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Instrument upgrades / New instruments

Fiber feed from 3.5m to SDSS/APOGEE


We propose to initiate a project to implement a fiber optic network at Apache Point Observatory such that the existing (and future) spectrographs on site can be supplied with target light from any of four telescopes. The first phase of this implementation will be to develop the site infrastructure to connect all of the telescopes through the addition of a new utility pathway running from the ARC 3.5m past both of the small aperture telescopes and terminating at the SDSS telescope. This will be accomplished through a combination of below-ground conduits, above-ground cable trays, and existing cable routing paths available at each telescope. This infrastructure will be used to implement a fiber run between the SDSS/APOGEE spectrograph, the ARC 3.5 m telescope, the ARCSAT 0.5 m telescope, and the NMSU 1 m telescope. We propose to route 270 of the 300 available APOGEE fibers to the TR1 bent Naysmyth port of the 3.5 m telescope. Of these 270 fibers, 217 will be grouped together to form a integral filed unit ~32 arcsec size on sky. The remaining 53 fibers will be placed outbound of the IFU to sample sky. The combination of the 3.5m aperture and the APOGEE spectrograph used as an integral field unit will open exciting new science opportunities at a relatively low cost for the capability, leveraging the existing APOGEE instrument. The remaining 30 APOGEE fibers will be routed to the two small aperture telescopes, with 15 fibers going to the focal plane of the ARCSAT 0.5 m telescope for a future instrument application and the remaining 15 will be routed to the focal plane of the NMSU 1 m telescope where they will replace and improve the existing fiber run.

The full paper can be downloaded here.

Assumed Parameters for APOGEE:

  • R=22,500 from 1.51 - 1.70 um, 300 2“ dia fibers in SDSS 2.5m f/5 focal plane
  • ~15% throughput –> S/N = 100/pix for H > 12.2 in t = 3 hrs
  • radial velocity to 100 m/sec

Possible 3.5m Feed Configurations

  • Single fiber: 0.7” at F/10, 1.4“ at f/5 (i.e., with focal reduction)
  • IFU 1.4” spaxels, 217 elements, ~39“ across, lenslet array in front for ~100% fill factor and f/5 conversion
  • MOS ~ 200-300 1.4” fibers (with lenslets) in 8' FOV (pretty tightly packed!) or ~30 Randomly targeted objects (e.g in 7-fiber bundles) in 8' FOV.

Science Cases Overview

John Bally, Kevin Bundy, Jon Holtzman, Don York, Jennifer Sobeck


  • Nearby Galactic Star and star clusters [Bally ~ See attached]
  • Nearby HII regions and post-main-sequence objects [Bally ~ See attached]
  • Young Massive Clusters (YMCs) and Super Star Clusters (SSCs) [Bally ~ see attached]
  • Massive Stellar Transient Nearby Galaxies [Bally ~ See attached]
  • Individual abundances in globular cluster stars from near-IR, perhaps from an IFU [Holtzman]
  • Integrated light of globular clusters
  • Dwarf spheroidals [Holtzman, Sobeck]
  • Dwarf galaxies [Bundy, Holtzman]
  • Extragalactic (Emission lines, Stellar populations, and Stellar Dynamics) [Bundy ~ See below]

Single object science

  • Abundances of Hipparcos sub-giants [Holtzman]
  • Radial velocity monitoring of late-type stars
  • supernova followup in H band
  • Survey of B[e] stars [Chojnowski]


  • The nature of star and clusters in the Central Molecular Zone (CMZ) and nearby starburst galaxies [Bally ~ See attached]
  • Blind Emission-Line Searches of Deep Extra-Galactic Fields [Bally ~ See attached]

Future Fiber Feeds

  • A Visual-Wave fiber bundle for the APO 3.5 meter [Bally ~ See attached]
  • Visual wave band IFU observations of nebula [York ~ See below]

Detailed Science Cases

A number of science cases have been put forward by John Bally in an attached document “Science Case for a~300 fiber connection from the APO 3.5m to the SDSS spectrograph” for details please see the attachments of this page.

