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RCAL-853: Apply velocity aberration correction to the WFI WCS #1354
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RCAL-853: Apply velocity aberration correction to the WFI WCS #1354
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@schlafly Current PR includes basically a duplicate of the JWST correction, that of applying the single scale as given by However, the questions in the issues relate more to whether Nominally, all these values are being calculated in RSDP. Are we tasked to provide those calculations, and hence also deciding whether the single scale_factor or the individual offsets are to be used in the correction? Towards this, I am getting some sample data and seeing about making some empirical estimates. |
Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #1354 +/- ##
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- Coverage 78.52% 78.14% -0.39%
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Files 117 117
Lines 7833 7852 +19
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- Hits 6151 6136 -15
- Misses 1682 1716 +34
*This pull request uses carry forward flags. Click here to find out more. ☔ View full report in Codecov by Sentry. |
Hi Jonathan, We will need to compute scale_factor. I think that's a simple function of the velocity of the spacecraft---some beta or gamma factor? I found the computation of that factor in the Webb code, and put the link in the issue. It depends on the speed of the spacecraft, which is more annoying to estimate. I think the easiest thing to do would be to pretend that the spacecraft were exactly at L2, so that the spacecraft's velocity is exactly the same as the Earth's velocity times (l2_distance_from_sun / earth_distance_from_sun), where I guess I'm ignoring effects from the moon but that seems very small. I had not appreciated until looking at your code that that the ra_ref and dec_ref correspond to the aberrated v2/v3 rather than the nominal v2/v3. Those ra/dec_ref come from romanisim at present and so I think it is straightforward to get that right. But I'm rather attached to the notion than ra_ref, dec_ref corresponds to v2_ref, v3_ref, which I claim this will break. Is that how things work for Webb? |
The previous comment generated much offline discussion. The summary and path forward is below; let me know if I got it wrong. Concerning where the computation of For JWST, as described, the RA_REF/DEC_REF are the "semi-aberrated" versions. These values are determined in Just recently learned (27Aug24) about the plans on back-porting the Level3/GAIA corrected solution to the Level 2 files. Details of that does not matter. Note that this process will invalidate all the wcsinfo keywords. For the moment, I believe the plan is to leave the |
It's fine to compute va_scale in romanisim (when the L1s are produced). Let's follow Webb for ra/dec ref. i.e., radec_ref only match v1v2_ref exactly for the boresight; the others don't match up exactly due to differential aberration but they're within a pixel or so. This is tracked in the WCS but not in the wcsinfo. I'll add an issue to rad (spacetelescope/rad#437) bringing up whether we want to add v2a/v3a_ref which would allow these to match up better, though still not perfectly after tweakreg has been run since we don't update the wcsinfo with tweakreg information. Re backporting the L2 tweakreg WCSes to the L1 files, we'll end up doing something like providing a pile of L2 WCS files separately from the rest of the L2s (which would include tweakreg, aberration tweaks) or just the tweakreg info plus code that turns the wcsinfo + tweakreg info into a new WCS (basically, repeating assign_wcs, aberration, ...). I don't think that invalidates the wcsinfo keywords any worse than they are already invalid in the L2s. Let's not update ra_ref/dec_ref yet and instead just continue following Webb. |
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Resolves RCAL-853
Closes #1192
This PR applies the velocity aberration correction to the WFI WCS.
Checklist
CHANGES.rst
under the corresponding subsection