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Firstly I am not an imager so perhaps someone else can comment on the use of an ED Barlow versus a 2x Powermate or telextender to double the focal length. Note in either case it will come down to what telescope you have and what you are trying to achieve by doubling the focal length as by doing so you also double to focal ratio and slow the speed of the system by 2x, eg. 25cm f5 (1250mm f.l.) becomes 25cm f10 (2500mm f.l.). Increasing the focal length is good for planets but not deep sky and your final focal length with a 2x amplifier may still not be enough to give you the required image scale.
I would think that you would definitely need a Powermate or telextender for use with the 42mm as a standard Barlow will increase the eye relief too much. However, I have not tried this eyepiece with a Barlow so cannot comment on how bad the eye relief actually is.
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