MegaMOS–CFHT Extreme Multiplex SpectrographSubmitted for CFHT Instrument Call 1/10/08.Version 6R. Content & T. ShanksDepartment of Physics, Durham University, UKSummary We describe the science and design case for a spectrograph for the CFHTprime focus that can deliver at least 10000 MOS slits over a 1.5° field in 0.7" seeing.This extreme multiplex capability means that more than 80000 galaxy redshifts can bemeasured in a single night, opening up the possibilities for large galaxy redshiftsurveys at z~0.7 and beyond for the purpose of measuring the Baryon AcousticOscillation (BAO) scale and for many other science goals. The design features sevencloned spectrographs and exploits the exclusive possibility of tiling the focal plane ofwide-field 4-m telescopes with CCDs for multi-object spectroscopic purposes. In~200 night projects, such spectrographs have the potential to make galaxy redshiftsurveys of ~16x10 6 galaxies at z~0.7 and 200,000 Lyman Break Galaxies at z~3 andthus may provide a low-cost alternative to other survey routes such as WFMOS andSKA. We describe a version of an extreme multiplex spectrograph, MegaMOS, forCFHT that uses 7 cloned spectrographs. The clones use a transparent design includinga grism in which all optics are smaller than the clone rectangular subfield so that theclones can be tightly packed with little gaps between the contiguous fields. Almostonly low cost glasses are used; the variations in chromatic aberrations between bandsare compensated by changing one or two of the lenses adjacent to the grism. The totalweight and length is smaller with a few clones than a unique spectrograph whichmakes it feasible to place the spectrograph at the prime focus.MegaMOS Science CaseWe take two examples for the science case; other examples are given by Content &Shanks (2008, arXiv:0808.2367).Emission and absorption line galaxy redshift surveys at z~0.7A prime cosmological goal for the MegaMOS instrument is based on its ability tomeasure 10000 galaxy redshifts per hour for i
correlation function. These features can be used as standard rods and allow tests ofcosmological models. In particular, such observations will allow us to probe theequation of state of the vacuum energy, p=wρ. Currently, the spike in the correlationfunction is tentatively detected in the 2dF Galaxy Redshift Survey of 250000 z~0.1galaxies and also in the SDSS redshift survey of ~75000 z~0.35 Luminous RedGalaxies. In future bigger galaxy surveys will be needed to measure the BAO scale athigher redshifts and hence track any evolution in the vacuum energy equation of statewith redshift. Instruments like MegaMOS therefore will have a crucial role to play inthe future of observational cosmology.There are enough galaxies at the magnitude limits quoted above to fill ~10000MegaMOS slits since galaxy count data suggest that there are ~4000 galaxies persquare degree at i
survey plan that would include the lower redshift z~0.7 galaxies discussed in Section3.1, in a total survey time of 250 nights, flexible scheduling might allow the 50 nightswith the best seeing to be used to observe fainter LBG targets with < 1 arcsec wideslits and the remaining 200 nights could be used for the z~0.7 galaxy redshift surveywhere 1-2 arcsec wide slits might be used. This 250 night survey plan might thenrealize redshifts for 200000 z~3 LBGs as well as for ~16 million z~0.7 galaxies.Given the excellent seeing at CFHT the prospects for large z~3 LBG surveys areparticularly promising for MegaMOS.MegaMOS Conceptual Design OverviewMegaMOS ConceptThe fundamental technology driver here is the cheap cost of CCDs, which now makesit possible to tile a large focal plane cheaply and, coupled with an efficient opticaldesign, still retain high image quality. As noted above, imaging surveys such asCFHTLS and Pan-STARRS are already exploiting the étendue of small aperturetelescopes with wide fields of view. Here we propose similarly to take advantage ofthe large étendue of 4-m class telescopes for extreme multiplex spectroscopy.1.5° Diameter(T=0.96T0)1.6° Diameter(T=0.82T0)30’25’SpectrographfieldFig. 1. The field of view of MegaMOS for CFHT. The 1.5° field is divided into sevenrectangular subfields. T0 is the transmission in the centre.Number of slitsThe conceptual design of MegaMOS exploits this promising combination of apertureand field of view to allow slits to be placed on ~10000 targets. This estimated numberassumes 1.2" width and 5-10" length; up to 25000 slits could be in principle targetedif the 5-10" slit width was reduced to a 1.2" aperture, with a few hundred such skyslots to map the sky over the whole mask. The multiplex gain of the instrument would
