MMT/Binospec: preliminary information

The Binospec spectrograph has been commissioned at the MMT telescope in fall 2017. 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 was successfully commissioned in fall 2017 and is scheduled for shared-risk science observing in spring 2018.

MMT / Binospec Resources:

Contents

Binospec overview: basics

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 gap of 3.2'. Two sheet metal slit masks held in a machined frame cover the fields of view. Each slit mask can hold over 150 slitlets, parallel to the long side of the masks. 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. Guiding and alignment are done through stars viewed through holes at each end of the slit mask. 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. The resolution per pixel is tabulated below. The wavelength coverage of a slitlet depends on the distance from slitlet to optical axis along the X direction (along the short side of the masks). From near (central) side of the mask to the far side, the change in coverage is approximately 900 A to the blue for the 600 gpm grating.

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. Masks with long slits are also provided; the widths available are 0.75", 1.0", 1.25", 1.5", and 5". One long-slit mask will always be loaded, typically the 1.0". If different long-slit widths are needed, please request these widths in your target catalog submission so they can be loaded ahead of time. 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 wavelengths used for the flexure control system limit the usable central wavelength settings to those given in the 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 (approx 6560 A for full wavelength coverage)
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. Here are on-sky throughput measurements made with the three gratings during commissioning, which should be taken as lower limits as the sky was not completely photometric. Three central wavelength settings are included for the 600gpm grating.

Throughput measurements

Exposure time calculator

SAO has built an exposure time / sensitivity calculator for Binospec and Hectospec. A preliminary version is available at this link: Binospec exposure time calculator , which also gives you an option to download the package.

A direct link to the spectroscopic exposure time calculator for spectra is: http://hopper.si.edu/etc-cgi/TEST/sao-etc . For imaging see this calculator (select MMTcam): http://www.cfa.harvard.edu/mmti/megacam/obs_manual/exptime.html

For standard stars and similar calibrations, be careful not to saturate the detector. A preliminary suggestion is an exposure of 15 seconds for a typical spectrophotometric standard at V=11. Please be aware that accurate coordinates in the Gaia frame and proper motions are necessary for standards; many standards have high proper motions.

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

Optical train of Binospec.

The paired fields of view on the sky are shown below in green - the separation between them is 3.2 arcmin. The yellow boxes are the guider patrol regions, and the blue box is the wavefront sensor patrol region. 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
shown on a sky image

Binospec on the sky. Green rectangles are the spectroscopic fields, yellow are the allowed positions for guide stars. The blue rectangle shows the wavefront-sensor patrol area. Red dots are targets; blue are guide stars; short cyan lines show the slits assigned to targets.

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. PIs/guest observers will typically not come to the telescope. PIs will submit observing catalogs and mask designs through the MMT queue scheduling interface. Prior to the run, PIs will get an email asking them to log in to the queue scheduler to upload catalogs and observation details.

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).

Target acquisition and coordinates

Binospec uses guiders offset from the main field of view, patrolling the yellow box regions in the figure above. Guide stars can be selected automatically from GAIA (preferred) or GSC2 catalogs. Thus, it is necessary to supply coordinates that are on the GAIA or GSC2 system. The correct coordinate system is critically important for both longslit and slitmask spectroscopy.

Targets for longslit spectroscopy will be offset onto the slit by acquiring stars onto the guiders, whose positions have been calibrated relative to the slit. Thus, your target position needs to be on the same system as the guide stars. If you provide a target position that is on the Gaia system, the guiders can bring the target directly onto the slit without any extra effort. This is strongly preferred as the way of acquiring a target. The preferred method of acquisition is very accurate, but your coordinates must be good and on the Gaia system. If the target has significant proper motion, these must be provided! Many standard stars have high proper motions.

It may be possible in future to tune the pointing up by tilting the grating to zeroth order and taking a direct image of the target through the slit. This would work for objects brighter than some magnitude (R~21). For faint targets, PIs could provide a nearby offset star on the same coordinate system as the target. However, these modes are not in common use now and providing Gaia-frame coordinates is preferred. Finding charts and other useful information about the target can be entered in the "notes" section of your observing catalog.

Acquiring using a slit-viewing camera requires a "mirrored" slit mask whose use has not yet been commissioned, so good coordinates that allow the above procedure to work are a requirement.

