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The Low-Dispersion Survey Spectrograph-2 Upgrade Project. A Brief History : WHT  Baade Upgrade Goals/Plans : Optics Grisms Detector (Miscellaneous) Upgrade Results : Optics (aka the commissioning report) Grisms Detector (Miscellaneous) Overall

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A Brief History : WHT  Baade

Upgrade Goals/Plans : Optics




Upgrade Results : Optics

(aka the commissioning report) Grisms




Science Examples: low-z galaxy dynamics (M. Geha)

faint galaxies at z~0.8 (D.Kelson)

GRBs (E.Berger/M.Gladders]

To Do: Detector


Future Improvements

A Brief History

-first deployed on the William Herschel 4m in the early 1990s

-loaned by Durham to Carnegie in exchange for telescope access and deployed on the Magellan-I telescope in early 2001

-had a number of unfavorable characteristics

  • generally poor transmission

-unfavorable plate scale of 0”.378/pixel

-significant distortions, >10% at edge of field

-smallish field of view of only 4.5x6.5 arcminutes

-poor resolution: the highest dispersion grism gaves a

maximum R~700 in the blue, and R~400 in the red

-inferior optical performance compared to best possible images

delivered by the telescope – additionally has chromatic focus issues


Naysmyth Port

Guider Bonnette

A Brief History

A Brief History

Includes no atmosphere or slit losses



1.3 m

Upgrade Goals/Plans : Optics








Upgrade Goals/Plans : Optics

Upgrade Goals/Plans : Optics

-new design developed based on variants of the GMOS collimator (Gemini), and the FORS camera (VLT). Design has fewer surfaces and elements and should give much better performance.


Plate Scale 0”.378/pix 0”.19/pix

Image Quality ~0”.2-0”.3 <0”.10

F.O.V 6’.5x4’.5 8’.25x8’.25+

-optics to be manufactured by IC Optical Systems (UK)

-desired coatings : broad-band extremely high quality as

design emphasizes throughput above all else



1.3 m

Upgrade Goals/Plans : Optics






3700A: 0”.16

3700A: 0”.07

6500A: 0”.17

6500A: 0”.06

8500A: 0”.34

8500A: 0”.06

10500A: 0”.35

11000A: 0”.09

Upgrade Goals/Plans : Grisms

-higher throughput, higher resolution grisms


-designs based on volume-phase-holographic (VPH) transmission gratings, and prism pairs

-2 new grisms ordered (one red, one blue) + spacer ring to make room inside instrument

-new grism holders designed by S. Smee at JHU, manufactured at OCIW

-for 0.75” slit (4 pixels)  R~1700

Upgrade Goals/Plans : Grisms

grating sandwich


Upgrade Goals/Plans : Detector

  • -desired device is a “red-sensitive” 4kx4k 4-amplifier single chip, 15 micron device. Arizona agreed to provide as part of contribution to Magellan.

  • -4kx4k required to get full science advantage

  • -15micron pixels give 0.189” sampling

  • 4 amplifiers give rapid readout and hence low overhead (more important here than in IMACS due to flexure, and expected long-slit use)

  • -dewar/electronics etc. to be provided by Carnegie

Upgrade Goals/Plans : Miscellaneous

-encoders on all 3 wheels to be replaced

-fix shutter instability

-new interface software to handle new encoders

and detector

-new imaging and spectroscopy filters : SDSS griz + spectroscopic order-blocking, + user filters

Upgrade Results: Timeline

-daytime re-engineering took place Feb 13-22, 2005, at Las Campanas

-M. Gladders, A. Uomoto, I. Thompson principles from Carnegie, with invaluable and extensive assistance from F. Perez and the mountain staff

-nighttime engineering commissioning Feb 23-24, 2005, with J. Mulchaey and M. Phillips in addition to the engineering team

Upgrade Results: Optics

-coating witness samples from Cleveland Crystals look to spec. Coatings are hybrid- solgel : CC deposited solgel over ICOS’ MgF ¼ wave undercoat

-interferometric testing looked excellent – optics accepted Feb 07, 2005 at ICOS, and shipped directly to Chile

