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Determine how far away from lens to put focuser

optics refractor
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#1 Couder

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Posted 23 January 2021 - 03:25 PM

I have a 3" lens in a cell, when I aim it at the Sun the focal length is right at 29" away. I have 3 focusers to choose from - I have no preference - 

1) focuser travel 2-3/4"

2) focuser travel 4"

3) focuser travel 5-1/2"

 

Do you put the focuser so the middle of travel is the focal point? I do mainly solar and planetary viewing if that helps. I've done this before but may have just lucked out.



#2 Redbetter

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Posted 23 January 2021 - 04:29 PM

For visual you have to figure out where the focal plane will be with a 1.25" and/or 2" diagonal in it.  The main problem arises if the OTA is too long and the focuser is too far out with a preferred arrangement.  In that case you won't be able to achieve focus even with the focuser racked all the way in, and particularly with 2" eyepieces that have their focal plane high in the system (e.g. 31 Nagler and some Ethos.) 

 

With my 2" mirror diagonal and 31 Nagler set ups there is still about 10mm on inward focuser travel available as memory serves (the extra helps when filters are added since they tend to add a mm or two.)  With focusers having  73 to 80 mm travel, I can still achieve focus with 1.25" prism diagonals and my 1.25" eyepieces at the other end of the range, with the focuser racked out near the end of its travel.  IIRC the RACI's sometimes focus a little past the end in specific configurations, so I have to pull the eyepiece out somewhat.   

 

Without a diagonal I cannot achieve focus since I do not have extensions for that.  So for the focusers and refractors I have the focal plane is not in the draw tube and is well beyond.  With a 5-1/2" travel as you have above I could probably still achieve focus without a diagonal if the focal plane was somewhere within the end of the draw tube travel.




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