Following on from my post #318, I started wondering if there was an "optimum" setup for my scope, in terms of a focal reducer and its effect on the spectra. So I ran a few calcs, and found some interesting results (I think
).
I decided to calculate the minimum band width I could resolve in a spectrum with the SA, for each FR with my scope. I found that regardless of which FR I used, the smallest bandwidth I could resolve with my scope is 6.25 Å.
At f/10, the maximum dispersion is 1.86 Å/pixel, but the resolution is 3.36 pixels; at f/6.3, the maximum dispersion is 2.963 Å/pixel, but the resolution is 2.1 pixels; and at f/3.3, the maximum dispersion is 5.7 Å/pixel, but the resolution is 1.1 pixels. These all multiply out to about 6.25 Å for my scope and camera.
Note: the above calculations ignore the effect of seeing and imperfections, too. So to do better than 6.25 Å I believe I'd have to introduce a slit to the system...
So the choice of which FR to use has to be from other factors:
- the length of extension tubes between the SA and the CCD; and
- the number of frames required to capture a given bandwidth.
For f/10, the maximum dispersion is achived when the CCD is 300mm from the SA. That's a long extension tube! And to capture , say, the whole visible spectrum of 4000Å would take about 5 or 6 frames!
For f/6.3, the max distance from SA to camera is a more reasonable 189mm, and the visible spectrum could be captured in about 3 frames.
For f/3.3 (if I could find a suitable FR for the job
) , the max distance would be 99mm, and the visible spectra could be captured in 2 frames of the DMK 21AU04.AS.
So I'm a long way from pushing the limits of the SA!
It would be interesting to see if my scope has enough focus travel to handle the extreme distance from SA to CCD.
I guess an experiment is the only way to tell.
For the time being (in the absence of an f/3.3 solution) the f/6.3 reducer still seems like the best setup for me ATM. Plenty of scope to explore yet!
Al.