Observing with SPIRAL
This webpage gives in depth information on how to operate SPIRAL, although it assumes that the user is already familiar with the AAOmega control task. Much of the basics will be taken care of by the Support Astronomer, and are reproduced here for their reference.
- SPIRAL Control Software
- Orientation on the sky
- Focusing AAOmega
- Twilight Flat Fields
- Calibration system - Arc and Flat Field Frames
- Target Acquisition
- Focus SPIRAL
- Standard Observations
- Science Observations
- Troubleshooting Issues
The SPIRAL control GUI is a cut down version of the AAOmega control task, tdfct. To run the software, log into aatlxy as aatinst and type spiral in a terminal window. The control task will be familiar to any 2dF/AAOmega user.
It is crucial to understand the orientation of SPIRAL on the sky to enable accurate acquisition and dithering between exposures.
Position Angle (PA) = 0 places the LONG axis of SPIRAL East-West.
For PA=0, the 2dfdr SPIRAL reconstruction is oriented such that :
|Telescope moved||Effect on 2dfdr SPIRAL image||Effect on SkyCat View|
|North||Moves star to the left||Moves star down a whole bank|
|South||Moves star to the right||Moves star up a whole bank|
|East||Moves star down||Moves star up within a bank|
|West||Moves star up||Moves star down within a bank|
If the PA has been manually adjusted set the new value in the control software using:
Commands->Set IFU Instrument Mount Parameters
from the SPIRAL control task Commands menu. This adds the relevant information to the image headers, and therefore allows the electronic log to pick them up.
As for normal AAOmega/2dF observations the spectrograph needs focussing
every afternoon. The process is identical to that for AAOmega/2dF
observations and is described in
Bais frames are useful for removing artefacts on the blue CCD. The red CCD is adequately debiased using the overscan region, and therefore does not require seperate bias frames.
To take baises, the dome lights need to be off or the dark slides closed. Take 30 or so every few days.
We slew the telescope with a low-level dome light on, to watch for fibre cable snags with the current fibre routing. The astronomer should assist the night assistant in remembering to check this, and also in remembering not to turn the lights on during exposures or read outs. What to look for is the fibre cable looping around or snagging on something. The most likely scenario is it looping around the end of the diving board (walkway access to the Cassegrain cage), and typically only on a big slew from the northern part of the sky southward. Slewing to Prime Focus Access should not present problems. The telescope has been manually locked out of observing south of the pole, however, which would overstretch the fibre bundle.
To determine fibre throughput calibrations Twilight Flat Field frames are taken STRAIGHT AFTER SUNSET and/or JUST BEFORE DAWN. These are taken with Observation type "Offset Sky" and a 60 arcsecond offset (e.g. North) between each of 3-5 exposures. Exposure times will increase exponentially from 10s in the seconds just after sunset.
The SPIRAL calibration system is in two parts, consisting of
- The Chimney lamp calibration system
- The IFU fore-optics calibration system
The chimney system is primarily used for the quartz-halogen flat field lamp. The IFU fore-optics system is used for arc lamp illumination. (The arc lamps in the chimney are too faint to be very useful with SPIRAL, with the exception of the He and Ne lamps. The He and Ne lamps cannot be successfully used at the same time as the IFU fore-optics CuAr, FeAr and ThAr lamps, as the fore-optics screen blocks the light path to the chimney lamps.).
Use the CuAr+FeAr arc lamps by default, but take a few sets of ThAr frames every now and then. It is best not to take all three together.
When arc or flat exposures are taken a pop-up dialogue will appear asking the user to indicate which lamp combination is in use. The user should select the appropriate lamps and then take the exposure.
Only leave the quartz flat lamp on for the bare minimum length of time, to avoid excessive heat load on the colour balance filter. Arc lamps can be left on for longer periods if required.
Flat field and arc frames are not required more frequently than every few hours for SPIRAL and are generally taken at regular intervals while slewing between targets.
The AAT pointing will not normally be sufficient for a blind pointing acquisition, even with the wide FoV of the IFU. The 2dfdr IFU reconstruction is used to define the APoff (see below) and to check acquisition on bright stars in twilight. However, for science target acquisition it takes too long. Direct CASS camera acquisition or Blind offsetting (if necessary) are the preferred methods of target acquisition.
The A&G system at CASS has a 45degree mirror system which can be interchanged with the SPIRAL feed. Readily identifiable astronomical targets can be acquired with the A&G camera, and accurately centred on the IFU within 5 minutes at the <0.7arcsec single lens level.
The Apogee acquisition camera has a field of view of about 2.8 arcminutes. The night assistant will need to flip the image in both axes to get the standard "north-up, east-left" configuration of a finder chart. The camera typically reaches 20th magnitude in about 10 seconds.
