Editing Apache Point Observatory (section)

=== ARC 3.5 m ===
[[File:D70050914 15 ApolloLLR.jpg|thumb|upright|The ARC 3.5-meter telescope]]

The ARC {{convert|3.5|m|in|abbr=on}} telescope is a [[Ritchey-Chretien]] reflector on an [[alt-azimuth mount]] with instruments attached at several [[Focus (optics)|focal points]]. Construction of the building began in 1985, but full operations of the telescope were delayed until November 1994 due to problems with fabricating the [[primary mirror]].<ref name="APOHIST" /> From 1991 until early 1993, the telescope was fitted with a 1.8&nbsp;m mirror, now located at [[Rothney Astrophysical Observatory]] under a [[cost-sharing agreement]].<ref name="REP92" /><ref name="REP94" />

There are a variety of optical and near-infrared instruments available for the 3.5 m telescope, including:
* The ARC [[echelle grating|echelle spectrometer]] (ARCES) uses a 2048 × 2048 pixel [[Charge-coupled device|CCD]] and has a [[spectral resolution|resolution]] of <math>R\sim 31{,}500</math>.<ref name="ARCES" />
* The Double Imaging Spectrometer (DIS) is a low-resolution optical spectrometer.<ref name="DIS" />
*KOSMOS, on long-term loan from NOIRLab, is a low-resolution optical spectrometer.
* The Near Infrared Camera/Fabry–Pérot Spectrometer (NICFPS) was developed at the [[University of Colorado at Boulder|University of Colorado]]. It uses a 1024 × 1024 H1RG [[HgCdTe]] infrared detector and a near-infrared [[Fabry–Pérot interferometer]]. It has many narrow band filters, including [[Molecular cloud|H2]], [[forbidden line|[Fe II]]], and [SiVI]. It is unique among astronomical Fabry-Pérot devices in that it is cooled with [[liquid nitrogen]].<ref name="NICFPS" />
* The Astrophysical Research Consortium Telescope Imaging Camera (ARCTIC) is an optical imaging instrument with a 4096 × 4096 pixel CCD.<ref>{{cite web | url=https://www.apo.nmsu.edu/arc35m/Instruments/ARCTIC/ | title=ARC 3.5m {{pipe}} Arctic }}</ref>
* TripleSpec (Tspec) is a near infrared spectrograph which provides continuous wavelength coverage over the range 0.94–2.46&nbsp;[[μm]] at moderate resolution (<math>R\sim 3500</math>, depending on the choice of slit).<ref name="TSPEC" />
*Agile is a high-speed imager with a 1024 x 1024 frame transfer CCD.{{cn|date=December 2021}}

The 3.5&nbsp;m telescope is also used by the [[Apache Point Observatory Lunar Laser-ranging Operation]] (APOLLO) lunar-ranging project.  The APOLLO laser has been operational since October 2005, and routinely accomplishes millimeter-level range accuracy between the Earth and the Moon.<ref name="APLO" /><ref name="2009BATT" />

Observations using the 3.5&nbsp;m telescope can be carried out remotely by observers using TUI, the Telescope User Interface, via the internet.<ref name="TUI" />