College of Mathematical & Physical Sciences
Department of Astronomy
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The Ohio State University College of Mathematical & Physical Sciences Department of Astronomy |
Erratum: 2002 March 19
Attendees: Tom O'Brien, Jen Marshall, Mark Derwent, Pat Osmer, Paul Byard, Darren DePoy, & Rick Pogge,
Schedule
Time once again for a periodic assessment of the schedule and an accounting of effort spent against progress. Tom and Darren will take care of this. So far, we're doing fine against the budget.
One concern on cost is that if we want to accelerate the schedule, we may have to out-source some of the machining. This will have a cost impact, so we should keep an eye on this and keep it in control.
We are starting to shift from doing mostly design work to a fabrication-limited stage of the project. At this point, getting on top of fabrication, especially small stuff we can out-source easily (and relatively cheaply), keeping the fancy stuff in-house, will be good vis-a-vis meeting the schedule.
Optics
The imaging flats have arrived from JML and look real good. Two are coated in aluminum (blue flats) and two in silver (red flats). Both coatings are protected with overcoats. One of these will be used for the grating mounting system tests (see below).
Paul has been working with Thermo/RGL to coming up with possibilities for suitable gratings for the R=8000 and possible higher-resolution modes (R=16000 with a nominal 0.6" slit is possible with this optical design). The issue is determining combinations of properties that meet our needs and that Thermo/RGL either (a) has in stock or (b) can actually make to order if we need a new master ruling. The latter exercise makes for an interesting sanity check as to what you can and cannot have made in gratings this size. We are also getting firm numbers for the low-resolution gratings we will be ordering in the coming months.
The low-resolution gratings available from Thermo/RGL from their catalog (i.e., replicas, no new rulings required) are as follows. These will be the low-res gratings for MODS. Resolutions are computed for 4 pixels per resolution element [effective slit width = 0.6/(anamorphic factor)], in practice we would probably bin the detector by 2 pixels along the dispersion and slit axes.
For the "R=8000" mode, there may be one useful grating in their catalog for the Red channel, but we would be using it in second order with order blocking filters. They are doing a workup on the efficiency (theoretical), and may be able to come up with a measurement. More about that when we hear more.
As for the blue side, there is no suitable grating in their catalog. We are working with Thermo/RGL to develop a design for a new ruling that can be made. A first-order grating would required 1600 grooves/mm and probably could not be made easily, or if possible not very efficient. A better option, perhaps, is to consider a second-order grating with ~800 grooves/mm and a blaze of ~22 degrees which would give R=8000 at 450nm. It will also require efficient order blocking filters.
Any higher resolution modes (R=16000 is one we are considering) would require cross-dispersion, and so be restricted to a short slit. A new rulings would be required for both channels. It is an interesting future upgrade path since with AO in a narrow field we could use a smaller slit and with our current optical design achieve R~30000. We hasten to repeat that this is beyond the baseline specification for MODS, and so only as a future upgrade. It also carries a substantially greater cost because it also requires an additional cross-disperser to use effectively, which itself (in all likelyhood) will require a new ruling for the cross-dispersing grism. It would not be a single ruling, however, because of the length (>300mm) of the lines. We might consider a 2x2 mosaic, but this adds expense and complexity to fabrication. Even if the mosaic can be made from stock rulings, the mosaic costs almost equal the new master ruling. There are also detector format issues. Nothing is easy with these big instruments. Thus a truly high-res mode of MODS is potentially feasible, but would need more work to develop into a complete plan, and a fair chunck of money to realize.
Mechanical
Tom started by showing us a good fabricated part that saves us time and machining. The piece is a standard 4-inch steel pipe with welded flange that is exactly like a part we use in our structural design that he can just order from McMaster-Carr (for example).
Currently working up a bid package for the structure fabrication. This exercise helps focus the preparation of the drawings and accompanying documentation now that the basic structural design is complete. The process should help reveal any loose ends. To seek a bid, we need to identify about 5 potential steel fabrication firms that could do the work (we know of 1 or 2 in Ohio already). Steel fabrication in this case means "steel carpentry", for example, a shop like the carpenteria in Lecco that did the LBT telescope swing arms, rather than a precision machine shop.
The specific part we are preparing for bid is the top structure, which is made of steel tubing and weldments. This is the structure that bolts to the Gregorian ring. The collimator structure ties onto this, and we will do that in-house: the bottom structure we need to build right away to evaluate the collimator mount design, and the long tubes that mate it to the top structure, are relatively simple and we can do the integration ourselves. These are the tubes that might be either steel or carbon-fiber, depending on how the various mechanical issues work out. The top struture is steel, and the tough part, and is is the largest single component of the instrument.
Tom has come up with an overall design that meets our mechanical requirements and is also turning out to be very nice for servicing the instrument. John Hill's suggestion that we handle the instrument with the telescope horizon pointing was an excellent one. On the "floor", we will also want to handle and work on MODS horizontally. The structure as designed works out very nicely in both cases (on the scope or in the lab).
In the shop, the current tasks are:
R. Pogge, 2002 March 6
Erratum added 2002 March 17