Since you are looking to travel to dark skies, then capabilities with DSOs
should be first and foremost. This means aperture is important. I also
believe that one needs to use a tripod, those table top jobs are too awkward
for serious extended use. In that event the tripod will need to go via
baggage.

There are lots of options here.

I have a Pronto, not a bad travel scope but it just does not have enough
aperture.

Same with the ETX-90 that others have mentioned, not enough aperture and it
also has a long focal length, limiting its field of view.

For my money, for under $1000, the best airline carryon is a C5 spotter on an
Alt-Az mount. This scope is 12 inches long, takes standard SCT accessories
such as a focal reducer (which hopefully your already have.) With the
reducer the C5 will work at about 800mm focal length and provide a 2 degree FOV
with a 32mm Plossl.

The entire OTA, complete with dew shield, finder and accessories will fit in a
regular size camera bag. The one I use is 15in x 9 in x 9 in.

Some of the 5 inch MAKs are also attractive but they have longer focal ratios,
are larger physically, and may or many not accept focal reducers.

As far as the mount goes, there are several possibilities, the Unistar series
are quite popular. I use a Bogen 3040 with a 3047 head with the scope mounted
"Side saddle." Works for me.

Other options for a scope under $1000 would be a 100mm F6 Achromat such as the
Orion/Synta scope on an alt-az mount. These can be airline portable.

A C-5 under dark skies does a pretty decent job of most DSOs. A couple of
years ago I spent a night roaming the Virgo galaxy with mine from a mountain
site, pretty nice going.

For scopes over $1000, there are several 4 inch APO refractors that should be
airline portable.

And then there are some foldup DOBs that are pretty amazing, the Teleport gets
a lot of aperture into a small package or one can home build a 6 inch F5 into
take apart/truss scope.

But for my money, the most aperture into the handiest, smallest package, the
C-5 Spotter is a hard one to beat.

Jon