u leak

Subject: There is a leak -- just how much does it matter -- your call
Date: Fri, 8 Mar 2002 16:25:43 -0700 (MST)
From: Buell Jannuzi

Hi Guys,

Well there is a red leak in the ctio SDSS u' filter -- at a very low level, but perhaps enough to explain the flat field counts Mauro reports -- I'm afraid I have not had time to think this through carefully. Below are attached the asci data files containing the traces (done in the center of the filter) made by Jim DeVeny. The good news is that in the band-pass the ctio SDSS u' filter is a good match to the tracing I have on file for the actual SDSS u' filter (measured in air -- not in vacuum as actually used). The bad news is that while the SDSS u' does not appear to have a red leak (actual measurements of that filter between 8100 and 1100 angstroms so nothing), apparently because in addition to the 1mm UG11 + 1mm BG38 there is a film to, and now I'm quoting the message I got in 1996 about this filter, "suppresses a strong leak around 7000 Angstroms and acts as an AR coating in the passband. Now, the ctio SDSS u' filter does not have strong leak at 7000 -- but does have a leak near H-alpha (0.04%, tiny), with an increasing leak longward of 8000, but again, not huge -- but very significantly different from the SDSS filter.

As I said, I'm not 100% sure the measured leak accounts for Mauro's measured count difference in the flats he took last night -- but given the tracing I'm hard pressed to understand how the ctio SDSS u' filter could yield more counts than the Harris U without there being a significant leak (one might have hoped that the poor red response of the CCDs might have helped, but it looks like they are hot enough in the red to see some of the leaked light).

Buell

Attached files:

Note that the two data files will differ ever so slightly in the region covered by both tracings, but the slit used was 5 times larger (lower res) in the second set up.