NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Inverting Scope
From: Alexandre Eremenko
Date: 2004 Oct 9, 23:11 -0500
From: Alexandre Eremenko
Date: 2004 Oct 9, 23:11 -0500
Dear Bruce, I am glad you purchased an SNO-T inverting scope, so I can discuss some specific points with you (maybe we should do it off-the-list, I am not sure). It seems to me that my inverting scope has the following problems: 1. It won't simultaneously focus on a remote object (a star) and the cross hair. From what I read about Kepler telescopes of this type I conclude that the cross hair assembly should be adjustible by changing its distance from the objective lens. This does not seem to be the case with my scope, but actually I am not sure. Do you think the black part of the tube which holds the cross hair can be moved somehow with respect to the objective lens? It has two holes which look like the holes for an "optical spanner wrench" but on the other hand, I afraid that this part is glued to the main tube, so I'm afraid to break it by trying to adjust. By the way, the cross hairs are barely visible or invisible when it is really dark... and I suspect they were not designed for Lunar distances. 2. Bright objects like stars and sometimes the sun are surrounded by some spots ("coronas") of light which look like hair growing from the object. If the object is exactly in the center of the field of view, and my eye is exactly on the optical axis, the star is visible in the middle of this "corona" of light. When I shift a little my eye or the sextant, the corona becomes excentric, and sometimes shifts completely to one side (so that the "hair of light grow" only on one side of the star). A professional astronomer explained that this kind of aberration is called "coma", and it is typical for Kepler scopes. Do you notice anything of this sort in your inverting scope? I found that this aberration does not influence much my star observations; the star at the vertex of its "coma" is sharp enough, but sometimes it obstructs my sun observations. (That's why I said once that the inverting scope seems to work better at night (I mean star-to-star distances) while the Galileo one is better for the sun, which is exactly contrary to the Russian SNO-T manual: "to use the inverting one in daytime and the Galileo one at night". I agree that the star scope (Galileo) gathers more light. But the 7x30 inverting one, despite its coma aberration seems to have better resolution and better permits to tell the moment when the images of two stars collide). > The wires are nearly 2 degrees apart in the field. > Two vertical, > two horizontal. Why the horizontal ones, I don't know. > But they do make it easy > to measure the angular distance between wires. Apparently these wires were fitted not for lunar distances but to help the observer to hold the objects in the middle of the point of view. That's why there are 4 of them making a square rather than a cross in the middle. > or do accurate > work with an artificial horizon, you'll find the > inverting scope invaluable. Could you explain more about how the cross hair help with artificial horizon? Do you mean a Davis-type or bubble art horizon? > My guess is, a right-side-up scope with a reticule > would be far more > expensive. And more importantly, it would let less light through. As I understand at least two extra lenses or prisms are required to make the image straight. (Contrary to what many people say, I experience no inconvenience with this inverting scope. Maybe because I never used a sextant and started learning with both types of scopes simultaneously. This shows that this "inconvenience" is just a question of habit.) Alex.