NavList:

Jim Marcuson recently told me of a modification he made to the SNO-M telescope. I think it is worth passing it on, as the SNO-M is a very good and somewhat undervalued instrument.

The telescope supplied with the instrument is a very fine 6 x 30 inverting one, with four wires in the field of view. Though an inverted view takes a bit of getting used to, it doesn't seem to cause too much difficulty to surveyors and astronomers, though the latter are generally much more familiar with the sky than the average amateur sailor. For such a sailor, the magnification seems too great, even though the field of view may be much the same as for a 3 x 30 Galilean 'scope.

My own approach has been to make a rising piece to fit a spare Heath and Co telescope to the SNO-M. Jim's approach is simpler. He has cut down the telescope tube and fitted a new eyepiece lens scavenged from an old pair of opera glasses. I thought it would be helpful to let interested members have a few dimensions, to help them in finding suitable lenses.

The focal length of the objective lens is 133 mm (7.5 diopters)and its clear diameter is 30 mm. It back focal length (the distance to the focal plane from the back of the lens) is 130 mm. To make a Galilean 'scope of x 3 power, you will need a negative (concave) lens of 133/3 = 44mm, say 45mm focal length. The distance between the lenses will then be 130 - 45 = 85. This leaves about 40 mm of telescope tube for the eyepiece tube, but you will have to reduce it by a few mm to allow for focusing. The diameter of the eyepiece tube is 26.98 mm, so I guess that the telescope tube was reamed out to 27 mm.

If you chose to go much lower than x 3 power, you begin to run out of telescope tube, so that by the time you are down to x2 power, you run into the rising piece, which cannot be repositioned easily.

A good source of lenses is www.surplusshed.com. Head straight for their lens finder and don't forget to specify a negative (concave, -ve) one for the eyepiece. It doesn't seem to matter whether it is plano concave or biconcave, nor is its diameter important, as the field of view is decided by the magnification and the objective diameter; roughly f.o.v. = diam/magnification.

Bill Morris
Pukenui
New Zealand
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------