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OSMOS User's Manual

[M51 Image] [OSMOS On Telescope] [QSO at z=6.4] [QSO at z=6.4]

The new R4K detector system was commissioned in September 2011. Click here for further details and pictures.

The multi-object capability was commissioned in May 2011. Click here for further details and pictures.


Quick Reference
Shutter Timing
Multi-Object Spectroscopy
Collimator and Camera Focus
User Interface and Prospero
Longslit Acquisition
Multi-Object Acquisition
Instrument Configuration
Flat Fields
Wavelength Calibration
Data Analysis
Technical Information


OSMOS (Ohio State Multi-Object Spectrograph) is a wide field imager and multi-object spectrograph that was commissioned at the 2.4m Hiltner telescope at the MDM Observatory in April 2010. OSMOS employs an all-refractive, zero-deviation design that projects a plate scale of 0.3"/pixel and a 20' diameter FOV onto the MDM4K or R4K detector systems. There is a 6-position slit wheel, a 6-position disperser wheel, and two 6-position filter wheels. The slit wheel may contain either long slits or customized masks that are laser cut into spherical, NiColoy masks that match the curvature of the 2.4m focal surface. Up to 50 to 100 slitlets should be feasible per mask. The current complement of dispersing elements includes a very low resolution triple prism and a high-efficiency, low-resolution VPH grism (R=1600, peak at 450nm). Anyone interested in supplying additional dispersers is encouraged to contact Paul Martini.

Click here for construction photos.

More information is available via these detailed instrument characteristics and these two papers:

Mechanisms and Instrument Electronics for the Ohio State Multi-Object Spectrograph by Stoll et al. 2010, SPIE, 7735, 154
The Ohio State Multi-Object Spectrograph by Martini et al. 2011, PASP, 123, 187
Please consider citing these papers in any publications that employ new data from OSMOS.

Quick Reference

Plate scale: 0.273 arcseconds per pixel
Unvignetted Field of View: 20' diameter circle, detector subtends a 18.5' x 18.5' square
Filters: All MDM Filters (4-inch filter cell) + New izY DES filters
Performance: See the magnitude zeropoints in the Performance section below
Image Quality: FWHM in UBVRI as a function of field angle for OSMOS only (atmosphere not included)
Finding charts: An easy way to generate them on the fly is with ds9 (Analysis -> Image Servers -> SAO-DSS)

Long slits: 0.9, 1.2, 1.4, 3, and 10 arcsecond wide slits are available (they are nearly 20' long). The 0.9" and 1.2" (both center and inner) are loaded by default. Please specify other choices on your setup form.
Triple prism: R=100-400 (highest resolution in the UV), 360-1000nm [Resolution vs. Wavelength for a 3-pixel slit]
VPH grism: R=1600 (0.7 A/pix), ranges of 310-590nm (peak at 500nm) and 390-680nm (peak at 640nm). See the Wavelength Calibration section for more information. [Predicted Efficiency (for a centered longslit)]
Acquisition: There is no slit viewer. See the Longslit Acquisition or Multi-Object Acquisition sections for more details on how to acquire objects for spectroscopy.
Multi-Slit Masks: Custom, laser-cut slits in electroformed spherical shells of NiColoy coated with Copper Oxide (CuO) black. Please see the section on Multi-Object Spectroscopy for information on mask design.
Orientation: Please specify either E-W or N-S orientation for slits (with the rotator at 0 degrees) in your setup form. Note that the wavelength solution zeropoint varies more with the N-S orientation due to instrument flexure.

Here is an OSMOS logsheet.

There are numerous useful Prospero scripts in the Scripts/ directory on hiltner. Observers are encouraged to look through these scripts. A brief guide to the most useful scripts may be written in the future.



Photometric magnitude zeropoints for OSMOS were measured in April 2010 with the MDM4K and in September 2011 with the R4K. These magnitude zeropoints correspond to 1 e- per second.

Vega magnitudes, MDM4K

AB magnitudes, R4K

Paul Martini's page on Useful Astronomical Data has information about Flux Zeropoints.


Basic information about wavelength coverage and resolution is available in the Quick Reference section. More details on performance will be posted shortly.


The OSMOS slit, disperser, and filter wheels require up to 3 seconds to change configuration and the instrument can consequently be reconfigured quite rapidly. Detector readout varies from tens of seconds to two minutes, depending on binning and whether the full chip is read out. Long slit acquisition takes approximately five minutes, although may require more time if there are problems with the telescope offsets. Here is a table that summarizes these overheads:
slit/disperser/filter wheel move: 3 seconds (max)
full detector readout binned 1x1: 134 seconds
full detector readout binned 4x4: 16 seconds
1k x 1k detector readout binned 1x1: 15 seconds
Long slit acquisition: 5 minutes (variable)
The next subsection contains more information about the detectors.


