1. NAC flat fielding and intensity calibration
Flatfields
Method
Examples
Open points
Intensity calibration
Definition
Results

2. Flatfields
Data:
Data taken with integrating sphere and calibrated lamps
Halogen in visible and IR
Xenon in UV
Data from July 2003
All raw images full frame (2096x2048) with amplifier B
All images taken with cold CCD (~190 K)

3. Flatfields
Method:
Select 4-5 images (must pass check for bad data and shutter problems)
Remove coherent noise (if necessary), subtract bias, cut off preclocked pixels
Create flat field pixelwise
At each pixel, discard values which deviate > 5 sigma from median at that pixel
Create flat field averaging the “good” pixels
Normalize so that the central 200x200 pixels average to 1

4. NAC FFP-UV/Far UV Obvious UV-pattern caused by the detector
No filter pinholes for the NAC
Some vignetting

5. NAC Neutral/Blue “Humbug” pattern nearly disappeared Vignetting
Straylight (?) at the edges

6. NAC FFP-vis/Orange Extremely homogeneous (consider the scale!)
No vignetting
Dust obvious

7. NAC IR/FFP-IR Not as homogeneous as in the visible
Little or no IR-fringing!

8. Open points
Should we improve the statistics of the flat fields by using more input files (as done at LAS)
Can improve S/N by factor (pixel-to-pixel variation calibrated down to below 0.1 % for some filters)
Needs understanding of systematic effects
Difference between flat fields in different lamp positions
Constant flat field in space?
Care is needed to avoid bad images (however, there were not many during the NAC calibration in 2003)

9. Intensity calibration
Definition of calibration factor
A = C/ I
A: Calibration factor [DN s-1 / W m-2 nm-1 sterad-1]
C: Count rate [DN s-1]
I: Irradiance of the lamp [W m-2 nm-1 sterad-1] at the central wavelength of the filter combination

10. Intensity calibration (2)
Calibration factor depends on input spectrum
This would also be true if integrated irradiance would be used in the definition
Knowledge about relative spectral response can in principle be used to predict calibration factor for different input spectrum
Nevertheless, calibration with solar analogs is essential for accurate science measurements with OSIRIS

11. Intensity Calibration (3)
Exposure time corrected for difference between commanded and real exposure time (assumed to be 2.7 ms)
Can be done more accurately (maybe)
Temporal change of irradiance of the lamps was monitored with photo diodes
Reliability of monitoring hard to access
Occasionally diodes were not used during calibration
Diode correction sometimes applied to calibration factors

12. Results (NAC) Solid lines: Prediction Symbols: Measurements
The large deviations are for the neutral density filter!!
Laboratory
Standard Star (ε Aqr)

13. Results Prediction and 2 measurements generally agree more or less
Complete agreement is not expected due to different input spectra
However, transmissivities calculated from measurements sometimes appear high
Prediction completely wrong for neutral density filter
Inaccurate filter curve ?
Wrong filter curve in data base?

14. Open points Error of intensity calibration hard to evaluate
Lab: Dominated by temporal changes in the output from the integrating sphere
Star: Calibration may be more accurate, evaluation needs more stars
Current best estimate ~5-10 %
Some differences between predicted and measured transmissivity are unexplained

15. Future work Compare LAS and MPS flat fields and calibration factors
Reassess calibration procedure to understand discrepancies
Analyse additional data from in flight calibration