Blind offsetting is a standard procedure when the target is ill-defined in an acquisition image (it may, for example, be too faint for a short exposure, extended with no clearly-defined feature or perhaps an emission-line source not easily seen in a broad band image) then the user will define a nearby bright (<18 mag) acquisition star. The acquisition star is then acquired using the CASS camera system and the telescope guiding started. The user then supplies clearly defined offsets from the acquisition star to the well-defined position in the target object. The guide probe is then moved to the correct position and the telescope offset so as to place the guide star at the offset guide probe position, hence the telescope is now tracking accuratly on a well-defined target position.
The night assistant must choose a telescope guide star that can be seen
by the guide probe when observing the acquisition star AND the final
target position. The user must therefore keep the offset as small
as possible to move the guide probe as little as possible. In
practice 1-10arcmins is usualy sufficient. Poor choice of a star will result in vignetting.
SPIRAL also needs to be focussed each night. This is achieved with the
following steps (if following on from an APOFF start at step 5):
Slew to a bright SNAFU star or your chosen standard star, preferably near zenith
The night assistant will acquire the star with the A&G camera in the usual manner, placing the star carefully on the cross hairs.
The night assistant should then quickly acquire a guide star, so as to ensure that telescope tracking is constant during the next 15 mins.
Take a flat field (~10s exposure) and an arc frame (~60s exposure) on normal data type (so that you can reduce the data), with ultrafast readout if you do not need these frames for science.
Ask night assistant to adjust focus in 5 steps of 0.2, 2 either side of the current value.
At each focus value, take an object frame of your SNAFU or standard star.
Once data is taken, reduce each frame
In 2dfdr use the Plot XY option in the IFU Map Tool under the Tools menu - this displays a histogram of the X and Y cuts at the position of your cursor. The best focus value has the narrowest histograms in X and Y. You are aiming for a FWHM<2 pixels
Ask your night assistant to move the focus to the best fit.
Take these with Observation type "Object" and the telescope guiding. You need ~2000 counts above the background (the spectra end up being summed so there is no need for 10,000s counts). The ESO list is good and will require ~300s exposures to achieve these counts.
There are a number of bad columns on the Blue CCD. The first set is at x~1024 the second at x~1610. For stellar population work, one must take care NOT to align these bad regions with a key diagnostic index. For example, if MgB is the principle line of interest, then one should place this at x~500.
Small dithers on source allow one to recover dead IFU pixels, and to mitigate the effects of the bad CCD columns on the Blue CCD. Use integer IFU lens offsets (i.e. 0.7 arcsec) dither steps North or South (for the default PA=0) as this moves the spectra into the next bank of 32 fibres on the CCD, whereas a lens dither East or West would move the spectra up a fibre within the current fibre bank, which does not move away from the bad columns. Integer IFU lens offsets (factors of 0.7 arcsec) make stitching the mosaic back together much easier.
If MOSAICKING large images the guide star chosen by the Night Assistant must be chosen such that the star will remain visible to the guide probe throughout the operation.
To Reset - just use reset, there is no power cycle as for AAOmega/2dF
The flap that comes in for arc calibrations sometimes fails, in which case there will not be data. Take another arc frame and it will fix itself.
- The SPIRAL light path involves many more possible blocks than 2dF:
- Primary Mirror Cover
- Chimney Calibration Flap
- This must be open to observe, but closed to reflect light from the flatfield lamp into SPIRAL. It is set automatically by the control task.
- Central Dust Cover
- This is closed during the day, but will block light from the flatfield lamp and the sky. Ask the night assistent or afternoon tech to open it for you.
- A&G Unit manual shutter
- This is usually opened when SPIRAL is placed on the telescope. The clearly marked knob is on the side of the AGU opposite SPIRAL.
- AGU Mirror Slide
- The mirror slide must be in the correct position to send light to SPIRAL (on Aux Port 2) or the acquisition camera (Port 1).
- Auxillary Port Filter Wheel
- The port where SPIRAL is mounted has a filter wheel within the AGU. If this is not set to clear, then full spectral coverage will not be possible. (Usually you see light in only one arm of AAOmega.) This is set by the night assistant.
- SPIRAL Calibration Flap
- This flap is internal to SPIRAL and reflects the arclamps into the instrument. Occasionally it fails to close for an arc frame, but should recover on its own. If it has not been connected to the air, or the pressure is wrong, then it will not operate correctly.
- The light path within AAOmega is shown in the AAOmega status dialog of the control task.
- Check you have set the cameras for AAOmega up correctly. If you modified the AAOmega setup, check you did not set the Blue camera to the Red wavelength by mistake (or vice versa). You often get little light with the wrong set-up (e.g. low-resolution set-ups with high-resolution gratings).
Sarah Brough (email@example.com)