Sean,
It is very easy to get infected with aperture fever when all you do is look
through the eyepiece. I recommend spending more time at star parties
watching setup and tear down of scopes in addition to looking through them.
>What is the limiting factor on how big a telescope one can fit into a
car (in my case this is a 98 Beetle)?
There are two major factors affecting portability. The first is the aperture
of the scope. That's the reason you don't see many scopes 25" - 36",
although we all enjoy the views through the eyepiece. The second factor is
the design of the scope. Get the measurements and weights of various scopes
you are interested in - they vary quite a bit.
>Eventually, when I get a garage, I would like to own something like that.
Sounds like the size of your vehicle isn't the only restriction. Where are
you going to store the scope? Or are you looking to buy in the future? If
so, consider buying another vehicle.
>Is it the ladder? The truss poles?
These can fit almost anywhere. The major factor is what you are willing to
lift and move. Generally this is the mirror box.
>Is there a big difference in optical quality between f/4 and f/5?
Yes and no. The greatest difference the f/# makes is in the height of the
eyepiece. That's why larger scopes favor smaller f/#'s. In a 20" scope, the
f/4 is 20" shorter than a f/5 - that's a huge difference. On the other hand,
you'll have to buy a coma corrector for a f/4. It is also more difficult to
make a good f/4, so you have to make sure you get a mirror from a good,
experienced mirror maker.
>At some of manufactures' web sites they mention 2" or 1.25" thickness
>mirrors. What is the difference between these choices? Durability?
You probably mean 1-1/2". You want the thinner mirror for lighter weight
(and more rapid cool down time). Again, in the larger sizes, the weight
difference is substantial.
All of the standard Obsession style portable scopes will be heavy. The
Starmasters are the largest and heaviest. They allow/recommend you remove
the primary mirror so you lift less weight. I don't consider this much of an
advantage. The last thing I want to do in the dark late at night when I'm
tired is to remove the heaviest, most expensive, and most important
component.
There are a number of lighter weight alternatives to the Obsession-style
scopes. If cost is no object, a custom scope can be much lighter and fit
into a much smaller space than commercial scopes.
It all comes down to what your goals and constraints are: storage and
transport area, weight you can comfortably/safely lift, cost, observe from
ground or ladder, etc. If you'd like to discuss more, please feel free to
e-mail me privately.
Albert