|
⇤ ← Revision 1 as of 2018-03-09 15:20:07
Size: 1252
Comment:
|
Size: 1468
Comment:
|
| Deletions are marked like this. | Additions are marked like this. |
| Line 13: | Line 13: |
| * this contains a list of vignetting values evaluated at 3 energies and 31 offaxis angles | * this contains a list of vignetting values evaluated at 3 energy bands and 31 offaxis angles * it is not clear what mean energy should be assumed for each energy band. |
| Line 15: | Line 16: |
| * A pseudo-Moffat distribution is found to give a reasonable match to the data: vign(oa) = [1 + (oa/theta0)**alpha]**beta | * A pseudo-Moffat distribution is found to give a reasonable match to the tabulated values, i.e. vign(oa) = [1 + (oa/theta0)**alpha]**beta |
| Line 17: | Line 18: |
| * This quadratic form is probably bad at energies above the upper tabulated energy band | |
| Line 18: | Line 20: |
| * The parameters describing the | * The parameters describing this fit are as follows: |
Modelling the vignetting function of eROSITA
This is a holding page to gather together notes on produsing an analytic vignetting model for eSASS tasks
Problem: The current method to represent the eROSITA vignetting function in the eSASS required interpolation from a tabulated grid of calibration measurements, this is computationally awkward, and can lead to discontinuities in the derived system response. Desired improvement: Compute an analytic description of the vignetting function of each eROSITA telescope module, and save the parameters of this model into the CALDB
First step
- Start from the vignetting description file supplied with the current version of SIXTE (share/sixte/instruments/srg/erosita_vignetting_v2.1.fits)
- this contains a list of vignetting values evaluated at 3 energy bands and 31 offaxis angles
- it is not clear what mean energy should be assumed for each energy band.
- this contains a list of vignetting values evaluated at 3 energy bands and 31 offaxis angles
- For each energy value we fit a simple model as a function of off-axis angle
- A pseudo-Moffat distribution is found to give a reasonable match to the tabulated values, i.e. vign(oa) = [1 + (oa/theta0)**alpha]**beta
- The values of theta0,beta,alpha are then interpolated using a quadratic expression to allow evaluation of the expression at any energy
- This quadratic form is probably bad at energies above the upper tabulated energy band
- The parameters describing this fit are as follows: