Barlow

I just purchased a used Orion Skyview XT-10. I'd like to know what barlow lens would work well with this unit . . . and perhaps what power level might be best to consider. I've used 2x and 3x barlows on a refractor before but never on a reflector. What do you think?

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Refractor or reflector objective lens makes no matter to the Barlow. Apply same as you did before. Whether 2X, 2.5X or 3X is selected kinda depends on your eyepiece collection & the multiplications vs duplications a Barlow yields & what you want. Example: if you had 25mm & 12.5mm eyepieces, a 2X Barlow only provides one additional magnification (25mm plus 2X Barlow = 12.5mm or a duplication of existing eyepiece) while a 3X Barlow with those same eyepieces allows two different additional magnifications (25mm & 12.5mm with 3X Barlow = 8.3mm & 4.1mm).

Do you still have your 2x and 3x barlows? If so, you're all set!

If not, the advice above that it depends on your eyepiece collection is spot on. OTOH, I've found it helpful to have both a 2x and a 3x (and even a 2.5x) so that you get to any mag you wish to!

The TV 2x 1.25" barlow is highly recommended, as are the Celestron Ultima and the Orion Shorty Plus.

I like the TV, found the Ultima a bit dark (it's more like 2.2x-2.4x), and preferred the Shorty to the Plus because it's threaded.

With your 10", you'll want to have four magnifications for sure--a very low power from a wide field eyepiece; then low, medium, and high. That combo will serve you well. The barlow can fill in between a couple and get you really high mags for planets or those exceptional nights.