Table 1.1.2

FOS Apertures

Designation

(Header 
Designation)

0.3

(B-2)

0.5

(B-1)

1.0

(B-3)

0.1-PAIR

(A-4)

0.25-PAIR

(A-3)

0.5-PAIR

(A-2)

1.0-PAIR

(C-1)

0.25 x 2.0

(C-2)

0.7 x 2.0-BAR

(C-4)

2.0-BAR

(C-3)

BLANK

(B-4)

4.3

(A-1)

Failsafe

Number

Single

Single

Single

Pair

Pair

Pair

Pair

Single

Single

Single

NA

Single

Pair

Shape

Round

Round

Round

Square

Square

Square

Square

Rectangular

Rectangular

Square

NA

Square

Square

Size

 (")

0.26 dia

0.43 dia

0.86 dia

0.09

0.21

0.43

0.86

0.21 x 1.71

0.60 x 1.71

1.71

NA

3.66 x 3.71

0.43 and 3.7

Separation
 (")

NA

NA

NA

2.57

2.57

2.57

2.57

NA

NA

NA

NA

NA

NA

Special Purpose

Spectroscopy and

 Spectropolarimetry

Spectroscopy and

 Spectropolarimetry

Spectroscopy and

 Spectropolarimetry

Object and Sky

Object and Sky

Object and Sky

Extended Objects

High Spectral 

Resolution

Surrounding 

Nebulosity

Surrounding 

Nebulosity

Dark and Particle 
Events

Target Acquisition 
and Spectroscopy

Target Acquisition 
and Spectroscopy



The first dimension of rectangular apertures is along the dispersion direction, and the second dimension is perpendic-
ular to the dispersion direction. The two apertures with the suffix designation "BAR" are bisected by an occulting bar 
which is 0.26" wide in the direction perpendicular to the dispersion.