Following our second telecon, here are some thoughts and reflections which could go towards our report. Although we have all pondered these issues a great deal, it has really not been easy to converge to the point where the choice is obvious. There are exciting - if rather different - aspects to both MOS and IFS multiplexing and I try to remain non-partisan about assessing them. I only wish I had a better understanding of how the MOS might work in practice! It would have been good to have had a groundbased instrument to look at at this stage. It is clear that there significant experience with the slicer-based IFUs and I know that the studies have gone through tough reality-checks regarding mass and volume.
We seem to have two major issues to address:
I'm sure that Jon, Santiago, Harry, Simon etc. have done the best job that can be done on (1). The numbers - and how we present them - will be very valuable.
I think we have a reasonable understanding of the options in (2) to first order. The MOS and IFS cover the parameter space in rather different ways. We know we can do the trades between field, crowding/overlapping, slit losses etc. for each choice of resolution. One thing we must be careful about - and it is a thought which has been expressed by several people (notably Simon) in different ways - is that there is a kind of 'logical loop' in the DRM process. We base the DRM on our idea of the Yardstick ISIM and, not surprisingly, start converging towards the Yardstick-type instruments to carry out the DRM.
There is a rather fundamental difference here between a wide field MOS and a smaller field IFS. The MOS design limits the maximum space density of sources which can be observed - because of crowding/overlapping - and is most 'efficient' when it is used near this space density (magnitude). This means that the instrument 'resonates' with the sky at some magnitude limit (for a given class of object) and quickly becomes less efficient away from this situation. I worry that there may be few such resonances within the total NGST program (DRM + what we have not thought of yet). The IFS, on the other hand, has no such resonance - which means that its peak 'multiplexing efficiency' is lower - but it may look considerably better when the integral is done over all the programs.
I have put some other issues in this comparison table. I have added my own selection of winners in pink (we could argue about) and strong winners in red (we'd probably all agree on).
| Characteristic | MOS | IFS |
| multiplex advantage | high at optimum space density poor for crowded fields unused detector at lower density (?) great flexibility |
just depends on space density of 'interesting objects' smaller total field excellent for crowded fields good for finding emission lines |
| optical throughput | nominal can suffer slit losses depending on object morphology/size |
nominal no slit losses |
| optical design | concern about spectrograph volume/weight for wide field |
multiple small spectrographs for wide field module |
| spectral resolution | can use multiple gratings | fixed in the slicer designs |
| stray light | a concern - some diffraction calculations have been done |
some practical experience with 3D and the IFMOS design |
| texhnical risk | MMA/MSA under development | slicer concept proved |
| operational risk | (several) moving parts | (few or) no moving parts |
| operational complexity | needs pre-imaging and high pointing/tracking precision but great operational flexibility |
point and shoot - low pointing precision needed |
There are two major issues that we are not in a very good position to judge ourselves. I think these should come out in the report as important questions which have to be addressed before any decision on multiplexing is made. These are, firstly, the implied mass/volume of the spectrograph(s) needed to achieve the level of multiplexing advertised and properly taking into account the diffraction problems and, secondly, the realistic estimation of the stray light reaching the detector.
I emphasise that, in my view, the achievment of ultimate sensitivity on a single source is the primary requirement with the degree of multiplexing coming in as second. I expect that some people may argue with this and say that it is the completion of the DRM which matters. I would answer that the requirement of sensitivity will not change over the next decade but the perception of the current DRM most certainly will!
| 21 October 1999 | Bob Fosbury | Send Comments | NGST home |
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