Single object possibilities

A single-object 3.5m feed is desirable for objects that are sufficiently spaced in the sky such that observing with the 2.5m does not offer benefits from wide field and for specific single objects of interest. The 3.5m would also potentially allow observations of fainter objects than can be done with the 2.5m. Note that the throughput gain comes from telescope area (3.5/2.5)^2^ = 1.96 (0.73 mag fainter) and also from the fact that the corrector at the 2.5m is estimated to absorb ~30% of the light in the H band (another 0.3 mag). On the other hand, there might be some additional light losses from a longer fiber run, especially if it were to involve multiple connectors. Also to avoid seeing losses that would be larger with the 1.4“ fibers even at F/5 from the 3.5m, one would want to consider a lenslet array in from of a fiber bundle, such that light was collected over multiple fibers without any loss between fibers. This might lead to additional throughput gains. But ballpark, a 3.5m feed would allow observations 1 mag fainter than at the 2.5m in the same time, e.g. S/N=100/pixel at H=13.2 in 3 hours, or S/N=100/pixel at H=11 in 1 hour.

One particular application might be observations of stars identified as subgiants once GAIA parallaxes become available. The great advantage of subgiants is that, with known distances, accurate ages can be determined. Ages in conjunction with the chemical abundances provided by APOGEE spectra provide a powerful tool for studying Galactic evolution. Subgiants are intrinsically fainter and bluer than the giants that are the targets of the main APOGEE survey. At M(H)~2, the 3.5m feed would be able to get good abundances to a distance of ~2 kpc. Of course, to build up a sample of a reasonable number of stars, one at a time, would require a significant amount of observing time.

Another application might be a survey of B[e] stars, which are B-type emission line stars that differ from classical Be stars due to the presence of forbidden emission lines and strong IR excesses. These features are attributed to a circumstellar dust component not present in the case of classical Be stars. The ~100 or so known B[e] stars are a heterogeneous group often found to be supergiants (sgB[e]), pre-main sequence (HAeB[e]), or compact planetary nebulae (cPNB[e]). However, the fact that ~50% of them remain unclassified (unclB[e]) is a testament to the complexity and richness of the emission line spectra, and also to the difficulty of conducting a uniform survey given the isolated nature of the stars. On average, B[e] stars are almost 4 magnitudes brighter in the H-band than in V-band, making them ideal targets for NIR spectroscopy. For more information, see [ Lamers et al. 1998], and Miroshnichenko et al. 2007 [ A] + [ B].


Prepared by Kevin Bundy


A 3.5m APOGEE feed coupled to large fiber-IFU could be very exciting in the context of extragalactic science. The overall sensitivity is a key challenge, but is worth more thought. Several science cases could be pursued further, but the instrument’s power for galaxy science could be enhanced if some changes in the spectrograph setup (wavelength range and resolution) were possible, namely a somewhat lower resolution and coverage in the J-band.

Emission Lines:

The current APOGEE wavelength range misses many strong emission lines, including Pachen Beta (1.282 um), H2, and several [FeII] lines in the J-band. This is too bad, because mapping narrow emission lines at high velocity resolution would be compelling for two reasons: 1) Less sensitivity to dust in the near-IR allows deeper probes of gas in dusty systems (e.g., LIRGS and ULIRGS) and 2) The [FeII] lines are diagnostics of electron temperature, density, and ionization field that arise in shocks and high-ionization regions (e.g., Thompson+95). They are therefore useful probes of AGN, winds, and shocks, phenomena that are often difficult to study again because of dust obscuration at bluer wavelengths (Iserlohe+13).

Stellar Populations:

Obviously, there are many spectral features however that provide chemical diagnostics of integrated stellar populations, including potential IMF-sensitive features that vary on the few percent level with IMF shape (requiring S/N~100, see Conroy & van Dokkum 2012). The spectral resolution required is about 6000-8000, so higher S/N could be achieved by spectral binning. The near-IR wavelengths are relatively unexplored territory for galaxy stellar population work, and APOGEE on the 2.5m provides a valuable “library” for interpreting it.

