The Ohio State University College of Mathematical & Physical Sciences Department of Astronomy

We received feedback on the question of the choice of fixed vs. continuously adjustable slits, and what slit sizes people thought were important. Thanks to Rogier Windhorst, Guenther Hasigner, Charles Steidel, and Craig Foltz for their comments and suggestions.

The summary is that a choice of fixed, carefully made slit masks, with sizes in steps like 0.3", 0.6", 1.0", 1.5" and a few larger [Note: these are just examples, exact specs are TBD] will suit most people's needs, and that while a continuously adjustable slit mechanism is "nice" it clearly adds expense and complication.

One interesting upgrade option we might consider is a module with a manually adjustable slit mechanism that is set in advance and then put into the slit mask holder (but not adjustable while on the instrument).

We emphasize that the complement of fixed slits is not just a set of pre-machined multislit masks with 1 slit, but high-quality, carefully machined slit modules. These will not be cheap, but will hopefully address all the issues of scattered light, edge quality etc.

Regarding the question of how good of a slit is required, Darren is investigating how to setup a test in the lab. We have an incoming graduate student keen to do instrumentation work that may be ideal for this project. More later.

Mechanical Design

Mechanical design progress continues. For the upcoming summer Tom is getting orders together to build a prototype of a grating tilt mechanism (each grating turret eventually holds 3 such mechanisms, plus one fixed flat). This will give us real numbers on the tilt precision and tolerances.

Tom is also ordering components to test the specifications of the slit mask mechanism.

Optics Bid Package

Paul has identified ~10 vendors to whom we should send the bid package. He has been getting the final design dimensions generated and handed off to Tom for drawings, and evaluating the surface wavefronts required. He uncovered a bug in Code-V in the course of this, but was provided with a workaround by ORA.

He has redone the vacuum deflection calculations for the corrector plate (which doubles as the camera dewar window in our working design). He puts in the computed vacuum deformation and then refocuses the camera. The bottom line is that he finds very little differences in the error functions between the reference (no deformation) design and the slightly re-focussed vacuum-deformed design. Attempting to pre-optimize the surface before vacuum deformation has little effect on the error function. He has not included the variations in corrector thickness in computing the vacuum deformation (it assumes a plane-parallel sheet not a curved, wedged surface). This still needs to be done.

This report was followed by a long somewhat tortuous discussion of the bid strategy we should adopt, what should or should not be in the bid package, etc. The bottom line here is that at our next meeting the main order of business will be to review a set of bid package materials that Tom and Paul will prepare during the intervening week and then decide on the next steps.

R. Pogge, 2000 April 29