Part I – using the SSON Transmission Grating Spectrograph


Updated 2016 January 14




Spectroscopy, Part II – Generating a profile using Visual Spec


Spectroscopy, Part III – Analysing spectra using Visual Spec


Spectroscopy, Part IV - Resources


Spectroscopy, Part V – The Miles catalogue of spectra




This tutorial describes how to use the SSON Transmission Grating Spectrograph (TGS) to obtain stellar, planetary and comet spectra.


Description of the TGS


How a TGS works is described in the Rigel TGS User’s Guide The Rigel TGS facility is no longer available but the SSON TGS works in exactly the same way. Figure 1 (from the Rigel Guide) shows how the light is split into its component colours by a diffraction grating. Figure 2 (from the Rigel Guide) shows the components of the TGS assembly. A simplified diagram as there are several spectra produced each fainter and further from the axis.



Figure 1. Transmission grating showing wavelength-dependent diffraction angle.


Figure 2. TGS components.


The purpose of the (3mm wide) slit is to block out stars near the same plane on the x axis that would be superimposed on the target stars spectrum.


How to use the SSON TGS(with slit)


This section is based on advice from Rich Williams and the SSON TGS User’s Guide and uses one of the stars listed in Table 1. Objectives are;

- to obtain several spectra over a range of magnitudes and compare with known data. 

- verify SNRs for various exposure times

- ensure images are not over exposed


Scheduling a job


Step 1: Object


Prior to selecting a target star ensure that;

- the star is actually well placed for imaging

- the Moon is not too close (>30º separation is recommended)


Go to the ‘Object Catalogue Page’, select ‘Stationary Objects’ and then ‘SKYMAP Bright Stars – 8882 Records’. Theta Aur(igae) is used in this example.


To reduce the number of stars listed select either a magnitude and RA range and select the ’Go’ button next to the RA selection boxes or a Magnitude and a search word and use the ‘Go’ button next to the Search Criteria box. Theta Aur is a magnitude 2.65 star so a magnitude range of 2 to 3, Ra range 5 to 7 hrs and the word ‘Aur’ were entered. The result is shown in the screenshot. Figure 3. Select Aur Theta-37A (the star is known as Theta Aur and 37 Aur).


 Figure 3. Target selection screenshot.


Step 2: Title


A suitable title ‘Spectroscopy’ and observer name ‘Roger Dymock’ were entered.


Step3: Date/Time (Optional)


I usually skip this step as, if the job is not run on the next night, it has to be cancelled and rescheduled. Missing this step avoids having to do that as the job is run at the next available opportunity. If the job is delayed check that the star is still well placed and that the Moon is not too close as mentioned earlier.


Step 4: Telescope/Filters


The spectra of Theta Aur shown in the SSO TGS User’s Guide were obtained with a 4 second exposure and varying focus offsets. Estimates for the exposure times of the other stars listed were based on their brightness compared with that star i.e. for each magnitude difference the exposure time is multiplied by 2.512 – Figures 4a and b.


Figure 4a. Exposure times



Figure 4b. Exposure times for brighter stars


On the ‘Observation Request Form’ an exposure time of  4 secs was entered in the ‘Duration’ box against the Filter Name ‘600-line/mm TGS WITH a slit’ – screen shot shown in Figure 5. A focus offset is required as the spectrum produced by the TGS is brought to a different focus depending on wavelength – Figure 2. Reviewing the spectra in the SSO TGS User’s Guide an offset of 500 seemed to be a reasonable compromise.


Figure 5. Observation Request Form


Step 5: Submit


On this page click on ‘Submit Schedule’. It is wise to check the email received confirming the job, example below,  to ensure your input was correct.


User: rogerdymock
Job ID: 35713
Title: Spectroscopy
Observers: Roger Dymock
Object Name: Aur Theta-37A
Ra/Dec: 5:59:43.27 / 37:12:45.31
Catalog: Stationary Objects Catalog
Epoch: 2000
Filters: S
Duration: 4
Time to repeat series: 1
Total Images: 1
Telescope: 1
UTC Date: N/A
TGS Focus Offset: 500
TGS RA Offset: 00:00:48
You will receive another email when your job is completed.





Images can be retrieved via Exavault and then unzipped. The RA offset positions the target star on the edge of the image – Figure 6. The Megastar chart, Figure 7, shows the CCD frames centred on the star and offset. The SSON TGS uses an approx. 3 mm wide ‘slit’ to block out other stars in or near the same plane on the X axis, the spectrum of which would be superimposed on the target object’s spectrum.


Theta Aur

Figure 6. Spectral image of Theta Aur (available here)


Figure 7. CCD frames centred on star and offset.


Viewing the image in Astroart shows that the maximum pixel value has all but been reached so the four second exposure time should not be exceeded for this star.


Once your images are available you can proceed to Spectroscopy, Part II – Generating a profile using Visual Spec Note that to analyse images with Visual Spec the zero image must be on the left so the image above would need to be flipped through 180º.