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2dF/AAOmega Frequently Made Mistakes


This potted summary of the most frequent howlers in multi-object observations aims to assist observers in avoiding making the same mistakes.

Summary of FMMs.


Not reading the FAQ and FMM

Turning up at 4pm on the first night of observing.

Guide stars not from the same catalogue as the targets

Guide stars too bright.

Guide stars have unreliable magnitudes.

Not enough guide stars.

No sky positions in input file

Bright stars in the configuration or the range of target brightnesses too large

Blue wavelength range too short, requiring offset sky observations

Spectrograph arms don't overlap, difficult to splice spectra

Observational Wavelength not set

Requested single configuration exposure too long

Not allowing time for reconfiguration

Not allowing enough time for standard stars.

Not having run configure before arriving to check your proposed setups look reasonable.

Putting TOO much effort into CONFIGURation before the run.

Nod and Shuffle observations

Complex configuration strategies for multiple re-observations of a field

The full story on the FMMs.


Not really reading the FAQ or FMM

Most people, on their first AAOmega run, make one of the mistakes outlined here or in the FAQ. If one reads these instructions carefully, and consults with the support astronomer early, most problems have simple solutions. This will improve your data quality.

Turning up at 4pm on the first night of AAOmega observing.

We generally start setting up fields for the night at 2pm in the afternoon, especially on the first night of a new project. So all the field configurations must be ready prior to this time. As field configurations must be tweaked for the current astrometry model it is strongly advised that observers turn up the night before.

Guide stars not from the same catalogue as the targets

The choice of guide stars is critical to the success of AAOmega. The most fundamental point to make is that the guide stars must be on the same astrometric system as the target sources. Simply using two independent catalogues which claim to be J2000 will not give good enough results. If they are not then one may get excellent acquisition of the guide stars, but totally miss the science targets. The astrometric accuracy has to be good to ~0.3arcseconds. Most people use the HST Guide Star Catalogue, or the APM catalogues. UCAC-2 or 2MASS have worked well in recent years, as proper motions are not yet a big issue. However, it is simply not good enough to just pick stars at random from these catalogues. The star coordinates need to be as measured from the same data from which you pick your targets. With SDSS this is rather easy, but do watch out for errors in the data in SDSS for bright stars.

Guide stars are too bright.

Bright guide stars (<13th mag) have high proper motions and should not be used unless they have had their positions measured in the last few years, or are PM corrected. They also suffer from bad centroids due to halo and diffraction spike effects which cause errors if positions are measured from sky survey plates. DON'T USE THEM. A case in point are UKST Bj plates some of which date from the 1970's. Better to use more recent R plates and to go fainter. AAOmega can work to 14-14.5th mag for guiding in dark of moon. In bright of moon we need stars in the range 12-13th mag and so you will have to make a careful selection of your guide stars.

Guide stars have unreliable magnitudes.

You may think it is OK to have dodgy magnitudes for your stars - after all they are only guide stars aren't they? WRONG. If your guide stars are too bright they will have bad positions and guiding will be difficult. We have frequently had people turn up with guide star magnitudes 1-2 mags out. How did this happen? They applied their galaxy photographic calibration to their stars.

Additionally, all guide stars should have a small range of magnitudes (<0.5mag). A large range of guide star magnitudes will cause a dynamic range problem with the guide camera meaning that only some of the stars from a given set can be used, the brightest or faintest being rejected.

Not enough guide stars.

The 8 guide fibres can not access the full field. If the stars in your inout catalogue are of uniform density you need about 20-30 per field to ensure all guide fibres can reach stars. While we can guide with 2 stars, we prefer 4+ and there is little reason not to use all 8. Do not deallocate valuable science targets in order to get every last guide star, but equally, do not only use 3 guide fibres to avoid having to do a bit of extra work. Good field acquisition is the single most important factor when observing with AAOmega.

No sky positions in input file

While it is possible to add a grid of sky positions to your configuration using the CONFIGURE software, if you do not add sky positions to your input file then it is remarkably easy for the field to be configured WITHOUT any sky fibres. This makes data reduction next to impossible as drcontrol can not do sky-subtraction!

Sky positions should be added to your .fld file. If you really want the standard uniform grid then it can be saved from configure (using File->lists) and pasted into the .fld file. However it is far better to add real blank sky positions, which you have looked at to check that they are blank using your input imaging data. It only takes one 1st magnitude star in a sky fibre to really ruin your data.

You need about 20-30 allocated sky fibres in order to get a good median sky spectrum. This means you need MORE than 20-30 BLANK sky positions in your input catalogue to get good sky subtraction without impacting the allocation of science fibres

Bright stars in the configuration or the range of target brightnesses too large

With 392 fibres all projecting onto the CCDs, inevitably there is some light from each target scattered across the whole CCD. AAOmega has been designed to minimize the effects of scattered light and reduce this crosstalk between different targets to an absolute minimum. However, it is not a good idea to allow the range of targets magnitudes in any given configuration to grow too large. A range of <3mags is typically best. While it is tempting to include some bright stars in any configuration, to allow simultaneous calibration of data, the magnitudes of such objects should be kept close to those of the targets to avoid scattered light compromising the science data. The acceptable magnitude range depends on what information is to be extracted from the spectra.

Blue wavelength range too short, requiring offset sky observations

The dichroic wavelength has been selected to place the strong airglow line at 557.7nm on the red end of the blue arm. If the central wavelength of the blue arc is set too short, particularly at high resolution, then one will lose the sky line in the blue spectra. This may well be what is required for the science program, particularly at high resolutions in the blue, however, it means that offset sky observations will be required to allow fibre relative throughput calibration and sky subtraction.

