Mutiplexed High Dispersion Spectroscopy at Keck

1. Multiplexed High Res Spectroscopy at Keck – J. Cohen (PI), H. Epps (Optical Design), M. Rich (Project Scientist) Keck instruments for optical spectroscopy – only HIRES has spec res (R) > 10,000. HIRES is 1 object at a time. Our competitors: ESO FLAMES/UVES has been extremely succesful (100 fibers Giraffe, R 17,000, 8 fibers to UVES R 45,000) MOE/IMACS/Magellan (R 21,000, 100 objects, 1 echelle order) is working well. Hectoechelle/MMT is working well (240 fibers, R 32,000) For large areas of science involving clusters, dwarf galaxies, Galactic bulge fields, etc Keck is no longer competitive

2. Mutiplexed High Dispersion Spectroscopy at Keck Our task: try to find a cheap solution via modifing a Keck instrument to get spec res 20,000, >20 objects Harland Epps looked at possible paths involving LRIS or Deimos Leverage off existing slitmask capabilities Only way – use a different dispersor in LRIS-R (new concept, pair of VPH gratings) and combine this with a smaller slit, 0.4 arcsec wide, to get to 18,000 spec res. LRIS-R CCD upgrade – pixel size OK. This scheme is quick (could do it in 1 year start to finish) and relatively cheap, less than 1 million total

3. Pair of identical VPH gratings, “tent” config., more than double dispersion.

4. Proposed Solution with LRIS-R No tilt or moving of disperser, has a fixed central wavelength covering ~320 A in first order, so several different VPH pairs would be needed. Not so good for extragalactic work as cannot cover a wide range in redshift without many VPH pairs, which would be expensive. Would gain by higher throughput of LRIS vs HIRES (about a factor of 2) , lose some light at slit, gain speed by lower disp than HIRES, could go at least 1.5 mag fainter than HIRES Only cheap way we have found.

5. LRIS Optical Performance Issues We believe that the LRIS-R camera will perform OK with such small slits as are required in this scheme because: There is no anamorphic factor, the beam is round Most of the intrinsic image size comes from chromatic aberrations over the broad bandpass of LRIS, but here we only need to maintain focus over less than 500 A. Will need to test optical performance of camera after installation of new LRIS-R detector.

6. Sketch of Location of VPH Pair inside LRIS-R

7. Concerns with the Proposed Modification of LRIS-R Need to move LRIS from summit to HQ, then back, to carry out necessary machining. Done to install LRIS-B, but this is a risk There is enough space to place a VPH pair in front of the grating turret but not enough for a jukebox holding several of these to enable remote changes of the disperser element (expect to change every night). Installation and changing of VPH pairs – access is very limited. Will have to be done on Nasmyth deck. Finding a way to do this that is safe and does not jeopardize other LRIS components is going to be challenging and require a very clever and experienced mechanical engineer. This is our major concern and that of the Keck operations group as well.

8. Concerns for proposed scheme (continued) Concerns about structural integrity of LRIS, flexure Bob Weber is the logical engineer to handle these issues, but is not available much until MOSFIRE is commissioned. Small slits are a concern, light loss, astrometry for slit mask design will need to be tuned up, as will alignment tools Fabrication of such small slits requires purchase of a laser mill, expensive (~200K, included in the 1M total cost), but would benefit all slitmask fabrication at Keck High throughput off the central wavelength of each VPH pair will require thin VPH gratings – presently being calculated. Need to consult manufacturers re feasibility. Must verify LRIS-R optics perform OK with small slits after red CCD upgrade

9. Other More Expensive Options If a scheme for installing and removing the VPH pairs that is safe cannot be found, or if this scheme is rejected due to other risks to LRIS, Investigate more expensive options: A new instrument designed for this purpose (~15 M) Or relaxing the spec res below 18,000, designing and building a new instrument that an even larger user community might find useful (cost ~15 M)

10. Recommendation The only way forward that will add this capability in less than 6 years is the modification to LRIS-R. This is the only way to proceed in terms of Keck’s limited funding as well. We suggest raising at least 80k for salary support for a mechanical engineer, and as soon as Weber can be spared, have him, or someone else as good as him, try to find a safe way to install and change out VPH pairs in LRIS-R that is acceptable to the Keck Operations Support Group.