The spectral resolution is basically determined by the slit or image slicer
used, though the maximum resolving power is limited by the data sampling of the
detector. Within this limit, the relation between the slit width and
resolving power is represented as:
slit width (mm)=
/resolution (R)
(ex.) resolving power
slit width = 0.2 mm
The image scale at the Nasmyth focus is 2.0 arcsec/mm, or 0.5 mm to 1 arcsec, thus the resolution of 90,000 corresponds to a slit that is 0.4 arcsec wide. The slit width can be set between 0.2 and 4 arcsec.
The amount of sky observed is determined by the slit length.
If the slit in use is too long, overlapping of orders may occur. The
maximum slit length for which orders do not overlap is given as a function
of wavelength in Table ![[*]](crossref.png)
.
It is also possible to observe in a longslit moge, where only one
echelle order is recorded, and other are blocked out by apropriate filters.
The maximum slit length is 60 arcsec (30 mm).
| Wavelength | Cross disperser | Maximum slit length | |
| (mm) | (arcsec) | ||
| 3100Å | Blue | 2.2 | 4.4 |
| 3500Å | Blue | 2.9 | 5.8 |
| 4000Å | Blue | 3.8 | 7.6 |
| 4500Å | Blue | 4.9 | 9.8 |
| 4000Å | Red | 2.4 | 4.8 |
| 4500Å | Red | 3.1 | 6.2 |
| 5000Å | Red | 3.8 | 7.6 |
| 7000Å | Red | 7.4 | 14.8 |
| 10000Å | Red | 15.1 | 30.2 |
Image slicers allow for observations with higher spectral resolution and S/N,
especially in poor seeing conditions. Their basic characteristics are summarized in
Table .
The entrance opening is different for each of them, but wider than the slit that gives
a similar resolution, so more light is collected. Comparison of the efficiency
of each IS with respect to slits of various widths, as a function of seeing, can
be found here.
The image of the opening is chopped into 3- or 5-element pattern. The relation between the angular size of the pattern element and spectral resolution is similar to the one above, however the effective R is somewhate lower, e.g. the IS #1, which is 0".3×5, gives R=110,000, whereas the 0".3 slit gives 120,000.
| IS #1 | IS #2 | IS #3 | ||
|---|---|---|---|---|
| Slice pattern | 0".3×5 | 0".45×3 | 0".2×3 | |
| Spectral resolution | R=110,000 | R=80,000 | R=165,000 | |
| Type | Bowen-Walraven type | Bowen-Walraven type + Focal Length Corrector | ||
| Entrance opening | φ1".5 circle | 1".35×1".35 square | 0".6×1".45 rectangle | |
| Elongation along slit length | 7".9 (λ > 5,000A for red settings , λ > 4,000A for blue settings) |
5".2 (λ > 3,800A for red settings, λ > 3,600A for blue settings) |
5".5 (λ > 4,000A for red settings, λ > 3,600A for blue settings) | |
| Material | Fused quartz w/AR-coating | |||
| Efficiency | > 83% (@4,000 ~ 7,000A) | > 93% (@3,800 ~ 8,000A) | > 73% (@4,000 ~ 8,000A) | |
| Manufactured by | OptCraft | |||
| Opened since | S11B | S13A | S14A | |
All ISes are pre-slit units, thus to avoid unnecessary light blocking, the slit width is set to 4 arcsec when an IS is mounted. Guiding is done with the SV.