All is back to normal and PEPSI is ready for observing in 23B after extensive work during the summer of 2023.
The two new STA1600 CCDs were integrated into the existing dewars at STA premises in California over the August time period. The new package has now much better bonding protection with the preamps closer to the amplifiers than before (and
much more), which reduces noise. All went smoothly at STA. AIP folks were participating in the final lab tests and did the focus alignment of the dewars’ windows (which are the last lens groups in both optical cameras of PEPSI), and then drove down both units by car from California straight to the mountain bypassing the obligatory x-raying at the airport. Thanks to their heroic road efforts both units arrived back safely and without any damage. Reintegration on the mountain succeeded on the weekend Aug. 26/27 and both dewars were cold on the 29th. After all, the first calibration images were taken and looked as expected from the CCD acceptance protocols earlier this year.
The normalized image of the de-focused flat field in CD35 in ADUs after amplifiers equalization in TAPLINEx of the Archon config file. September 6, 2023
Above is an example de-focused flat field image (taken with CD3&5, i.e., blue and red CCD) with bias subtracted and normalized to a spline fit to remove the blaze function and the spectral orders. You can see all defects immediately then. Due to the enormous real estate (12k x 10k; incl. the overscan regions), there are several hot rows on the chips plus the usual coating defects. Residual fringing is still visible on the red image (because not been corrected yet). The bias level is at 1000 ADUs on both, full well at 65k. ROT remained at 80s. Ilya is in the midst of the final characterization and a document will be made available soon. Gain and RON tables were calibrated and implemented into SDS4PEPSI as part of the automatic data reduction. Gain is on average 2.2, RON on average 3 e-/amp. There is no visible fixed-pattern noise structure seen so far, the red-device hot spot is gone.
Pending tasks include observing our usual flux standard stars in photometric conditions in order to revise the ETC. This is work in progress and planned during our restart period.
Besides the CCD exchange, the Red cross-disperser (CD) lift was replaced after the Blue one had already been exchanged in June. All allowed CD combinations are available again.
The fiber-to-Image Slicer coupling was cleaned and realigned and a refractive-index-matching oil inserted. Alignment is somewhat better than before (one missed slice in one of the 7-slice R=250,000 configurations was recovered) but is still PEPSI’s largest light-loss location. We are working on a replacement of that unit that then will make each individual fiber adjustable wrt its Image Slicer entrance (recall that every science fiber has its own Image Slicer grouped in two blocks SX-DX on a joint optical carrier). All modes are operable at this time.
SDI-POL: We discovered that the polarimeter’s main retarder plate, a multi-layer quarter-wave plate from Astropribor, got loose in its mount so that it experienced an extra tilt whenever rotated. This produced very subtle effects in the spectrum in Stokes V (Stokes QU were unaffected) and prevented us from routine SDI operation in the past. The problem is now fixed but we had no time yet to resume SDI-POL operation because the entire POL calibration must be redone and implemented in SDS4PEPSI (planned for late September, early October).
Amazing work by the PEPSI team and LBTO support to make this PEPSI overhaul possilbe.
