• Only fibre types 'S' (sky) and 'P' (program) fibres are combined. This includes cases in which a fibre has been disabled part way though a field observation, so only good data is combined. Other fibres such as unused and parked fibres have all values set to zero.

• All the fibre table extension information is properly propagated. Additional fibres are numbered beginning from the last fibre of the first frame. So for AAOmega, the first 400 fibres will be from the first frame, and fibre 401 and beyond will be additional fibres from subsequent frames (if any).

• Variances are handled properly.

Reduced File Extensions

• Multiple configurations of the same field can be combined together when objects are in common. Notice this can result in more spectra than the instrument can produce in one exposure.

• Only fibre types 'S' (sky) and 'P' (program) fibres are combined. This includes cases in which a fibre has been disabled part way though a field observation, so only good data is combined. Unused/parked fibres have all values set to zero.

• The first spectra will be all those from the first frame in the combine including unused/parked and sky fibres. Any additional spectra will be only sky and program spectra from objects in subsequent frames and not present in the first frame. If the data combined are all from the same configuration there will be no difference in the fibre count.

• All the fibre table extension information is properly propagated. Additional fibres are numbered beginning from the last fibre of the first frame.

N.B. Currently the combined file exposure time is NOT set properly. Exposure time is given in only one place, the value of the FITS header keyword 'EXPOSED'. This exposure time applies to all fibres within the file. When files are combined, this keyword is copied from the first frame. To fix this situation the fibre table structure must be modified to have a new column. This column would give the actual exposure time (in seconds) for each fibre spectrum. See Exposure Time Handling in the bug list.

Combining Control

Control of the combining process is done using the widget on the 2dfdr GUI Combine tab. There are widgets to control

Selecting whether the combine is done based on matching object name or object location is done with the COMBNAME argument list member. This appears as the Combine Matching Name widget on the Combine tab on the 2dfdr GUI. Setting Combine Matching Name to true causes that to happen. Setting it false causes the combine to be based on match object locations.

Combining Multiple AAOmega Data Sets containing a Common Subset of Targets

There are plans for an extra array to be added that gives the actual exposure time (in seconds) for each spectrum.

If the data combined are all from the same configuration there will be no difference between the new combining and the old fibre number combining (apart from that unused/parked fibres will have all their values set to zero).

Important Note on 2dF Fibre-Pivot Number and 2dfdr Fibre Number

There are two very important, and very different, numbers needed to recover the information for which object each fibre was allocated: Fibre slit position AND 2dF Fibre-Pivot position. For the most part there is a one-to-one correspondence between these number sets. i.e. the fibre at AAOmega slit position 1 (at the bottom of the CCD image, ...red.fits[0][*,1]) will map directly to 2dF Pivot position 1, and fibre 400 will map to Pivot position 400. (Note 400 is a guide fibre and so maps to a blank space at the top of the CCD image, ...red.fits[0][*,400]). However, during manufacture or repair of the AAOmega slit units, it is possible for the order of fibres in each of the AAOmega slits to fall out of synchronization with the 2dF Pivot numbering (2 slit blocks on each plate are like this as of April 2007). It is not practical to mechanically alter either position so each of the conversion betwen the two is propagated in the binary table extension (Pivot position).

In the primary image .fits[0] (and also the variance array .fits[1]) the fibre at the bottom of the image, which is the fibre at slit position 1 (...red.fits[0][*,1]), corresponds to the first row in the binary table extension .fits[2]). The table contains a column entry, PIVOT, which gives the 2dF pivot position for this fibre. This is the fibre number seen be the Configure software. The very top fibre in a CCD image corresponds to the very last entry in the binary table (which will be an AAOmega guide fibre in the case of a single AAOmega data set). There is ALWAYS a one-to-one correspondence between each spectrum position in the image and the binary table. There is typically a one-to-one correspondence between slit position and 2dF pivot position but with a number of known mismatches and discontinuities which are tracked via the PIVOT column of the binary table.

The fibre table is described below.

The extensions can be accessed in a number of ways given below.

Fibre Table Columns

How to Access the Fibre Table Information

File Extensions

The table below gives a summary of the 2dfdr output file content, for either an individual reduced frame ....red.fits files or a combined reduced frame combined_frames.fits file. The file is a standard Multi-Extension FITS file (FITS MEF).

An Important Note on 2dF Fibre-Pivot Number and 2dfdr Fibre Number

Combining Multiple AAOmega Data Sets which contain a Common Subset of Targets

An improvement to the combining algorithm (as of March 2004) is that instead of simply combining based on the fibre number in the reduced data frames (i.e. combine fibre 1 in frame 1 with fibre 1 in frame 2 etc.), the 2dfdr combine algorithm now combines data based on object name. This has a number of advantages:

• Multiple configurations of the same field can be combined together in the case where some, but not all objects are in common.
• Only sky (S) and program (P) fibres are combined. This includes cases in which a fibre has been disabled part way though the observation of a field, so that only the good data is combined.

This can result in final combined data frames in which there are more than spectra than the instrument can produce in one exposure, so that all objects are contained, whether they are contained within every frame or only a sub-set of the frames. The first spectra will be all those from the first frame (including any unused/parked and sky fibres), while any additional spectra will be extra program (only) spectra from objects not present in the first frame. All the fibres extension information is properly propagated in this process and an extra array is added listing the actual exposure time (in seconds) for each spectrum. If the data combined are all from the same configuration there will be no difference between the new combining and the old fibre number combining (apart from that unused/parked fibres will have all their values set to zero).

Fibre Extension Binary Table Columns

How to Access the FITS Binary Table Information

An Important Note on 2dF Fibre-Pivot Number and 2dfdr Fibre Number

Examples Accessing the FITS Binary Table Information

An important note on 2dF Fibre-Pivot number and 2dfdr Fibre number

Combining multiple AAOmega data sets which contain a common subset of targets

Binary Table columns

Example for accessing the fits Binary table information

With Configure

Within 2dfdr

2dfinfo

IRAF

IDL

file='combined_frame.fits'

AAOmega File Format (or, which spectrum is which object?)

Download the software

When analysing data from an AAOmega run, one needs to be able to map the combined spectra returned from the reduction task back to individual objects from the input catalogue. All the relevant information is contained within the combined output file(s). The page that follows attempts to describe how to recover this information. The process is simple, once one follows the logic. However, it can seem rather convoluted at first sight. Please send any questions, comments and suggestion you may have on how to make this information more digestible, to Rob Sharp (rgs@aao.gov.au).

The table below gives a summary of the 2dfdr output file content, for either the individual frame ....red.fits files or a combined_frames.fits file. The file is a standard Multi-Extension FITS file (FITS MEF).

>IRAF format

>Contents

>WxN image containing the spectrum  (W pixels long) for each fibre of the N spectra

>First Extension

>.fits[1]

>WxN image containing the Variance array for the data

>Second Extension

=.fits[2]

>binary table, with N rows, one for each fibre

Each Row contains information for each object such as RA, Dec, 2dF Pivot number and more