The Sharpstar 94mm f/5.5 ED APO Refractor Telescope If you are looking for an amazing apochromatic, short focal length refractor that is great both for visual and imaging use, the Sharpstar 94EDH won’t disappoint.
This brand new telescope is the third in our Sharpstar EDPG family. With an aperture diameter of 94mm and a focal length of 517mm (f/5.5), this telescope will provide color-corrected views well out towards the edge of the field of view.
What is an Apochromatic Telescope?
94EDPH objective cell is a triplet air-spaced APO lens that includes two pieces of ED (extra-low dispersion) glass. Many refractors, especially those with one or two lenses, exhibit lots of colors in the stars (especially at the edge of the field of view). This is due to various wavelengths coming to a focus at different distances from the lens. While one color may be in focus, the others may not, and that will show up as a tiny rainbow or individual violet or yellow colors. This is known as chromatic aberration.
Triplets (three lenses) overcome this by focusing all colors of the spectrum to an equal distance. When there is no chromatic aberration, or at least very little, you have an apochromat. If they are also made from ED glass, all the better because this type of glass has a low dispersion, meaning light passes through it very easily without many internal reflections. Taken together, this lens cell provides sharp views with no color problems.
The short focal length provides a wide field of view and makes the telescope very portable. The short tube is lightweight and easy to pack for remote observing trips.
How to Turn this Telescope into an Astrograph
Astrographs are telescopes specially configured to take images. They are a little different from visual observing telescopes because with today’s large-format cameras, all short focal length telescopes can distort stars towards the edge of the field of view which will be very noticeable in a large imager that covers the entire field of view.
To correct for that, an additional optical element is added called a field-flattener. This is a highly curved lens that brings slightly out of focus stars at the edge of the field into a perfect focus. This effect is usually too small to notice under visual use, but in a camera, this needs to be corrected.
Special purpose-built astrographs have the field flattener permanently mounted inside the optical tube assembly (OTA), but by adding one to the focuser just ahead of the camera, you’ve just turned this telescope into an astrograph.
Available Focal Reducer Supports Full-Frame Imaging
With very large frame cameras, vignetting may be noticeable. To combat this, an additional focal reducer will reduce the original focal ratio to f/4.5, supporting full-frame imaging.