Alpy 600 spectrograph

First results

Christian Buil
 

Introduction

Here are some characteristics of the spectrograph:

- Very compact and lightweight.

- True slit spectrograph with a specially designed optical configuration, with a removable entrance slit, a collimator, a grism dispersive element and an objective lens (a complement lens is provided in standard to use a DSLR as a camera).

- Resolving power R = 600 @ 6500 A (with a 23 microns slit)

- Typical spectral domain: 3700 A - 7400 A

- Can accept f/4 beam without vignetting (high luminosity spectrograph)

- Standard T2 camera interface (M42x0.75)

- Many opportunities for interface to the telescope (31.75 mm, T2, ...). Ability to fix a photographic objective lens for wide field spectrography or an optical fiber.

- In option, a guiding module and a calibration module are available.

More about performances of Alpy 600 spectrograph here.

On the left, the exterior aspect of base module. The size is similar to a simple telescope eyepiece. At the center, we can see a set of interchangeable slits (given in the standard configuration). On the right, a slit etched on a reflective substrate supplied with the guide optional module. For the scale, the Alpy 600 spectrograph is coupled to an ATIK 460EX CCD camera.

The base module interfaced to a flip-mirror system.

The optional high quality guiding system, recommended for advanced observations. The guiding camera is here an Atik314L+, but you can also choose any other model (low cost CCD, video camera, ...).

Alpy 600 can be used at the direct focus of fast modern refractors (apochromatic), at the f/10 focus of SCT telescope, and more... Here we use a standard SCT focal reducer to observe at f/6.5 (better luminosity, less sensitivity to seeing and guiding error).

The optional calibration module is added in front the guiding module (the white box). A simple 12 V power is required. ALPY600 system is presented here in its most complete configuration.

The most economical disposition for Alpy 600. The base module is attached to the telescope from a standard 31.75 mm interface. A flip mirror help to find and center objects on the spectrograph entrance aperture (hole or slit).

 

The guiding module

The guiding module.

Alpy 600 spectrograph on Takahashi FSQ-85ED refractor (D = 8.5 cm - F/5.3).
Remenber, the use of a spectrograph is much simpler than using a small telescope. And this often leads to more accurate results. A big telescope is not always the best solution for a given problem.

Alpy 600 spectrograph on a Celestron 11 telescope (D = 28 cm).

Alpy 600 spectrograph on St-Caprais (D = 94 cm, F/4.45) telescope during observation of asteroid 2012 DA14.

On the left, we have the image delivered by the guiding module. The camera used for this function is here an Atik314L +, a solution quite luxurious, but also very efficient. Use of more economical camera is possible, of course. The vertical line is the image of entrance slit. The slit size is 23 microns x 3 millimeters. Note the excellent image quality provided by the system, a very important point for accurately guiding and identifying faint objects. On the right, the corresponding image of Messier 42 spectrum . The presence of a grism in the Alpy 600 system induce a slight curvature of the spectral lines (corrected during processing). The exposure time is 3 x 180 seconds with an Atik460EX camera model (driven by Audela software). Some sky pollution lines are visible, merged to M42 spectrum (click here for identify spectral lines).
 

The corresponding calibrated spectral profile. Note the UV part of the spectrum and thanks also to high UV sensitivity of Sony ICX694AL CCD detector (camera Atik460EX).

The calibration module

The Alpy 600 system can be completed by a calibation optional module, whih three functions:

1 - Ar/Ne/H lamp for spectral calibration ;
2 - tungsten lamp for flat-field ;
3 - shutter.

Remote control is possible.

Spectral calibration spectrum delivered by the module (Ar/Ne/H spectrum).

Flat-field spectrum. The oscillations along the spectral axis are from the CCD responsivity (Atik460EX camera). The signal in the UV part is faint and it is a problem for calibrate this part of the spectrum (sum of many flat-field image is necessary for increase signal to noise ratio).
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Slit or Slitless

- A narrow slit mode (currently 23 microns). This forces the adoption of Alpy optional guiding module. The advantage is that the spectral resolution is now limited by the slit and not the seeing, it is possible to calibrate more precisely, and faint objects are more accessible.

Up image is an Alpy 600 spectrum of Messier 42 nebula obtained by using a large slit (300-microns), in a quasi-slitless mode. Monochromatic details of the nebula are clearly visible at position of the main emission lines. But at the same time, the spectral information is blurred. The spectrum is taken with a FSQ-85ED Takahashi refractor and artificially colored. For the down image, ALPY600 is now used with a narrow slit (23 microns) and the optional guiding system module. The spectral resolution is significantly improved and the sky background is much darker (remember, I observe in urban condition). The telescope is a Celestron 11 used at f/6.4.

Slit and slitless modes: variation around NGC 2392 planetary nebula

2D spectrum of NGC 2392 nebuba (Eskimo planetary nebulae) taken with a small telescope, here a FSQ-85ED Takahashi refractor (D = 85 mm) in quasi-slitless mode (300 microns slit). Up image show the sum of 15 x 120 sec. exposure. Mid image is the situation after numerical sky removal. The lower image is a high contrast negative view. Note that the spectrum is very sharp from left to right.