then be even greater. The estimate of 10000 targets also assumes that the spectrawould have a length of 200 slit image widths on the detector.DetectorRemovable box(one per band)MaskFig. 2. The preliminary design of a single spectrograph for MegaMOS; seven of theseare needed to cover the 1.5° field of CFHT. The box containing the disperser ischanged when changing of band. This design is for 3 bands from 0.42 µm to 0.92 µm.Spectrograph at prime-focusTo use the large 1.5° field or more, the spectrograph must be at the prime focus. Thisput some constrains on its characteristics, mostly putting some limits on its weight
and length. A unique spectrograph would easily be too large and too heavy so anarray of smaller spectrographs was chosen instead. Our design contains 4spectrograph units in each of the 1° field projects NG1dF for AAO and XMS forCalar Alto. Although a similar design could be use for MegaMOS, we propose adesign with 7 slightly larger spectrographs due to the larger field of view of itscorrector. Each unit has its own detector. An even larger number of smaller unitscould be used to take full advantage of CFHT's exceptional seeing. Figure 1 showsthe present baseline design for MegaMOS. The subfields are 110 x 130 mm wide atthe corrector focus which corresponds to 25' x 30' on the sky. This size is designed for4k x 4k detectors.While there is more vignetting outside the 1.5° field, there is still a lot of light gettingthrough in the 4 corners of the spectrograph so this region is not lost. One difficultywith many spectrograph units is to place these units near each other to minimize thegaps in the field. Ideally, all optics in a spectrograph unit would be smaller than thesubfield width. This would permit to pack the units tightly. This is achieved in ourdesign by placing a field lens as near as possible to the input focal plane to bend thebeams toward the centre (Fig. 2). Mechanically, the spectrograph can be made as onestructure or each unit can be a separate structure. A combination of both is alsopossible. While a unique structure would make the spectrograph more rigid for aspecific weight, it makes alignments more difficult.Low cost spectrographFor this concept to be practical, it is necessary that the price is maintained sufficientlylow. A first part of the solution is to avoid the complex mechanism and spectrographshape that comes with reflective gratings. A transparent disperser combining atransparent grating, as a grism or a VPH, with prisms permits to make the unitsstraight (Fig. 2) and makes it possible to place optics very near the disperser which inturn reduces the size of the spectrograph and reduces the aberrations. Differentdispersers will have different prism angles and materials depending on the band andspectral resolution. For low spectral resolution, a grism or a grism plus a prism woulddo. For high resolution, VPH glued to prisms would be the preferred option.The second part of the solution is to use low cost glasses. This however makes itdifficult to design the spectrograph achromatic. To resolve this problem, we takeadvantage of the need to change the disperser when changing of band. In our solution,one or 2 lenses are changed with the disperser. These lenses can be made small andthin and would fit in a small box with the disperser (Fig. 2). A special care must betaken when designing them to avoid the need of tight alignments of the disperserboxes which would increase the cost. One or more of the boxes can be made of lensesonly and used for imaging; alternately, the disperser alone can be replaced by someoptics in each box.Mask changing mechanismTo maintain a low cost, there must be as little mechanisms as possible. The basicconcept is to use only one disperser per night and to have it changed by hand duringthe day. It may however be necessary to have some imaging capability for acquisition.
