# Investigate PSF problems found in MEDS generation

XMLWordPrintable

#### Details

• Type: Story
• Status: In Progress
• Resolution: Unresolved
• Fix Version/s: None
• Component/s:
• Labels:
None
• Team:
Data Release Production

#### Description

In creating MEDS files for HSC data, Erin Sheldon found some problematic PSF models. This ticket is to investigate where the problem is coming from.

#### Attachments

1. e1_res.png
39 kB
618 kB
3. problem_psf.png
21 kB
4. rho1_1am.png
53 kB
5. rho1.png
46 kB
6. Screen Shot 2021-01-13 at 3.19.22 PM.png
110 kB
7. size_res_vs_size.png
66 kB
8. size_res.png
39 kB

#### Activity

Hide
Bob Armstrong added a comment -

Here is an example PSF image of the problems that are being seen.

Show
Bob Armstrong added a comment - Here is an example PSF image of the problems that are being seen.
Hide
Bob Armstrong added a comment -

There appear to be a couple of issues going on:

• Cases where there are too few stars to constrain the psf model. Things get especially bad when the PSF is interpolated into regions where there are no stars. This will be problematic in HSC for the some of the outer CCDs that are heavily vignetted. We could consider requiring a minimum number of PSF stars for an image to be used.
• There are artifacts in the PSF models due to the attempt to mask out neighbors. Below is an example of the full postage stamps on the left and the masked image fed to psfex on the right. We could potentially avoid these by using the deblended images or by removing them out. Some preliminary numbers from HSC suggest that 75% of stars are isolated.

Show
Bob Armstrong added a comment - There appear to be a couple of issues going on: Cases where there are too few stars to constrain the psf model. Things get especially bad when the PSF is interpolated into regions where there are no stars. This will be problematic in HSC for the some of the outer CCDs that are heavily vignetted. We could consider requiring a minimum number of PSF stars for an image to be used. There are artifacts in the PSF models due to the attempt to mask out neighbors. Below is an example of the full postage stamps on the left and the masked image fed to psfex on the right. We could potentially avoid these by using the deblended images or by removing them out. Some preliminary numbers from HSC suggest that 75% of stars are isolated.
Hide
Bob Armstrong added a comment - - edited

I've tried to test the impact of the masking problem in the data by doing a separate run where I throw out any star that has masking. To increase the number density of stars I set the reserved fraction to zero. This results in roughly the same number of stars as the original settings. I processed 40 randomly selected i-band visits and then compared some different diagnostics.

Rho1 averaged over all visits:

Rho1 @ 1 arcmin for each visit:

This shows very little difference when using the masked objects.

Show
Bob Armstrong added a comment - - edited I've tried to test the impact of the masking problem in the data by doing a separate run where I throw out any star that has masking. To increase the number density of stars I set the reserved fraction to zero. This results in roughly the same number of stars as the original settings. I processed 40 randomly selected i-band visits and then compared some different diagnostics. Rho1 averaged over all visits: Rho1 @ 1 arcmin for each visit: This shows very little difference when using the masked objects.
Hide
Fred Moolekamp added a comment -

I came across similar issues in DM-28294, where ci_hsc has regions of the image with a partially masked out PSF in the I-band:

Show
Fred Moolekamp added a comment - I came across similar issues in DM-28294 , where ci_hsc has regions of the image with a partially masked out PSF in the I-band:
Hide
Fred Moolekamp added a comment -

Related to the above comment, there is a patch in ci_hsc to adjust the minStellarFraction to 0.7 in validate.py when scarlet is used as the deblender. This should be modified to 0.9 for all cases when this ticket is closed, or a new ticket can be opened and assigned to me and I'll make sure that ci_hsc is working properly with the new PSF model.

Show
Fred Moolekamp added a comment - Related to the above comment, there is a patch in ci_hsc to adjust the minStellarFraction to 0.7 in validate.py when scarlet is used as the deblender. This should be modified to 0.9 for all cases when this ticket is closed, or a new ticket can be opened and assigned to me and I'll make sure that ci_hsc is working properly with the new PSF model.
Hide

Could anyone (Bob Armstrong or Fred Moolekamp ) post a list of exposures and locations where I can find these bad PSFs? I'm guessing this issue exists even outside of MEDS file creation. PSF ellipticity seems to have little effect on whether these the masked star images are included or not, but I am concerned a bit about the size residuals.

Show
Arun Kannawadi added a comment - Could anyone ( Bob Armstrong  or Fred Moolekamp ) post a list of exposures and locations where I can find these bad PSFs? I'm guessing this issue exists even outside of MEDS file creation. PSF ellipticity seems to have little effect on whether these the masked star images are included or not, but I am concerned a bit about the size residuals.
Hide
Fred Moolekamp added a comment -

Look at the ci_hsc i-band PSF. That's the worst one that I've come across. It's worse in some parts of the image than others, but I can't recall offhand what locations in the exposure are the worst, so you'll just have to poke around it a bit.

Show
Fred Moolekamp added a comment - Look at the ci_hsc i-band PSF. That's the worst one that I've come across. It's worse in some parts of the image than others, but I can't recall offhand what locations in the exposure are the worst, so you'll just have to poke around it a bit.

#### People

Assignee:
Reporter:
Bob Armstrong
Watchers:
Arun Kannawadi, Bob Armstrong, Fred Moolekamp, Matthew Becker