ChroTel data products

ChroTel observes the Sun pseudo-simultaneously in three channels at Ca II K, H-alpha and Helium 1083. The nominal cadence is 3 minutes. The three channels are recorded 10 seconds apart by means of a filter changing unit on the same 2k x 2k Kodak KAF-4320E CCD sensor. The following table lists the basic properties for each channel:

  Calcium II K H alpha Helium 1083
Filter center wavelength (nm) 393.4 656.2 1083.0
Filter passband FWHM (nm) 0.03 0.05 0.14
Exposure Time (ms) 1000 150 250
Acquisition mode single exposure single exposure 7 scansteps

The exposure times are approximate values that might be changed depending on the observing conditions.

We currently produce science-ready level 1.0 FITS data along with JPEG compressed preview images. Level 1.0 data is created from the raw level 0 data with a standard dark/flat correction procedure.The preview images are rotated such that solar north always points upwards. They are also scaled to the same size of the solar disk since ChroTel has a different image scale for each filter channel. The helium preview images are contrast enhanced by subtracting the continuum from the helium 1083 line core. Additionally, we create movies for each day.

Before 2012, data was stored in plain FITS files with only a very basic set of meta data. The older data is currently being re-processed to match the current data format as specified below.

Level 1.0 FITS data

ChroTel level 1.0 data is made available in gzipped fits files. Encoded in the filename is the most important meta data. The file name structure is: chrotel_{ha,ca,he}_lev1.0_yyyymmdd_hhmmss.fits.gz.

The FITS header contains a conforming WCS header so that ChroTel data can easily be aligned with data from different instruments using WCS aware sofware such as ds9 or SSWIDL. We use the WCS header structure proposed by W. Thompson (2006). SolarSoft documentation has a description of the WCS keywords. We also recommend the SolarSoft WCS Tutorial for a quick introduction on how to work with WCS compliant data in IDL.

In case of the helium channel, a separate WCS header is stored in the fits header for each scanstep. The corresponding WCS keywords for each scanstep are suffixed with a letter ranging from A to G. The first scanstep is indexed with an A, the last one with a G.

