I recently practiced Polaris sights with an old instrument (GTS-2B) that does not have built-in reticle illumination. I get reasonably repeatable results during twilight, but when it gets too dark to see the crosshairs, I have trouble finding illumination that does not interfere with the measurement.
The "light on white paper inside the lens shade" trick is not convenient.
I bought a miniature "book light" at the dollar store and covered its one LED with red plastic. Clipping it on the instrument handle, I could point it into the objective lens and get a nice picture of crosshairs on a red background with the star shining in reasonable contrast. I thought I was set.
But I find that the star azimuth appears to change +/-20" depending on the position of the light, which blocks about a 3/4 inch diameter spot of the lens. Not acceptable. Not understood, either, why this happens.
There is an illuminator shoe on the side of the instrument and the manual lists an optional accessory illuminator (that ran on 2 D cells!) but does not picture it or discuss it. I can't figure out how to rig one of my own. A little flashlight produces barely enough light and the focus is weird so I see dust specks as big blobs in the illuminated field. I must need a lens system with the flashlight?
Can someone educate me?
Bill - It has been awhile since I've operated GTS 2B, but as I remember there is flip down mirror to illuminate the vernier. If the Topcon is like my old Lietz TM 20C then a light shined on that mirror will also illuminate the reticle.
Let me know if this works.
Scott
I use another version of the little LED book light to clip onto the mirror and illuminate the scale. It doesn't do anything for the reticule.
I'm thinking the shift when I move the light out in front may be some super-magnified parallax in the crosshairs rather than shifting the star image. Same result in azimuth reading.
> I'm thinking the shift when I move the light out in front may be some super-magnified parallax in the crosshairs rather than shifting the star image. Same result in azimuth reading.
Okay, the effective aperture of your instrument isn't the full width of the objective lens, its whatever is leftover after the obscuration of the EDM optics is removed. So if you're adding a 3/4 in. assymetrical obscuration to the telescope aperture, that may be the problem.
I'd experiment with shining the light at an angle into the telescope, but taking care that the light source is well off the central axis of the telescope and well away from it.
