Update the config files for aligning HST images

[stscijgbot-hstdp]

Issue HLA-1233 was created on JIRA by Jennifer Mack:

The instrument teams request changes to the alignment software configuration files, including both the  fitgeometry and the minimum number of matches required for a successful fit.

https://tweakwcs.readthedocs.io/en/latest/source/imalign.html

1.) fitgeom:  change from 'rscale' to 'rshift'

The plate scale of WFC3, ACS, and WFPC2 images is well-calibrated using the current distortion solutions.  Changes on orbital timescales (due to breathing) and on longer timescales (due to changes in focus) are tiny, with errors <<0.1 pixel.  Furthermore, scale terms due to velocity aberration are well modeled and corrected for in the CD Matrix when 'updatwcs' applies the VAFACTOR during pipeline processing.

Simplifying the fit geometry when aligning to catalogs could significantly improve the quality of the alignment, especially for datasets with a relatively small number of matches.  (Rick White confirms that the HLA pipeline did allow for scale terms... just a simple shift+rotation)

2.) {}minobj{}{}:  increase from 6 to 10{} 

Currently 6 matches is required for an 'rscale' solution and 4 matches for a 'shift' solution.  We would like to remove the shift-only option and increase the required number of matches to 10 and use the 'rshift' geometry instead (see #⁠1) 

 

From Michele De La Pena in team_burgundy slack channel:

This is straightforward to implement as it is controlled by a configuration file.  However, there are some checks in the code, so this will also need to be examined.

[stscijgbot-hstdp]

Comment by Steve Goldman on JIRA:

Jennifer Mack Please let me know if you have a good dataset(s) to test this ticket on. 

[stscijgbot-hstdp]

Comment by Steve Goldman on JIRA:

I was able to confirm that code change results in the desired effect using the program: jczf55 which has 6 nmatches for one of the files (hst_14212_55_acs_wfc_f606w_jczf55iy_flc.fits).

[stscijgbot-hstdp]

Comment by Jennifer Mack on JIRA:

Cool. I was still trying to find an example...  but looks like you found one.

[stscijgbot-hstdp]

Comment by Jennifer Mack on JIRA:

We need some distortion experts to weigh on the estimated accuracy and stability of the distortion solutions. 

Specifically, is there any evidence that fitting a scale term (in addition to a shift and rotation) is desirable for either instrument, and if so, how many Gaia matches would be needed to ensure an accurate fit, e.g. > 50 ?  

Tagging individuals here. We need your input before this change is implemented in the next software update in 1-2 months.

Tri Astraatmadja Roberto Avila  -ACS

Varun Bajaj  - WFC3

[stscijgbot-hstdp]

Comment by Jennifer Mack on JIRA:

Steve Goldman

I wanted to verify that these changes in the fitting logic will apply to:

• all of the alignment code, not just HAP products.
• both RELATIVE (HST-to-HST) and ABSOLUTE alignment (HST-to-Catalog).

 

[stscijgbot-hstdp]

Comment by Tri Astraatmadja on JIRA:

With six parameters, minobj = 10 is adequate if ordinary linear regression is used and there are no outliers in the data set. However, if weighting of individual sources is applied to make the regression more robust against outliers (or if using sigma clipping), it is advisable to increase it. Perhaps twice the number of parameters to make sure if there is adequate number of stars after some are downweighted or rejected… so minobj = 12 if rscale is used?