SCUBA: FCF Analysis Jan 2002 - Remo Tilanus

Since the end of December 2001 typical 850um FCFs of SCUBA have been in the 250-300 Jy/V range, instead of the normal 190-220 Jy/V range (remember lower is better). The appearance of high FCF's seemed to coincide with a day that a mirror got reset and the dish was re-adjusted. However, subsequent holography shows the dish actually improved and initial tests of the optical alignment of the mirrors show that it probably is not very far out, at least not enough to explain the increase in the FCFs. A close look at the FCFs suggest that the problem started not on the night after the two adjustments, but in fact on the subsequent night. This in turns suggests that the appearance of high FCFs is an unrelated event.

Following a suggestion from Wayne, today I checked the history of the internal calibrator signal on H7 as shown by the noise observations. The two plots below show the measurements since Nov 1, 2001 and in a window around the relevant dates. From the plot it is immediately clear that something happened not on the night immediately after the adjustments, but one night later (Sat 12/30 UT: red line in the plot; be sure to expland the plots!) as already was suggested from the FCF record.

Fig 1. Plot of the internal calibrator as measured by H7 (white dots) as a function of time. The noise of H7 is plotted in blue (offset by 2000). The green squares show base temperature of SCUBA as represented by the mixing chamber resistance (scale on left). A change from 13.0 to 14.0 kOhm corrsponds to a change in temperature from 65.1 to 62.35 mK. The yellow lines indicate the mini-warmups, the green line the day the mirror and dish were reset, and the red line the day the FCFs went high (12/30/01 UT). Expand the plots to show all details!

The plots of the calibrator record strongly suggests that a problem internal to SCUBA is causing the high FCFs. This points to another event: on Dec 27 HST (before observing Dec 28 UT) a mini-warmup was performed. Contrary to previous mini-warmups this fall, contamination re-appeared very quickly, on the night of Dec 29 UT and moved across the arrays very fast to be off by the end of observing on Dec 30 UT. In fact, the detailed plot shows the noise on H7 (the blue points) to be high at the start if first shift on that day, but off H7 by the start of 2nd shift. The mini-warmups affect the base temperature, but the variation seen on Dec 27 is rather typical and doesn't seem a cause for concern.

While I do not understand the exact cause and effect, another mini-warmup seems a sensible next try. Suggestions are welcome.

Plotted the internal calibrator signal for C14, the center bolometer of the SW array. It closely follows the trend as seen for H7, except that the calibrator signal drops 2 days later not until 01/01/2002. Significantly this is not until the 'migrating noise' has moved across C14. The shown plot has an additional red line at new-year UT. The noise moved across C14 immediately after that. I think we can firmly conclude that the high FCFs are internal to SCUBA and related to the mini-warmup performed on 12/27/01 HST. It will be interesting to see what happens after today's mini-warmup.

Fig 2. Plot of the internal calibrator as measured by C14 (white dots) as a function of time. The noise of C14 is plotted in blue (offset by 7000). The green squares show base temperature of SCUBA as in Fig. 1. The yellow lines indicate the mini-warmups, the green line the day the mirror and dish were reset, and the red lines the day the LW and SW FCFs went high (12/30/01 UT and 01/0101/02 UT respectively). Expand the plots to show all details!

Two successive mini-warmups have faled to restore the FCFs as judged from the internal calibrator signal. This and problems with the He fills as well as inclement weather have prompted the decision to move the scheduled full warmup forward from the end of February. Since the problems are closely associated with the migrating He film, we expect that a full warmup will cure the problem. Full warmups of SCUBA may have to be scheduled 4 instead of 3 times a year.