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refractor tube measurements

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#1 Thomas Marshall

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Posted 23 May 2020 - 06:19 PM

When putting together a refractor OTA, does stopping "Down" a lens assembly say, - from 127mm or 130mm to 120mm Shorten the needed tube length? OR merely change the focal ratio? I have a OTA for F6/130mm = 780mm fl. and I'm trying to figure if I can gain a few inches by stopping down a F7/130mm or F7.5/127mm lens a little bit, in conjunction with a focus tube extender and/or helical spacer at the focus end to get me to the Goldilock's Zone focus wise. Thanks for any insights. 



#2 randcpoll

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Posted 23 May 2020 - 06:39 PM

The focus point of a given lens combination never changes. If you reduce the aperture you are just increasing the focal ratio.


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#3 Couder

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Posted 23 May 2020 - 06:43 PM

Yes, I had a 6" F15 lens (90" focal length) that was badly chipped on the edges. I cut the outer part off to make it a 2" lens. Still 90" focal length, but now a  2" F45.



#4 Thomas Marshall

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Posted 23 May 2020 - 08:08 PM

The focus point of a given lens combination never changes. If you reduce the aperture you are just increasing the focal ratio.

Lately, it seems I've gotten some disappointing answers that dampen my hopes, on certain ideas. But, - it is what it is, - that's what I needed to know. I guess I'll just have to plan on finding a lens that fits that tube, or buy a longer tube and/or focuser extender to fit whatever lens I am able to find/afford. Thanks for the info. 



#5 sunrag

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Posted 23 May 2020 - 08:51 PM

The singlets seem to change focal length when you stop them down (possibly because it focuses each color to a separate focal plane).  I have a 6" singlet with 2600 mm focal length. I can see that it has a very deep focal plane with each color coming to focus at different distances. The longest one was blue and shortest was dark red. When I stopped it down to 3", the focal plane became much tighter, and overall focal length reduced by at least 100 mm.




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