The Ohio State University
College of Mathematical & Physical Sciences
Department of Astronomy

MODS Team Responses to LBTOSWG

1999 April 5

At the 1999 March meeting in Columbus, the LBTOSWG identified three specific action items for the MODS team. The MODS team agreed to provide their responses to these items by the end of March.

The LBTOSWG's specific requests to the MODS team were the following:

Evaluate an unobstructed longer focal length (approximately 700 mm) camera for both sides (red and blue) and performance in both binned and unbinned modes.
Elaborate on the "extended field" (5' × 6') concept. Include commentary on the likely availability and cost of the atmospheric dispersion corrector for this larger format.
Provide updated throughput and quantum efficiency curves based on a range of realistic gratings and CCD coatings.

The MODS team responses to these issues appear below.

(1) Preliminary analysis of image characteristics for a 700 mm focal length camera for MODS

The camera is a modification of the one discussed at the meeting last month. The focal length was increased to 700 mm and the camera was decentered to allow an unobstructed design while imaging on to a 8K detector in the dispersion direction. The detector size in the cross-dispersion direction is 4K with 2K used for a 4 arcminute slit height. If the field is extended to 5 arc minutes in the slit direction the used area will be increased appropriately.

The results below show the diameter of the circle containing 80% of the image point spread function (D₈₀) values in arcseconds for different slit heights as a function of wavelength with the dispersed spectrum covering the 4000 pixels .

**D₈₀ for 700 mm Camera**
Slit height (arcmin)	Wavelength (nm)
Slit height (arcmin)	564	530	499	460	434
2	0.20	0.25	0.19	0.27	0.21
1.3	0.13	0.15	0.16	0.16	0.11
0.6	0.13	0.13	0.14	0.12	0.10
0	0.15	0.12	0.14	0.09	0.13
0.6	0.17	0.10	0.10	0.05	0.16
1.3	0.20	0.10	0.13	0.10	0.20
2	0.25	0.15	0.22	0.23	0.29

The D₈₀ values are less than 0.3 arcseconds for all points in the spectrum. Near the center of the slit the image quality will allow narrow slits to be used if adaptive optics are used to improve the image quality through the atmosphere. The above values correspond to a resolution of R = 8000 when 4-5 pixels are binned in the dispersion direction, and double this resolution with a narrow slit with 2 pixels binned.

Overall the performance of this camera is similar to the shorter focal length camera discussed at the meeting. Furthermore, it provides a higher throughput since it is unobstructed by the detector.

(2) Performance of the "extended field" mode

The issue is image quality over the extended field. The Table below gives the image quality in terms of D₈₀ as a function of displacement from field center in the V band.

**Variation in D₈₀ across MODS field**
Y Displacement (arcmin)	X Displacement (arcmin)
Y Displacement (arcmin)	-3	-2	-1	0	1	2	3
3	0.69	0.63	0.48	0.51	0.49	0.64	0.91
2	0.64	0.36	0.24	0.18	0.24	0.39	0.69
1	0.51	0.24	0.17	0.15	0.13	0.27	0.56
0	0.47	0.22	0.18	0.16	0.13	0.23	0.53
-1	0.54	0.28	0.19	0.17	0.15	0.31	0.60
-2	0.72	0.45	0.30	0.25	0.31	0.46	0.80

In this mode the D₈₀ image diameter grows to as much as 0.46 arcseconds in the corner of a 4 arcminute field and reaches a values of 0.9 arcseconds at the corner of a 5 × 6 arcminute field. The field is limited on one side by the proximity of the camera corrector to the collimated beam.

The image diameters for the different wavelength values when used in a multislit mode have not been evaluated individually. However, the values in the first table closely track the sizes and are usually less because the monochromatic images in the spectrum are not broadened by lateral color. Therefore the imaging mode D₈₀ values give a good indication of the spectral values for a multislit mode. This assumption will be verified in future analyses.

(3) Revised MODS Throughput Calculation

The goal of this study is to predict the throughput of the MODS spectrograph. The atmosphere, telescope and the slit are not included. All surfaces are assumed to be clean and new. In general all values are taken from actual measurements. The model for the sol-gel coatings are is the Sandia Labs base catalyzed fixed index process not the acid catalyzed graded process studied at OSU. The dichroic is assumed to be 85% efficient in both transmission and reflection which appears to be conservative. A filter is included in the red channel to block second order "blue", in many configurations this will not be required. Available designs for the ADC require glasses that would limit the throughput at the extreme blue end. Other combinations of glasses will be investigated for better throughput. The blue response is shown both with and without the ADC.

ADC:

Both surfaces of the ADC are coated with a sol-gel coating tuned for an index equal to the square root of the index of fused silica. The internal absorption is for 25 mm of LLF 6 glass. Note that the absorption will be a function of field position and zenith distance. No allowance is made for the index mismatch between the fused silica and LLF6 glass.

Field Lens:

Both surfaces of the field lens are coated with a sol-gel coating tuned for an index equal to the square root of the index of fused silica. The pupil position may be acceptable without a Fabry lens or it may be possible to use the rear surface of the ADC to control pupil position. Thus it remains to be determined if the field lens will be required.

Dichroic:

The dichroics is assumed to have and in-band reflectivity of 85% and the first surface is assumed to have and in-band transmission of 85%. The rear surface is assumed to be coated with same sol-gel coating used for the previous surfaces.

Collimators:

The blue collimator is coated with aluminum while the red collimator is coated with silver.

Gratings:

The blue grating is coated with aluminum. The blaze efficiency published by Milton Roy for their 600 groove/mm 5.2 degree blaze grating is used. The P and S curves are averaged. No correction is made for the departure from Littrow condition. The red grating is coated with aluminum. The blaze efficiency published by Milton Roy for their 600 groove/mm 8.6 degree blaze grating is used. The P and S curves are averaged. No correction is made for the departure from Littrow condition. Improved performance could be obtained with a silver coated grating in the red channel.

Camera Correctors:

The corrector for the blue camera is coated with ZC&R's broad band coating. The corrector for the red camera is coated with the Sol-gel coating.

Camera Primary Mirrors:

The primary mirror of the blue camera is coated with aluminum. The primary mirror of the red camera is coated with silver.

Camera Field Flatteners:

The field flattener for the blue camera is coated with ZC&R's broad band coating. The field flattener for the red camera is coated with the Sol-gel coating.

CCDs:

The quantum efficiency for the blue detector is taken from the room temperature measured values for the Steward CCD lab processed Lick-Loral 3 CCD L4-W9-(1,0). The quantum efficiency for the red detector is taken from the low temperature values for the LBNL deep depletion CCD measured at Lick Observatory.

throughput calculation

Note added 2000 Feb 14:
A recalculation of the response has been done using a more efficient blue grating. You can view it here.

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Updated: 2000 Feb 14 [rwp]