Below are some of the telescopes I have used for observing and astro-imaging.
For astro-imaging the most important considerations in picking a telescope are:
Focal Ratio: F/4 is fast, F/10 is slow.
for me this is the most important factor.
With an F/4 scope I can image the Andromeda Galaxy unguided with 1 minute exposures.
With F/10 you require additional equipment for guiding and more time to capture the images.
Focal Ratio is calculated by dividing the Focal Length by the Aperture,
E.g. a 200mm (8") Telescope of 1000mm Focal Length has a focal ration of F/5.
Aperture: 6" and larger is required for most Deep Sky Objects.
With a smaller scope, you can obtain excellent wide field images of clusters and larger objects
such as The Orion Nebula.
Focal Length: 600mm is very short, 2,000mm is quite long
The longer the focal length, the larger the image will appear on your camera.
But the longer the focal length, the more accurate your tracking and guiding will have to be.
To summarise, if you want to begin astro-imaging by the easiest route, use a
small telescope with fast focal ratio. A 150mm (6") Newtonian at F/4 or F/5 would be inexpensive,
easy to handle and would not require a large expensive mount.
Likewise a 100mm (4") refractor at F/4 or F/6 would also work well.
Larger, longer focal length scopes are required for Deep Sky Objects (DSO) such as galaxies.
These scopes require larger more expensives mounts which track accurately.
Guiding is usually employed in longer exposures of fainter DSO.
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