Detailed question:
Unfortunately the AstroSolar® filter I’ve ordered from you doesn’t fit (just for a bit) on my telescope even though it should have worked perfectly. The clamping isn’t enough for the third bolt to fit. Even though the next filter size fits perfectly, it has a too big opening for me. So if I can’t get the right filter opening, what can I do?
Answer:
Please look at the filter carefully. If the clamping doesn’t allow for the filter to fit straightly in front of the telescope, we would suggest that you, or gladly us, would slightly file the bolts with a round outwards so that it fits. As a last resort, you could even discard the slots, and simply drill three other holes with 6.5 mm diameter and attach three centering bolts without the slots. There are accessories to close the 6 holes.
The objection to the bigger opening is legitimate. A significantly larger filter opening can cause scattered light from the edges of the telescope lens and unwanted reflections. This would reduce the contrast of the image. It’s best for the telescopes and other lenses that the filter opening is slightly smaller than the diameter of the opening, which lets light in.
However, one should not consider such conditions to be right for every kind of optical systems. For example the primary mirror of a Newtonian reflector telescope is at the lower end of the tube, almost half of the telescopes focal length away from the opening. If someone wants to use the full diameter of the primary mirror with a filter attached at the front, the filter diameter should be 10-15 mm larger than the diameter of the primary mirror, so it doesn’t reduce the light collecting area. This recommendation is more valuable for very short focal length Newtonian systems where the incident light is sharper.
In SC-systems and in other mirror systems with multiple folded light-path, the primary mirror sits less than 1/3 of the focal length away from the main opening and the actual Bernard Schmidt system requires a smaller Schmidt plate diameter than the main mirror. In this case the same diameter as the main mirror is required – that’s why we have these ASTF-filter widths.