Availability and Creation of Sky Coordinates

• WCS in KOA Data
• DEIMOS
• ESI
• HIRES
• KCWI
• KPF
• LRIS
• LWS
• MOSFIRE
• NIRC
• NIRC2
• NIRES
• NIRSPEC
• OSIRIS

WCS in KOA Data

The majority of the raw data in KOA do not have a World Coordinate System (WCS) solution mapping array pixels to sky positions. For KCWS, MOSFIRE, NIRC2, and OSIRIS, however, approximate WCS solutions have been created by using the telescope pointing information and not by registering stars on the array. As a result, users should not rely on these solutions if accurate astrometry is required. This document describes the WCS status for each instrument (see the table below for a summary) and useful information for users who need to determine more precise solutions.

DEIMOS WCS Information

No attempt is made to apply a WCS solution to DEIMOS data. For those wishing to compute an approximate WCS for DEIMOS images, the following information from the DEIMOS website should be taken into account.

Stitching the extensions

The layout of the extensions in the final mosaic depends on if the data are spectra or images. The layouts for both are presented here.

"Up" on the array

"Up" on a DEIMOS image is given by: PA_image = EL + ROTPPOSN + PARENTEL. The slit is rotated 90 degress with respect to PA_image such that PA_slit = PA_image + 90.

Pixel scale

The plate scale for DEIMOS is fixed at 0.1185 arcsec/pixel

ESI WCS Information

No attempt is made to apply a WCS solution to ESI data. For those wishing to compute an approximate WCS on ESI images, the following information from the ESI website should be taken into account.

Pixel scale

The plate scale is fixed at 0.1542 arcsec per unbinned pixel.

HIRES WCS Information

HIRES only takes spectrographic data so the application of sky coordinates on the raw frames would be difficult. Consequently, HIRES data only contain pixel coordinates in their headers.

KCWI WCS Information

WCS solutions are added to the headers of KCWI focal plane data (KOAID's starting with "KF") as part of the normal FITS writing operation during observations at the telescope. WCS solutions are based on the instrument and telescope information and should not be used for precision astrometry without refinement to a known stellar field. KCWI spectroscopic data (KB* and KR* files) only contain pixel coordinates in their headers.

KPF WCS Information

KPF only takes spectrographic data so the application of sky coordinates on the raw frames would be difficult. Consequently, KPF data only contain pixel coordinates in their headers.

LRIS WCS Information

After 2009, the LRIS red channel was upgraded to use two separate arrays. Each array is read out by two amplifiers, resulting in four extensions in each FITS file. No attempt is made to apply a WCS solution to these data. For those wishing to compute an approximate WCS on LRIS images, the following information from the LRIS website should be taken into account.

Stitching the extensions

When displaying LRIS files in DS9, users should open the file with File->Open As->Mosaic IRAF. This will extract the data sections from all four extensions and stitch them properly. For the red channel, the extensions will be displayed in the order 2,1,4,3 and extensions 1 and 3 will have their X coordinates flipped. For the blue channel, the extensions are displayed in the order 1,2,3,4 with 2 and 4 having flipped X axes.

"Up" on the array

The position angle of North is related to the header keyword ROTPOSN by the equation PA = ROTPOSN + 90.

Pixel scale

LRIS has a fixed pixel scale of 0.135 arcsec/pixel for both the blue and red channels.

LWS WCS Information

No attempt is made to apply a WCS solution to LWS data. For those wishing to compute an approximate WCS on LWS images, the following information from the LWS website should be taken into account.

"Up" on the array

• ROTMODE = 'stationary': position angle = ROTPOSN + 178.75 + PARANTEL
• ROTMODE = 'position angle': position angle = ROTPOSN + 178.75

Pixel scale

LWS has a fixed plate scale of 0.0847"/pixel for image data. For spectroscopic data, the plate scale changes slightly to 0.080"/pixel. Spectroscopic data can be identified by the keyword GRANAME, which will be set to either "LRES" or "HRES" for spectroscopic data. Any other value indicates imaging data.

MOSFIRE WCS Information

Creation of the MOSFIRE WCS

WCS solutions are added to the headers of all MOSFIRE data as part of the normal FITS writing operation during observations at the telescope. The sky coordinates presented in the spectral data can't be used to locate sky positions; they are there to provide a guide for the area of the sky that was covered by the slitmask for that observation. WCS solutions are based on the instrument and telescope information and should not be used be precision astrometry without refinement to a known stellar field.

NIRC WCS Information

No attempt is made to apply a WCS solution to NIRC images. For those wishing to compute an approximate WCS on the NIRC images, the following information from the NIRC website should be taken into account.

"Up" on the array

NIRC can be used in two different rotator modes: one in which the rotator is held stationary during the observation (causing the sky to rotate on the image throughout a sequence) or one in which the rotator moves to offset the sky rotation (the image on the array is kept at a constant position angle). To determine where North is pointed on the array, you need to need which rotator mode (ROTMODE) was used, as well as a few other header keywords.

• ROTMODE = 'stationary': position angle = ROTPOSN + 178.75 + PARANTEL
• ROTMODE = 'position angle': position angle = ROTPOSN + 178.75

Pixel scale

NIRC has a default plate scale of 0.15"/pixel. However it is also possible to insert an image converter that will increase the resolution to 0.0206"/pixel (the total FOV is decreased accordingly). Users can identify images in which the image converter has been inserted by the keyword MTR5RAW; this keyword will have a value of 49700 when the image converter has been used.

