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

Attendees: Pat Osmer, Darren DePoy, Bruce Atwood, Paul Byard, Tom O'Brien, Jerry Mason, Jen Marshall, Dan Papallardo, Dave Brewer, & Rick Pogge,

Entre Act: LBT Aluminizing

Bruce returned from a meeting in Milan and gave us a brief slide show on progress on the mirror cell/bell jar system for the LBT primary mirror alluminizing. Full vacuum testing will be done at the Ansaldo plant in Milan later this month.

AGW Issues

While in Milan, Bruce also had a chance to talk with Jesper Storm from Potsdam (and others) about the Acquisition, Guiding, and Wavefront Sensing (AGW) system. Some highlights:

• The guiding and "slow" WF sensor layout has now evolved into a system with a smaller footprint than before , but we will still not be able to accommodate it at the straight-through Gregorian focus with MODS.

• The imager for the AGW system will be two standard Steward built TEC cameras using a Marconi frame-transfer CCD (512x1024 with a 512x512 viewing area). There are two images per AGW system, one for acquisition & guiding and a second for slow wavefront sensing ("slow" here means time sampling).
  • The acquisition & guiding system is comprised of reimaging optics, a filter wheel, and the camera for guiding on a stage that provides (X,Y,Focus) motions.

  • The slow WFS uses a second camera fed by a dichroic and a selection of three microlens arrays, currently 3x3, 6x6, and 12x12. The slow WFS is used to set the shape of the secondary mirror and to provide gross information about the primary mirror support system for optical alignment of the telescope.

Of relevance to MODS is that we would need to provide some way to feed light to a slow WFC camera for the Gregorian focus, probably from a pick-off above the slit (the current design for the WFC feed optics assumes the light comes from above the instrument focal plane). We need to learn more about the Steward camera to see if it can be used with our proposed behind-the-slit A\&G system. "More Data are Needed..."

Calibration

Rick handed out copies of a Draft Calibration Protocol (71KB PDF) for MODS that he and Pat have drawn up. We will discuss this at next week's team meeting.

Note, this document was replaced by the final Calibration Protocol Document on 2004 Aug 25 [rwp]

Acquisition & Guiding

Rick presented a picture describing how the MODS science field and behind-the-slit offset guiding field would project onto the sky. The image was generated using tools in the SAOImage DS9 astronomical data visualization application (which we intend to adopt as the display app for MODS). This screenshot (196KB) shows the MODS instrument field for a long-slit superimposed onto a digitized sky survey image of a potential target (the Seyfert 2 galaxy Markarian 573, located in the south galactic cap). For reference, a 1-arcminute guider probe field of view has been centered on a likely guide star.

The MODS guiding field is a 2x6 arcminute region offset by 35-arcsec from the edge of the science field. This gap is where the view is blocked by the slit mask support frame. The upper limit is set by where the slit mask cassette mechanism blocks the view. The regions around the other 3 quadrants outside the science field are occupied by the red and blue grating mechanisms (these flank the science field at 3-o'clock and 9-o'clock, respectively), and below (6-o'clock) by the dichroic/flat changer.

Image quality will degrade towards the corners of the guide field in a predictable way, and we need to design in sufficient focus latitude to refocus the guide camera optimally for the choice of guide star. Paul Byard will be working out the change in image quality across various guide probe fields.

A fancy application for observing planning that will let th eobserver superimpose the MODS fields on DSS images of targets for arbitrary slit position angles is in the works. For now, Pat and Rick will use these pictures to help them work out the A&G protocol for MODS, to be presented (hopefully) next week.

Sharing is Good, Part 3: VIRMOS Laser Machine Report

Jen Marshall reported on the VIRMOS laser machine as described in various papers and web pages. The machine is a StencilLaser 600 which uses an Nd:YAG laser. The VIRMOS unit can work a 600x600mm slit mask. The LUCIFER team has proposed to buy the StencilLaser 200 unit, which can work a 200x200mm slit mask. The machine is capable of making any size or shape slit wider than 100 microns. For MODS we envision that our smallest slits at first will be 0.6-arcsec wide (~400 microns), but want to preserve the capability to use 0.3-arcsec (~200 micron) slits later in small fields of view in the best seeing or tip/tilt corrected modes. The MODS optical design is such that 0.3-arcsec slits project to 2 unbinned pixels.

After some discussion of the properties of the machine and the slits it can make, a number of specific issues raised regarding how suitable it would be for MODS:

1. The MODS widest FOV is 6-arcminutes square, which projects to approximately 220mm square. We would like to be able to machine masks of this size, which are larger than those needed by LUCIFER as we understand it. Can a machine be acquired that will let us machine such masks?

2. LUCIFER (at last report) intends to bend its masks into a cylinder, but for the full MODS field, we need slit masks curved in both dimensions with a center-to-corner sag of ~12mm (1-meter radius of curvature). To do this requires a 3-axis machine (or more if the beam needs to be kept orthogonal to the surface). Is the proposed StencilLaser machine upgradable to a third (and perhaps fourth and fifth) axis?

3. Is LUCIFER going to use the same invar/black paint mask material as VIRMOS, or something else?

4. Can this machine work other materials, specifically graphite epoxy or aluminum?

5. Laser technology is advancing quite rapidly, so might it not be better to wait a year or two to purchase the machine? In particular, solid state lasers may make this sort of short MTBF flashlamp systems obsolete relatively soon. It is also possible that the price will be less.

MODS PDR

The PDR text for all parts are now due February 14 (next Wednesday). Darren and Rick will collect drafts, answer powerpoint questinos, request (or make) corrections, and harrass slackers.

See last week's report for info on the powerpoint template and PDR task assignments.

The next MODS meeting will be Tuesday, February 13 at 3pm in the Astronomy Conference Room.

R. Pogge, 2001 February 7