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Mark at Beaufort

Inexpensive f5 refractor improved by cardboard aperture mask

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Ok, I guess I didn't realise that the objective lens is actually different.  Do you mean it's a lower quality lens?  Likely to have more imperfections?  If I was to take my existing telescope and a grinder and cut the barrel down to 400mm, making it into an f/5, would it simply not work?  (Newbie questions, sorry!).

The objective will have a fixed focal length, cutting the barrel of the scope will just mean you can't focus, the reason the f number changes is that it is focal length diveded by objective diameter, thus decreasing the diameter increases the f number but the focal length stays the same.

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The objective will have a fixed focal length, cutting the barrel of the scope will just mean you can't focus, the reason the f number changes is that it is focal length diveded by objective diameter, thus decreasing the diameter increases the f number but the focal length stays the same.

Oh of course, that makes sense -- it's basically the same as a prime lens in photography.  Doh, sorry :-)

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Just to bump this thread and check with you guys again. Ive the opticstar 80mm frac (reason for it cos its small) and baader film. if i buy a HW and polarised filter, will it give me improved views? Thanks

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F11 80mm is a far more sensible option for the technology these sorts of achromatic scopes use. In real terms the old 3in F15 refractor gives a true indication of where they need to be if the usual magnification range for a given aperture is to be expected. Go to 4in and they need to be longer and so on. I believe there were some 6in designs that could still be F15 but gave stronger colour halo's around stars.

Optical systems don't scale up well as any problems scale up as well. The F ratio remains the same so the diffraction spot does as well. Conversely they improve if scaled down. That is why using an aperture stop can improve things. There may also be edge defects but the same effect would be noticeable even if there wasn't. The only fix is more complex designs within limits and the use of different glasses - ED's etc.

John

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Just to bump this thread and check with you guys again. Ive the opticstar 80mm frac (reason for it cos its small) and baader film. if i buy a HW and polarised filter, will it give me improved views? Thanks

The F/5 Optistars can have some optical issues that are not easily remedied without replacing the objective lens. These fast achromats are best used for what they were designed for which is low to medium power, wide angle views.

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