Spectroscopy

An Introduction to Spectroscopy with the Seestar S50

Spectroscopy is the study of starlight and the information hidden within it. While I’m far from an expert, I was curious to explore this fascinating field. The Seestar S50 has proven to be a remarkable tool—an innovative device made even more powerful thanks to community-led modifications and clever hacks.

Through using this engineering marvel, I’ve learned an incredible amount. The community surrounding the Seestar S50 has been both generous and inspiring. In the famous words of Isaac Newton, "If I have seen further, it is by standing on the shoulders of giants." One such giant is Kai Yung. Though we’ve never met, his contributions have been invaluable and truly inspirational.

To help others get started, I’ve put together this quick how-to guide focused on the practical side of capturing stellar spectra. Full credit for this process goes to Kai Yung.

Resources and Links:

• YouTube channel (by Kai Yung):
  https://www.youtube.com/watch?v=aY4EHAjl0nk&t=1480s
  
  
  

• Facebook group:
  https://www.facebook.com/groups/373417055173095
  
  
  

• GitHub repository:
  https://github.com/smart-underworld/seestar_alp
  
  
  

• What You’ll Need:

• Grating Material:
  I sourced mine from Kai’s Facebook group. It's possible to find similar diffraction gratings elsewhere, but I recommend checking there first.
  
  
  

• Seestar ALP Software:
  Available for free via GitHub, this custom-built software gives you full control over the Seestar from a computer. I currently run it on a Raspberry Pi. It’s a vital tool for capturing the specific dropped frames required for spectral analysis.
  
  
  

• Astrometric Stacking Program:
  You’ll also need software capable of stacking frames and aligning them with star positions. Any tool that allows FITS processing can work here.
  
  
  

• BASS Project (Basic Astronomical Spectroscopy Software):
  This free software is essential for extracting and analysing the spectrum from your FITS files. You’ll need to join the associated Facebook group to access it.
  
  
  

As an example, below is a single 10-second frame of the star Arcturus, loaded into BASS for analysis. From this, we can begin extracting and interpreting the star’s spectral signature.

Next, we need to convert the image to colour. To do this, go to Tools > De-mosaic Bayer.

Once the image is in colour, it should be rotated horizontally. Navigate to Tools > Rotate Image and apply the necessary rotation.

After rotation, the image is ready to be cropped to isolate the spectrum.

Now that the image is correctly oriented, we can proceed to crop it. Simply right-click on the image and select the crop option to isolate the desired area. 

Save the file as a 16-bit PNG linear image—it's important that the image remains unstretched, however I have also used JPEGs as I play with the software, Im not quite sure why the image has to be unstetched as I am still learning.

Next, load the cropped image into BASS Project. Open the PNG file using the RGB channel, and you should see a view similar to the one shown below.

Next, we need to level the spectrum. Go to Image > Rotate, select Draw Angle, then apply the adjustment to all open images. 

Once the image has been aligned, the next step is to tell BASS exactly where the spectral signal is located. To do this, go to Selection > Set Binning Region, and highlight the area containing the spectrum. 

We also need to subtract the background by setting Subtraction Regions 1 and 2. Repeat this process for each of the RGB channels. Once complete, you should see a result similar to the example below. 

We now need to calibrate our graph and compare it with other spectra.  Chatgpt is a helpful resource for questions about specific stars.  I changed the star I analysed because I was somewhat disappointed with my original graph, so below is the spectrum of Vega.  Using ChatGPT, I can determine what we should expect to observe.   What you see in Vega’s Spectrum

Strong Balmer absorption lines:

Hα = 6563 Å

Hβ = 4861 Å

Hγ = 4340 Å

Hδ = 4102 Å

My spectrum might not be that great.  It tells me that the best reference is Reference Spectrum Name is A0V, which we use for an overlay.  Go to Tools | Reference Spectrum.  Does it line up, may not quite, maybe I need a better spectrum, looks promising around 450 mark, maybe??