ATM Light diminuation in Barlows?!

[Carl Woebcke <carlw@telis.org>]

Hellooooo-oh out there,

A fellow ATMer e-mailed me the following:

"I have found that in testing [variable stars] I lose .3 to .5
magnitudes on the faint end of the scale using a barlow.  If I use a 5mm
eyepiece I can see to a limit of 15.0 for example, but using a barlow
and 10mm eyepiece I can only see 14.5 magnitude.  So I've lost .5
magnitude.  For me that is significant in that I frequently look at
stars that are known to be at or near their minimum.  Many times, the
star is not even visible and I am hoping to catch it on the way backup
at the lowest possible magnitude."

Has anyone else had this magnitude loss using a barlow?  I wonder if the
type of barlow, the difference between the 5mm and 10mm eyepieces, or
some other parameter than just the extra glass contributes to this
effect?

Sincerely,

Carl Woebcke
carlw@telis.org