16" dob

[Ptarmigan]

how to test your optical vs physical axis position http://www.skyandtelescope.com/howto/diy/3306876.html

Nice find

It says"If necessary, move the spot to the correct position or put another, larger piece of tape on top of it."

Job done !

It seems to me that the purpose of a centre spot is to make the job of collimating a scope quick and easy

(especially if it is a 'sensitive' type of 'scope)

when a star test is not available, like before dark, or when condx dont allow !

That is the first time I have seen ref. to adjusting the marker position, after the fact, ,  or  "on test", ,nice find, thank you.

[Beulah]

EDIT: I had to modify my response as I had replied with the wrong brand of scope in mind. My apologies.