For multi-slit masks, the PI will provide a list of targets on GAIA or GSC2 coordinate systems. The BinoMask slitmask design program will add suitable guide stars and cut alignment holes in the slitmask, through which the guiders will image the stars and tune up the alignment of the mask.

Slitmask design

Slitmask design will be done by PIs using the BinoMask 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.

Please see the BinoMask slitmask design interface tutorial webpage for a more detailed walkthrough of using the software.

You will be running a web-based mask making program called BinoMask which will take a text file as input, containing your targets to be observed. If your objects are on the GAIA coordinate system, binomask can add guide and wavefront-sensor stars for you. If not, you must create a second catalog with such stars on the same coordinate system as your targets, extending to at least 0.4 degree in radius from the center of your target field. For guide stars, please select a magnitude range of r=14-18; for WFS stars, magnitude r=11-14.

The format for the catalogs is an ASCII file with columns. You may separate entries either with a comma or a tab. The first line of the file is a header:

name, ra, dec, magnitude, priority, pm-ra, pm-dec, epoch, type
The following lines contain entries for those columns.

Only ra and dec are required, but name, magnitude, & priority are also recommended. (1=highest priority, then 2, 3, etc). Coordinates may be in decimal degrees or sexagesimal. Type should be 1 (or “target”), 2 (“sky”) or 3 (“standard”).

A queue scheduling interface similar to the MMIRS user interface allows PIs to submit mask designs and track their program progress. The materials/machining cost for slitmasks will be determined at a later date.

Mask making is integrated with telescope scheduling software. Thus if you have been granted time, you will receive an email from scheduler@mmto.org with instructions to log in, make masks, and submit catalogs. You may also use the mask making software outside of that framework. To run the BinoMask software visit scheduler.mmto.arizona.edu/BinoMask/. Slit widths are also set by the user. As an aid, the MMT web site states that the median seeing is in the range of 0.77" - 0.85" based on data from 2003 to the present. 1 arcsecond slits are a good starting point.

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.

You may get the error “slits can not be made” which often occurs because the rotator setting is not correct. Try adding or subtracting 180 from the present setting.

It is good practice to avoid mask position angles that run into the rotator limits (which are near +180/-180). Binomask will try to avoid PAs that hit the limits in the middle of your observing window. In practice, the limits mean that to avoid hitting the limit at transit, for masks that are north of the MMT (Dec > +33), you should avoid position angles near 0, and for masks south of the MMT (Dec < +33), you should avoid PAs near +180 or -180.

Queue scheduling

In order to obtain the maximum scientific impact from Binospec, we will to continue the queue scheduling that has proven successful with Hectospec, Hectochelle and MMIRS. Queue scheduling offers the considerable advantage that we can flexibly adapt the observing program to the current conditions. We will reserve conditions of perfect transparency and superb seeing for those projects that can take best advantage of excellent conditions. Queue scheduling also allows observations to be obtained at low air-mass and allows continuous observation of high priority objects. The instrumeht will be operated by MMT staff queue observers, and proposers are not generally expected to travel to the telescope.

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.

The raw data are stored as multiextension fits files; each CCD has 4 amplifiers, each of which is stored as an extension. Data taken are archived at the CfA. A pipeline under development (in late 2017) will be used to process the data and provide wavelength calibrated, relative-flux corrected spectra. In progress: the archive data system will send you a notification the day after any of your data is taken, at which time you may download the raw data. A few days later, you will receive another message concerning the reduced data. We are also developing an addition to the MMT queue catalog management pages that will allow you to view available data files and download them from your catalog page.

Example images

These images were taken during Binospec commissioning in fall 2017.

Single exposure

A multislit single exposure of 600 sec. Blue wavelengths are toward the center; red is to the left in the left frame and to the right in the right frame. Note the cosmic ray hits.

Combined exposure

Combination of 4 exposures. Ca H and K absorption lines are visible as a doublet near the blue end of several spectra.

exposure with many short slits>

A 600 sec single exposure on a mask with many short, 2 arcsec long slits, using the 600 gpm grating

direct g band exposure

A direct imaging exposure, 50 sec in g band.

Other questions?

We will update this page as information is finalized during Binospec commissioning and preparations for the first semester of operations in 2018A. Please contact Benjamin Weiner, bjw@mmto.org, for further information.


Page created by
Benjamin Weiner, bjw@mmto.org
with contributions from Nelson Caldwell. Last modified: Thu Jun 7 23:30:47 MST 2018