, you need to put its value into 'instrpars'. Type 'instrpars' in the Arcon window, replace the values for `base focus' and 'reference temperature' and exit. This may need to be repeated after your first set of observations, because the night has cooled down in the meantime.
Only Nescafe is available, a fate worse than death in some circles. An espresso machine is coming: do
10. Help! Heelpp!
You did all this and everything is in shambles. If it is not a technical problem (e.g., CCD falling off the bench, mirror rolling down the mountain), and everything else fails, call extension 203 or 278. Your friendly Hydra scientist will come to the rescue via Telecon...
There are substantial overheads: 30min to configure, about 3min readout and about 3min to remove/place the calibration lamps for attached flats and arcs.