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Post by Dragon Man on Jan 22, 2015 6:40:28 GMT
If you are considering using a Refractor for Video, here is a quick and simple guide I made to show the vast difference in light gathering capabilites between 3 different refractors. The ED80 is an 80mm f/7.5 semi-apochromat so it will give sharper stars and almost no false colour, but at a loss of light gathering due to the small aperture. A 120mm f/5 Achromat, although only 40mm wider in Diameter than the 80mm, gives more than twice the amount of light gathering capability of an 80mm! But also at the expense of Colour problems. Achromats are famous for false colour artifacts like Chromatic Abberations and Violet Fringing on bright stars. I have also included a 150mm (6") f/5 as a I know a few people that use them (including myself). The increase over the ED80 is amazing. Although the diameter is LESS than only twice that of the 80mm the light gathering area increase is around 3 1/2 times that of the 80mm! Here is the visual guide I made showing the differences in Aperture, the area (in mm) of aperture, quality of stars, and brightness of background sky. If using an Achromat, also add violet fringing around bright stars. (Image not to scale but comparative sizes are accurate)The Blue text tells the telescope used. The white numbers are the area of aperture in millimetres.
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Post by Rick in NWArk on Jan 22, 2015 16:59:55 GMT
Great illustration, Ken. A lot of people may talk about light gathering increases but I've never seen it illustrated with stars and limiting magnitudes before.
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Post by ChrisV on Jan 10, 2016 9:55:34 GMT
Interesting read .... I've been looking at some skywatcher achromats. Which of the following aperture/FL would be better ? 80 x 400mm 102 x 500 120 x 600
The 80/400 would have a wide FOV, but the 120/600 more light ? I'm confused. Or are there other factors such as worse aberation with the shorter FL.
Chris
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Post by Dragon Man on Jan 10, 2016 11:01:27 GMT
The 120 x 600 would be excellent. Either the Saxon or the Skywatcher 120mm f5 Achromat (both come from the same Synta factory). I see that Andrews has them for $529 Make sure you get one with a Crayford Focuser because the models with the standard Rack & Pinion are woeful 120mm giving you slightly more than twice the light gathering ability of the 80mm, which will reduce the length of integrations needed, and 600mm focal length is just about right to go with or without focal reduction. These are some of my results with that exact scope (go to Achromats > 120mm): ballaratman.wix.com/mallincamIt's a pity no-one seems to sell the Skywatcher 6" (150mm) f5 Achromat at a good price any more. For a small amount of $$$ more you get a lot more telescope! Andrews has them in the over-priced 'Black Diamond' version for $1299, but I have seen them in normal version for around $800. (normal version just means no fancy paint job on the tube and no finderscope or tube rings).
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Post by Rick in NWArk on Jan 12, 2016 17:10:46 GMT
In my opinion, for wide-field deep sky applications: - maximize aperture to get the most light - minimize the focal length to get the largest field of view - this combo naturally gives you lower F/ ratio and thus lower exposure times
As to which to sacrifice first, you need to bring in $$$ and weight vs mount capacity and APO vs Achro to the conversation.
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