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Jim Thompson wrote:

> I wonder where my right eye's biological optics stand compared to
> experienced sextant-users.
>
> Stars appear to me in my sextant telescope as very slightly fuzzy
> pinpoints of light with a few sharp radiating lines projecting out of
> them.  The effect is less with higher magnification, but still there.
> This makes finding dead center of the star image a little difficult
> for me.  What do you see?

I don't usually see any astigmatism on stars unless it's introduced by the
optics.

> Venus last night was simply a bigger star-like image.  Frank Reed
> suggested putting the center of Venus on the moon's limb and
> accounting for SD, but there is no way that I could see that with my
> biological and physical optics.  Is that average or bad?

Venus is too bright for me to see a disk right now. If I had a neutral
density filter it would probably be a lot easier. I also get some
astigmatism with Venus that makes it difficult to place on the moon, both
with a sight tube and with my homemade 5x36 scope. This increases the
time it takes to place Venus, and subsequently leads to the increasing
shakiness of the image (due to arm fatigue).

> Even with my 6x40 telescope, measuring index error on a fainter star
> results in an imprecision of +/- 0.1-0.3' on successive readings, in
> part because I cannot precisely see where the centers of the two
> images exactly coincide. I usually take about a dozen readings for IE,
> and always screw the index image down to the horizon image.

How far off are your successive readings for IE? With stars I use a single
sight.

Ken Muldrew.

   

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