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OSU College of Arts and Sciences Department of Astronomy

Boller & Chivens CCD Spectrograph (CCDS)
at the MDM Observatory

CCDS News

2009 March 14
qccds is a CCDS Quick-Look Tool based on pyraf that performs quick spectral extractions of CCDS data with IRAF apall. Written by John Thorstensen and available at MDM.

2008 November
A new 1200x800 CCD (OSU Loral C) has been installed in the CCDS dewar. It has lower readout noise (3.5e-) and better cosmetics.

2006 November 1
New grating commands have been introduced to make setting Zero-th order and resetting the mechanism easier. The grating tilt system has also been recalibrated.

2005 September 1
Jen Marshall and Darren DePoy installed and commissioned a new CCD-based slit viewing camera to replace the aging ISIT camera. Please read the CCDS Acquisition Camera User's Manual (476k PDF) for instructions on how to use this camera.

2005 February 1
Jen Marshall at OSU provided revised measurements of the properties of the 1800 line/mm grating based on observations at the 2.4m.

2003 November 10
The OSU copy of the CCDS Manual became the official copy on 2003 November 10. All other copies you might find on the web are guaranteed to be out of date after this date.

2002 January 10
The grating drive mechanism has been repaired. However, the angle of zero order as read on the grating drive mechanism dial on the spectrograph now reads -6.03 degrees instead of about 0.0 degrees as before. The new encoder zero point has been updated in the Prospero software. Grating tilt operations from Prospero should continue to work as before.

The timeout problem associated with the below-slit filter slide and the positioning of FILTER 5 has been fixed. Users should no longer encounter the timeout message when selecting FILTER 5.

Contents:

Summary

The Boller and Chivens CCD Spectrograph (CCDS) is a conventional optical grating spectrograph operating in the 3200-9500Å region. It is used on the MDM Observatory 2.4-m and 1.3-m telescopes as a facility instrument. It was acquired by the Ohio State University through a grant from the National Science Foundation in 1970 for stellar spectral classification programs and was used for about 28 years (1970-1998) on the Perkins 1.8-m telescope on Anderson Mesa in Arizona. It was moved to MDM in 1999.

Seven diffraction gratings are available providing a range of spectral resolutions of 550 to 9100. The slit width is continuously adjustable between 0.5 and 13 arcsec. The current science 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 typically 7 electrons and the gain is nominally set at 2.1 electrons per ADU. Lamps located in the MIS box at each telescope are used for wavelength and flat-field calibration. With the lowest dispersion grating in first order, the peak system quantum efficiency is about 19% at 5200Å.

A thermo-cooled SBIG ST-9 CCD camera is used to image the polished entrance slit jaws of the CCDS for target acquisition. In brief 5-second integrations it is possible to acquire point sources as faint as V=20th magnitude. Please read the CCDS Acquisition Camera User's Manual for details.

Pictures of the CCDS: On 2.4-m, Northwest, Southwest, Southeast, Northeast

CCD Characteristics & Parameters

Measured 2008 Oct 15 [rwp] Detector format, binning, and overscan regions are user selectable options.

Gratings and Spectral Information

The table below summarizes the characteristics of the gratings available for use with the CCDS. The values tabulated for the dispersion, spectral coverage, and spectral resolution have been calculated but appear to be accurate when compared with actual data obtained with the CCDS. We also give the approximate collimator focus measured at 20°C for as many gratings as we have information. Updates will be forthcoming.

Grating
(grooves per mm)

Blaze
(order, angle, wavel)

Dispersion
(Å/pixel)

Coverage
(Å)

Resolution (Å)
1 Arcsec slit
87 µm @ 2.4-m

Resolution (Å)
1 arcsec slit
47 µm @ 1.3-m

Approx
Collimator
Focus
@20°C

Notes

150

1st / 2.15 / 4700

3.29

3650

8.2

4.4

850

c

158

1st / 3.63 / 7530

3.16

3650

8.1

4.1

 

a, c

350

1st / 4.30 / 4026

1.33

1592

3.4

1.9

850

c

600/1

1st / 8.63 / 4700

0.79

953

1.9

1.0

910

c

600/2

2nd / 22.03 / 5875

0.41

495

0.8

0.4

 

b, c

1800

1st / 26.75 / 4700

0.275

330

0.825

0.3

 

b,c,d
Notes:
(a) The 158 gpm grating is currently mounted. Please contact OSU as to its availability.
(b) Undersampled at the 2.4-m telescope.
(c) Undersampled at the 1.3-m telescope.
(d) Measured at the 2.4-m by J. Marshall, 2005 Feb 1.

