CONTENTS

INSTRUCTIONS

Description
Care and maintenance
A quick look
First light
Using the locking ring
Troubleshooting
Processing
Wavelength Calibration
Displaying the results graphically
Tips for recording spectra of faint objects
Visual Use
Further advice and information

FREQUENTLY ASKED QUESTIONS

APPENDIX I:   BRIGHT STAR SPECTRAL TYPES
APPENDIX II: TWENTY BRIGHTEST WOLF RAYET STARS
APPENDIX III:  TELLURIC LINES (O2)
APPENDIX IV:  HYDROGEN BALMER LINES

STAR ANALYSER 100 INSTRUCTIONS

Description

The Star Analyser 100 is a high efficiency 100 lines/mm transmission diffraction grating, blazed in the first order. It is mounted in a standard 1.25 inch diameter threaded cell, to be compatible with most telescopes and accessories. It has been designed to make the production of low resolution spectrum images of a wide range of point like astronomical objects as easy as possible. It complements a wide range of types of camera used in astro-imaging. It can however also be adapted for visual use. A locking ring is also supplied to lock the grating in the desired orientation.

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Care and maintenance

The Star Analyser has been designed and built to give many years of trouble free service.  It cannot be dismantled by the user.  To protect the delicate diffraction grating surface, it has been sealed between anti-reflection coated glass cover discs. The sealed unit is fixed in the cell in the correct alignment.

As with any optical device, care should be taken to protect the optical surfaces.   It should be stored in its protective box when not in use. 
(After use, any dew which may have condensed on the device should be allowed to evaporate before closing the box). 
Avoid touching the glass. 
Any dust should be removed with a blower brush or clean oil free canned air.  
More stubborn marks and fingerprints can be removed with care using conventional lens cleaning techniques. 
(Lens cleaning fluids should be used very sparingly to avoid the risk of seepage of the fluid between the glass elements)

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A quick look

If you hold the Star Analyser up to you eye and view a compact source of white light through it, you will see the light source flanked by a series of rainbow spectra, stretching away in both directions. One of the pair of spectra closest to the light source will look significantly brighter than the others.  This is the blazed first order spectrum and is the one we are aiming to image, along with the straight through view of the light source (the zero order). If you look at the edge of the filter cell, you will see a white line marking the direction of the blazed first order.  This will help identify the right spectrum and line it up in the field of view of your camera.

The blazed first order spectrum dominates the other orders in this view of a low energy light bulb.

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First light

Set your telescope and camera up on a bright star and centre it in the field. 
(Spectral type M stars are a good first target as they show nice broad spectrum lines which are easy to see. 
Alternatively type A stars show narrow dark absorption lines die to Hydrogen. 
You can see a list of the spectral types of all bright stars down to mag +2.5 in Appendix 1).

Screw the Star Analyser fully onto the nosepiece of your camera.
Unscrew it slightly until the direction indicated by the white mark is on the horizontal axis of the camera and to the right as you view from behind the camera.
When you replace the camera you should still be able to see the image of the star but it will be somewhat fainter. (Adjust to bring the star back into focus).
Move the telescope slightly so that the star image is on the left edge of the frame.
You should then see the spectrum of the star spread out horizontally across the field, with increasing wavelength (blue to red) running from left to right.  
(Note that because the light from the star is now spread out over many more pixels, you may have to increase the exposure to see the spectrum clearly. 
You should now be able to see some lines in the spectrum. If not, adjust the camera settings and focus slightly until you get the sharpest, clearest spectrum.
Congratulations! You have recorded your first spectrum with the Star Analyser.


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	Images of star spectra taken using a Toucam Pro webcam Delta Virginis (type M, top) Vega (type A, bottom)

Using the locking ring to fix the orientation of the spectrum

A threaded locking ring is supplied which can be used to lock the Star Analyser in the desired orientation. 
The ring is screwed into the internal thread of the camera nosepiece and the Star Analyser screwed in until it locks against it.
By adjusting how far the locking ring is screwed in, the Star Analyser can be locked in the required orientation.
If desired, the ring can be fixed in position with thread locking compound to ensure repeatability between observing sessions.
(First check that the presence of locking ring does not affect the use of any other accessories you may want to screw into the nosepiece and be sure to use the removable type of locking compound in case you wish to remove the ring in the future.)

