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 Leibniz-Institute for Solar Physics in Freiburg, Germany, at the Spanish Observatorio del Teide, Tenerife, Canary Islands. The ChroTel filtergraph has been developed by the Leibniz-Institute for Solar Physics 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
- Kentischer et al., ChroTel: a robotic telescope to observe the chromosphere of the Sun, Proc. SPIE 701413-701413-11 (2008)
- Bethge et al., The Chromospheric Telescope, A&A 234, 2011