The above graph shows the measured grating efficiencies as reported by the manufacturer as a function of wavelength. Users should note that these curves were measured in a specific laboratory setup (Littrow configuration) and thus may not be applicable to the CCDS. Note that the 1800 gpm grating has a strong polarization dependence on its measured efficiency curve that is not shown above and which is based on unpolarized light. They are provided here for relative comparison only. Quantitative performance estimates should use the information provided below based on the spectra of standard stars with the CCDS.

Performance and Throughput

Throughput:
The sensitivity function shown below is the convolution of contributions from the Hiltner 2.4-m telescope and its reflective surfaces, the spectrograph and its optics, and the detector. It was derived based on spectra of the standard stars BD+28 4211 and Hiltner 102 obtained under photometric conditions on August 21, 1999. To avoid light losses which would affect the measurement, the slit was widened to 13 arcsec. The 150 gpm grating was employed and was centered at 4400 Å (blue), 7970 Å (red), and 5500 Å (green) respectively. Exposure times were 100 s in the blue, 240 seconds in the red, and 100 seconds in the green. 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.

qeff.gif (6564 bytes)

Count Rates:
We have measured the counting rate of the CCDS for each grating at the 2.4-m telescope and the results are presented in the table below. The count rates given for the 1.3-m telescope are estimates and simply reflect the difference in aperture with respect to the 2.4-m. After we obtain actual measurements at the 1.3-m we will replace the estimates given in the table with the measurements. The count rates in the table are given in e-/sec/Å in an extracted spectrum at 5556 Å for a star of monochromatic magnitude m(5556) = 10.0 at unit airmass. In the plot below, we show graphically the count rate as a function of wavelength for each CCDS grating normalized such that m(lambda) = 10.0 at unit airmass in order to illustrate the wavelength dependence.

 

Count rate (e-/s/Å) for m(5556) = 10.0 mag
at unit airmass for CCDS

Grating

Hiltner 2.4-m

McGraw Hill 1.3-ma

150

730

214

350

592

174

600/1

650

220

600/2

840b

246b

1800

688c

202c
Notes:
(a) Count rate estimated from 2.4-m count rate based on ratio of collecting areas.
(b) Count rate at 6436 Å. Designed for use at H-alpha.
(c) Count rate at 5710 Å.

Note: the 900l/mm grating depicted above is no longer available.

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 10 to 1100 microns. Both jaws move in the direction of the slit length for changes in slit width. The jaws remain parallel up to widths of at least 900 microns. For larger openings, some tapering of the slit might be noticed.

The slit width is controlled by the setslit <n> command where <n> is the slit width in microns. The following table lists the physical slit widths in microns at the 2.4-m and 1.3-m telescopes respectively for some popular slit widths in arcseconds. The unvignetted slit length at the 2.4-m telescope is 5.2 arcmin while at the 1.3-m telescope we believe it is approximately 10.0 arcmin, however, it has not been measured yet.

Width
(arcsec)

Width (microns)
@ 2.4-m

Width (microns)
@ 1.3-m

1.0

87

47

1.5

131

71

2.0

174

94

3.0

261

141

5.0

435

235

7.5

653

353

10.0

870

470

12.0

1044

564

CCDS filters

The CCDS has two internal filter slides and both positioned through the software. The above-slit filter slide, called the prefilter, carries 4 neutral density filters made of quartz which can be used to alter the intensity of the lamps in the MIS box. They can be used for slit targets as well but target acquisition using the slit-viewing CCD camera will be difficult or impossible. The four filters are as follows: (1) ND0.5 (1.25 mag), (2) ND1.0 (2.5 mag), (3) ND1.5 (3.75 mag), and (4) ND2.0 (5 mag). For normal operation, the prefilter slide is moved out of the telescope beam with the prefilter 0 command. The prefilter positions are summarized in the table below. Each filter measures 0.35 x 1.5 inches with a maximum thickness of 0.20 inches.

The below-slit filter slide is referred to as the filter. It carries five order separation filters to block light from higher orders. They are called (1) LG350, (2) LG370, (3) LG400, (4) LG450, and (5) LG505.. 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. The filter slide is moved out of the telescope beam with the filter 0 command. The filter positions are summarized in the table below.