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Troubleshooting

The spectrum seems very faint

Are you sure you have found the right spectrum? Move the telescope until the star image is on the right of the field. You should just be able to see a faint spectrum running from right to left. It should be much fainter than the spectrum on the other side of the star image. If it is brighter, rotate the Star Analyser through 180 degrees.

The spectrum is too long to fit across the field.

(Take care not to confuse the infra red end of the wanted spectrum with the blue end of the next order spectrum which will be much fainter but will overlap.)

This is because the dispersion (The amount by which the light of given wavelength is deflected) is too high. The Star Analyser100 is designed to work with the majority of cameras and nosepieces.
You may however run into this problem with cameras with smaller sensor chips and/or longer than average nosepieces. For example, most Toucam owners may find it difficult to include the complete infra red end of the spectrum. (See the FAQ for more information on the optimum distance). 
There are several solutions to this.

a)	Rotate the Star Analyser so the spectrum falls diagonally across the camera field. This increases the available length by 25%.  The spectrum image can easily be rotated back to horizontal with any image processing package.
b)	Reduce the distance between the Star Analyser and the sensor by using a shorter nosepiece.  (Some camera suppliers can supply shorter nosepieces which are designed primarily for use with focal reducers and where focus travel is limited).
c)	Move the star image off to the side of the field.  (Note this has the disadvantage of  making the calibration more difficult, as the star image represents the zero wavelength point (see calibration).

The spectrum is rather short

You can increase the length of the spectrum by adding optional spacers to increase the grating to sensor chip distance. See FAQ 13 & 14 for more information

I cannot see any features in the spectrum

Not all stars show clear features in their spectra.  Some features can be quite subtle and only show up after further processing of the image. 
Spectral types M and A probably show the most obvious lines.  (See appendix I for a list of bright stars and their spectral types).  If you don't have any luck, try a different star or try processing your spectrum image further as described in the Processing section.

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The features are indistinct and smeared out

This simple type of slitless spectrograph depends on the target being almost a point source.
If it is not (for example if the seeing is bad or you are using a long focal length) then sharp features in the spectrum can be smeared out and become indistinct. 

If you are using a focal length of over 2m, a focal reducer will often help to sharpen the spectrum features.

I cannot focus all positions along the spectrum at the same time

There will be a slight shift in focus due to the increasing angle of the light beam from blue to red so you may find you have to compromise focus slightly at the far ends of the spectrum.  If you are using an achromatic refractor, the focus errors will be larger due to chromatic aberration of the telescope. Particularly with short focal length achromats, you may find you can only focus part of the spectrum at a time.  See the FAQ for more information on focussing and other factors which affect resolution.

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Processing

	The image of the spectrum can be enhanced to improve the visiblity of the features in various ways, using your favourite image processing package.
Vega spectrum broadened and processed to enhance the visibility of the lines

Rotate the image so that the spectrum is horizontal with the star image on the left.
Produce a strip spectrum as follows:-

1.   Produce a strip spectrum as follows:

Crop the image so it just shows the strip with the spectrum (it will typically only be a few pixels high).  Resize the strip to the same width but only one pixel high, then resize again to the same width but 30 pixels high.

2.      The standard image processing tools can be used to improve the spectrum. Adjust the image to give the best visibility of the features.   The image can then be sharpened to make faint lines more visible or blurred to reduce noise.

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Wavelength Calibration

The distance along the spectrum can be calibrated in nm or angstroms per pixel (10 A = 1 nm) so that the wavelengths of the features can be determined.   A rough calibration can be made using the following formula.

Dispersion (A/pixel) = 10000* pixel size (um) / [grating lines/mm * grating to CCD distance (mm)]

e.g. for the Star Analyser 100 at 35 mm distance from a Toucam Pro webcam, the dispersion would be

10000 *  5.6 /  [ 100 * 35]  = 16 A/pixel

Here the wavelength axis of the Vega spectrum has been calibrated using the atmospheric Oxygen line. The Hydrogen Balmer lines were then identified."
v:shapes="_x0000_s1056

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For greater accuracy, the wavelength of known lines identified in the spectrum can be used. This need only be done once for a given configuration, provided the geometry remains the same. 
Appendices III and IV lists the wavelengths of the Hydrogen Balmer lines and lines from Oxygen molecules in the atmosphere, which can often be identified in spectra and used for calibration.
The distance from the centre of the zero order star image to the identified line is measured in pixels and the result divided into the wavelength of the line to find the dispersion.
The distance along the spectrum can then be calibrated using the zero order star image as the origin.


