Post by Troy PigginsHow does one stop down the aperture of a fixed aperture scope?
The bare scope is f/10. With the 2.5x powermate it becomes an
equivalent f/25. I haven't heard of people using those masks
you're referring to. I'll look into it. Thanks.
A simple round hole cut into some opaque plastic, cardboard, or thin
aluminum sheeting will suffice. Placed over the opening of your telescope.
For a refractor this is easy (no central obstruction). Just place it
concentric with the optical axis.
For a reflector the choice is not so easy. The secondary mirror's size is
optimized for the light path and f/ratio.
Larger reflector telescopes can use an aperture mask offset to one side, so
as to use an unobstructed region of the mirror between the outside diameter
of the primary and the outside diameter of the secondary, and situated
between the spider-vanes. Consider too the number of spider-vanes you have.
If 4 vanes you will have to cut your mask smaller so its diameter fits
within an open quadrant between any two spider-vanes.
The huge plus of this for planetary imaging is that now you have an
obstruction-free telescope. Of reduced aperture but for bright subjects and
due to "seeing" problems this can be a huge plus too. Many people buy 12"
or larger reflectors with the intent to only use it as a stopped-down
off-axis planetary imager. (8"-10" telescopes too, but you then start to
lose resolution due to primary size alone when stopped-down off-axis.)
There is a huge cost-savings in buying pre-fabricated easy to make
manufactured telescopes much greater than the size needed, as opposed to
buying or building an off-axis (asymmetric) reflecting telescope design
(see below), or prohibitively expensive refractor of those diameters which
is now fraught with CA problems.
With the aperture offset you are no longer plagued with diffraction from
secondary mirror and its spider supports. Since this is a reflector, you
now have a telescope that is free of all chromatic-aberration, making it
much better than a refractor of the same size (large and astronomically
expensive refractors bought with planetary imaging in mind). Special
asymmetric reflector telescopes are designed this way, but grinding and
figuring the offset curvatures are extremely difficult and many ingenious
methods were tried and found to try to circumvent this fabrication problem.
One of the more ingenious is to grind an achromat corrective lens for use
with a standard parabolic mirror set at an angle. This achromat ground to
the proper figure by using a creative method found for the home telescope
builder, but then you introduce CA problems. Often, to simplify things,
they'll just buy a much larger pre-figured mirror and then cut it up into 2
or 3 smaller offset-telescope primaries. (I don't think I could bring
myself to do that, even though I have the means. It would be like cutting a
favorite child into 2's or 3's.)
By using an offset aperture mask on a large telescope you now have the best
of 3 worlds. An exceptional planetary imager (the same as a prohibitively
expensive asymmetric reflector telescope), no CA problems as exists in all
refractors, and when the mask is removed you now have a very very nice
deep-sky light-bucket.
Aside: This is precisely why I chose the size telescope I now have (16"
dia.). The 16" also not chosen arbitrarily due to costs nor other issues.
When researching I found that due to even the most pristine seeing
conditions (unless I am on a mountain-peak), that without adaptive optics
the resolution of this size telescope is the same as that of Mt. Palomar's
200" telescope. The weakest link now being caused by the atmosphere itself.
There was no appreciable gain in resolution by buying larger. Light-grasp
yes, resolution no. (Keep in mind too, this was before image-stacking
became popularized to increase resolution. And since I was going to
primarily use it for visual astronomy this didn't enter into my
decision-making equations. Then, nor now.)
Another plus to an offset mask is that you can rotate the aperture-mask to
find a "sweet spot" of your mirror where the figure is the most pristine.
This can greatly improve on its 1/8th to 1/20th wavelength of light
tolerance across its whole surface.
For smaller telescopes you can try an aperture reducing mask placed
concentric with the axis of the telescope, but then the smaller you stop
down the aperture the more that diffraction becomes an issue due to the
larger percentage of central obstruction vs. the useful light path.
Experiment.