Note:
Insertion of an order separation filter moves the focus of the CCDS approximately -50 units as measured, on the collimator dial or defined by the setfocus command, with respect to the best focus without a filter.

Position

Filter

PreFilter

0

Retracted

Retracted

1

LG350

ND 0.5

2

LG370

ND 1.0

3

LG400

ND 1.5

4

LG450

ND 2.0

5

LG505

 

Plot of the LW350-LW505 filter transmission curves

FILTER=*HI* Condition:

The CCDS mechanism control computer (the "IE") can occasionally get into a state where it reports the filter and/or prefilter ID as "*HI*". This means that the controller thinks either filter insert mechanism has been pushed into the high limit switch. In fact, it probably isn't, but no matter, it thinks it is, and as a safety measure it refuses to allow you to set the filter or prefilter, with ensuing confusion.

If this happens, issue the following Prospero commands 

   FILTER RESET
or
   PREFILT RESET
as required to clear the condition. These commands override the high-limit safeties and drive the filter/prefilter mechanism back to the "zero" or "home" position. Once the (bogus) high limit assertion is cleared, you can move the filter and/or prefilter to the desired positions.

Comparison Lamps & Spectral Templates

Emission-line Lamps currently available for wavelength calibration include HgNe, Ne, Ar, and Xe. They are enabled through the MIS software and can be used individually or in any combination. We hope to add a HeAr or HeNeAr Cd hollow cathode source in the near future. Templates and line lists for the MIS box emission line lamps are given below for display and download. Hardcopy plots are also available in the CCDS notebook in the respective control rooms. The line lists can be combined in any combination. Their format is IRAF compliant. Below are some notes regarding use of the lamps:

Hg lamp:
This lamp requires as much as 30 seconds to warm up. After warm up, the HgNe spectrum will be replaced by a strong Hg spectrum only. When using the Hg lamp it is necessary to use an ND1.0 filter to cut down on the intensity of the light. With the 150 gpm grating the exposure time is at most a few seconds. Observers should start with the ND1.0 or perhaps ND1.5 filter if they are unsure of the proper filtering for their chosen grating and slit width. The Hg line list includes 2nd order blue lines visible in the 1st order red. For low resolution applications in the 3200-7000 Å region, the Hg and Ne lamps provide a good array of strong lines.

Hg Plots: (3300-6900 Å), (enlarged), (6550-9900 Å), Hg linelist

Ne lamp:
The Ne lamp contains strong lines in the red beginning at 5700 Å. A filter should not be necessary. The Ne lamp should be used in conjunction with the Ar lamp to construct a dense comparison lamp spectrum spanning the red and blue spectral regions respectively, but be careful balancing the exposure times since the Ar lamp is considerably weaker. The Ne lamp is not useful below 5300 Å.

Ne Plots: (5300-6800 Å), (enlarged), (6500-9900 Å), (enlarged), Ne linelist

Ar lamp:
The Ar lamp is relatively weak and does not require a filter. An exposure of 120 sec with the 600/1 grating centered at 4700 Å gave an adequate spectrum for cross-correlation with a longer exposed template.

Ar Plots: (3300-6900 Å), (3300-5300 Å), (5000-7000 Å), (6550-9900 Å), (enlarged), Ar linelist

Xe lamp:
The Xe lamp provides two clusters of bright lines: one at roughly 4500-5000Å, and the second at between 8000 and 10000Å, with many lamps in between. It is a useful lamp to use for blue spectra where you need to fill in the numerous gaps in calibration line coverage below about 5000Å.

Xe Plots: (3300-7000 Å), (enlarged), (4000-6000 Å), (6550-9900 Å), (enlarged), Xe linelist

All Lamps:
You can download a complete set of PostScript versions of these plots and the ASCII format line lists ready for use in IRAF using the link below
ccds_lamps.tgz (128k gzipped tar file)

Flatfields:
The MIS box lamp set includes an incandescent light bulb that produces a continuous (essentially thermal) spectrum. This source can be used to illuminate the slit and thus to obtain internal flatfields. At present the lamp is far too red to construct flatfields for the CCDS in the blue parts of the spectrum. The dome screen can also be used but has the same limitations as the internal lamp. We hope to replace the internal lamps with higher intensity quartz-halogen lamps in the near future.

Operations Manuals

Additional Documentation & Information