Boller and Chivens CCD Spectrograph (CCDS)

This page is currently under construction, but comments are welcome.

Last updated: January 7, 1998

Instrument Scientist and Staff Contact: R. Mark Wagner, Department of Astronomy, Ohio State University; Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, (520) 779-0106, email: rmw@lowell.edu

Contents:

• Summary
• Optical Layout
• Pictures of the mounted CCDS
• CCD characteristics and parameters
• Gratings and spectral information
• Performance and throughput
• Slit width adjustment
• Order sorting filters
• Comparison lamp templates
• Operation
• Mounting and preparation instructions
• Additional documentation

Summary

The Boller and Chivens CCD Spectrograph (CCDS) is a conventional optical grating spectrograph operating in the 3200 - 9500 Å region for use on the Perkins 1.8-m telescope of the Ohio Wesleyan and Ohio State Universities at the Lowell Observatory. It was acquired by the Ohio State University through a grant from the National Science Foundation in 1970 for stellar spectral classification programs. Eight gratings are available providing spectral resolutions between 400 and 12,000 assuming a 2 arcsec wide entrance slit. The slit width is continuously adjustable between 1 and 20 arcsec. The detector is a Loral 1200 × 800 CCD utilizing permanent backside charging and a dual-layer anti-reflection coating which provide high quantum efficiency between 3200 and 9500 Å. The CCD read noise is 7 electrons at a gain of 2 electrons per ADU. Both FeNe and HeAr lamps are available for wavelength calibration. An internal quartz-halogen lamp provides a continuous spectrum for flatfields. At the lowest dispersion, the total system quantum efficiency is about 11% at 5500 Å. An ISIT camera records the image provided by the telescope on the polished entrance slit jaws and for an integration time of a few seconds, point sources with magnitudes just below the POSS limit can be seen over most of the 3 arcmin field of view. A signal-to-noise ratio of 10 per pixel can be achived in a 1 hour exposure for m(5556) = 19.5 mag with the 150 lines per mm grating under conditions of good seeing and transparency.

Optical layout

Pictures of the mounted CCDS: Southeast view; Northeast view; Northwest view; Southwest view

CCD characteristics and parameters

• Designation: OSU W19-(1,1)
• Manufacturer: Loral
• Thinned and packaged by the Steward Observatory CCD Laboratory
• Mounted and interfaced by the OSU Imaging Sciences Laboratory
• Dimensions: 1200 × 800; 15 µm square pixels
• Features Permanent backside charging; no UV flooding required
• Dual-layer anti-reflection coating for broad wavelength sensitivity (400 Å HfO₂ + 900 Å MgF₂ coatings)
• Measured W19-(1,1) quantum efficiency curve
• Read noise: 7 electrons
• Gain: 2 electrons per ADU
• Linearity: Linear to A/D saturation at 65,535 ADU
• Operating temperature: -113 C
• Orientation: Shorter wavelengths are at smaller column numbers; East on the slit corresponds to smaller row numbers
• Spatial scale: 0.56 arcsec per unbinned pixel
• Binning and overscan regions are user selectable

Gratings and spectral information

Notes: The 158 gpm and 1200 gpm gratings are currently not in holders. Perspective users should contact RMW as to their availability and performance..

Performance and throughput

• Count rate: The Perkins telescope and CCDS with 150 gpm grating centered at 4900 Å yields a net count rate of 108 ADU/sec or 216 electrons/sec in an extracted spectrum at 5556 Å for a star of m(5556) = 10.0 mag at unit airmass assuming no light losses. As we receive more information on the performance of the CCDS with other gratings, we will include those results here as well.

• Sensitivity function: The sensitivity or response function for the CCDS utilizing two different CCD detectors on the Perkins telescope is shown below. The function is the convolution of contributions from the telescope and its reflective surfaces, the spectrograph and its optics, and the detector. It was derived based on spectra of the standard star Hiltner 102 obtained under photometric conditions on December 12, 1996 just after installation of the new Loral 1200 x 800 CCD. To avoid light losses which would affect the measurement, the slit was widened to 20 arcsec. The 150 gpm grating was employed and was centered at 4500 Å and 7300pÅ respectively. Exposure times were 120 s in the blue and 60 s in the red and no order separation filters were used. The stellar spectra were extracted from the frames with sky subtraction and were corrected for the effects of atmospheric extinction, but were not flat fielded. The blue and red response functions matched to better than 1% in the overlap region. The curve shows the effects of the blaze of the grating and the response of the CCD. With this combination, peak efficiency is about 11% at 5900 Å, but the instrument has excellent blue response as well by virtue of the dual-layer AR coating on the CCD. We do not have data for all gratings at all possible wavelength settings.

