Different Scope for Different Objects

[CanisMajorTom]

Can someone explain to me why some eyepeices and some scopes can yeild better views than others (without having respect to aperture or FL).

Refractors seem to be the way to view planets rather than SCT's, why?

[Radar]

I'm in the dark on this one mate.

[Astrolounge]

with respect to scopes, the central obstruction in SCT's and Newtonians affects the contrast to some degree depending on what percentage the obstruction is compared to the primary, normally around 18-25%.
because refractors have no central obstruction they show higher contrast and clarity but then of course they are usually a smaller aperature and therefore suffer from less light gathering ability.
planetary observing requires longer focal lengths for magnification and this is better achieved with SCT's and Newts because they have the aperature to cope with the high power, although there is a compromise on clarity.
ep's are another matter, they are becoming so crammed full of exotic glass they are hard to keep up with although the shorter the f/l particularly in Newts the higher quality ep is required to cope with the added mirror curvature, whereas longer f/l SCT's are much more forgiving.

[Radar]

Yeah, my Takahashi Epsilon is F3.3. And I'm told that I should really be using a low power Nagler to yield nice views. Though to be honest, I rarely get a chance to look through it. On the occasions I set it up, it is usually astrophotography only.

Good answer AstroLounge. :thumbsupmate:

[AstroTasmania]

A Takahashi Epsilon is designed specifically as a photographic scope, not a visual one. My Tak Epsilon 180ED f/2.8 suggest 12 mm as the lowest EP to use for visual, going to a 25mm for instance shows up the large elliptical as a dark central region.

The Takahashi Dall-Kirkham Cassesgrain has one of the smallest central obstruction of around 28%, a properly designed Dall-Kirkham can deliver the finest image of any Cassegrain design. A standard f/10 SCT is around 38%, which makes them a poor photographic scope. An apo refractor will give you perhaps the best sharpness and contrast, however they cost around $1000 per inch and reflectors less than $100 per inch. In days gone by the standard 3" refractor was f/15 to reduce the chromatic aberrations, and produce a highly magnified but dim image of the brighter objects. To-day those same 3 & 4" scopes are around f/5 or f6, which makes them easier to mount.

For bright solar system work a long native focal length is best f/10 and more, for dim deep sky a shorter focal length, f/5 is better. You may have noticed that owners of f/10 SCT's have to use focal reducers for deep sky work, which is not the best option. A fast Newtonian will do better on deep sky.


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