wpo - stellar spectrograph #1

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Foth...Tessar...Tessar in colour....text & images (c) Maurice Gavin - 2000-2003

This is my versatile wpo-spectrographdesigned around the Littrow principle where a single lens serves as collimator and camera lens.

2000 July 20 - These are the first light spectrograms via a simple spectrograph that has been on the back-burner for some years based around the Starlight Xpress range of monochrome and single-shot colour cameras of the eyepiece design.

Any of these camera i.e. MX5m/c, HX5, MX9 and MX7c may be interchanged with minimum adjustment.   Regular SLR camera lenses of 50mm or greater focal length can be swapped to change the dispersion/resolution. A  x2 tele-extender [Barlow] will double the focal length of any primary lens.

The decision to initially omit the slit proved the right one even though no convenient comparison source [for wavelength calibration] can be added.  Rather than the time-consuming procedure to align a star onto a slit [~2"arc wide projected onto the sky] any star within the telescope field can be captured.  It relies on point-like stellar images at the telescope focal plane being sufficiently small to form sharp spectra.

Results are beyond expectations - compare the b Cygni and XY Lyrae spectra via the SBIG spectrometer and this devise.

2000 Dec 28: Winter stars BU Tau [Alcyone], zeta Tau and M42 show emission of the Ha  line - zeta Tau shows a red/blue shift in this line.

2000 August 20:  SX MX9 + 135mm lens outperforms the SBIG spectrograph ! - tonight the spectrograph lens exchanged to give a dispersion scale of 0.6A/pixel - some 80% better than the SBIG hi-res performance of 1A/pixel even though no slit was used on the WPO spectrograph. The red bars on the adjacent spectrograms of b Cyg take a similar swath of spectrum from both instruments.
More spectra at 0.6A/pixel dispersion...

In simple terms the spectral dispersion [or scale of the spectrum] is based on three factors:-

1] focal length of lens forming the spectrum.

2] number of lines or groves in the grating measured in lines per mm.

3] size of the detector pixels.

The highest spectral dispersion and resolution comes from a long focus lens, 1200 l/mm grating and small pixels - upper right Table 1 ie HX5 camera, 300mm fl lens and 1200 l/mm grating.  Long exposures necessary limiting its use on faint stars.

Conversely the largest swath or coverage of spectrum comes from a large CCD, short focus lens and 600 l/mm grating - lower left Table 1 ie MX9 camera 28mm fl lens and binned pixels.  The latter example will also have the lowest resolution extending into the IR region when used with the 600 l/mm grating permitting lo-res spectrograms of faint stars in reasonably short exposures covering the complete visual spectrum [4000A - 7500A].

Note: all the quoted dispersion values under each lens refer to the 1200 l/mm grating [at slight variance to other dispersion values quoted on this page] with the spectrum coverage in upper shorter coloured rainbow bar These numeric value must be doubled for the 600 l/mm grating shown as the longer lower rainbow bar where resolution is halved but spectral coverage is doubled.   Binning of pixel [HX5 and MX9] lowers the dispersion and resolution but not the swath of spectrum covered.  As an approximation a dispersion of 1A per pixel = resolution of about 2A - 3A.

Using a HX5 camera, 58mm fl lens and 1200 l/mm grating gives a similar dispersion to the SBIG spectrograph in hi-res mode ie ~1A/pixel with ST7 and 9um pixels.  A blazed grating using the 1st order spectrum assumed throughout.

How it works - These initial spectra taken with a 58mm focal length F/2 camera lens and a 1200 lines/mm reflective grating yielding an image scale of 1.23A/pixel on the Starlight Xpress single-shot MX5c camera.  The full colour spectrum of XY Lyrae [Mira type variable in the field of Vega] proves attractive.  The colour information can be discarded to present line profiles [and spectrograms] in monochrome.

First shown by the author at the BAA Exhibition Meeting in London in 1998, this compact and light-weight modular design is based around the Starlight Xpress range of CCD cameras but could be applied to most CCD cameras.  It uses a tiny plane pick-off mirror [or right-angle prism] to send starlight from the telescope onto the grating via an SLR camera lens. The lens covers a focal plane of 43mm diameter [36mm x 24mm format] and the prism and CCD are offset by equal amounts off the the lens/grating optical axes.  Both the telescope focal plane and CCD chip are at the common infinity focus of the camera lens.  Starlight projected onto the grating is thus parallel or collimated.  As the spectrum occupies a few rows of pixels across the CCD, vignetting from the pick-off mirror is negligible.  Increasing the lens focal length increases the resolution of the spectrograph. The pick-off prism must be rotated slightly to account for the increased distance of the grating from the CCD.  The optional slit is imaged at the focal plane full size [ratio 1:1] and thus typically has a width equal to a pixel or so.

More spectra via the WPO spectrograph