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The slit spectrocope is made from ideas taken from the Christian Buil page: 

The spectrograph has a classic design using a 600-line / mm reflection diffraction grating from Edmund Optics, a collimator also with Edmund Optics elements and a second-hand lens. The assembly mounted on a plywood box and adjusted so that the focus is suitable for the QHY IMG2 Pro camera.

The data used for calculations (formulas and spreadsheet at are:

  • Telescope:

    • Aperture: 200mm

    • Focal distance : 2000mm

  • Collimator:

    • Aperture: 25mm

    • Focal distance: 100mm

  • Lens:

    • Focal distance 50mm

    • Speed f/=1.7

  • Difraction grating:

    • Lines per mm: 600

    • Width: 30mm

  • CCD camera pixel size: 6.45 microns and number of pixels 1360.

  • Total angle between incident ray and diffracted ray for 5500 Angstroms: 38º (angle of incidence 29º, angle of dispersion 9º)


The calculated data are:

  • Dispersion: 2.12 A/pixel

  • Range: 2900A

  • Resolution: 707

  • Spectral power: 7.7A

The real data basically coincide with the expected ones. The dispersion is of the same order as the theoretical one (about 2.2A / pixel), the somewhat larger real range (about 3000A), and the spectral resolution power of the order of 10A.




























On the back the spectrograph has an adjustment of the diffraction grating angle to be able to capture different ranges of wavelengths. A rod ending in a point indicates the central wavelength according to the angle of the diffraction grating.


The most complicated part of the design was the slit of variable width, made with two blades and a mechanism that by tightening a screw allows to adjust the separation from about 2mm to approximately 0.05mm.

The assembly is coupled with a tilting mirror and an eyepiece to facilitate the centering of the lens especially when using the slit in its narrowest position.

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