• Signal-to-noise ratio versus apparent magnitude: The plot below shows the performance of the CCDS between 9.5 and 20.5 mag as measured by the signal-to-noise ratio (SNR) as a function of m(5556) at unit airmass for an integration time of 1200 s or 20 minutes. The calculation assumes the 150 gpm grating, no light losses, the quantum efficiency curve and count rate derived above, and a dark sky brightness of 21.8 mag per square arcsec in the V-band.. The turnover in the curve at fainter apparent magnitudes is the result of increasing contributions to the SNR due to the sky and readnoise. In practice, the continuum of a point source between 19 and 20th mag can just be detected in an exposure time of 20 minutes in a dark sky at our lowest dispersion. Again, we do not have data for all gratings at all possible wavelength settings.

Slit width adjustment

The slit consists of two 2.5" long polished and aluminized jaws. This is a biparting slit and the jaws are continuously adjustable over a range from 5 to 1200 microns. Both jaws move in the direction of the slit length for changes in slit width. The jaws remain parallel up to widths of approximately 500 microns. For larger openings, some tapering of the slit will be noticed.

The adjustment of the slit width is controlled by a micrometer located on the west side of the spectrograph (see west view). This micometer reads the slit width directly in microns. A knurled ring at the base of the micrometer barrel is a lock and when rotated fully CW locks the micrometer barrel against accidental rotation. To release the lock, turn the ring to the full CCW position. Each rotation of the micrometer translates the slit 50 microns.

The following table lists the micrometer settings corresponding to several popular slit widths in arcseconds. For example, for a width of 2 arcsec and micrometer setting of 120, one would (1) close the slit by rotating the dial CCW until it is past zero, (2) rotate CW until zero is reached and resistence is felt from the dial, (3) rotate CW two full turns (=100 microns or units), and (4) rotate CW 20 microns or units thus stopping at a setting of 120. Slit widths smaller than 2 arcsec should not be used at the Perkins telescope since they result in a significant loss of light with typical 2-2.5 arcsec seeing.

Order sorting filters

At present, eight order separation filters are available to block light from higher orders. They are called LW350, LW360, LW370, LW400, LW420, LW450, LP495, and LP600. All of these filters are long-wave passband filters. The number in the filter designation identifies the wavelength in nanometers at which 50% maximum transmission occurs. LP495 is a yellow cut-on filter and its half-power turn-on wavelength is at 4950 Å and provides blocking to ~1 µm. LP600 is a red cut-on filter with half-power turn-on wavelength of 6000 Å. Over the range of sensitivity of the CCDS it is redundant with LP495 for the most part. For blocking solutions in the red, LP495 should be the filter of choice. Note: Insertion of a order separation filter moves the focus of the CCDS approximately -50 units as measured on the collimator focus with respect to the best focus without a filter.

Plot of the LW350-LW450 transmission curves

Plot of the LP495 and LP600 transmission curves

Comparison lamp templates

Operation

• IC/WC computer startup
• Prospero software startup
• Changing gratings
• Calibration lamps
• Taking and storing spectra
• Focus procedures
• Observing with the CCDS
• ISIT acquisition camera and instructions
• CCDS FAQ

Mounting and preparation instructions (authorized personnel only)

• Click here for an installation diagram
• NOTE 1/7/98: The CCDS with new W19-(1,1) CCD should NEVER be UV-flooded.
• Balance information: AB = 66; CD = 87; slide weight = 31.25; Add 37# weight to south side as marked above
• Approximate telescope focus: 947
• Approximate guider focus: -28

Additional documentation and information

• OSU Prospero documentation
• Perkins telescope Sun SparcStation 2 computer and peripherals
• IRAF telescope primer
• OSU Astronomy Department
• Lowell Observatory
• MDM Observatory