Spectrograph arms do not overlap, difficult to splice spectra

AAOmega is a dual-beam system, with red and blue spectrograph arms. The standard dichroic change over is at ~5700A. In most default modes of operation at low and medium resolutions, the system is set to allow a small overlap between the two arms to splice the full spectrum together. The user should be certain that splicing is not required before requesting a central wavelength which does not give an overlap between the two arms. See also the note on the 5577A sky line in the blue arm.

Observational Wavelength not set

In order to achieve a wide field of view and good image quality over that entire field of view the 2dF prime focus corrector suffers from Chromatics Variation in Distortion (CVD). This means that while the Atomospheric Distortion Corrector (ADC) accounts for the effect of the atmosphere on your target object's white light apparent positions, the prime focus corrector moves your target on the field plate as a function of wavelength. The effects can be quite large, up to 2 arcsec in the worst case when considered over the full wavelength range accessible to 2dF and over the full 2degree field. 2dF knows about CVD and so you must specify for what wavelength you want 2dF to put the fibres in the correct position. This must be the compromise which best suits your program goals (e.g. 400nm for Ca H+K and the Balmer lines, 860nm for Ca Triplet work, 600nm for low-resolution broad-band redshift measurements with the 570nm dichroic or 670nm for low-resolution broad-band redshift measurements with the 670nm dichroic).

Requested single configuration exposure too long

The atmosphere acts as a giant, time variable (due to changing Hour Angle, HA) chromatic lens. The 2dF top end is equipped with an Atmospheric Dispersion Corrector (ADC) which corrects for the chromatic component, but atmospheric refraction changes the plate scale of the 2dF field plates as a function of HA (or rather Zenith Distance, ZD) and there is no way to account for this stretch of the field (Differential Atmospheric Refraction, DAR) during an observations. Each 2dF configuration has a specified mid-point for which the field is set-up, accounting for the effects of DAR at that time. For a full 2degree field, a configuration is typically valid for +/- 1hour either side of this mid-point. Smaller fields are valid for longer, fields at high airmass are only valid for short time periods. Losses due to fibre position mismatches (this is in no way an error within 2dF, it is the Earth's atmosphere which is at fault) can be significant outside of this time window. This problem is discussed further on the Chromatics Variation in Distortion (CVD) page. An excellent paper discussing the effect is Newman 2002 PASP 114 918.

Not allowing time for reconfiguration

For a full field reconfiguration (i.e. to remove the old configuration and replace it with a new one) the reconfiguration time is of the order 40mins.

Not allowing enough time for standard stars.

Allow 15 minutes per standard, and even longer if you want to observe the standard in multiple fibres. This is how long it takes to get the star down a fibre and observe it. Also note that while we can defocus the stars a little (and in fact we probably need to for bright stars to avoid saturation in the red before giving good blue counts) we cannot defocus over a 2degree field plate and so standards are typically done with a special configuration which will take 10-20mins to set up on the field plate. Standards should typically be fainter than 4th mag (and note that some of the Lick standards are not) and brighter than 12mag for low resolution work. We have little experience with higher resolutions yet. Much fainter and you need a rather long exposure time to get high count rates. Also, note that while it is possible to do good relative flux calibrations (spectral shape) it is not possible to do absolute flux calibration with fibres due to unknown variables such as fibre placement or seeing losses.

Not having run configure before arriving to check your proposed setups look reasonable.

It can take some considerable time to prepare good 2dF configurations. For example having 400 targets in the central arcminute will not work due to crowding! It is best to download configure from the 2dF ftp site and prepare in advance.

Putting TOO much effort into CONFIGURE-ation before the run.

The 2dF astrometric model on the telescope will differ ever so slightly from that before the run. You will no doubt lose 5-10 fibre placements due to collisions. It is not worth worrying about the finer details of configuration until the correct astrometric models are available. i.e. don't run 'Allocate' and then do manual tweaking too early! As long as we have rough guidelines in advance of numbers of objects configurable that usually suffices. Contact your Support Astronomer or 2dF observer if you have any questions.

Nod and Shuffle observations

By using Nod and Shuffle observations, whereby the telescope is nodded rapidly between the target and the night sky while at the same time shuffling the charge around on the CCD to retain the integrity of the observation, AAOmega has demonstrated Poission limited sky subtraction. However, with dedicated sky fibres it is possible to obtain 1% sky subtraction accuracies with AAOmega. Due to the way Nod and Shuffle must be implemented, there is an increase in read noise and background sky noise plus usually a reduction in the number of targets that can be observed. The upshot is that N+S observation will tend to give lower S/N spectra for a given exposure time when compared to dedicated sky fibre observations (by a factor of between sqrt(2) and 2). For short exposure (less than 4 hours) there is little to be gained from N+S observations. N+S is ideal for multi-night observations when simple stacking of many dedicated sky fibre observations has generally been shown to be limited in ultimate depth by systematic error in the reduction process. Current investigations suggest that one could in principle integrate forever using N+S and increase the sensitivity according to Poission statistics.

A guide to setting up Nod and Shuffle Observations

Complex configuration strategies for multiple re-observations of a field

Some observing programs require repeat observations of the same field in order to observe the required number of target objects. In such cases it is often desirable to pad the target list with lower priority targets as filler for individual observations, and then to cull higher priority targets form the list once they have been successfully observed. The best approach for setting this up can depend on the nature of the observing program. Rather than trying to engineer a complex set of setup files unaided, contact your support astronomer and explain your requirements. The chances are we have run such a program before and can offer advice on the best way to construct the input files.

A guide to setting up complex configurations

Sarah Brough (sb@aao.gov.au)