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Displaying the results graphically

The image of the spectrum can be converted to graphical form using suitable software.

This technique is very effective for revealing faint features which might otherwise be missed in the image. The software may also have tools to help with calibration and identification of features in the spectrum.   It is also possible to correct for variations in intensity of the spectrum with wavelength due to the response of the camera sensor.  Examples of software for spectrum analysis include AIP4WIN and Visual Spec.  The user is recommended to refer to the instructions supplied with their specific software for further information.

The spectrum of red giant delta Virginis converted to graphical form using Visual Spec software
(uncorrected for the spectral response of the camera)"

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Tips for recording spectra of faint objects

Before imaging a faint object, set up and focus on a bright star first. The spectrum produced can also be used as a calibration check. 

In crowded star fields, unwanted star and spectrum images can interfere with the spectrum you want to record. Rotate the camera so that the spectrum misses them.
If you have trouble with star trailing on long exposures, orientate the grating so that the spectrum is at 90deg to the direction of drift. This will prevent the drift blurring the spectrum.


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Visual Use

The Star analyzer can be fitted directly to the bottom of an eyepiece threaded for filters.  Note that, since the device is designed primarily for small camera sensors, the spectrum image will be rather short.  The length of spectrum can be increased by increasing the distance between the Star Analyser and the eyepiece. This can often be accomplished by attaching the Star Analyzer to the incoming end of a star diagonal.  Note that these configurations require increased inward travel of the focuser, which may not be available with some telescopes, particularly Newtonians.


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Further advice and information

There is an on line community of Star Analyser users where you can get advice, exchange ideas and share results at  http://groups.yahoo.com/group/staranalyser.

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APPENDIX I 

BRIGHT STAR SPECTRAL TYPES

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APPENDIX II

TWENTY BRIGHTEST WOLF RAYET STARS

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SAO	WR	R.A.	Dec	Type	Mag(V)
219504	11	08:09:32	-47:20:12	WC8	1.74
227425	79a	16:54:59	-41:09:03	WN9ha	5.29
252162	48	13:08:07	-65:18:23	WC6	5.88
238353	22	10:41:18	-59:40:37	WN7h	6.44
238394	24	10:43:52	-60:07:04	WN6ha	6.49
227328	78	16:52:19	-41:51:16	WN7h	6.61
69402	133	20:05:57	+35:47:18	WN5	6.7
172546	6	06:54:13	-23:55:42	WN4	6.94
227390	79	16:54:19	-41:49:12	WC7	6.95
49491	140	20:20:27	+43:51:16	WC7pd	7.07
227822	90	17:19:29	-45:38:24	WC7	7.45
69592	136	20:12:06	+38:21:18	WN6(h)	7.65
251264	40	11:06:17	-65:30:35	WN8h	7.85
69755	138	20:17:00	+37:25:24	WN5	8.1
69833	139	20:19:32	+38:43:54	WN5	8.1
238408	25	10:44:10	-59:43:11	WN6h	8.14
69677	137	20:14:32	+36:39:40	WC7pd	8.15
186341	111	18:08:28	-21:15:11	WC5	8.23
69541	134	20:10:14	+36:10:35	WN6	8.23
251296	42	11:10:04	-60:58:45	WC7	8.25

The Seventh Catalogue of Galactic Wolf-Rayet Stars (van der Hucht, K.A. 2001)

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APPENDIX III

TELLURIC LINES (O2)

Fraunhofer Band	Wavelength A
a	6276 - 6287
B	6867 - 6884
A	7594 – 7621

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APPENDIX IV

HYDROGEN BALMER LINES

Alpha	6563
Beta	4861
Gamma	4340
Delta	4102
Epsilon	3970
Zeta	3889
Eta	3835

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