-installation of optics went (mostly) smoothly apart form some minor on-the-spot machining

- To date: best on-sky images are 0”.42 (2.2 pixels) full field (seeing limited) and 0”.30 (1.6 pixels) off a pinhole mask (sampling and laser limited)

Upgrade Results: Grisms

-grisms and mounts designed in collaboration with JHU

-grisms shipped to Campanas last week of January and installed on on wheel with new counterweights

-3 positions in wheel, 3rd position has old medium-red grism

-lab measurements of grisms are very good and to spec, except for poorer than hoped for performance in the far blue

Upgrade Results: Grisms

Red VPH Grism @ JHU

Upgrade Results: Detector

-detector is a 4kx4k 15micron pixel CCD from UA, based on normal silicon and a typical thickness (19 microns).

QE curve is better than the old 2kx2k CCDs previously used on LDSS-2, but is not particularly red-sensitive. The chip is cosmetically excellent.

-initial belief was that this is a 4-amp device, but testing during and after commissioning shows clearly that it is really a 2-amp device

-while an improvement, the lack of 4-amps and red sensitivity are concerns

-new compact dewar runs on a cryotiger : ease of use

Upgrade Results: Miscellaneous

  • new filters from Asahi (Japan) are excellent

  • -minor ~1% ghosting from filters – cosmetically striking but not a signifcant scientific problem

  • - In addition there are (so far) 4 new user filters, with more expected

Upgrade Results: Overall

-total sepctroscopic throughput is as expected, modulo unknowns such as the precise reflectivity of M1-M3. Gain over old system is dramatic!

Upgrade Results: Overall

-total imaging throughput is also excellent and competitive with MagIC at all wavelengths except for the UV (not measured yet)


g 27.44 27.82

r 27.51 27.61

i 27.13 27.17

Airmass=0, 1e-/sec zeropoints

Upgrade Results: Overall

-after the upgrade work and two engineering / commissioning nights the instrument’s mechanical and controls performance was as good as before, save for some problems with the shutter

-shutter problems came about due to efforts to fix prior mechanical problems with the shutter. The new problem is electronic and has been identified and corrected.

-new user manual reflecting changes to the instrument was written during the commissioning and is now in use. Changes tweaks and improvements continue…

Initial Science Results

(first use 5 days after initial commissioning)

B&C spectrograph

LDSS-3 spectrograph

Comparison to the B&C

Dwarf galaxy @ H-alpha, courtesy M. Geha

Longslit Use

(M. Geha)

MultiSlit Use



I =22.44+-0.04

190 minutes

Integration, typical conditions


lines only

S/N=1-2 per pixel ; 1.18Å/pix ; ~100 km /sec uncertainty,

(D. Kelson)



Emission and absorption


VPH Red Grism

15 min integration

Typical conditions

(E. Berger/M. Gladders)

To Do: Detector

-current detector is neither red-sensitive nor 4-amp

-this is now the primary limit to throughput

Peak Throughput: 41%

Peak Throughput: 42%

25% Edges: 4400Å-7800Å

25% Edges: 4400Å-9900Å

10% Edges: 3800Å-9200Å

10% Edges: 3900Å-10200Å

To Do: Fixes

-the shutter is working properly now, but solution is temporary. The problem is understood and harware

For permanent fix is on order (in Chile already?). Final fix shortly.

-hardware and software upgrades to the wheel controls are planned for the near future (next few months)

-the shutter and the wheels are two outstanding problems areas from BEFORE the upgrade. Once fixed, the burden of this instrument on mountain staff should decrease significantly

To Do: Future Improvements

  • -experience already demonstrates that filter changes will be needed regularly. Two improvements would facilitate this:

    • New door to access the filter wheel

    • New wheel to make filter insertion simple

  • -similarly grism changes are likely to happen, though less frequently

    • New door to access the grism wheel

    • Modified mounting hardware to access bolts

To Do: Future Improvements

  • -Two further grisms would improve science possibilities:

    • 300ln/mm VPH mid-band: replace old medium red

    • 1200ln/mm far blue/UV

-further improvements to the object acquisition software to minimize overheads