Ghost of ThAr

[bjfultn]

Faint traces of the saturated ThAr lines are visible in dark frames taken immediately after. See the example below. Is this image persistence? The lamp not shutting off completely before the dark exposure starts? Something else?

[awhoward]

Let's definitely understand and correct the underlying issue.

We should also have a way to detect this, perhaps with a cross-correlation function in the x-direction. This could be a QC test for dark frames.

[howardisaacson]

We discussed this at Operations today and made the following conclusions:

• This is not due to a light leak. The octagon positions during the dark confirm this.
• Steve showed evidence that it is persistence, identifying details where the brightest ThAr lines of the preceding ThAr show a pattern in the subsequent dark frame.
• Immediately before the problematic dark are the autocal-thar-hk. They have a 60sec vs the nominal 10sec exposure time. Moving forward the sci/cal/sky fibers will be closed during the autocal-thar-hk, giving a few more minutes for the persistence to clear before the dark is taken. We need to follow-up this change to see if it worked.

[joshwalawender]

I deployed a small change to the OBs for the cals. The images preceding the affected darks are the last ThAr frames which have exposure times set for the HK detector. In the new scheme, those frames will have the Sci and Sky source select shutters closed, so the science detector images will effectively be darks. Nothing else will change, but for the ~5 minutes (3x 60s exposures + readout) prior to the dark, the science detector will not be illuminated by the ThAr light. The change will take effect this afternoon (April 23, 2024), so we should see if this makes a difference in a few hours.