Jim,

I'm hoping to be able to view Mars with reasonable high magnification
and would appreciate advice and suggestions. It happens that Mars will
be closer to earth on my birthday next month than it has been in a long,
long time, so I want to at least give it a try. Here's the plan:

Don't wait for your birthday. Mars is very close right now and is
providing plenty of good viewing for those that are out late enough.
I'd say it's a very good object for two months around the opposition
date, uh, I mean your birthday.

In addition to missing some viewing opportunities -- and as Alan says,
the greater chance to get good seeing -- you're missing out on
experience and training your eye/brain system that will be extremely
important in seeing all the detail your scope will present. With the
image wobbling and some detail approaching the limit you can detect,
experience helps you filter out those good moments of steady air and
discern detail that's just glimpsed. Our eyes our great detectors and
our brains are the best image processors yet but it needs to learn what
to do with the image.

I had a novice at a telescope a couple nights ago that didn't see much
on Mars with a quick glance, just like her companions (including some
amateur astronomers!). But this lady was different. She stayed at the
eyepiece asking about details she'd overheard us more experienced
observers discuss and asked about what she should see and how to see
it. I told her to stay and the eyepiece with a short break once in a
while to relax. And I told her how to eventually acquire details that
you can't see at first. The detail she began to describe to me was
amazing for a newbie. She was seeing detail in the melt band of the
south polar cap that I didn't think anyone else would see (I'd been
observing the same two nights out of the previous three). I can't
stress enough how much time at the eyepiece matters.

1) Using my 16-inch, 4.5/f Dob light bucket, I intend to use an off-axis
aperture mask, with an opening of about 6 inches, to enhance the viewing
at high magnifications.

This will reduce your scope's potential resolution to that of a 6-inch
scope. Leave off the aperture mask and enjoy your 16-inch. Don't worry
about coma at f/4.5 -- you're only using the center of the field for
Mars anyway. I've viewed Mars and other planets in larger instruments
many times and would never consider masking them off.

2) I have a Tak LE 5 mm, and a Nagler 9-mm. I plan on trying both of
these, and, if seeing is unusually good, adding a 2X Barlow.

I'm no eyepiece expert but these eyepieces aren't slouches and will do a
good job for you.

3) I plan on using an equatorial platform to assist in keeping the
planet in view for at least a few minutes, permitting family and friends
to see it before it's out of the fov.

Very good idea.

4) I will try some filters, but really don't know what would be the best
choice.

Red to enhance dark markings, like Wratten #25. Some use blue to
enhance the polar cap but I haven't tried that myself. Others here know
more about filters than I do, though, so I hope they chime in.

What magnifications might be
expected with this scope

Depends on the seeing. And the quality of the optics, of course. If
seeing and the optics are "perfect" you can go to 1000x. But don't
count on it. You'll probably spend most of your time at 200x to 300x,
up to 500x if the seeing in your location is very good. Just try it out
every time you go out and see what's good for that night. You'll soon
learn, too, that it varies from night to night and even moment to
moment. Also, sometimes a higher power will help with some detail even
though the overall image seems worse. And sometimes lower power is
better for what you're looking at. Just keep that in mind and try
different magnifications.

, and is the use of an aperture mask equivalent
to increasing the effective focal length, and magnification? If so, by
how much?

It will increase the f-ratio -- the ratio between focal length and
aperture (because the aperture is decreased) -- but the focal length is
unchanged.

Are good seeing and low levels of light pollution of
significant importance when viewing bright objects such as the planets?

There are three things that are important in observing planets --
seeing, seeing and seeing. Light pollution doesn't matter (unless you
mean there's someone with a flashlight in your face). At low power you
may find you're better off in a lighter location so your eye will stop
down a little. Dome lights are usually turned on at observatory
telescopes for just that reason. But seeing is critical.

(Should I plan to travel to a remote location?)

If you can get better seeing somewhere else then by all means go there.
Otherwise, I'd stay near the refrigerator. :-)

(incidentally, I also have an f8 -inch
(Chinese) refractor that we might try also)

Yes, do! How the different scopes perform is very interesting and
having them side-by-side on a good planet with colors is a great way to
go. I find it a lot of fun to scope-hop like that.

Mike Simmons