Difference: AAOmegaFileFormat (7 vs. 8)

Revision 82008-07-01 - RonHeald

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

When analysing data from an AAOmega run, one needs to be able to map the combined spectra returned from the reduction task back to

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< 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.

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> individual objects from the input catalogue. All the relevant information is contained within the combined output file(s). The page that follows describes 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.

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Extension	IRAF format	Contents

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Primary image	.fits[0]	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]	FITS 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
...	...	Details to be added
Seventh Extension	`.fits[7]`	Sky spectrum Stored as a FITS binary table, with W rows. Each row contains the one element of the 'typical' sky spectrum used in the data reduction ('typical' because for a combined frame it is not obvious how the final sky spectrum for each fibre should be represented here). Note: The variance information is correctly propagated, the sky spectrum is not presented here for this purpose.

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Primary	.fits[0]	WxN image containing the spectrum (W pixels long) for each fibre of the N spectra
Variance	.fits[1]	WxN image containing the variance array for the data
Fibre Table	.fits[2]	FITS binary table, with N rows, one for each fibre. Each row contains information for each object such as RA, Dec, 2dF
Other Extensions		The file contains several other extensions which are generally only useful with in depth analysis of the data. They are not necessarily in order and are accessed by name. They are described here.

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< The extension can be accessed in a number of ways shown below.

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> The primary extension in the MEF FITS file is a WxN image where N is the number of spectra represented. This is 400 for AAOmega data, 392 science fibres and 8 guide fibres. Unused science fibres and Sky spectra, are included in the output file along with the guide fibres spectra, even though the spectra contain no information, as this is seen to simplify book keeping, and is a small disc space overhead. In the case where multiple sets of AAOmega datasets, which contained a subset of common objects, have been combined, the format is a little more complex, and is explained separately below.

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< The primary image in the MEF FITS file is a WxN image where N is the number of spectra which are represented. This is 400 for AAOmega data, 392 science fibres and 8 guide fibres. Unused science fibres and Sky spectra, are included in the output file along with the guide fibres spectra, even though the spectra contain no information, as this is seen to simplify book keeping, and is a small disc space overhead. In the case where multiple sets of AAOmega datasets, which contained a subset of common objects, have been combined, the format is a little more complex, and is explained separately below.

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The variance extension is a WxN array. Each member contains the variance for the corresponding element in the primary extension.

The fibre table is described below.

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

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

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< 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:

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> 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 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.

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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).

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Fibre Extension Binary Table Columns

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Fibre Table Columns

Column	Column name	Description
1	Name	Object name from the `.fld` file

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How to Access the FITS Binary Table Information

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How to Access the Fibre Table Information

This list is not exhaustive. If your favorite option is missing, send an e-mail to Rob Sharp and it will be added to the list.

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< -- RonHeald - 23 Apr 2007

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> -- RonHeald - 01 Jul 2007

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