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Reducer/flatteners-focal length or focal ratio?

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#1 microstar

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Posted 23 January 2013 - 08:01 PM

TeleVue advertises their TRF-2008 for 400-600 focal length scopes. WO advertises their newest Flat-6 for f/6 scopes of 66-81mm aperature (particularly 70-72mm). Why the difference? A 500mm focal length scope could be 50mm f/10, surely that wouldn't work with the TRF-2008? The 76mm and 85mm scopes it is designed for are f/6.3 and f/7 respectively. Why don't they mention focal ratio specs as well? Isn't a reducer/flattener effective over some combination of focal length and focal ratio? I'd appreciate if someone could help me understand what to expect from different reducer/flattener specifications.
...Keith

#2 Eddgie

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Posted 23 January 2013 - 11:28 PM

As I understand it, a field flattner is optimized for a specific range of "Radius of Curvature" or "RC" of the telescopes best focus.

The manufacturers of the flattenters know that most telescope makers do not provide the RC of their telescopes.

In general, a refractor has a negative radius of curvature (the field curves away from the observer) that is about 1/3rd of the focal length.

For example, a 600mm focal length refractor would have a radius of curvature of about -200mm (RC = -200mm).

A 400mm focal length refractor would have an RC of about - 133mm.

This means that the TRF - 2008 should give acceptable results with any telescope that has an RC of beteen -133mm and -200mm.

So, it is not really the focal ratio, but the radius of curvature, which can be calculated by dividing the focal length by 1/3rd (this is approximate, but close enough).

Again, this only applies to doublet and triplet refractors (and assumes that they have not been aspherized). the 1/3rd forumla does not apply to Petzval scopes or othter specialized flat field designs.

That is they way I understand it (based on my reading of the book "Telescope Optics"), but since very few telescope manufacturers publish the RC of their scopes you can only approximate it, and the flattener manufacturer gives a guideline in focal length becuase most reflectors will be doublets or triplets and if they have this range of focal lengths, they know the RC will be about -130mm to -200mm.

#3 microstar

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Posted 24 January 2013 - 12:45 AM

Great explanation "Eddgie". Thanks!

So if I understand correctly, RC has to do with focal length and not so much focal ratio for doublet and triplet designs. So another way of expressing the WO Flat-6 numbers (f/6 66-81mm aperture) is that it should be suitable for a RC of -132mm to -162mm based upon the 1/3 estimate, i.e. focal lengths of 396 - 486mm?
...Keith

#4 Eddgie

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Posted 24 January 2013 - 09:22 AM

I am not at all famaliar with the telescope you mention, but based on my understanding of field curvature and focal reducers, the Televue flattner should work with the scope if it is a doublet with an RC between -130mm and -200mm.

The TRF 2008 is sold as being suitable for Televue 85 and Pronto.

The TV 85 (using the 1/3rd formula) would have an RC = 198mm.

The Pronto would have an RC = -160mm.

If the calculation suggests that the scope you have has an RC in this neighborhood, I don't see why it would not work.

My advice though is to simply send a note to Televue or call them. I am sure that they can give you a more definitive answer as to whether the focal reducer will be suitable for your particular scope.

Anyway, hope this helps.






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