how to recover "the lost light"
One of the problems that it has to solve when photographing deep sky objects with
large focal lengths is the search of a suitable guide star.
That is not always so easy, either with traditional or with "off axis" guiders.
The ideal for the guiding would be in fact a sufficiently luminous star, possibly within the himself field that is being photographed.
The case occurs that the interference filters, already sufficiently spread, reflect a great amount of light, which goes generally lost.
Why not to use it for guiding?
In the case of a Newtonian configuration, the interference filter, placed parallel to the focal plane, while allows the formation of a filtered image in the Newton focus, reflects simultaneously a great amount of light who, passing through a hole in the diagonal, and with interposition of a small lens, forms a second increased image, in that could be called "anti Newton focus". The execution of the hole in the small secondary mirror is easier on could imagine.
More comfortable and interesting could be the application of this principle in a Newton-Cassegrain configuration.
In this case, the reflecting surface of the filter would have to be convex, so that the telescope forms a filtered image in the Newtonian focus, and a increased one in the Cassegrain focus, generated by the reflected light, for better and more comfortable guiding.
Experiences done using as secondary, for a Cassegrain, spherical convex surfaces (of common, lenses, aluminized or not), instead of a hyperbolic small mirror, have given excellent results, demonstrating that a hyperbolizing is not strictly necessary for this use.
It is superfluous to add that the "condicio sine qua non" for a good operation, to avoid flexions, is the rigidity of the system.
Personally I could make until now only the Newtonian configuration, not being easy to obtain a convex interference filter.
I assure nevertheless that to have in a telescope two Newtonian foci is a nice comfort, also for other uses.
Theoretically it would be possible to solve the problem of the convex filter with the interposition of a negative lens of low power, properly treated. One is to prove.
Who has the possibility of making a Newton-Cassegrain configuration?
Rome, april 1997