The solutions to this problem have been:
- Use a Barlow lens to increase the focal length of the system. This allows focus to be achieved, but at the expense of brightness of the image and field of view.
- Move the position of the mirror further up the tube. This is a drastic solution, and is particularly difficult with the Omni XLT 150.
- To use the direct camera connect threads on the focuser that allow a camera T-ring to be screwed to the end of the focuser tube. This has the advantage of allowing the camera to be brought to focus. Another, important consequence of using the direct connection method is that the camera cannot fall out of the focuser as could happen with a 1.25" adapter, if the tightening screws have become loose. Moreover, with direct attachment, there is less potential for vignetting. However, there is a huge disadvantage to the direct attachment method: it is not possible to use a light pollution filter, which makes a critical difference if the sky suffers from even slight light pollution. Moreover, if the user wishes to use any other filter, the same problem arises.
- There are also, some astronomy cameras that do not have a filter-threaded nose piece. With some scopes this is not a problem because a threaded extender tube can be used. However, with many Newtonians this is not possible because the camera is now held too far out to achieve focus.
We show here, an experimental method of allowing a 1.25" light pollution filter to be used with direct camera connection. This is a solution that could be adopted by Celestron (or any other manufacturer) at very little cost, as a standard component of the focuser.
First, a Barlow lens was disassembled so that the middle tube, which is filter threaded, could be removed and used as an internal filter holder in the focuser tube.
A thin shim of plastic tape was placed around the middle tube and it was inserted tightly into the focuser tube and fixed in place with superglue.
The filter threaded Barlow tube was pushed far enough into the tube to allow the 1.25" adapter to be screwed back into place for normal eyepiece work. This adapter has to be removed for filter insertion and direct camera attachment.
After removal of the 1.25" adapter, the Light Pollution filter can be screwed into the Barlow tube threads.
The T-ring can then be attached as usual, ready for direct camera attachment.
Or the 1.25" adapter can be replaced, with the filter in place, for use with a camera that does not have a filter thread as in a camera tested recently.
It is unlikely that this procedure will introduce any more vignetting into the image than would have been produced if the scope had enough back focus to allow a 1.25" adapter to have been used.
In my view, any scope with the facility to attach a DSLR directly to the focuser, should have a filter threaded section inside the focuser to allow the use of filters, in particular, light pollution filters.