telescopeѲ          ▪▪▪▪                                             CONTENTS Houghton-Cassegrain comparison       12. THE EYEPIECE


Majority of the telescopes in use are those made for general astronomy. However, a telescope for general purpose may be limited in its ability to serve for some special purposes, such as observing outside of the visible range (infrared, radio), or observing particular astronomical object with special properties, such as the Sun. Among various specialized instruments for solar observations (coronagraph, spectroheliograph, etc.), probably the most interesting for an amateur is a telescope specialized for use of the H-α (hydrogen alpha) filter. Blocking the rest of abundant solar radiation makes possible observing of a variety of solar features, otherwise less pronounced or invisible (prominences, filaments, solar eruptions, etc.). 

11. Solar H-α etalon telescope

The H-α solar telescope can either use H-α filter placed in front of the objective, or H-a etalon placed inside telescope, combined with a blocking filter in front of the objective (for astrophotography of emission nebulae, such filter can be mounted close to the image w/o use of blocking filter, but otherwise it is avoided due to the heat-related risk). For the optimum performance, such filter requires near-collimated light, hence a telescope with H-α etalon located behind the objective needs special arrangement providing a collimated section within the light path. It can be created in a simple arrangement of three singlet lenses, two positive and one negative, as shown below.

The advantage of the etalon arrangement is that the filter can be manipulated in order to increase, or modify performance. For instance, double etalon will further narrow the passband; tilting the etalon slightly shifts the passband, allowing optimizing the passband to the detail of observation, and so on.

This simple arrangement cancels all aberrations except field curvature and some residual astigmatism (chromatism, of course, is not corrected, but it is of no consequence operating at a single spectral line). Despite the best field being strongly curved, the 0.7-degree field is still well within diffraction limited even at the edge, due to the small linear field extent.

Width of the collimated section is a function of the front-to-mid lens separation: the smaller separation, the smaller width, and vice versa. The flat-field correction somewhat improves with the smaller separation, but not significantly. For any given separation, the width of collimated section can be also widened by using stronger glass for the mid element. It also improves field correction but, again, only by 10-15%, or so. Houghton-Cassegrain comparison       12. THE EYEPIECE

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