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How-to's Accessories Reviews

Astrophotography - The Telescope

David Goldstein (dagoldst)

Keywords: astrophotography, equipment, telescope, lenses, dagoldst

This is the third part of the series on Astrophotography.

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.


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David Goldstein (dagoldst) on August 6, 2019

Thanks Bob, interesting read! CCDs are still being used in some of the dedicated astrocameras but CMOS is basically replacing them with their lower noise floors and greater QE. Anything like the MKID is so specialized at this point as to not be that applicable to the average astrophotographer, but it certainly is interesting technology - it requires super conductors to function at all. Here's a link directly talking about the tech...

Robert B. Williams (firebasebob) on July 28, 2019

Very impressive use of your equipment. You might find the article in the current Smithsonian Air and Space magazine interesting. It focuses on the revolution coming in sensors made with MKID's rather than CCD's. Don't know if it will ever trickle down to consumer level, but it sure will be interesting to see what it does to astronomy.