Star Analyser spectra optimised acquisition

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The easiest way to acquire a star spectrum is to interpose in the convergent beam of the telescope, not far in front of the detector, a diffraction grating. The grating StarAnalyser is particularly effective because characteristics of its dispersive power (a grating of 100 grooves / mm), its high efficiency (blaze) and low price. The device can be significantly improved by combining to the grating itself a prism which disperses the light in a inverse maner. It is well known as an optical setup called "grism". An optical property of the system is to cancel the chromatic coma for wavelength not deflected by the device. The theory is described here.

A grism setup in a convergent beam.

Another interesting improvement and less known is to complete the device by a filter that removes infrared radiation (an "order" filter). For example, the UV / IR  rejection filter of Baader company.

Optical transmission of Baader infrared cut filter. The filter becomes almost opaque beyond the wavelength of 700 nm.

The optical system of the grism in the convergent beam supplemented by an infrared cutoff filter.

The IR-cut filter eliminates part of the spectrum where the order 1 is mixed with a diffracted order 2. In fact, a StarAnalyser grating, because of this mix of orders can not be used for all wavelengths above 750 nm due to the spectral sensitivity of CCD cameras in the current blue. The addition of the IR cut filter limits the spectrum to the segment directly usable, which makes the images less confusing. Finally, it becomes possible to exploit the order 2 (the prism optimizes the quality of the spectrum in the green for order #1, but also in blue for the order #2).

The assembly may be mounted in the turret filter holder of a CCD camera. In the example below we show the situation of QSI-583 camera. The setup efficiency is demonstrated on a small high quality refractor: the Takahashi FSQ-85ED (D = 85 mm F / 5,3).

The acquisition setup.

Part of raw image below is typical of spectra acquired with this device. The observed object is the star HD123299 of spectral type A0III and magnitude 3.7. The exposure time is of 7 seconds (this value is chosen to avoid saturation of the spectrum). It may be emphasized that the setting is poor: the dispersion axis is not horizontal, which is a fault.

Raw spectrum of HD123299 taken the StarAnalyser + prism + order filtre.

We have 16 similar spectra of this star. The spectra are processing with StarAnalyser ISIS tool:

The procedure is described in detail here. Now the result:

Verification that no bright object is present in the part of the image where the sky background is evaluated. The dialog box control give the part of the image used for sky evaluation en binning. The dialog can be opened from the "General" tab (this is now a permanent window as you do not want the close). Note that ISIS has made a high quality and complete processing automatically. The spectrum is now properly oriented. Moreover profile spectrum is extracted automatically (see below).

Detail of the 2D spectrum

The useful spectrum at order 1 is confined to a band from about 3700 A to 7000 A. The cut in the blue is in large part induced by the spectral CCD response. The sharp cutoff in the red is related to the use of the IR-cut filter. Thanks to this filter, the spectrum order +1 does not overlap the ordrer +2 spectrum. The latter shows many fines lines in the blue part of the spectrum (Balmer series). They are particularly clear because the grism setup adopted optimizes the quality of the spectrum in this region for the order +2.

Below the spectrum of the star HD123299 with a contrast visualization for reviewing order +1:

ISIS produced simultaneously raw profile @raw.dat:


Below the spectral calibrated spectrum of the star HD123299 (the proper  instrumental transmission is also removed):

    On the left, the spectrum of HD123299 corrected for the instrumental spectral response.
Right, comparison with the spectrum expected of HD123299 - in red (from the base of Miles ISIS).

Here's a sample of spectra acquired with the device described above. First, the spectrum of the supernova SN2012aw in the galaxy Messier 95. The nearly full Moon was very close to the object for the date. Added to the light pollution of the observatory (Castanet-Tolosan, nearly Toulouse city), noise picture of the sky background is consequently high, which does not prevent detection of the supernova:

Spectrum of the supernova SN2012aw. For more resolved spectra, click here. Note the Halpha in emission in
the diffuse spectrum of the galaxy M95.

The spectrum of the nova Sgr 2012 4.4 May 2012 while the object is at magnitude V = 12.8:

The spectrum of the nova Sgr 2012 is mainly dominated by an intense Halpha emission. Expsoure of 10 x 180 seconds, while the object is very low on the horizon.
For more information on this subject (spectra LISA), click here.

The spectrum of the star Mira R Leo, a very red object visually:

The spectrum of planetary nebula Messier 57:

Spectrum of Messier 57 (exposure: 17 x 240 seconds). At the top, normal display.
The spectrum is dominated by the Halpha line and forbidden ionized doublet oxygen two times (green).
Down, a negative display with high contrast: more monochromatic images of low intensities were found.

The spectrum of hot peculiar stars Wolf-Rayet 134 and Wolf-Rayet 135:

WR134 (V = 8.0) and WR 135 (V = 8.5). For more details on these stars click here. Note the finespectra delivered by the system. Exposure: 18 x 100 seconds.

Spectrum of the symbiotic star CH Cyg - 9 poses x 90 seconds (more details on this star here):