Technical Data - Optics

The Basic Elements 

Lhires III is a spectrograph optimised for high-resolution spectroscopy with amateur-sized telescopes. The resolving power, expressed as λ/Δλ, is around 17,000 in the red, and is therefore capable of resolving spectral details smaller than 0.4 Angstrom units in the vicinity of the hydrogen H alpha line. With a 1-hour exposure, the spectra of stars down to visual magnitude 6 to8 with the popular SCTs of 200mm to 300mm diameter and an S/N of 100 can be obtained (see the performance table below). The spectrograph can be connected to most other standard types of telescope, including refractors and Newtonians, provided that these instruments are slower than f/8, in order to collect all the light and maintain throughput.  Several type of reflection gratings are available, allowing the user to vary at will the trade-off between spectral resolution and the limiting magnitude attainable. The nominal configuration for Lhires III is based on a holographic grating with 2,400 grooves per mm.    

LHIRES is an acronym for Littrow High REsolution Spectrograph. A Littrow-mounted spectrograph is one where the same optical component, in this case a 200mm lens, acts as both the collimator to render the light beam parallel, and the focusing (or imaging) lens to bring a focused image of the spectrum onto the detector. This design eliminates the need for separate lenses for the collimating and focusing compared to "classical" spectrograph mountings such as the Czerny-Turner or Ebert configurations. Lhires III is therefore compact and economical to produce.  The weight of Lhires III is 1.3 kg.

Inset Optical Schematic

The usable field where the spectrum is obtained has been deliberately restricted in order to allow the use of a simple achromatic doublet as the objective lens. The image quality will allow you to obtain a resolving power of 17,000 only in the highest resolution option for Lhires III (i.e., 2400 lpm grating and a slit width of 25 μm over a linear mm field approximately. 

Connection to the telescope is either by a (standard) 50.8mm (2 inch) adapter or the regular screw adapter for Meade and Celestron SCT's. Both of these adapters are supplied with the basic version of the spectrograph.

Lhires II (the precursor to Lhires III) mounted at the principal focus of a Takahashi FS-128.
The CCD camera being used is an Audine.

The adapter on the camera side is a Pentax-type M42 T-ring (42mm x 0.75 screw thread).   There is sufficient back-focus with Lhires III so as to allow the use of a DSLR as the detector (the T-ring is obviously brand-dependent as so is not supplied). Adapters for the Audine and SBIG cameras are supplied with the basic kit as well as a 1¼ inch (31.75mm) adapter for observing spectra with an eyepiece, a low-light level video camera or with a webcam such as the ToUcam, for example.   

The entrance slit to the spectrograph is highly polished and reflective and is inclined at 10º to the optical axis of the telescope. The slit plays the role of a mirror for objects whose light is not actually passing through the jaws. Light from this polished surface is reflected back to a guide camera (not supplied) via a 1¼ inch (31.75mm) adapter ring. Alternatively, it is possible to employ a webcam, video camera of a second CCD camera for automatic guiding.

The Lhires II prototype at the focus of a Celestron 11.  A Watec 120N video camera is used for guiding.
This type of integrating camera is capable of guiding on stars down to about mag. 10 with this configuration.

The spectrograph employs a fixed slit. However, it can be removed and its width can be varied.

 
Here we can see an image from the guide camera (here a Watec 120N).  The entrance slit is just discernible as a darker horizontal line against the slighter brighter sky background. On the left, the star is not on the slit. On the right, it is correctly positioned on the slit. The star's intensity has been strongly reduced since most of the light is now passing through the spectrograph (where it should go!) An algorithm has been developed for guiding on the halo (blurred) image of a star (a virtual slit)

Focused light from the telescope passing through the slit meets the flat 45° mirror and is sent the collimator lens, focal length 200mm, 30mm diam. The collimated beam then passes to the plane reflection grating that is mounted on a pivoting support. Varying the spectral range of the spectrograph is achieved by rotation of the grating by adjusting a precise micrometer screw. 

The grating is interchangeable.  The "standard" type has 2400 grooves per mm, giving a spectral dispersion of 0.115 A/pixel in the red, for a camera using the Kodak KAF-04XX family of CCD chips (Audine, ST7,...).  Less dispersive gratings are available, the groove density falling in proportion to light intensity of weaker and weaker objects in order to obtain a spectrum.      

The dispersed light from the grating passes back through the lens and is focused on the detector. This could be an astronomical (cooled) CCD camera, a DSLR or even a webcam. However, for most applications in stellar spectroscopy, only a cooled CCD camera will have the required sensitivity. A few application can make use of a video camera for example, one of them being solar observations, where the Lhires III can easily be converted into a spectroheliograph for observation of the solar chromosphere. Visual observation of spectra with an eyepiece is also equally possible. Lhires III, therefore, can be considered and excellent teaching tool to support the teaching of physics in schools as well as the principles of astronomical optics.

Here we see the 2-dimensional spectrum of the Be star Gamma Cas. The very broad, intense line in the centre is the H-alpha line in emission.
The several narrower dark lines on either side are produced by atmospheric water vapour absorption in the Earth's atmosphere.

Note Maurice Gavin's web page comparing differents spectrographs to Lhires III.

Perfomances

Limiting Magnitude

Calculated for an exposure time of  1 hour (12 x 5min exposures) for two signal-to-noise ratios at Dl of 50 and 100.

2400 l/mm Grating
Star type B0V - CCD KAF-0400
Slit Width: 25 µm - Seeing : 4 arcsec.
Resolving Power (R) : 17000
Sampling (KAF-0400) : 0.115 A/pixel

1200 l/mm Grating
Star type B0V - CCD KAF-0400
Slit Width: 25 µm - Seeing : 4 arcsec.
Resolving Power (R) : 5800
Sampling (KAF-0400) : 0.345 A/pixel

300 l/mm Grating
Star type B0V - CCD KAF-0400
Slit Width: 25 µm - Seeing : 4 arcsec.
Resolving Power (R) : 1300
Sampling (KAF-0400) : 1.493 A/pixel

See also on this page the ETCL tool: "Exposure Time Calculator Lhires"...