OSMOS may be used with either the MDM4K or the R4K detector. Both are 4k squared devices with 15um pixels. The MDM4K has better sensitivity in the blue and the R4K has better sensitivity in the red. Here is a QE comparison figure that is based on data from the manufacturers. The R4K also exhibits substantially less fringing, but does have a higher rate of radiation events (a.k.a. cosmic rays).

Region of Interest

One may reconfigure either detector to read out a specific 'Region of Interest' (ROI), rather than the full detector. This mode has the advanges of faster readout and smaller image files and may be particularly valuable for judging when to start twilight flats, focus sequences, and spectroscopic observations. Configuration files for 1k square, 2k square, 1x2k, and 1x4k regions centered on the detector are presently available. To implement an ROI for the MDM4K in Prospero type:

pr> call roi1k
pr> call roi2k
pr> call roi2x1k
pr> call roi4x1k
and type
pr> call roi4k
to return to full frame readout. For the R4K detector the commands are instead:
pr> call rroi1k
pr> call rroi4k

Note that the 2x1k ROI is well-suited to single-object spectroscopy with the triple prism and the 4x1k ROI is well-suited to single-object spectroscopy with the VPH grism. (The old 'init' versions of these ROI scripts were replaced with the present set of 'roi' scripts in October 2011.)

Shutter Timing

OSMOS uses a Prontor/E100 shutter. This is a leaf shutter that may introduce timing errors in short exposures. To attain better than 1 percent measurement precision, one should employ exposures longer than 10 seconds (including for twilight flats). The figure below demonstrates the shutter timing error for a 1s exposure. In this image the peak at the center of the field is approximately 5 percent higher than at the edges.

Multi-Object Spectroscopy

Mask Preparation

Observers interested in multi-object masks presently should contact Paul Martini at least two months before their run.

OSMOS masks may be designed with a software package called OSMOS Mask Simulatior (OMS) OMS is a slightly modified version of MMS, which is the mask design software for MODS (MMS is in turn a slightly modified version of LMS, which is the mask design software for LUCI). In addition to the OMS web page, the web pages for MMS and especially LMS contain additional documentation.

Mask alignment is performed with alignment stars. These stars will have square apertures in the mask at their locations relative to the slits. While in principle only two are necessary to solve for the rotation and translation offsets required for mask alignment, at least four are recommended to guarantee a good solution. To speed the mask alignment process, these stars should be in the central 4x1k region of the detector so that this ROI may be used to align the mask. These stars should also be well distributed across the field to maximize the lever arm for the rotation offset calculation.

The outputs of OMS include a file in gerber format (.gbr extension) that contains instructions to the laser cutting machine on how to cut the mask and mask description file (.oms extension) that describes the mapping between targets and the slit mask. Note that it may take up to a month to arrange for the masks to be cut. Observers will consequently need to send their final mask designs to Paul Martini at least a month in advance of their run to guarantee that the masks will be ready. An image of an OSMOS mask in a cell is shown here

The cost per mask is still TBD, but will likely be $150 to $200. This cost covers the electroformed spherical shell and the labor for the laser machine operation.

Multi-Object Observations

Multi-object observations are similar to longslit observations, with the exception that the rotator angle must also be set very precisely (to within 0.01 degrees). This can be achieved with custom mask alignment software available at the telescope and is described in the section on Multi-Object Acquisition.

Guidelines for Calibration

The present MIS calibration system does not uniformly illuminate the entire OSMOS FOV. A reasonable solution is to simply take very long exposures during the afternoon until sufficient signal is obtained in all of the slits for both arc lamps and spectroscopic flats. Twilight spectroscopic flats could be used to remove the non-uniformity of the illumination by the MIS flat lamps. See also the section on Flat Fields for more information.

The zeropoint of the wavelength solution will likely drift between the afternoon calibations and the nighttime observations due to instrument flexure. Sky lines could be used to compute this zeropoint offset. See also the section on Wavelength Calibration for further information on specific dispersers.


