NavList:

Paul,
you raise an interesting point that led me to see what happens with other sextants. Almost without exception they allow direct vision around the edges of the horizon shades when using a Galilean star scope of large aperture and do not do so when using the "inverting" or Keplerian scope. The exception (and I did not go through my whole collection) was the war time C Plath with triangulated frame. Many allow light over the top of the horizon mirror and some makers, notably Tamaya and Filotecnica, added a baffle to the top of the horizon mirror to prevent this happening.

One solution is to place a slip of masking tape over the telescope objective so as to cut out the offending dazzle. You can find out which part to mask by sliding a slip of card over the objective until the offending sector is masked, and then apply a piece of tape to the objective lens mount.

Your suggestion that the shades may have been mounted upside down led me to look at some pictures of SNO-Ts currently for sale and they all have the shades mounted as shown in your photo (and mine). I have just tried mounting the shades upside down, and this may in fact be the right way up, because the offending sector is then adequately shaded (see attachment). There is still the possibility of a blast of light beneath the index shades, but this is only the case with the top-most shade which in my instrument is number 4, the lightest.

In the next few days, I will add a post to my blog at www.sextantbook.com to show the hesitant how to mount the shades upside down/right way up. This is not the first time that sextants have left the factory "all wrong". Tamaya put out a lot of sextants with the collimating rising piece wrongly made and assembled. (See "Tamaya collimation blunder" at www.sextantbook.com).

Bill (W.J) Morris
Pukenui
New Zealand
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