MMT/Binospec: preliminary information

The Binospec spectrograph will be installed at the MMT telescope. Binospec is a multislit imaging spectrograph with two 8' x 15' fields of view, wide wavelength coverage, and high efficiency. There are two identical all-refractive spectrographs, one for each field. Each channel has several gratings and filters.

This page provides preliminary information about Binospec for potential users and proposers. Binospec is currently scheduled for commissioning in fall/winter 2017; pending commissioning, it may be available for shared-risk observing in spring 2018.

MMT Resources and official pages:

Binospec overview

Binospec is installed at the f/5 Cassegrain focus of the MMT. It has two fields of view that are 8' x 15' with a small gap of 1.7'. Two sheet metal slit masks held in a machined frame cover the fields of view. Each spectrograph has 3 gratings and 4 griz filters, and a 4K x 4K CCD detector. Two guiders and a wavefront sensor are used for guiding and image control. The spectrographs have active flexure compensation by moving the CCDs.

Each side has a 4K x 4K deep depleted E2V CCD detector with 15 micron pixels and a scale of 0.24 arcsec/pixel.

Slitmasks will be machined by a laser cutter at the MMT. Slitmasks will be configured by software written at SAO and running on a server hosted at the MMT. Users will log into a web interface to interactively design masks and submit them. Masks must be configured ahead of time to allow time for machining. Long slits will also be provided (of width ?). 10 slitmasks can be loaded simultaneously. Masks can be changed during the daytime (and potentially during the night pending approval of this mode and training of staff).

Spectral properties

Binospec has 3 gratings for each spectrograph. The grating resolution, approximate simultaneous wavelength coverage, and other properties including blaze, anamorphic demagnification, and pixel scale are given in this table for the 270, 600, and 1000 lines/mm gratings. The 600 line grating is shown at three different tilts. The usable central wavelength settings are given in a second table. The pixel scale along the slit is the normal 0.24 arcsec/pixel; due to anamorphic demagnification, the pixels per slit in the spectral direction are larger, so a 1 arcsec slit projects onto approx 3.2-3.7 pixels depending on the grating and tilt. The resolution listed is a FWHM (R = lambda/dlambda).

Grating lines/mm Order Blaze angle Angle of incidence Anamorph Demag. Coverage (A) Dispersion (A/pixel) Pixels per 1'' slit Resolution in 1'' slit
270 1 5.5 28.0 1.08 3900-9240 1.30 3.75 1340
600 1 16.0 33.2 1.17 4500-6960 0.60 3.47 2740
600 1 16.0 36.1 1.22 6000-8480 0.61 3.32 3590
600 1 16.0 38.5 1.27 7255-9750 0.61 3.20 4360
1000 1 13.75 37.1 1.24 3900-5400 0.36 3.27 3900

Grating lines/mm Allowable central wavelengths
270 5501 - 7838 A
600 5146 - 8783 A
1000 4108-4683, 5181-7273, 7363-7967, 8153-8772, 8897-9279
Ghosts may be troublesome with 1000 gpm grating at central wavelengths 5600-8500 A, worst near 7100 A.
Throughput of 1000 gpm grating will be low in the red.

Sensitivity / exposure times

Binospec is expected to be several times more efficient than Hectospec due to throughput and improved sky subtraction. This plot shows the calculated throughput as a function of wavelength for the 600 lines/mm grating. It will be updated when there are on-sky measurements from commissioning.

Throughput plot

Exposure time calculator

SAO has built an exposure time / sensitivity calculator for Binospec and Hectospec. The preliminary version is available at this link: Binospec exposure time calculator .

Binospec configuration

A model of the optical train of Binospec is shown below, illustrating the single slitmask at the top, and the paired spectrographs, filters, gratings, and detectors.

optical layout

The paired fields of view on the sky are shown below in yellow - the separation between them is 1.7 arcmin. The green boxes are the guider patrol regions, and the orange box is the wavefront sensor patrol region. The labeled dimensions in mm are in the telescope focal plane, not the CCD plane. The slitmasks cover the guider regions and holes are machined into into the masks at the location of guide stars, to allow for slitmask alignment and guiding.

Fields of CCDs and guiders

Observing procedures

Binospec observations will be done in a queue, similar to Hectospec and MMIRS. The queue will be administered by MMT staff and the instrument will usually be operated by MMT queue observers.

Binospec can hold 10 slitmasks, and these can be changed out freely during the day (unlike MMIRS), and potentially at nght. However, there is a lead time for machining masks.

The grating tilt controls wavelength coverage. We anticipate that a limited number of standard grating tilts will usually/initially be used, to ease calibration requirements and configuration changes during the night. The instrument can be changed from spectroscopic to imaging mode quickly, and will have g, r, i, z filters for each side. We do not anticipate initially offering mixed observing between the sides (ie imaging + spectra, or two different gratings).

Slitmask design

Slitmask design will be done by PIs using a web based interface. Masks will be machined with a laser cutter on the mountain by SAO staff. Masks will need to be submitted well in advance of the start of queue observations for a semester.

Note that the differential atmospheric refraction across the 15 arcminute field is significant, so that the location of slitlets depends on the hour angle at which the mask is observed. In practice this means that the mask machining would change slightly depending on whether the mask is observed when HA is East or West. Thus the date and time of observation specified during mask design are significant. Observable hour angles will be taken into account during queue scheduling.

An interface similar to the MMIRS user interface will allow PIs to submit mask designs and track their program progress. There will be a materials/machining charge of ? ($250 ??) per slitmask.

Data reduction

Binospec will have a data pipeline, written in IDL by SAO staff and publicly available, that is similar to the MMIRS pipeline. The SAO TDC is anticipated to return reduced data to the PIs. The timescale for data reduction is TBD. Availability of quick look reductions or quickly returning raw and reduced data are recognized as desirable for some programs and will be addressed. A quicklook for queue observers to assess data quality is in progress.

Other questions?

We will update this page as information is finalized during Binospec commissioning. Please contact Benjamin Weiner, bjw@mmto.org, for further information.


Page created by
Benjamin Weiner, bjw@mmto.org
Last modified: Tue Sep 5 14:35:02 MST 2017