If no practical alternative method can be found, it would be necessary to have achanging mechanism with 2 positions, most probably a slide.The problem is different for the mask; many of them are necessary every night. Thetwo main alternatives are a wheel or a jukebox like mechanism as in GMOS. Thewheel would support a smaller number of masks than the jukebox but is simplermechanically.Fig. 3 The arrangement of MegaMOS’s 7 spectrographs in a ‘circular’ pattern in the1.5° field-of-view allows contiguous tiling of a survey region.Spectral resolutionThe basic concept is for a resolution of 10Ǻ between wavelengths of 0.52 µm and0.72 µm with a 1.2" slit. A detector size of 4k x 4k is necessary to take advantage ofthis slit size and smaller on better seeing and on star observation. The resolution thenbecomes 7Ǻ with the same design and 0.88" slits which corresponds to 2 pixels on thedetector. Higher resolution are however possible by changing the disperser. Theprisms associated with a high resolution disperser would need to have a larger interiorangle but the present angle is quite small so it is perfectly possible to at least doublethe resolution. We have now demonstrated this in other projects.Much higher resolutions are also possible but request some significant changes to thedesign. The camera and collimator could not be maintained in a straight line; thecamera would need to be at an angle. This is feasible with a spectrograph that has nomore than 6 units but would be problematic with the central spectrograph of
MegaMOS which is completely encircled by other units. To have a sufficiently largebandwidth at high resolution, it would probably be necessary to have a camera anglethat can be changed. This would of course increase the complexity and the cost of thesystem.Image qualityIn the present preliminary design, the average image quality over 0.42 µm to 0.92 µmis 0.6" (50%EED). A better image quality will be necessary in the final design to fullytake advantage of the good seeing and to permit acceptable tolerances. Still, thepresent design is an excellent first approximation.Wavelength rangeWe have demonstrated so far for other projects a system with 3 bands, each with itsown disperser+lenses box. Their wavelength ranges are respectively 0.42 µm to 0.52µm, 0.52 µm to 0.72 µm, and 0.72 µm to 0.92 µm. There is also an imaging capabilityin a 5% bandwidth and a higher resolution (3A) for a short band. There are then 5boxes in this design. It is however possible to have other bands. One choice is to havea range with a maximum of 1.0 µm instead of 0.92 µm. Another would be to havehigher resolution in up to 1000A bandwidths to see between the OH lines.~ 400 mmspectrographfield lensmaskcorrectorFig. 4. Geometry of the MegaMOS multi-slit mask, field lenses and spectrographsbehind the 1.5° corrector.
CostWe have not made a detailed cost estimate for MegaMOS. However, we can make apreliminary estimate based on the approximate cost estimates for NG1dF at the AAT.Seven E2V 4kx4k CCDs plus controllers would cost US$1m and seven clonedspectrographs would cost a maximum of US$420k, even assuming no furthereconomies of scale accrue. Mechanical hardware would cost US$370k. This gives atotal hardware cost of US$1.8m. Taking the total costs including labour to be 3x thehardware cost gives a total cost estimate for MegaMOS of US$5.4m. This iscomparable to the cost of WIRCAM.Large IFU optionThe type of spectrograph design used for MegaMOS originated in a design of integralfield spectrograph using a very large IFU called the Million Element Integral FieldUnit (MEIFU). It uses a new innovative design based on the concept of "microslicelens array" which permits to place far more spectra on the detector than the usualdesign of microlens IFU. Such an IFU could be added to the spectrograph as anoption. It would replace the corrector and have its own small corrector for the IFUfield. The resulting field would be extremely large compared to what is presentlyavailable or in planning. It would be composed of hundreds of thousands of "spaxels".Table 1 shows a few possible parameters. Note that the spectral element of resolutionis not a spectral pixel but the width of the slit image. More than one of these cases canbe obtained by using some exchange mechanism. It is especially very simple totransform the system into one for adaptive optics by changing the magnifying opticsin front of the IFU field.Table 1: A few set of possible parameters for a large field IFU in front of MegaMOSspatial elementor "spaxel"0.44" x 1.20"0.44" x 0.88"0.44" x 0.44"0.10" x 0.10"Field-of-viewspectral elementof resolutionbandwidthnumber of"spaxel"5.3' x 3.6' 10 Å 2000 Å 130,00011.2' x 7.5' 10 Å 300 Å 570,0004.6' x 3.1' 7 Å 2000 Å 130,0009.6' x 6.4' 7 Å 300 Å 570,0008.0' x 5.4' 3 Å 220 Å 400,0003.2' x 2.2' 7 Å 2000 Å 130,0006.8' x 4.5' 7 Å 300 Å 570,0005.7' x 3.8' 3 Å 220 Å 400,0000.74' x 0.49' 7 Å 2000 Å 130,0001.5' x 1.0' 7 Å 300 Å 570,0001.3' x 0.86' 3 Å 220 Å 400,000ConclusionGiven these huge, order of magnitude multiplex advantages both over VLT/Keck forfaint galaxy surveys at z=3 and over 2dF for brighter galaxy surveys at z=0.7, wesuggest that MegaMOS opens up clear route for CFHT to set the world standard insurvey spectroscopy. A feasibility study has been carried out for MegaMOS which
seems promising. We now propose the MegaMOS instrument for a Phase A study forthe CFHT programme, “Instrumentation for 2013 and beyond”.AcknowledgmentsWe thank Dick Bond (CITA, Canada), Ray Carlberg (Toronto, Canada), KarlGlazebrook (Swinburne, Australia), Olivier LeFevre (Marseille, France), RaySharples (Durham, UK), Christian Veillet (CFHT) and Howard Yee (Toronto,Canada) for helpful discussions and encouragement to produce this study.