2D spectrum of NGC 2392 captured in quasi-slitless mode with a Celestron 11 telescope used at f/10. It is a one shot of 20 seconds exposure. The spatial structure of the nebula is now well detailled (note difference between [O III] image (left) and Halpha image (right)).

Now we use the narrow slit mode. NGC 2392 is observed through a 23 microns slit. The spectrum contrast and spectral resolution are increased.
The telescope is a FSQ85-ED refractor. The science camera is an Atik460EX model used in binning 2x2. Exposure of 4 x 300 sec. in urban conditions.

The same object, NGC 2392, observed by using Alpy 600 a biggest telescope: a Celestron 11 at f/6.7. Up, a standard positive visualization, down, a high contrast negative represention (note presence of very faint emission lines). Exposure 6 x 180 seconds.

NGC 2392 spectrum taken with LISA spectrograph and a Celestron 11 f/6.4 telescope. Exposure 15 x 300 seconds.
Comparison with Alpy 600 is instructive. LISA offers a better spectral resolution (R=900 for LISA, R=600 for Alpy 600) for the major part of the spectrum. The exception is for near UV, where Alpy 600 gives very sharp line profiles down to 3800 A (better chromatic aberration correction). For example, ALPLY600 is probably a good choice to observe CN lines in cometary spectra (near 3880 A). Detectivity comparison between LISA and ALPY600 is not easy because the difference of spectral resolution and dispersion. The performance seems pretty close if the spectrographs are fixed to telescope with a diameter between 100 and 300 mm. But LISA is equivalent to a x0.63 focal reducer (for example, the f-ratio of telescope + spectrograph is here f/4.2). In comparison Alpy 600 magnification is nearly unity. As a result, the detectivity of LISA will be higher than Alpy 600 on telescopes larger than 300 mm (i.e. for the same spectral resolution, the slit width of LISA can be larger).

The spectrum of NGC 2392 is here obtained with a Star Analyzer grating in the convergent beam of a Celestron 11 telescope (CCD camera is a QSI-532 model and the exposure time is 15 x 30 seconds). Alpy 600 and SA spectrographs are not really comparable. SA may acquire the spectrum of many objects simultaneously, it is a true field spectrograph. The possibility of Alpy 600 are really more limited in this area, even using a very large slit. In contrast, the Alpy 600 spectra are considerably more detailed because difference of intrinsic resolution power and it is a true slit spectrograph, capable of isolating very small regions in the sky. For example, Alpy 600 can more easily obtain spectra of supernovae because it is possible to separate the star from the host galaxy with more efficiency. Paradoxically, Alpy 600 is also easier to use because the calibration is more automatic and slit acquisition more tolerant (if you use narrow slit option of course). In the over side, SA is perhaps a better spectrophotometer because the absence of slit (insensitivity to atmospheric refraction), but sky background can also induce some photometric distorsions.

  

Aspect of the guiding field during NGC 2392 acquisition sessions with Alpy 600.
Left, with the FSQ-85ED refractor (D = 85 mm, F = 450 mm).
Right, with the C11 SCT (D = 280 mm, F = 1800 mm).
The vertical dark line is the image of spectrograph entrance slit (23 microns wide).

A typical faint surface object observation

 
Spectrum of a section of planetary nebula Messier 97 (Ownl nebula). This faint surface object magnitude is not easy from my polluted city observatory (not visible in the guiding camera!). The Alpy 600 equipped with guiding module is attached to a Celestron 11 telescope (f/6.4). Up, a  raw 600 seconds exposure. Note presence of thermal dark signal and lines distorsion ("smile" effect consequence of grism configuration). Middle, sum of 9 elementary exposures (1.5 hour total integration time), after preprocessing and distorsion correction (ISIS software). Down, the final 2D spectrum after sky parasitic signal removal. Note also the capture of central star spectrum.

Messier 97 final spectral profile (after correction of instrumental spectral response).
 

Science:  Variable stars survey

Teach astronomy:  Data acquisition and methods for an educational project

Example #1:  Stellar chimical composition (measure of metallicity)
(inspired by the book "Stellar spectral classification", R. Gray, C. Corbally, Princeton University Press (see page 282)).

Example #2: Redshift measurement. The quasar 3C273 is a very easy object with a moderate sized telescope.
Here a Celestron 11 (D = 28 cm). Alpy is equiped with the 23 microns refective slit. Atik 460EX CCD camera for acquisition (CCD at -15°C). The exposure time is 1h50m.

Example #3: Extragalactic astronomy. Observe active galaxies, supernovae and measure Univers expension.

Example #4: Observation of atmospheric refraction. The observed objet (HD73495) is at low elevation relative to horizon (20.1°). Note the cuvature of the spectra (the atmosphere act as a dispersive element, i.e. blue rays are more deviated then red rays).

The design team of Alpy 600. From right to left, Olivier Thizy (Shelyak), François Cochard (Shelyak), Christian Buil (author of this paper).

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