9.3 Spectral Element Movement Overheads

For any COS exposure, including target-acquisition exposures, any change of spectral elements incurs an overhead. A transition from FUV to NUV requires movement of OSM1 to the NCM1 position, followed by a possible OSM2 movement. On the other hand, a transition from NUV to FUV requires only the movement of OSM1 from NCM1 to the desired FUV grating and cenwave. Table 9.3 gives the times required for movement between OSM1 spectral elements. These depend mildly on cenwave, so typical values are shown. Table 9.4 gives the times for movement between OSM2 spectral elements. These do not depend on cenwave.

All COS visits start with OSM1 at the G130M position (with central wavelength 1291 Å) and OSM2 at the MIRRORA position. These gratings are highlighted in Table 9.3 and Table 9.4. OSM1 and OSM2 move sequentially, so the total overhead is the sum of the two separate overheads. The time required to move from one optical element to another is independent of the FP-POS settings.

Table 9.3: Overhead Times for Motions Between OSM1 Spectral Elements.

Movement time(s) fromto G140Lto G130M1to G130Mto G160Mto NCM1

G140L

194

165

205

114

G130M1

198

156

158

G130M

172

117

119

G160M

212

162

122

165

NCM1

120

154

121

159

1 Use these entries if the G130M cenwave is 1055 or 1096.


Table 9.4: Overhead Times for Motions Between OSM2 Spectral Elements.

Movement time(s) fromto G230Lto G185Mto G225Mto G285Mto MIRRORAto MIRRORB

G230L

210

141

176

105

99

G185M

204

136

102

170

176

G225M

136

141

107

101

107

G285M

170

108

102

136

142

MIRRORA

100

175

106

141

71

MIRRORB

94

181

112

147

77