Galaxies for Fun Astronomical CCD photos on peculiar galaxies and other astronomical stuff.
www.galaxies.se - by Ivar Hamberg
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SPECTROGRAPH CHARACTERISTICS
The spectrograph was built in the late 70's, and I put the optical parts inside an aluminum frame. The collimator is an Edmund Opitcs 30 mm diam. and 200mm f.l. achromatic lens. Not optimal, but it does the job.I have several gratings to choose among, and also the possibility to use a prism. Early on I used a traditional camera with film, and a visual check of the slit position through an eyepiece. Nowadays the spectrum is picked up by an 85mm Takumar camera lens and an ASI183MM CMOS camera.
The spectrograph is built with an adjustable slit. This consists of 2 parts of reflective metal. The basic design can be found in Albert G. Ingalls classical book on Amateur Telescope Making, ATM III p 145, where a lovely drawing by Russel W Porter is shown. I constructed the slit unit using aluminum and brass metal. It is mounted at a slight angle, so the star field can be reflected and viewed with a separate guide camera, an ASI120MINI-camera.
DISPERSION
The illustration shows the spectrograph mounted on the Ritchey-Chrétien instrument.
The adjustable slit is typically used at some 20-50um opening. It is always a fight to get in as much light as possible through the slit (widerning the slit), while still keeping high resolution (narrowing the slit). The slit opening setting is a compromise.
The ASI183MM camera has a detector width of 5496 pixels. The sensitivity is between some 3800 Å and up beyond 8000 Å. Pixel size is 2.4um. Together with the utilized lens elements, grating properties, and mirror coatings, the usable spectral range is typically from 4000 Å to 8000 Å. The blaze wavelengths for the gratings are in the mid visual band, 5000 Å.
Using different gratings, I get these approximate values of dispersion:
GRATING lines/mm | Dispersion Å/pixel | % of full chip width | Spectrum coverage | Approx Resolution | Spectral Resolution R | Approx Limiting Magnitude |
150 | 1.7 | 45% | 4000-8000Å | 8 Å | 625 | 10 |
300 | 0.84 | 90% | 4000-8000Å | 4 Å | 1250 | 9 |
600 | 0.42 | NA | 2000Å | 2 Å | 2500 | 8 |
1200 | 0.21 | NA | 1000Å | 1 Å | 5000 | 7 |
Slitless | ||||||
SA100 / SA200 | 5-15 | <50% | 4000-8000Å | 20-50 Å | 100-500 | 10-12+ |
SPECTROGRAPH SLIT
The spectrograph is built with an adjustable slit. This consists of 2 parts of reflective metal. The basic slit design can be found in Albert G. Ingalls classical book on Amateur Telescope Making, ATM III p 145, where a lovely drawing by Russel W Porter is shown. I constructed the slit unit using aluminum and brass metal. It is mounted at a slight angle, so the star field can be reflected and viewed with a separate guide camera, an ASI120MINI-camera.
RESOLUTION
Measuring FWHM of a narrow spectral line in a reference spectrum gave about 3Å of resolution (FWHM=3Å) for the 300 lines / mm grating, and a narrow slit. The 1200 lines / mm grating yields around 1 Å in resolution. It is of course dependent on careful focusing, slit width and how well different parts of the spectrum are focused by the lenses used.
RESPONSE FUNCTION
By taking a spectrum of VEGA and dividing it with a reference spectrum of the same star, I get this approximate response function for the system Telescope-Grating-Detector, including all mirrors and lenses used. It is mostly due to blaze characteristics of the 300 L/mm grating and the detector of the ASI camera.
