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Information on NOT FITS files

The NOT FITS headers and file formats are currently being reviewed. It is foreseen that a small subset of the new keywords described in the draft FITS requirements will be implemented in the NOT FITS files in autumn 2004.

Known features, funnies and fixes

We are aware of several short-comings of the NOT FITS headers. Below you find a list of some notable deviations from standard FITS usage, and some tips to overcome these. Several of these can be remedied by applying not2mef to data files. Not2mef can also be applied retroactively to data from previous periods.

Please share your experiences and tricks regarding NOT FITS headers and data reduction packages (stp [at] not.iac.es), so all NOT users can benefit from them.

NOTCAM WCS headers

DescriptionNOTCAM WCS headers are incomplete for NOTCAM data taken in ramp-sampling (non-destructive readout) mode. This is an error in the image acquisition software.
Affects IRAF imcombine
Work-around Remove the offending WCS headers (CRPIX1, CRPIX2, CRVAL1, CRVAL2, CDELT1, CDELT2) from the FITS file.
Fixed by not2mef? YES. Not2mef processes ramp-sampling data (wich are currently being recorded in a data cube) into multi-extension FITS (MEF) and incorporates nominal WCS information.

Use of DATE-OBS and TM_START keywords

Description Currently, DATE-OBS records the date at the end of detector read-out in stead of at the start of the integration. The comment field is empty (for FIES and MOSCA), or misleading ('Start of observation' for STANCAM, 'End of observation' for ALFOSC). TM_START correctly records UT at the start of the integration. For exposures that start integration before midnight, and finish read-out after midnight, DATE-OBS lists the date after midnight.

For NOTCAM TM_START does not correspond to the the starting time of the exposure itself, but to the reset frame readout preceding the actual exposure.

Affects
Work-around
Fixed by not2mef? Yes. Not2mef merges the information from DATE-OBS and TM_START into the DATE-OBS keyword in ISO8601 yyy-mm-ddThh:mm:ss.s format recording the start of the integration. A new keyword DATE-AVG has been introduced for the midpoint of the integration.

Free-format character strings (ao TCSTGT, most string-valued MOSCA, NOTCAM, FIES keywords, FILTER keyword value for STANCAM)

DescriptionThe position of the opening quote of several string keyword values is different from column nr 11. The use of free format character strings is compliant with the FITS standard, but not recommended.
Affects IRAF hselect
Work-arounds
  • Do the selection outside IRAF.
  • Change the format with hfix.
Fixed by not2mef? Yes. Not2mef records all FITS headers in the recommended formats.

MOSCA data format

DescriptionThe data from all 4 MOSCA detectors are combined into a single FITS image. The exposed pixel data from the different detectors in the resulting image are separated by a few lines of pre- and overscan. This offset does not correspond to the physical gap between the detectors in the array. Because of insufficient memory in the detector controller, the pre- and overscan are limited to a few lines.
Affects
Work-around
Fixed by not2mef? Yes. Not2mef copies the single image data into MEF format (image data for each detector are recorded in a separate FITS extension) and adds several FITS headers, including nominal WCS information.

NAXIS3 incorrectly present for MOSCA and FIES

DescriptionMOSCA and FIES data have conflictive mandatory headers regarding the dimensions of the data array. Data are recorded in a 2-dimensional array (NAXES equals 2), yet NAXIS3 is also specified. This is an error in the image acquisition software.
Affects
Work-around Ignore NAXIS3 for MOSCA and FIES data
Fixed by not2mef? Yes.

DETNAME and PIXSIZE values incorrect for ALFOSC fast-photometry after 2003-09-24

DescriptionThe values for DETNAME and PIXSIZE in ALFOSC fast-photometry headers still reflect the previous ALFOSC detector (CCD7) which was retired in September 2003. The fast-photometry mode of ALFOSC is using non-standard image acquisition software. It is not known when this will be updated.
Affects
Work-around See detector information
Fixed by not2mef? No.

CDELT2 missing for ALFOSC fast photometry

DescriptionThe WCS keyword CDELT2 (increment in second axis) is not recorded in ALFOSC fast photometry mode, while CDELT1 (increment in first axis) is. The fast-photometry mode of ALFOSC is using non-standard image acquisition software. The corrected version of this software is due to be implemented in December 2005.
Affects IRAF imcombine
Work-around For unbinned data, add CDELT2 = 1 to the FITS header (for binned data CDELT2 = binning-factor).
Fixed by not2mef? No.

Use of character " (double-quote, ASCII code 34) in APERTUR keyword values

DescriptionThe value field of the APERTUR keyword may contain a " (double-quote, ASCII code 34) meaning arcsec. This character is part of the standard ASCII character set, and therefore its use within a string valued keyword is allowed by the FITS standard. (bug reported to IRAF)
Affects IRAF imgets
Work-around
Fixed by not2mef? No.

Use of character # (cross hatch, ASCII code 35) in AFILTER, BFILTER, FILTER, GRISM keyword values

DescriptionThe value field of the AFILTER, BFILTER, FILTER and GRISM keywords may contain a # (cross hatch, ASCII code 35) meaning number. This character is part of the standard ASCII character set, and therefore its use within a string valued keyword is allowed by the FITS standard.
Affects
Work-around
Fixed by not2mef? No.

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