HDS Image Slicer

For detailed information about this image slicer, please refer Tajitsu et al. 2012, PASJ, 64, 77.

In order to collect more photons into a narrow slit for high spectral resolving power, the image slicer (IS) can be used. The IS reformats the 2D-image of a pinhole with 1''.5 diamteter (in the case of IS #1) in the F/12.6 focal plane of the telescope into a series of narrow slits which is imaged on the spectrograph entrance slit.

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)
(λ > 3,800A for red settings,
  λ > 3,600A for blue settings)
(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

In some cases, the proposal requesting image slicer may be switched to normal-slit observations (and vice versa) if the observatory judges the requested observation can be accomplished in either mode, due to operational reasons. This applies to both normal and service programs.

The HDS ISs are basically designed for brighter targets (<∼15 mag).
Since we have never had actual results for fainter targets and need to make sure the performance of the IS, we recommend to use the IS for relatively brighter ones during the shared-risk mode operation.
For fainter targets, observers must give care to the following points.

  • Due to the spread of the spectrum along the slit length direction, the increase of total dark currents and read out noises in allover pixels cannot be ignored for fainter targets. Please check carefully the resultant S/N ratio in HDS ETC for your targets.
  • With the IS, the telescope guiding during exposures usually uses the light of the target itself leaking from the IS's pinhole (see below).
  • The telescope focus must be adjusted not for the slit viewer (pihhole) but for the IS's optics (the difference is ~ 0.4mm in the secondary mirror of the telescope.). Therefore, the telescope guinding might be difficult for fainter targets.
  • The offset auto-guider camera does not have its indemendent mechanism for focus shifting. So, if we use the offset auto-guider, the guiding might be unstable during the IS observation.
  • Considering these problems for guiding and limited space along the slit length direction, it is difficut to remove sky background from the spectrum taken with the IS.
  • The IS is manually inserted into the beam and bolted by hand just before the HDS entrance slit unit. Therefore, the insertion and the retraction of the IS during the night operation might not be recommended. It takes 10 minutes at least, including the changes of foci of the telescope and the collimator of the spectrograph.

    Optical transmittion

    Optical Transmittion of the Image Slicer
    measured at the center of each echelle order, using flat lamp frames.

    Expected observing efficiency

    Fraction of light that enters into the spectrograph
    The actual gain with respect to a standard observation through a narrow slit depends on the value of the seeing.
    Use HDS ETC to check more detailed gain for your desired targets.

    2D raw spectra

    2D spectral format (IS #1)
    Basically, the spectrum with the IS can be extracted as one large aperture with 7''.9 slit length on the IRAF reduction package.

    Slit Viewer guiding

    Slit Viewer (SV) image (IS #1)
    The star on the left side is a companion (separation is ~3 arcseconds).

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