Wednesday, February 20, 2008

On choosing a narrow band solar telescope

Let's talk about how to choose a suitable telescope to use with H-alpha narrow band filters.

First of all, refractor should be a very nice choice. Objective H-alpha filter has the expensive etalon in the front, and therefore, you don't want to have any secondary mirror to block this valuable aperture. Of course, you can use your H-alpha as offset filter in a large aperture scope to get an unblocked objective, however, that would make the scope unnecessarily large.

Given a refractor is good, we shall see what kind of refractor is suitable. Normally, we will first look at the chromatic aberration, however, this is not the case for narrow band usage, since you're essentially looking at monochromatic light, and therefore, chromatic aberration is unimportant here.

Having said that, other forms of aberration should be well controlled, to name a few: spherical aberration, astigmatism, etc. A good achromat will do the job.

Since you will want to tune the etalon via some sort of tilting mechanism, you will want a short tube refractor so that you hand can access the tilting mechanism while looking through the eyepiece. So how long should be suitable depends on the length of your arm.

Finally it's the focal ratio of the telescope, the rear end blocking filter would perform better at slower focal ratio, and usually f/10 is around enough, I mean you won't get much better performance with even slower telescope. And f/8 should be fairly nice as well, so it's actually a balance between the point above. Notice that the focal ratio might not be the native telescope focal ratio. For example, a 70mm ~f/6.8 refractor, if used with a 40mm etalon in the objective, it will become a ~f/12 telescope since the aperture is stopped down.

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