NIRC2 WCS Information

Creation of the NIRC2 WCS

The World Coordinate System (WCS) information is added by KOA when the data are being prepared for ingestion. The WCS information is determined from the keywords RA, DEC, and ROTPOSN, using an instrument PA zero-point of 0.7 deg (keyword INSTANGL) with a correction of -0.252 deg for the narrow camera (Yelda et al. 2010), and pixel scales of 0.009952, 0.019829, and 0.039686 (arcsec/pixel) for the narrow, medium, and wide camera, respectively. In addition, for vertical angle and stationary rotator modes, the parallactic angle (PARANTEL or PARANG) and elevation (EL) are also used to determine the rotation of the image.

Changes to the FITS Headers

When NIRC2 data are prepared for the archive, new keywords are added to the FITS header to provide additional information about the sky position, weather, wavelengths, and observing program. An error in this preparation code was discovered that formatted several of these keyword values (see below for the list) as string values rather than numerical values. This format error causes certain IDL programs (like the FITS file viewer ATV) to fail because the KOA-added WCS header keyword values are not in the expected and proper format. The common FITS viewer DS9 can read in these FITS files and display sky coordinates based on the WCS, but shows 'nan' for the "Physical" pixel coordinates.

As of 29 September 2014, all NIRC2 headers in the archive have had this error fixed and are correctly read and interpreted by FITS reading software. If you have downloaded NIRC2 from KOA prior to 29 September 2014 and are negatively impacted by this error, then we encourage you to re-download the corrected data files. You could also try to fix the formats of these keywords (listed below) using whatever software with which you are most comfortable (eg., python, IDL, WCStools). The data values for these keywords are valid; only the format is incorrect.

The header keywords whose values were changed from string to numeric values are:

• WAVEBLUE
• WAVECNTR
• WAVERED
• PIXSCALE
• PIXSCAL1
• PIXSCAL2
• CRPIX1
• CRPIX2
• LTM1_1
• LTM2_2
• CD1_1
• CD1_2
• CD2_1
• CD2_2
• WXDOMHUM
• WXDOMTMP
• WXOUTHUM
• WXOUTTMP
• WXPRESS

Null WCS Keywords

For approximately 1% of the NIRC2 data, the data preparation software is unable to compute a valid WCS based on the information in the header. Typically this occurs when one or more of the header keywords required to compute the WCS are either not present or have invalid values. For example, in the NIRC2 file N2.20010803.33588.fits, the camera name, which indicates the pixel scale, is set to 'Unknown' rather than the expected 'narrow,' 'medium,' or 'wide.' These random keyword glitches occur at the time of observation, making it very difficult to determine what the correct values should be.

NIRES WCS Information

WCS solutions are added to the headers of NIRES data as part of the normal FITS writing operation during observations at the telescope. WCS solutions are based on the instrument and telescope information and should not be used for precision astrometry without refinement to a known stellar field. The WCS provided in NIRES spectroscopic data (NR* files) is meaningless and should be ignored.

NIRSPEC WCS Information

After the October 2018 Upgrade

WCS solutions are added to the headers of NIRSPEC data as part of the normal FITS writing operation during observations at the telescope. WCS solutions are based on the instrument and telescope information and should not be used for precision astrometry without refinement to a known stellar field. The WCS provided in NIRSPEC spectroscopic data (NS* files) is meaningless and should be ignored.

Before the October 2018 Upgrade

Spectroscopic data only contain pixel coordinates in their headers. In addition, no attempt is made to apply a WCS solution to the slit viewing camera (SCAM) images (NC.* files). For those wishing to compute an approximate WCS on NIRSPEC SCAM images, the following information from the NIRSPEC website should be taken into account.

"Up" on the array

The orientation of SCAM images depends on whether or not the image was taken with the AO or not. For non-AO data (ISAO = "no"), the SCAMPA header keyword provides the position angle for North on the array. For AO data (ISAO = "yes") there is an error in the headers and so the correct value of SCAMPA is given by SCAMPA = ROTPOSN + 94.0.

Pixel scale

Like the orientation, the plate scale for SCAM also depends on whether or not the AO system is in use. For ISAO = "no" the plate scale is 0.178 arcsec/pixel. For ISAO = "yes" the plate scale is 0.0168 arcsec/pixel.

OSIRIS WCS Information

The World Coordinate System (WCS) information is added to the spectroscopic data cubes by the OSIRIS Data Reduction Pipeline (DRP) during KOA processing. The WCS is determined from the keywords RA, DEC, SSCALE (spec pixel scale), SFILTER (spec filter), and PA_SPEC (spec position angle). If any of these keywords are missing or not populated with valid values, then the WCS in the data cube will be incorrect. In addition, since the WCS solution is based on keywords and not the objects detected in the field, all positions should be taken as approximates and not absolutes.

Data from the OSIRIS imager also have an approximate WCS inserted in the headers. Similarly to the spectrograph WCS, these are generated by position and rotation keywords and not by objects in the field of view. These, too, can also have missing WCS information if the keywords used to create them are invalid.