NavList:

There is a big sunspot group on the Sun right now (download the "3d Sun" iPad app published by NASA if you want to see them on the far side of the Sun, too... yes, really). I tried spotting it today. It was clear and showed structure using a 7x35 telescope on an Astra IIIB but this spot group is mostly penumbra right now, so relatively inconspicuous dark umbral areas. It doesn't stand out well. I also tried viewing it through an octant (c.1840) with no telescope. I could not detect anything though the shades were just about the right density for easy viewing. Even steadying myself againt a pole, I could not see the sunpot group at unit magnification.

Experimenting with the octant reminded me of something that may have been missed in the discussion of the pair of sight "peeps" found on old octants. As was discussed, you use one hole to view the Sun or other body on the mirrored side, and the other to view it on the un-mirrored side of the horizon glass. But there's more to it than that. The separate holes preserve line of sight collimation. When you look through the left peep, you can easily move around until you see the Sun on the right (mirrored) side. But you're not supposed to do that. The peeps are there so that you look at specific points on the horizon glass. The error resulting from lack of line of sight collimation is not great, and for altitudes less than about 45 degrees, it doesn't matter. Naturally when you have an instrument with a telescope, the scope itself provides the collimation if its mounting is adjusted properly. And if it's well-built, sliding it or out on its mounting preserves that collimation.

Finally, it was quite noticeable today that the sight peeps on the octant improve focus. It's a cheap pair of eyeglasses for moderate near-sighted vision.

-FER
PS: another entertaining NASA iPad app: "ISS Live".

----------------------------------------------------------------
NavList message boards and member settings: www.navlist.net/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]navlist.net
----------------------------------------------------------------