OSMOS will typically be ready for operation when observers arrive for their first night. The MDM staff will perform the following tasks before a run, but they will be necessary for observers to follow in the event of a power failure or lightning shutdown:

Turn on the IC (Computer Room) and type 'O' for OSMOS and choose the appropriate detector.
Turn on the Instrument Electronics Box
Turn on the Head Electronics Box
The next step is to launch various control clients and the User Interface on hiltner. From the "Data Acquisition" menu on the desktop, select the following options in the order they are listed:

Then type startup in Prospero to load the current instrument configuration. The last step is to place the collimator and camera lens barrels at their nominal position. This is described in the next section.

Finally, make sure that the OSMOS instrument hatch is open. OSMOS has its own instrument hatch that is separate from the MIS hatch. The lever to open and close this hatch is on the opposite side of the instrument from the access doors for the slit and filter wheels.

There is a separate web page with Detailed Startup and Usage Information.

Collimator and Camera Focus

OSMOS has two internal focus stages, one for the collimator and one for the camera. If the IEB power is cycled, these stages need to be reset with the following commands:
pr> colfoc reset 2300
pr> camfoc reset NNNN
where NNNN is the temperature-dependent focus value for the camera. At 15 C the best camera focus value is 5800. At 25C the best value is 5300. These commands take approximately 5 and 10 seconds, respectively. More information on temperature dependence will be posted here as data become available. Instructions on how to recalculate the focus of the collimator and camera will be posted in the near future.

User Interface and Prospero

OSMOS is operated with the Prospero User Interface written by Rick Pogge. Please see his extensive online documentation for general information about how to use this software. Below is a brief synopsis of the most commonly used commands.

Basic Commands

The instrument configuration is controlled with six basic commands:

pr> slit N
pr> disp N
pr> filter1 N
pr> filter2 N
pr> colfoc FFFF
pr> camfoc GGGG
The slit, disperser, and filters take an integer argument [1-6] that corresponds to the desired aperture position. Executing the command with no arguments will return the current position of the wheel. The collimator and camera focus stages are controlled with the colfoc and camfoc commands, respectively. An integer argument to these commands (i.e. the FFFF or GGGG variables above) moves the appropriate stage to that absolute position in microns.

Data Acquisition

pr> go take an exposure
pr> exp T set the exposure time to T seconds
pr> object name set the object name to name
pr> filename name set the filename to name
pr> print wheel print the current population of wheel [slit, disp, or filter]
pr> call script run the Prospero script named script.pro
pr> snap take an image but do not save it to disk
pr> newext num change the file number to num

The tedit command may be used to update the tables that map wheel positions to their contents (e.g. that position 6 is open). Please do not edit these tables without good reason! And please also double check these tables (ideally with the help of the observatory staff) if you suspect the tables are incorrect.

Prospero Scripts

Prospero has a powerful scripting capability that can be used to automate many common tasks. Examples include mask alignment (via oalign.pro), telescope focus (via focus4k.pro) automated, guided dither sequences for deep imaging (via four.pro and nine.pro, and changing the ROI (e.g. roi1k.pro, roi4k.pro). These and many other scripts are available in the Scripts/ subdirectory on hiltner. Further details on how to write Prospero scripts are available in the Prospero online documentation. Should the OSMOS-related scripts discussed here become compromised, here is a backup .tgz archive of some scripts.

Telescope Focus

A quick way to get close to the best focus is to find a reasonably bright star, switch to the 1k ROI, turn on movie mode (type the movie command), and focus by eye until the star appears reasonably in focus. (Type stopmovie to exit movie mode.)

A simple Prospero script called focus4k.pro is available to help refine the telescope focus once it is close. This script takes a sequence of 5 focus frames in steps of 10 units starting with an input, minimum value. The syntax is:

pr> call focus4k focmin
where focmin is the desired starting value. Note that it is important to set the instrument configuration (filter, ROI, etc.) before you start this script.


A convenient way to identify guide stars for OSMOS is with JSkyCalc24mGS by John Thorstensen. The new Fingerlakes Guide Camera can be used to guide with stars as faint as 16 mag, although 13-14 mag stars are recommended.

Instructions on how to use the guider are available in these Detailed Startup and Usage Instructions. More details about how to use the guider are available from the guide to Autoguiding and Acquisition at MDM by John Thorstensen.

JSkyCalc24mGS is also used for spectroscopic acquisition, which is described further in the next sections on Longslit Acquisition and Multi-Object Acquisition.

Note that the OSMOS FOV includes nearly the entire unvignetted footprint of the MIS, which means that guide stars need to be near (or ideally in) the partially vignetted beam for imaging (or multi-slit spectroscopy) that employs the full FOV. The box labeled "Probe can block detector inside this region" in the figure at the top of the JSkyCalc24mGS manual is much smaller than the equivalent, circular region for OSMOS.

