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I like viewing double stars. I currently have a 6-inch reflector but would a refractor work best? What would you recommend for a budget of £300?
Observing double stars is a very popular aspect of astronomy and an area where amateurs can contribute greatly as very few professional observatories make such observations these days. Observational data is vital for increasing our understanding of stellar evolution, so amateurs can carry out real science as well as enjoying the wonderful sights.
A 6-inch reflector will produce some great views of double stars but a good refractor is likely to give a better observing experience as there is no spider vane to create diffraction spikes which can cause issues with very close doubles. A long focal length refractor will give excellent views and make focus easier to achieve but this needs to be combined with as large an aperture as you can afford. Larger apertures will reveal fainter companions but, just as importantly, they will have a higher resolution allowing you to split closer doubles. Long focal length refractors require a substantial mount to counter the effects of wind shear in particular.
Unfortunately, large aperture refractors can be quite costly, so a very popular, lower-cost alternative instrument for double star observing is the 5-inch Maksutov-Cassegrain. This instrument has a wide aperture and a long focal length of around 1,500mm in a short physical length and would be an excellent choice within your budget. The Sky-Watcher Skymax-127 MaksutovCassegrain or Orion Apex 5-inch Maksutov-Cassegrain telescopes would be excellent choices. It is unlikely that the ‘clipping’ you’re observing is caused by the positioning of the eyepiece in the eyepiece holder. However, eyepieces incorporate a field stop which consists of a metal or plastic ring that defines the edge of the field of view as part of their design. If correctly placed at the focal plane within the eyepiece, a field stop produces a welldefined circle which avoids a gradual drop-off in the fidelity of the view. The field stop limits the apparent field of view of the eyepiece which in turn limits the true field of view observed through the telescope.
If you know the diameter of the field stop, you can calculate the true field of view through the telescope in degrees by dividing the eyepiece field stop diameter by the focal length of the telescope and then multiplying the result by 57.3.
I have trouble seeing unclipped images through high maginification eyeplaces Is the position of the holder incorrect? TINA COX
The Sky-Watcher Skymax-127 MaksutovCassegrain is a good, low-cost choice if you want to see double
Field stop A field stop gives a crisp edge to the field of view