The verb "to telescope" conveys in a picturesque way the meaning that an elongated object slides into itself, so that it becomes smaller. I'm looking for an equally attractive verb to convey the opposite meaning, i.e., that an elongated object extends.
Untelescope has been used for this.
Searching with Google, though seems to show that @Eugene's suggestion of telescope out is quite a bit more common. If he posted this as an answer I would upvote it.
(Transformed from comment at the suggestion of Peter Shor): Mitch had the right hunch when he wrote:
I wonder whether 'to telescope' could refer to both directions, getting smaller and getting longer
This prompted me to look on the web and indeed, there are a number of web pages that do use "to telescope out" (verb) in the way suggested by Mitch, for example this page on troubleshooting for a materials handling crane, which lists the following problem condition: 12. BOOM DOES NOT TELESCOPE OUT OR TELESCOPES OUT SLOWLY.
As an aside, while I'd be comfortable using "to telescope out", I'd feel more comfortable using "to collapse" in place of "to telescope in".
The term to telescope implies extending or contracting in length from within. It was coined in analogy of a telescope's barrel.
Telescopic effect refers to the capability of changing in length (+/-) by 'growing' or 'shrinking' along one axis, so to speak.
To convey specific meaning, we have therefore to say telescope in or telescope out as the case may be.
Telescoping (mechanics): Telescoping in mechanics describes the movement of one part sliding out from another, lengthening an object ... (from entry on Wikipedia)