keyword value comment/reference
SPEC_RP 1300 for high resolution gratings (G*H)
250 for low resolution gratings (G*L)
From the FOS Instrument Handbook, version 6
105 for FOS/RD Prism at 6000A
104 for FOS/BL Prism at 6000A
Calculated from information taken from the FOS FAQ
http://www.stecf.org/poa/FOS/fos_faq.html.
Full table for the Prism is:
wavelength(A) FOS/RD FOS/BL(A/pixel) lambda/delta-lambda
1500 0.24 0.31 6250 / 4839
3000 6.6 6.6 455 / 455
6000 56.9 57.7 105 / 104
9000 114.8 140.7 78 / 64
SPEC_ERR ! error in the wavelength
Blue side
grating error (A)
G130H 0.25
G190H 0.37
G270H 0.52
G400H 0.77
G570H 1.11
G160L 1.72
G650L 6.28
PRISM 3.61
Red side
grating error (A)
G190H 0.36
G270H 0.51
G400H 0.75
G570H 1.09
G780H 1.43
G160L 1.66
G650L 6.36
PRISM 3.56
SPEC_ERR needs to be calculated from the values of delta-lambda (in
A/diode) given in Table 1-3 for the FOS Instrument Handbook, version 6,
using the accuracy of the zero point of the wavelength scale. FOS ISR 67
gives the accuracy of the zero point of the wavelength scale as < 0.25
diodes.
For the Prism, there is no published dispersion value in A/diode. The
value given above is for a wavelength of 6000A, so dispersions of 56.9
and 57.7 A/pixel (from the FOS FAQ), and assumes an x substep of 4 (i.e,
4 pixels per diode).
STAT_ERR 3 ! the flux error in percent (%).
Comments: All references stress the need to have well-centered targets
in order to achieve the highest flux accuracy. The FOS Instrument Handbook
gives a limiting accuracy for FOS/BL of 1.6% and for FOS/RD of 2.0% The
value we quote, 3%, given in ISRs 144 and 125, is applicable to all FOS
observation, grating or prism, pre-COSTAR or post-COSTAR.
references: FOS Instrument Handbook, version 6, section 3.3, page 49.
FOS ISR 144 - post-COSTAR
FOS ISR 125 - pre-COSTAR
SPECSYS HST CENTRIC ! wavelength system (HELIO, GEO, etc.)
reference: The FOS FAQ (http://www.stecf.org/poa/FOS/fos_faq.html)
Comment: From the FOS FAQ:
Q: What type of velocity correction is made to FOS data? What is the
maximum amount of shift to heliocentric wavelengths at 2000A?
A: There is no on-board velocity correction made to FOS data. The FOS
is unable to resolve wavelength shifts of the order of HST's orbital
motion. The GHRS does have high enough resolution to be affected by
these velocities and its detectors are operated accordingly. However,
if wavelength shifts of ± 30 km/s are significant to your
measurement then the Earth's orbital motion should be removed
post-facto.
Dorothy Fraquelli
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