IRIS technical information#WARNING: the information given below was obtained from notes taken by Peter Young at an IRIS meeting in January 2013, and it may not be up-to-date or accurate.
IRIS will be launched on 28 April 2013 sometime in May 26 June 2013. The commissioning phase will last about 30 days, and then the 30 day science plan will start.
IRIS consists of a single telescope that feeds a spectrometer and a slitjaw imaging camera. There is also a small guide telescope bolted to the main telescope that is used for pointing.
The slitjaw imager has a single CCD and a filter wheel. The filter wheel is used to select C II, Si IV and Mg II line core and wing (four filters). The FOV is 175" x 175". Note that images and spectra will be obtained simultaneously.
The FUV spectrometer has two wavelength bands of 1331-1358 and 1390-1407 A, and the NUV spectrometer a single band of 2782-2834 A. The 0.33" wide slit has a length of 175".
The spectral pixels have size 0.028 A.
Plate scale: 0.17".
Data volume and storage#
IRIS has 43 Gbits of storage, and approximately 56 Gbits/day downlink rate.
Any pointing up to 4 arcmin off-limb is allowed.
The guide telescope has a an offset relative to the main telescope and it varies with temperature. An offset look-up table will be used to make sure the telescope pointing is correct.
The spacecraft can roll from -90 to +90 degrees, thus allowing the slit to be pointed at any angle. The default slit position will be north-south. At certain times of year certain roll angles are not allowed based on the position of the Moon. The roll angles that are ruled out are between 60 and 90 degrees. The roll is performed by using a star tracker.
For feature tracking, the PZT mechanism (mirror tip-tilt used for rastering) will be used to track a solar feature across the field-of-view. When the feature reaches the edge of the field of view, the telescope will be re-pointed. This jump will not be completely accurate, leading to a jump in the position of the feature.
(This section indicates some expected features of the IRIS data that may affect data analysis. EIS has similar features and so experience from EIS may be useful for interpreting them.)
Emission lines on detector will be tilted and curved. A 2nd order polynomial will be used to fit them.
During ground testing the spectra were found to move on the detector in both the X and Y directions as the instrument temperature was changed. Motions in Y direction were larger and were 10s of pixels. The temperature changes in orbit will be much smaller. The approximate shifts of the spectra with temperature are 1.5 pixels/C and 0.5 pixels/C in the vertical and horizontal directions, respectively. The thermal model expects +/- 1.1 C temperature variation during orbit.
The slit has two fiducial marks that basically appear as black dots in the final images. These will help with alignment.
There are particles on the Slitjaw Imager CCDs that will appear as black spots in the images. These are likely to move during launch.
The flatfields show intensity variations of 10% left over from the CCD annealing process (?).
The slit spectra show intensity variations along the slit (due to non-uniform slit width?). Burn-in of the emission lines with time is expected.
AEC and flare mode#
AEC will apply to SJI images. The number of pixels above the threshold will be counted. Above the critical value the exposure will be reduced by a specified amount. Further decrements will eventually lead to a critical exposure time at which the spectrometer observation study will be changed (or just exposure time reduced for existing study?).
Level 0 files will have flatfield removed, and dark current and pedestal subtacted.
iris_prep converts files to 1.5 or 1.6 (I'm not sure what difference is). iris_unprep allows level-1 to be converted back to level-0. Level 1.5 can not be un-prepped.
An Oslo convertor yields the level-2 file, which is analogous to an EIS file (i.e., a FITS file containing a raster).
A level-3 file contains a sequence of rasters (so lambda-T-X-Y) that are suitable for study through the CRISPEX software (Solarsoft).
Both NUV and FUV spectral windows will be in a single level-2 file. It's not clear how the slitjaw images will be stored. These may be in separate files.
The slitjaw camera has a spatially-varying PSF due to some dust in the filter and deconvolution routines are planned to give cleaned images.
Study and raster restrictions#
At most 8 FUV and 6 NUV wavelength windows can be taken.
The height of the slitjaw images is forced to be the same as the spectral windows.