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The Star Analyser 200
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The Star Analyser SA200-F Concept
I developed the original SA100 Star Analyser back in 2005 and since then it has introduced several thousand amateur astronomers worldwide to the fascinating field of spectroscopy. Over this period, the equipment used by amateurs has evolved with larger aperture telescopes, bigger camera sensors and close coupled filter wheels becoming more common. The Star Analyser SA200 model was developed in 2014 with these users in mind.
Using the same Paton Hawksley high efficiency blazed diffraction grating technology as the SA100, but with double the line density, the SA200 gives approximately the same length of spectrum but at half the spacing from the camera sensor.
A new low profile design allows the SA200-F to be used with a wider range of filter wheel models and when used with a threaded collar can potentially even be adapted for use in wheels designed for unmounted drop in filters.
Examples of the SA200 used in other applications
Although the main application of the SA200 is where the optimum spacing between grating and camera sensor for the SA100 cannot be achieved, there are other applications where the SA200 can be used to advantage.
The SA200 used as an objective grating mounted in front of a camera lens
The SA200 used in the SEPSA eyepiece projection slit spectrograph
A “junk box” fully collimated slit spectrograph design using the SA200
Is the SA200 a replacement for the SA100?
Most definitely not! For a given length of spectrum the SA100 still marginally out performs the SA200 when used in the standard configuration between the telescope and camera. For most users the SA100 is expected to continue to be the model of choice, particularly for those using cameras with small sensors, giving optimum performance for example with the SA100 mounted on the camera nosepiece. In circumstances though where it is not practical to achieve the optimum spacing from the camera sensor required for the SA100, the SA200 gives an opportunity to improve performance.
What is the optimum spacing and when should the SA200 be used ?
The on line calculator www.patonhawksley.co.uk/calculator(ticking the box for the SA200 or SA100) will calculate the dispersion (in Angstrom/pixel) for a given equipment setup. This figure, combined with the guidelines and help messages there gives an indication of the optimum distance between the grating and camera sensor and whether the SA100 or SA200 is the best choice. For many applications the SA100 gives the best performance but to see where the SA200 can in certain circumstances be the better choice we can delve into the theory behind the calculator in a bit more detail.
The maximum resolution achievable using the simple arrangement of a grating mounted in the converging beam in front of the camera sensor is limited by various optical factors to around 30-40 Angstrom. To be able to achieve this resolution however the spectrum must first be spread out sufficiently to overcome two other limitations.
Firstly the resolution cannot be better than 2 pixels (The Nyquist sampling condition) so to achieve a resolution of say 36A, we need to aim for18A/pixel or less.
Secondly the resolution cannot be better than the length of the spectrum covered by the star image. So for example if the star image is 3 pixels in diameter (FWHM, measured in focus without the grating in place) and we want to achieve a resolution of 36A, then we need to aim for 36/3 = 12A/pixel or less, first checking that the sensor is large enough to fit the spectrum and zero order in the frame at this dispersion. (Note that the star image size depends on the focal length of the telescope and the atmospheric conditions. Good collimation, focussing and measuring targets when they are higher in the sky all help here)
For typical amateur setups, it is usually the second condition which is harder to meet and the following examples illustrate typical situations where the SA200 can help.
The Star Analyser mounted in a filter wheel
Filter wheels are typically positioned as close to the camera sensor as possible to minimise vignetting in astrophotography. This distance is usually less than optimum for the SA100, limiting the spectrum resolution. Consider for example the spectrum below of P Cygni (in red), taken with an SA100 mounted in a filter wheel 30 mm from the camera sensor (an ATIK ATK314L+ camera with 1390 x 1038 x 6.45um pixels) The telescope is a 280mm SCT at f10. Seeing was ~2 arcsec, giving a star image size of 4 pixels.
The dispersion is 22A/pixel which would potentially give a resolution of 44A based on the first (Nyquist) criterion but according to the second (star size) criterion, the resolution will be limited to ~ 88A, which agrees with what was seen in practice and is well below the potential of the Star Analyser, as might be expected for this close spacing used with a long focal length telescope.
If however the SA200 is used instead, (blue spectrum) then the dispersion becomes 11 A/pixel and the resolution limit based on star image size limit is now ~ 44A, close to the maximum potential resolution using the Star Analyser, confirmed by the increased detail seen in the blue spectrum.
(The green spectrum shows the result of achieving the same dispersion but this time with an SA100 at double the distance. The resulting resolution is slightly higher than when using the SA200 at half the distance, confirming that provided the optimum distance can be achieved using the SA100 then this remains the best option)
Below are the same results represented as colourised 2D spectrum images
The Star Analyser on a large telescope
Consider a setup consisting of a 400mm aperture f10 telescope equipped with a camera with a large sensor (eg a Kodak KAF 3200 with 2184 x 1472 x 6.8um pixels) and seeing of 2.5 arcsec (7.1 pixels FWHM).
Using the calculator we find that to meet the star image size condition of 36/7.1 = 5A/pixel we would need a spacing of 135mm for the SA100. Such a large distance may be difficult to arrange, whereas the SA200 mounted at half the distance (68mm) would be easier to accommodate and would give similar performance.
Note that at this high dispersion, a large sensor is needed to fit the zero order and spectrum in the frame and the effect of field curvature will be greater making it more difficult to keep the full length of the spectrum in focus.
Mounting the SA200-F in filter wheels
With a significantly lower profile (7.7mm total height, 5.2mm above the thread compared with 11.2mm total height, 7.7mm above the thread for the SA100), the SA200-F can be mounted in filter wheels designed for 1.25 inch filters without risk of fouling the wheel housing. (Note that the low profile design means that, unlike the SA100, there is no thread available above the grating to screw on other accessories.) The SA200 can be aligned with the camera sensor and held in the correct orientation for example with PTFE tape on the thread or a spot of hot melt adhesive.
Provided there is sufficient clearance, the SA200 can also be mounted in wheels designed to take unmounted drop in filters. An internally threaded collar fixes the SA200-F in position in a carrier plate, for example made from black styrene sheet (plasticard), cut to the size of the unmounted filter. The grating can be orientated correctly and held in position by the threaded collar.
Additional spacers of can be added above or below the carrier plate as required to increase the total thickness and place the SA200 at the optimum height to clear the wheel housing. It is even feasible to fabricate a carrier which lowers the position of the SA200 below that shown here if neccesary, by laminating the material (bonded using polystyrene cement). Provided a clearance of at least 7.7mm (The total height of the SA200) is available within the filter wheel it should be possible to fabricate a suitable carrier. Users would need to check their particular model of filter wheel to ensure there is enough clearance. Paton Hawksley, the manufacturers of the Star Analyser offer a kit consisting of a threaded collar and plates for the user to trim to size.
A similar technique can also be used to mount the SA200 in a blank 2 inch filter cell for wheels designed for 2 inch screw in filters
Alternatively a 2 inch diameter 1.25 inch to M48 threaded adapter for example from Teleskop Service or Agena Astro can be used provided there is enough clearance.
(Note that, since the light cone from the star is only dispersed into a spectrum beyond the grating, any vignetting of the field due to the smaller aperture of the SA200, compared with filters used for imaging, is not a problem provided the zero order image of the star is placed within the unvignetted area.)
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