[janoskiss (Steve H)]

I don't expect that Optex barlow to be very good at all. A cheap barlow will disappoint. A good eyepiece in a good barlow will not. E.g., a UO HD ortho in Televue barlow is hard to beat or even match for optical performance unless you sacrifice comfort (e.g., get an ortho with painfully short eye relief), or spend top dollar on a premium EP like a Pentax XW.

Orion Deluxe barlows are also worth considering. I have an unbranded version of the 2" model and it's excellent (same as UO 2" 2x barlow). The 1.25" under $100 would be a steal if it's as good as its bigger brother - with FMC vs MC lenses there's no reason why it shouldn't be.

BTW. You will not have much use for a 3mm EP in an 8" Dob, if that's what it's for. In good seeing 5mm is near ideal for that scope. 4mm will get used very rarely in exceptional seeing. 3mm virtually never, except for star testing and maybe if you're into splitting very close doubles which may require close inspection of overlapping Airy disks.