Telescopes are like lenses for cameras – there are many choices in focal length, aperture, etc. One of the main differences is what constitutes wide angle vs telephoto. For instance, 480mm is actually very wide angle in AP. 2000mm is getting to medium telephoto.
To calculate telescope focal length, the formula is: objective lens diameter times the native f/stop. So 115mm times f/7= 805mm. A reducer is rated in the following way - .08x, or 80% of the native f/stop. Current telescope 115mm times (f/7 times .8) = 644mm @ f5/.6.
There many kinds of telescopes. They tend to be lens or mirror-based designs with some mirror/lens hybrids as well.
A partial listing of telescope designs:
I have been using refractors since I first started, primarily due to ease of use. Generally, they do not require collimation – i.e. an internal alignment/calibration of the internal components to the focal plane. Also, since refractors are typically shorter focal length than folded optic designs, they tend to be more forgiving of tracking errors as well as faster for their size – my current telescope is f/5.6 @644mm with the focal reduce/flattener. Finally, longer focal length telescopes that you begin to get into with folded focal length designs are recommended for more experienced astrophotographers that can manage tracking and guiding at long exposures without any visible trailing of stars in the image.
My current telescope – An Astro-Tech 115EDT – has a 115mm objective, ED lens elements, (FPL51), and is what is known as an apochromatic triplet, (3-lens) design. It is very good at controlling chromatic aberration which with the high contrast objects like stars against a night sky tend to be troublesome for AP. Just as an aside, mirror-based designs do not suffer from chromatic aberration, but as noted above have other challenges.
The AT115EDT provides a 2.5 inches diameter focuser and can work with both 2.5” and 2” field flatteners. For my modified Nikon D5300 camera, I also purchased a 2 inch .8x reducer/flattener that is ideal for up to APS sensors – it does however vignette on full frame.
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