Star Analyzer Prism Myths

Tim Stone

There seems to be a significant uptick lately of interest in the Star Analyzer Prism. This prism is an accessory designed specifically for use with the SA100 grating. Because I had some conceptions of the purpose of this prism that turned out to be simply and utterly incorrect, I thought I might relate those conceptions and hopefully clarify what the prism is actually for.

When I decided to make the jump, I had to decide what hardware I wanted to use. I didn't have a grating, and so I needed to buy one. I bought the SA200, under the basic notion "If 100 is good, 200 is better." This not entirely true. It's similar to saying if a little screwdriver is good, a big old honking screwdriver is better. Like the screwdrivers, the SA100 and 200 are both good tools, and useful for what they are good for. It turns out, the 100 would have been a better learning grating for me. If you're just venturing into this space, I would recommend starting with the 100, and then adding the 200 if or when you decide you want to do something that tool is better for. But that's not what this post is about.

I also bought the 3.8 degree prism. I thought it magically increased the resolution I would attain with my SA200. I was giddy with the thought that I could simply screw a wedge-shaped piece of glass onto my grating, and get even -better- results. At first, I acquired some spectra with my SA200 without the prism. I can tell you I was so thrilled to see absorption features, I could hardly contain myself. Imagine my thrill at the thought of doing EVEN BETTER than that with my prism. I screwed it on, acquired a spectrum from one of my previous targets, and... I didn't see any difference at all.

I thought I'd been ripped off. The thing clearly didn't do what I thought it would. Why did I spend all those $$ for it? In my disillusionment, I put it away and just concentrated on using the grating. That was the first correct thing I did.

Now, a couple of years later, I have come to understand the purpose of the prism, what it does, and what it does not do. Let's start there: what does it not do?

  1. It does not significantly improve your dispersion or resolution.
  2. It does not necessarily make your spectra better. I say -necessarily- because it -can- help in one way, I'll talk about in my list of what it -does- do.
  3. It is no substitute for quality data acquisition (e.g. focus, exposure), processing (e.g. stacking), and reduction (e.g. calibration) skills.
Let me dwell on point #3 for a minute. To even -see- the effect of the prism, you must be able to attain good focus. Focus is the #1 problem I had with my spectra. Even when I thought for sure I was in focus, I was way out of focus. I was chronically out of focus, no matter how little or much spacing I had between my prism and my sensor. If you don't know how to get excellent focus, then that's the only thing you should be working to perfect. Everything depends on that. (Hint: when the spectrum strip is in focus, the zero order star is not.)

Now, what -does- the prism do?

  1. It makes the spectrum strip distinctly non-linear. This is how prisms work. This introduces a lot of bother in the process of calibrating your spectra. You can no longer calibrate on the zero order and a well defined line. It won't work that way anymore. You will need to do at least a three-point calibration. If you're not really good at calibration, stay away from the prism. Once you can get excellent focus, then concentrate on becoming great at calibrating on two points. Graduate from needing the zero order as one of your points, to using two lines, one at the bluer end and and one at the redder end, to do your calibrations. Get good at this.
  2. For the bother (should I say "pain?") of non-linear calibration, you get a reduction in aberrations at a specific location in your spectrum image. The prism moves the spectrum strip toward the center axis of the optical system, where theoretically, aberrations such as coma and chromatic are null. The wavelength where the aberrations are null is determined by the grating and the prism deviation angle. For the SA 3.8 degree prism, the deviation angle is about two degrees. With the SA100 and the prism, at the grating/sensor spacing they were designed for, the aberration null wavelength should be about 6500A. With my SA200 and the prism at the spacing I use, the null is around 4300A. 

Once you've learned to acquire excellent data, process it well, and reduce (calibrate, normalize, etc.) it accurately, perhaps it is time to add a prism to your mix. It seems the null wavelength shifts with your grating/sensor spacing, so you will have to experiment some to position the wavelength you want to study at the null point. While you will not really have any higher resolution than without the prism, you will have lower aberration around the null point, and so your results there will likely be closer to the maximum for your system. The difference, though, is not nearly as dramatic as you might think.

Having been deeply involved in deep-sky astrophotography for years, I had gotten to the point where little things mattered to me. I was fine-tuning my deep-sky images in ways only I could see. When I stepped into spectrography, I had forgotten what it was like to be way down low on the learning curve, and naturally thought I understood everything well enough to continue to care about the little nitpicky things. It took me most of the first year to finally come to the realization that I didn't know anything about what I was doing, and needed to just go back to the fundamentals everyone here on this forum was trying to tell me about:

  • Practice, practice, practice
  • Learn to get awesome focus
  • Learn to get exposure correct
  • Learn to calibrate
  • Stick with bright stars
  • Learn with A stars first. 
  • Learn the cooler stars after the A stars.
  • Start tuning your spacing to see if you can get better results
  • Do some awesome stars like P-Cyg, a little dimmer, but still reachable
  • Do some Be stars or Mira variables
  • Now, are aberrations bothering you? Add a prism.

Last, let me offer my apology if this seems patronizing or pompous.I don't intend any offense. I don't know it all. I'm still only two years into this avocation. I may have some details wrong in this post. If so, I hope the forum will correct me! But... I'd just hate it if people had the same misconceptions I had about the prism and, like me, made decisions based on those misconceptions. If you're new, just know, there is a place for the prism, but you don't need it to take fantastic spectra!.

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