Longslit Acquisition

OSMOS does not have a slit viewer. Instead, targets are placed in a slit with the use of telescope offsets to the known position of the slit projected onto the sky. The object can then be imaged through the slit after acquisition to confirm that it has been acquired.

Guider offsets are much more precise than telescope offsets at the 2.4m and consequently it is much more efficient to offset the guide box the desired amount and then move the telescope to put the guide star back in the guide box. There are two algorithms for longslit acquisition. One uses a pyraf code by John Thorstensen and this is described in the Detailed Startup and Usage Instructions.

Alternatively, one may use the -l option in oalign.py, which is also used for Multi-Object Acquisition. (There is also a Prospero script called olsalign.pro that performs a similar function to oalign.pro.) Usage of these scripts is described in the next section on Multi-Object Acquisition.

Finally, the old long slit acquisition procedure describes how to perform these steps more manually and less efficiently.

Multi-Object Acquisition

Acquisition of the targets in a multi-object slit mask requires very precise translation (on order 0.1" in x and y) and rotation (on order 0.01 degrees) offsets. This requires an image of the slit mask (which should be obtained immediately prior to alignment to minimize flexure) and an image of the field. The relative positions of the alignment boxes and the alignment stars may then be used to calculate the translation and rotation offsets.

There are two scripts that aid in efficient mask alignment: The Prospero script oalign.pro and the python script oalign.py. These scripts are meant to be used in conjunction with one another. oalign.pro is a Prospero script (note the .pro extension) that facilitates the acquisition of the images necessary for mask alignment and the proper configuration of the instrument and oalign.py is a python script that is used to calculate the offsets needed for mask alignment. The rotation offset is sent via the Rotator GUI and the translation offsets are best sent by moving the guider stage. (Please read the Guide to the new MDM Hiltner Rotator for usage instructions for the rotator.) oalign.py must be run in a separate terminal from Prospero.

A step-by-step guide to a mask alignment is provided in this OSMOS Mask Alignment Cookbook.

Instrument Configuration

The instrument configuration will generally be set by the MDM staff before the start of an observing run. Use the print command in Prospero to view the current configuration of a given mechanism, for example:
pr> print slit
pr> print filter
pr> print disp
For all mechanisms, including camfoc and colfoc, typing the command with no arguments returns the current configuration. This information is also listed on the Prospero Status Window. See the information on the User Interface and Prospero for more information.

Filter and slit exchanges

Filter and slit exchanges should only be performed by, or in consultation with, Observatory staff. You should contact them with any special configuration information in advance of your run.

Flat Fields

Imaging Flats

Twilight flats are the most reliable flats to calibrate imaging data. A good strategy for evening twilights is to use movie mode and 1k Region of Interest until the sky is no longer saturated. For example, if you are using the MDM4K type:

pr> call roi1k
pr> movie
To exit movie mode and return to full array readout type:
pr> stopmovie
pr> call roi4k
For the R4K the equivalent ROI commands are call rroi1k and call rroi4k.

Note that exposure times longer than 10s are recommended for flats to avoid significant Shutter Timing errors. Note also that for the standard BVRI set (Harris set), the order of the filters from most sensitive to least sensitive is IRVB.

Spectroscopic Flats

The MIS flat lamp is useful for the removal of small-scale spatial variations along the slit, although they are not useful for large-scale variations because they do not uniformly illuminate the slit. Spectroscopic twilight flats should provide an adequate measure of large-scale variations along the slit.

Wavelength Calibration

Triple Prism

The best wavelength calibration source for the triple prism is a bright, compact planetary nebula. None of the MIS lamps offer a good selection of unblended lines over the entire wavelength range accessible with the prism. The best wavelength solution obtained during commissioning employed a 4th-order spline and yielded an rms of about 1nm. Here is a screenshot of the identify task in IRAF with planetary nebula lines labeled and here is a list of the emission lines. Many of these lines are actually blends, particularly in the red, and therefore there is room for improvement.

