Difference: AAOmegaFileFormat (3 vs. 4)

Revision 42007-04-16 - RonHeald

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

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> 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.

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< 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).

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< 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).

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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).

Extension	IRAF format	Contents

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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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< The extension can be accessed in a number of ways. A number of examples are provided below.

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

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

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

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< There are two very important, and very different, number which one must understand in order to recover the information on which object each

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> There are two very important, and very different, numbers which one must understand in order to recover the information on which object each

fibres was allocated to. Fibre slit position AND 2dF Fibre-Pivot position. For the most part there is a one-to-one correspondence between these number. Usually the fibre at AAOmega slit position 1 (bottom of the CCD image) will map directly to 2dF Pivot position 1, and 400 will map to 400 (note, 400 is a guide fibre and so maps to a blank space at the top of the CCD image). However, during manufacture or repair of each of the AAOmega slit units, it is some times possible for the order of fibres in each of the AAOmega slits to fall out of synchronization with the 2dF Pivot numbering (2 slit block on each plate currently (April 2007) are like this). It is not practical to mechanically alter either position so each of the two fibre numbers (slit position and Pivot position) are propagated in the binary table extension.

In the primary image (and also the variance array, stored in the first) the fibre at the bottom of the image, which is the fibre at slit position, corresponds to the first row in the binary table extension (the second FITS). 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.

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

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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).

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Binary Table columns

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Fibre Extension Binary Table Columns

Column	Column name	Description

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1 Name Object name, as given in the .fld file

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1 Name Object name from the .fld file

2	RA	Right Ascension from the `.fld` file
3	Dec	Declination from the `.fld` file
4	X	2dF field plate X co-ordinate (in microns)

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6	Xerr	Reported error in X in final fibre placement
7	Yerr	Reported error in Y in final fibre placement
8	Theta	Angle of fibre on field plate

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Type

Fibre type (P-program, S - Sky, U-unused etc...)

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Type

Fibre type, F-guide, N-broken, P-program, S-sky, U-unused

10	Pivot	2dF fibre pivot number
11	Magnitude	Object magnitude, as given in the `.fld` file
12	PID	???

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???

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Examples Accessing the FITS Binary Table Information

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How to Access the FITS Binary Table Information

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< This list is not exhaustive. If your favorite option is missing, send an e-mail to Rob Sharp (rgs@aao.gov.au) and we'll add it to the list.

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> This list is not exhaustive. If your favorite option is missing, send an e-mail to Rob Sharp and we'll add it to the list.

With Configure

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< -- RonHeald - 14 Apr 2007

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> -- RonHeald - 16 Apr 2007

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