I’m an Italian PhD student and a Rubin in-kind collaborator, at the moment I’m working on the estimation of etendue and total system throughput in the context of the technote SITCOMTN-151. For our simulations we need to use the filters limiting wavelength and we referred to the LSE-30 document, but we get a bit confused about upper/lower limits with percentage 97 and 103. Can you help me understanding it better and calify what do they refer to? Overplotting the upper/lower red/blue values on the filter curve looks like this and we were wondering which of the two combinations is the most correct for our purposes.
Are 97% and 103% related to the subtended area under the filter curve or simething else?
Although not my area, I was curious to see if I can help a bit, and then if needed find a Rubin staff member with the right expertise here.
To start, it sounds like you were referred to LSE-30. Since that is a requirements document and doesn’t (necessarily) represent the as-built system, it might not be what you need for SITCOMTN-151? I say that because based on your linked plot, it looks like you’re using the measured transmittance as in Fig 5 of SITCOMTN-151, and not, e.g., the baseline throughputs.
Next - and maybe you already know this, but just to establish what’s in LSE-30 for readers - the section “u-band Response Envelope” in LSE-30 sets the specification that “The area weighted mean u-band filter response normalized to the in-band average (as measured between u_inBandBlue and u_inBandRed) shall lie between the upper and lower envelopes defined in the tables below.” And that’s where the u_upperBlue(1.03), u_lowerBlue(0.97) etc. values are defined.
On page 176 of LSE-30, Document-16295 is cited for the definition of the area weighted mean response function, and that document starts off with a caveat that “The algorithm described here is primarily meant for the purpose of evaluating the filter witness samples … [and] is not intended to be used for acceptance testing of the final filter” which makes sense in the context of LSE-30 as providing the specifications for the system.
So maybe that helps to clarify, maybe not – but for the question of what might be most correct for your purposes, it might help to hear more specifically about the purposes?
Hi @MelissaGraham , Thanks a lot for finding the time to reply to this post.
Following what you said I understand that the two percentage refer to the area subtended by the transmission curve normalized, so obviously the 103% limits are wider than the 97% limits. I’m fine and agree with that up to the moment I look to the 0% limits that are even wider than the 103% ones. At this point it make no sense to consider that limits as the ones at which the normalized area is the 0% of the one calculated using the u_inBandRed and u_inBandBlue, but it seems that the % is referred to the response value of the curve. The 0% limits are so wide that the filter response has already dropped to 0 at that wavelengths, but how do we justify seeing the filter having a 103% response in that scenario? My concern is just about understanding the reference of the % and see if it is really what I’m looking for.
Probably to solve ambiguity I can use the as built filter curves as in SITCOM-151 and compute by my self the wavelength at which the subtended area is the same as the expected one, but for doing this I wuold need to make the computations listed in Document-16295 by myself and so be exposed to possible errors. I would prefer to use tabulated values (even requirements and not as built data) rather than number I made up by myself to be more error proof.
I suspect the main issue here is that the very specific quantities represented by inBandBlue and inBandRed are not appropriate for your needs. Although they were used to define the requirements specified in LSE-30, Document-16295 describes them as “not intended to be used for acceptance testing of the final filter or for estimating field dependencies of the as–built filter bandpass at the focal plane”. Whereas you mention that “for our simulations we need to use the filters limiting wavelengths” and that you’re “working on the estimation of etendue and total system throughput” for the as-built system (as in SITCOMTN-151).
It would be very helpful to hear more specifically about your use-case, and what your simulations need in terms of values derived from the filter throughputs.