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You need a negative lens of -23D.
This is how I obtained the result: 130mm/3=43.33mm
This is the focal length of the eyepiece. 1000/43.33=23.
It is interesting whether you can find such a lens in the Surplus Shed.
Alex.

> I apologize if that was a high-end question because I’m working on the
> simple end of this subject.  I have what I think is a 3x upright scope
> missing the exit (eye) lens and all I know at this time is that the
> existing 18 mm objective lens focuses on an object at about 130 mm.  I’m
> trying to select a replacement exit lens without buying every ~18 mm lens
> that Surplus Shed offers.
>
> My impressions at this time are that a magnifying lens converges the rays
> of light to a point at the focal length.  If you “look” at the rays
> before the focal length, the image will be erect.  If you put a suitable
> concave lens in before the focal length, you make a Galilean telescope
> with a erect image. See the attached “Telescope Finders”  from an old
> Edmunds paper.  Note that on the first scope the objective focal length is
> 171 mm and the overall scope length is just over 5” ( 127 mm).  The
> exit/negative/concave lens captures the light rays before the focal point
> of the objective lens.
>
>  If you “look” past the focal length, the image is inverted.
>
> I think that this means that a two lens scope could be either erect or
> inverting depending on the lens focal lengths and locations.
>
> It gets much more complicated than that.  See:
> http://www.physicsclassroom.com/Class/refrn/u14l5db.cfm
> http://answers.yahoo.com/question/index?qid=20091015204404AAFguIT
> http://physics.bu.edu/~duffy/semester2/c28_lenses.html
> -	And many more.
> Regards, Noell
>
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> Attached File: http://fer3.com/arc/imgx/TelescopeFinders.pdf
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> : http://fer3.com/arc/m2.aspx?i=125819
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