The main challenge is sensitivity, as shown in estimates below. Following up bright nearby galaxies from the ATLAS3D survey is worth further consideration however. The typical H-band surface brightness of these sources at 1 Re is about 20 mag per sq.arcsec, compared with APOGEE’s target of muH = 13.4 mag per sq.arcsec (H=12.2) at a S/N~140. Stellar population modeling (without IMF sensitivity) combined with fiber stacking and longer integrations could make achieving S/N~20 feasible in annular bins, however.

Stellar Dynamics:

The high spectral resolution naturally draws one to studying dynamically cold systems, where, for example, R~10,000 is sufficient for measuring out-of-plane galaxy disk velocity dispersions (5-10 km/s). This is the basis of exciting work by the DiskMass Survey (Bershady+10) to “weigh” the mass of galactic disks. Again, surface brightness is a challenge. A fairly bright galaxy in their sample reaches 18.0 Hmag/sq.arcsec at R~10’’. Their CaII Triplet focused observations aimed to achieve S/N~10. But, absorption lines at APOGEE wavelengths are much weaker. However, the advantage would be the ability to potentially trace kinematically a different stellar population and, again, better penetrate the dust in the near-IR.


Here are some down-and-dirty (and possibly suspect!) scaling arguments on the expected S/N. I’d be very curious if there are further gains with #5, and the benefits of a J-band option (#6) that would open access to more emission lines. With reductions in S/N targets, longer exposures, and by smoothing in wavelength, we start to approach the surface brightness limits argued for above.

Scaling S/N arguments, background limited:

  1. Start with S/N = 140/pix for H > 12.2 (Vega, from 2MASS) in t = 3 hrs (on the 2.5m)
  2. Larger 3.5m mirror (just D3.5/D2.5): S/N=100 at H=12.6 (muH = 13.8)
  3. Reduce S/N to 20: ΔH=2.1 (H=14.7) Reduce S/N to 10: ΔH=2.9
  4. Smooth in wavelength by a factor of 10 (Resolution now ~2000). Goes as sqrt(smoothing): ΔH=1.25 (H=16, muH = 17.2)
  5. Gains from less moon illumination (dark time), no MW background: ??
  6. Move to J-Band?
  7. Large (much larger?) fiber apertures: ?

More about APOGEE applicability for extragalactic studies

Prepared by Dmitry Bizyaev

Absorption spectra:

We dedicated a few APOGEE fibers in order to investigate what we can get for extragalactic studies in the frames of an APOGEE ancillary program in 2011. We have observed centers of M31, M32, and M110 in single APOGEE fibers (2” diameter) with the 2.5m telescope. Below is a table of the signal-to-noise obtained with one “visit” (67 min exposure time, typically with a plenty of Moon light). In addition to the center-of-galaxy exposures, a few fibers were allocated at “SDSS plate collision distance” (typically 80 arcsec) from the center of M32 along the minor and major axes. Figures below show combined spectra of the centers of M31 and M32, unbinned in the spectral direction, some 10 visit combined. The most prominent lines are Fe, Mg, Si, Al, and some molecular bands (CO).

Table: Signal-to-noise per pixel from extragalactic objects with APOGEE on 2.5m

Object SB(H),mag/sqarcsec SNR/pix, 1 visit (67 min)
M32 center 11.42 350
M31 center 11.76 178
M110 center 15.12 7

This is an option proposed by Bruce to use as much existing infrastructure as possible to join APOGEE to the 3.5 m. It would involve using existing conduits and would integrate into the Echelle focal plane utilizing the Echelle guider. I am assuming we would still correct the f-ratio to f/5, which is one of the remaining high cost items in this option. The two obvious advantages of this system are that it is far cheaper than any other option proposed, and it does not require forest service approval as no modifications to the site is required. The primary affects are as follows. 1) There is no field rotator on NA1. So the system would only be available for single object point source work no MOS or IFU mode would be available. 2) Throughput would be reduced due to the longer fiber run an extra ~12% (~10% to ~22%).

1/20/15 - I've updated the budget for the 'bare bones' option based on input from Mark and Bill - NKM

instruments.txt · Last modified: 2022/03/11 00:29 by holtz