Medium Blue VPH Grism

In addition to a longslit through the center of the field, two other slits are available that are +/- 30mm (about +/- 5.7 arcmin) from the center slit (but parallel to it). These offset slits allow one to take advantage of one of the unique properties of VPH gratings, namely that the wavelength of peak diffraction efficiency varies as a function of the angle of incidence inside the grating with respect to the fringe plane. These offset slits are referred to as the inner slit and outer slit to differentiate them from the center slit and these names refer to their position in the slit wheel, i.e. the slit closest to the axis of rotation is the inner slit. Here are the wavelength ranges and approximate wavelengths of peak efficiency for these three positions:

Inner Slit (or Red Slit)
Full range on detector: 390-680nm
Approximate efficiency peak: 640nm
Center Slit( or Blue Slit)
Full range on detector: 310-595nm
Approximate efficiency peak: 500nm
Outer Slit( or "Not too useful" Slit)
Full range on detector: 220-490nm
Approximate efficiency peak: 420nm

Matthias Dietrich has created a series of Wavelength Calibration Figures and Tables for the MIS lamps. Please see this information for further details.

Spatial Extent

The calibration lamps in the MIS do not fully or uniformly illuminate the entire OSMOS field. This will result in lower illumination at the ends of the slits. There is approximately a factor of two decrease in a long slit lamp at 5 arcminutes from the center of the field. Some points that are farther off axis still appear completely vignetted. Complete vignetting does not occur for the long slit, but may impact multi-slit observations. Below is an image of the Ne lamp through the U filter that shows the illumination pattern.

Data Analysis

There is no comprehensive data analysis package for OSMOS; however, Jason Eastman has written an IDL package that should be suitable for processing imaging data. His package is described at the bottom of his manual for the 4k detector.

John Thorstensen has also adopted his qccds.html script for OSMOS. OSMOS-specific documentation should be available in the near future.

The IRAF script bias4k.cl may be useful for quick-look bias subtraction at the telescope.

Technical Information

Here is a collection of technical information:
Telescope Installation and Removal Instructions
Recommended Specifications for New Filters
Instrument Characteristics


The slit [or disperser or a filter] wheel does not return a valid position

Move the wheel to the nearest position with the command
[aperture] reset
Where aperture is slit/disp/filter1/filter2.

The connection to the IC is lost

This may happen due to a timeout in waiting for the IC to respond. If this does occur and an exposure is in progress, let the exposure finish. It will still be written to the disk. To reestablish the link type:
PR> restart
If this is unsuccessful, try the steps outlined next in The IC stops responding
The IC stops responding

Follow the lightning shutdown procedure (for the IC only) and restart the software. Note in particular the following steps:
Select 'O' on the IC computer (in the computer room) when you restart the IC
Restart the OSMOS programs on hiltner as described in the Startup section

If this does not work, repeat the lightning shutdown procedure but also power off the HE on OSMOS.
Note that the HE is the red electronics box mounted to the side of OSMOS and its power switch is next to the digital temperature display
Power the HE back on and continue the startup procedure described above

Instrument Status Window in Prospero does not appear

Type startup in the Prospero window.

No connection to the guider camera

Step 1:
Close and reopen Maxim DL. If this does not work go to ...
Step 2:
Shut down the PC by holding the power button down to fully power it off, and then start it up again. If this does not work go to ...
Step 3:
Close Maxim DL, go into the dome, and physically disconnect the guider camera power supply and then reconnect it. The camera is located on the North side of the telescope (Control Room side). If you climb a ladder near the MIS hatch, you should see the power cable coming out of a small box labeled FLI on the East side. Disconnect and reconnect this cable and then go back into the control room and restart Maxim DL.

Shutter does not open fully in cold weather

The shutter has not opened fully in very cold weather. This problem was reported in early January 2011 and manifests as partial to complete vignetting of the detector. The temperature at the time was approximately -7 C and, while these conditions are rare at the site, observers are strongly encouraged to be alert to this problem under comparable conditions.

The connection to the IC is lost

Once the connection to the IC was lost during a call to roi4k. The error messages were:

Error: Requested Operation Timed Out
ERROR: Could not get IC status
**ERROR: Cannot get the data-taking system status
This was corrected by typing restart in the Prospero window.


Principal Investigator: Paul Martini
Co-Investigator: Rick Pogge
Mechanical Engineer: Mark Derwent
Optical Engineer: Ross Zhelem
Graduate Student: Rebecca Stoll
Electrical Engineer: Dan Pappalardo
Software Engineer: Ray Gonzalez
Undergraduate Student: Man-Hong Wong


OSMOS has been generously funded by the National Science Foundation and the Center for Cosmology and AstroParticle Physics at The Ohio State University. Additional support has also been provided by the Department of Astronomy at The Ohio State University and the Department of Physics and Astronomy at Ohio University.
The National Science Foundation The Center for Cosmology and AstroParticle Physics The Ohio State University Ohio University

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Updated: 2012 April 10 [pm]