ACAM operational walk-through

• Pre-observation planning:
  • Determine the observing configuration required, using information provided in the online user manual. In particular, the observer has to plan what filters and dispersers to use (sometimes in combination), whether any of the narrow-band filters need to be installed in the slit unit, what narrow-band-filter tilts are required, what focus offsets to use, the CCD window, binning and readout speed, and (for spectroscopy) what slit width and rotator position angle to use.
  • Calculate exposure times needed, taking into account the information provided by ING's exposure-time calculator (SIGNAL), and the user-documented technical constraints on minimum (shutter speed) and maximum (flexure, cosmic-ray counts) exposure times.
  • Plan the order of observations, taking into account the position of the targets in RA; restrictions on airmass for different targets and types of observations, and overheads for the various operations (both manual, and under computer control) e.g. switching between filters, taking arcs, readout.
  • Prepare finding charts (with the appropriate size in arcmin).
• Afternoon checks:
  • Move a mechanism, check that this is reflected on the MIMIC.
  • Insert a filter, take an exposure, check that the CCD is responsive to light.
  • Insert a disperser, illuminate the slit with arc light, take an exposure, check that arc lines are visible, and that the FWHM is OK.
• Pre-observing:
  • Input catalogue of target positions to TCS
  • Configure TCS for ACAM
  • Use default pointing model (probably), since the field of view is large.
  • Take bias frames and twilight imaging flat-fields through required filters, and with appropriate CCD configuration.
• During observing:
  • Configure ACAM, check the light-path mimic.
  • Autoguide.
  • Configure the CCD, take an exposure (e.g. bias, arc, flat, run or glance), save and set CCD parameters, inspect the data with iraf, in the same way as for other instruments.
  • Review the observing log, to check e.g. how many exposures of current target obtained in each filter.
• Post-observation data reduction:
  • Plan data reduction on basis of information in online observing logs, which must reliably record all the usual target and instrument-specific information, in particular which filters and/or dispersers are in the beam.
  • Check the configuration/observing details for individual exposures, using the information in the FITS headers, in particular that relating to the positions of the slit-unit components and the filter wheels, and the CCD configuration. This information may also be parsed automatically, e.g. by iraf, to search for calibration frames.

An override observation of a gamma-ray burst might be triggered at short notice, and will probably be carried out by an observer with limited experience of ACAM.

Example science goal: accurate (+- 0.1 mag) photometry and astrometry (+- 0.1 arcsec) of the GRB in B and R bands. It is assumed that the filters are already mounted in ACAM.

Procedure:

• Slew telescope to science target.
• Switch A&G flat to ACAM position.
• Configure TCS for ACAM.
• Change telescope focus to default for required filter (since speed of response may be more important than accuracy of focus).
• Deploy R filter in ACAM (no other deployable ACAM components in light path).
• Take test 30-sec exposure, check that the field matches the finding chart, and that the image looks in-focus (FWHM matches DIMM seeing).
• Take 600-sec science exposure, check that GRB is detected.
• Deploy B filter, take 600-sec exposure.
• Slew to standard star, take short exposures in each band, check that they are not saturated.
• Takes bias frames (this can happen while observing with other instruments) and flat-fields in B and R bands (e.g. in dawn twilight, or dusk the following day).

This would probably be observed in visitor mode, and the observations might take several nights.

Example science goal: imaging several z < 0.1 galaxies in Halpha light, to determine where the star formation takes place. The galaxies have angular diameters 1 - 8 arcmin.

Procedure:

• Afternoon:
  • Select the narrow band filter required for each galaxy (i.e. the one that transmits Halpha light at the given redshift).
  • Decide which filters go in the wheels, and which in the slit unit, depending on the size of field required.
  • Mount the filters in the wheels (or slit unit), replace the wheels in ACAM.
  • Calculate, and set, the filter tilt to optimise the central wavelength of the bandpass.
• Slew telescope to first science target.
• Deploy required narrow-band filter in ACAM (moving slit slide, or filter wheel). There should be no other deployable ACAM components in light path).
• Set default telescope focus for this filter.
• Take test 7-sec exposure, check that the field matches the finding chart.
• Fine-tune the focus (if required) by taking a series of (possibly-windowed) expoures including star images, at a series of telescope focus values, and selecting the best focus value.
• Take science exposure.
• For each target, slew WHT to the new coordinates, deploy required filter, focus (if necessary), take exposures, as above.
• If required, slew to standard star, take short exposures through each filter, check that they are not saturated.
• Takes bias frames (this can happen while observing with other instruments), and flat-fields through the narrow-band filters (e.g. in dawn twilight, or dusk the following day).

As with gamma-ray bursts, an observation might be requested at short notice.

Example science goal: characterisation of SN type, and determination of approximate redshift, via measurement of the strong broad emission lines in the spectrum. It is assumed that a disperser is already mounted in ACAM.

Procedure:

• Slew telescope to the SN.
• Deploy a broad-band filter in ACAM, take an acquisition image.
• Offset the telescope to place the SN at the x value on the CCD appropriate for the chosen slit (these values will be tabulated). In future this step might be via an acquisition gui similar to that developed for OASIS and ISIS.
• Deploy the required slit (e.g. 1 arcsec, or to match the seeing) and pre-slit mask in ACAM.
• Deploy required disperser (and order-sorting filter, if required) in ACAM. Remove any other filters from light path.
• Set default telescope focus for spectroscopic mode.
• Take a short exposure, check that a spectrum is obtained.
• Take science exposure.
• Switch on arc lamps, take an arc exposure (few sec).
• Slew to spectrophotometric standard star, acquire it, take a short exposure (possibly with a wide slit), check that it is not saturated.
• Takes bias frames (this can happen while observing with other instruments) and tungsten flat-fields.

As for any ING instrument, regular quality-control checks will be required, basically to confirm that the performance (1) meets the science requirements, and (2) is not degrading. Some quality-control work will require operations which won't normally be carried out during routine setup or observing.

Example of quality-control work: checking that the delivered PSF and geometric distortions are acceptable.

Procedure:

• Insert multi-pinhole mask in the slit slide.
• Move the mask into the light path.
• Insert broad-band filter in the light path.
• Apart from the mask and filter, no other deployable ACAM components should be in the light path.
• Illuminate the slit mask with continuum light.
• Take an exposure, check that the pinhole images are not saturated.
• Measure the image PSFs, check they are acceptable.
• Measure the geometric distortion, check that it is acceptable.