Wide-field, triple spectrograph with R=5000 for a fast 22 m telescope

Roger Angel, Steward Observatory1st draft, December 4, 2002

 

 

 

Summary

This wide-field, multi-slit spectrograph is for a telescope with primary mirror of 18 m focal length, effective diameter of 22 m and maximum envelope of 25 m. The secondary is 2.9 m diameter and forms an f/10.2 RC focus 14.4 m behind the primary vertex, where the field is divided into segments by a 3-sided reflecting pyramid. Reflecting collimators of focal length equal to the 2.1 m radius of curvature of the RC focal surface form 195 mm pupils with angular magnification of 128.

With a 937 grooves/mm gratings blazed at 50, the R product is 5000 arcsec at =1520 nm in 1st order and 760 nm in 2nd order. There is a factor 2 anamorphic demagnification in the dispersion direction. Each camera with 500 mm entrance aperture and 11.4 field radius records objects over a 120° segment of a 12-arcminute annular field with spectral range, for example, 690-830 nm. 300 spectra could be recorded simultaneously, assuming slitlet lengths of 5 arcsec.

Summary (continued)

 

The RC telescope field aberrations are small, with 100% energy encircled in 0.24 arcsec diameter at the edge of the 12’ diameter field. The collimator yields 80% encircled energy within 0.32 arcsec averaged over the field, and in the worst case, 0.41 arcsec. We have not designed a camera, but have simply assumed the 11.4 field radius of DEIMOS, with entrance aperture increased from 295 to 500 mm. To maintain uv cover this would require very large uv glass and CaF2 blanks. Cameras to cover the 0.45 – 1 m

range will likely be possible and even affordable. A broad spectrum of glasses is available, and the resolution requirement is reduced because of high angular magnification at the pupil. Separate, smaller aperture cameras would be adequate for lower resolution spectra in the ultraviolet.

22 m Davison telescope concept

• Primary mirror with 6 off-axis and 1 central 8.4 m segments

• 18 m primary focal length• collecting area 374 m2

• equivalent diameter 21.8 m• largest outside dimension

25.3 m

Complete optical diagram

(one channel)

Ray diagram for high resolution gratings, single slit per segment

The 5.5 m diameter instrument (at the collimator surfaces) fits easily at the direct RC focus between the C rings

Sources placed in 1/3 segment of 12’ diameter RC field (0.90 m)

Plate scale 1.24 mm/arcsec

All fields and wavelengths 690, 725, 760, 795 and 830 nm

52 22 cm footprint at R=5000 grating, showing all fields.

All fields, all wavelengths at 0.5 m camera aperture

Focal plane with focus shift to show vignetting by 0.5 m camera entrance

830 nm (magenta)

690 nm (blue)

22.8º diameter

Spectra in focus

Spot diagrams for all fieldsbox is 1 arcsec (y axis dispersion) by 0.5 arcsec x axis

Y-axis enclosed energy(1 arcsec=161 m)

Prescription (dimensions in meters)

Surface radius spacing diameter conic

1 primary (stop) 36 16 25 – 1.001233

2 secondary 4.303 27.598 2.867 – 1.339863

3 fold mirror flat 0.8

4 (RC focus) 2.144 2 0.901

5 collimator 3.98965 2.1 1.157 – 1.966114

6 grating flat 0.7 0.553

7 paraxial camera 0.5 0.5 (0.5 m f.l.)

8 image flat 0.202

Aspheric terms on the collimator: R6 0.016293531

R8 – 0.072999173

R10 0.17282961

R12 – 0.1590606