Celestron Omni 2x Barlow Lens (1.25")

Yes, the Celestron Omni 2x Barlow Lens (1.25") will work with the 10mm and 25mm Eyepieces that come with Celestron Astro Fi telescopes, as those are 1.25" eyepieces.

Mathematically, when you double the size of an image, the doubled image has on 25% of the brightness of the original. So, assuming the Barlow is truly 2X (they usually aren’t quite what they’re listed as, but close) the image is dimmed by a factor of four. There is also a very slight loss of light transmission when you put another piece of glass into the system. That’s pretty much negligible. Bear in mind that our perception of brightness is not always linear. Particularly because the eye readily adapts to small changes in brightness. Most people aren’t going to notice a huge difference in brightness when they put in a Barlow, even though it is a lot dimmer on paper. Considering that most people use Barlows on planets or the moon, loss of brightness is rarely an issue. (Unless you’re trying to see Neptune or Uranus) An important factor to consider is the resolution limit of the telescope. A handy rule of thumb is that you can magnify 50-60X for every inch of aperture. Most planetary targets will be plenty bright if you stick to that rule. You could probably push if a bit further on the Moon.