ChroTel Level 1.0 FITS keywords

Keyword Type Unit Notes
DATE string time UTC time of FITS file creation
DATE-OBS string time UTC time start of observation
ORIGIN string - fixed to 'KIS'
TELESCOP string - fixed to 'ChroTel'
INSTRUME string - fixed to 'ChroTel'
OBJECT string - fixed to 'Sun'
CONTENT string - fixed to 'Intensity'
FILTER string - one of 'Calcium', 'H-alpha', 'Helium'
WAVELNTH float nanometer filter center wavelength
FILTFWHM float nanometer filter passband FWHM
EXPTIME float seconds Exposure time of the observation
BUNIT string - fixed to 'ADU' -- data is stored in D/A units
DATAMIN int ADU minimum data value
DATAMAX int ADU maximum data value
DISKMIN int ADU minimum data value on solar disk
DISKMAX int ADU maximum data value on solar disk
DISKMEAN float ADU mean data value on solar disk
DISKMEDN float ADU median of data values on solar disk
DISKRMS float ADU standard deviation of solar disk values
WCSNAME string - WCS keyword, fixed to 'Helioprojective-cartesian'
CTYPE1 string - WCS keyword; fixed to 'HPLN-TAN'
CTYPE2 string - WCS keyword; fixed to 'HPLN-TAN'
CRPIX1 float Pixel WCS keyword; reference pixel, center of solar disk on the x-axis (FITS convention: first pixel has index 1)
CRPIX2 float Pixel WCS keyword; reference pixel, center of solar disk on the y-axis (FITS convention: first pixel has index 1)
CUNIT1 string - WCS Keyword; unit of the x-axis
CUNIT2 string - WCS Keyword; unit of the y-axis
CDELT1 float arcsec WCS Keyword; x-axis plate scale at reference pixel
CDELT2 float arcsec WCS Keyword; y-axis plate scale at reference pixel
CRVAL1 float arcsec WCS Keyword; x coordinate at reference pixel
CRVAL2 float arcsec WCS Keyword; x coordinate at reference pixel
PC1_1 double - WCS keyword; (1,1) element of the image transformation matrix
PC1_2 double - WCS keyword; (1,2) element of the image transformation matrix
PC2_1 double - WCS keyword; (2,1) element of the image transformation matrix
PC2_2 double - WCS keyword; (2,2) element of the image transformation matrix
HGLT_OBS float degree sub-observer heliographic latitude, same as SOLAR_B0
HGLN_OBS float degree sub-observer stonyhurst longitude (always zero)
CRLN_OBS float degree sub-observer carrington longitude
RSUN_REF float meter reference radius of the Sun
RSUN_OBS float arcsec observed angular radius of the sun in arcsec
DSUN_OBS double meter distance to solar center
SOLAR_B0 float degree solar B0 angle at DATE-OBS
SOLAR_P0 float degree solar P0 angle at DATE-OBS
IMG_ROT float degree rotation of the ChroTel focal plane wrt to geocentric north
ROT_OFFS float degree filter-channel specific offset to IMG_ROT. apply rotation by IMG_ROT + ROT_OFFS + SOLAR_P0 to get the solar disk north-up
AZIMUTH float degree telescope pointing azimuth angle
ELEVAT float degree telescope pointing elevation angle
TT_RMS float counts Tip-Tilt mirror RMS motion
PSD_LL float counts light level on PSD for the guiding system
GLOCKED boolean - true if telescope has locked on sun
CCDTEMP float deg celsius CCD temperature
BKPLTEMP float deg celsius Kamera backplane temperature
CCDPRESS float Torr CCD chamber pressure
CCDGAIN float electrons/ADU CCD gain; electrons = value * CCDGAIN
FITSFILE string - filename of this dataset
CREATOR string - name and version of the chrotel data reduction code
CDRCVERS string - version of the data reduction code
DATAVERS string - data format version; will be changed if the FITS header changes
CDRCREV string - revision ID of the data reduction code
LEVEL string - data processing level
REALTIME boolean - true if data was created on-site in realtime mode
OBSNFILE string - lev0 observation file
DARKFILE string - lev0 dark file
FLATFILE string - lev0 flat file
FLATDARK string - lev0 dark flat file
HISTORY string - comments on data processing procedure
DATASUM string - data unit checksum
CHECKSUM string - HDU checksum

Data access

The main ChroTel data archive is located at KIS in Freiburg and can be browsed via ftp or http. ChroTel data is also indexed by the Virtual Solar Observatory. Data access URLs:

The lev1.0 data are transferred from the observatory to the archive in Freiburg in near-realtime. The nominal lag is less than three minutes. Level 0 data and movies are transferred over night and should be available the next day.

For changes on the data format and instrument status information, please also look here.

Data handling example

Using SSWIDL, you can easily obtain ChroTel data in a programmatic way from the VSO. As an example, we query the latest H-alpha image with vso_search, download it and rotate it such that the solar north points upwards. The image scale is adjusted such that the radius of the solar disk is 1000 pixels.

IDL> res = vso_search(inst='chrotel', wave='6562', /latest)

IDL> a = vso_get(res, filenames=f)

IDL> d = readfits(f, h)

IDL> angle = sxpar(h, 'SOLAR_P0') + sxpar(h, 'IMG_ROT') + sxpar(h, 'ROT_OFFS')

IDL> cx = sxpar(h, 'CRPIX1')-1

IDL> cy = sxpar(h, 'CRPIX2')-1

IDL> rsun_pix = sxpar(h, 'RSUN_OBS') / sxpar(h, 'CDELT1')

IDL> scl = 1000.0 / rsun_pix

IDL> rotimg = rot(d, angle, scl, cx, cy, cubic=-0.5)

Data policy

ChroTel Data is freely available for scientific and educational use. Published work based on ChroTel data should include an acknowledgement like:

ChroTel is operated by the Kiepenheuer-Institute for Solar Physics in Freiburg, Germany, at the Spanish Observatorio del Teide, Tenerife, Canary Islands. The ChroTel filtergraph has been developed by the Kiepenheuer-Institute in cooperation with the High Altitude Observatory in Boulder, CO, USA.

If you work with ChroTel data, we would be glad to learn about it. Please contact us via e-mail: chrotel@leibniz-kis.de

Papers suitable for citation