Dispersion

Cross-Disperser Unit

Cross-dispersion is used to separate the the echelle orders on the detector. For bHROS, the cross-dispersion is provided by a set of 60^o fused silica prisms. This method was chosen because prism cross-dispersion is about a factor of two more efficient than grating cross-dispersion (owing to the absence of parasitic orders) and also conveys a number of other advantages (i.e. being virtually stray light free and providing more uniform order separation).

The cross-dispersion system provides a minimum order separation of 3.00 mm in the camera focal plane. This separation provides sufficient inter-order spacing for (1) adjacent sky and object spectra in dual-fiber mode or (2) one broad object spectrum in single-fiber mode. The minimum order separation occurs at a wavelength of approximately 780 nm, with the order separation being larger than this at longer and shorter wavelengths.

Echelle Unit

The main dispersion of bHROS is achieved by an echelle grating. The grating has a ruled length of 408 mm and a blaze angle of approximately 63^o. The echelle has a ruling of 87 g/mm. The echelle has two degrees of motion (tip and tilt) to scan the echellogram on the detector in order to configure the spectrograph for a specific wavelength range.

Echelle Positional Accuracy

Currently, the echelle can only be absolutely positioned within 50 pixels in both X and Y directions. This corresponds to approxamately 1/4 of an order in the spatial direction and 0.5 - 1 Angstrom in the spectral direction. This is largely due to the mechanical design of the echelle system. For programs that require a greater level of positional accuracy, please contact Steven Margheim, the bHROS Instrument Scientist.

Additionally, there is an additional uncertainty introduced by the optical model used to determine the grating settings (see below).

Echellogram and Interactive Applet

The echellogram produced by the dispersers is shown below. Many lines of astrophysical interest are indicated. A java applet is available to interactively examine the coverage available at a given grating setting (used to produce the image below) and can be launched in a separate window by clicking the button below.

The orders displayed by the applet roughly trace the centers of the single orders produces in single fiber mode and trace the center of the two orders produced in the dual fiber mode. Therefore the width of the orders in the applet does not represent the true order width observed. Additionally, the model becomes less accurate overall (order tilt, position, etc.) further from the center of the echellogram. While the model is useful for determination of echelle grating settings and coverage obtained, one should not overly rely on it. 

Click the image above for a larger version

Useful Settings: An incomplete list of settings that contain two or more lines of astrophysical interest is given below. Clicking on a link displays the echellogram for that setting in a separate window.

• 6708 and 6104 Li
• K I, Ca IRT (just one line), Pε
• Hγ, Hδ, He I

Last update August 30, 2006; Steven Margheim