barlow lens?

Question

i want to upgrade a 2x barlow and think that i've found just the one.

my telescope is an 8" Celestron dobsonian reflector f/6

i have caught sight on the Celestron Ultima Barlow Lens (2x - 1.25"). the Ultima Series Barlow (2x - 1-1/4") is an air-spaced, 3-element apochromatic design with a 27mm clear aperture.

Here comes my question:
Do apochromatic barlows work better on refractors or are they just as good for slow reflectors like my telescope? I have a normal barlow and it works OK. not exceptional viewing just OK. would an apo barlow be better?

Another question I'm wondering about. What does "27mm clear aperture" mean in the barlow product description?

Answer 1

"Apochromatic" doesn't count for much in a barlow, as a normal barlow doesn't introduce any significant color error to begin with. The Celestron Ultima does have a good reputation, though that probably has more to do with good polish, coatings, and quality control. Without more information, it's hard to say if a better barlow will give you a noticeable improvement. If you find that a 10mm plossl is much sharper or clearer than a 20mm plossl plus your barlow, then upgrading the barlow will probably help.

The clear aperture will give you some idea of how long a focal length you can use in the barlow without vignetting. 27mm is enough for virtually any eyepiece, as that about the maximum field stop size for a 1.25" eyepiece.

Answer 2

According to Terry Dickinson, there are no particular advantages to 3-element (apochromatic) Barlow lenses as opposed to 2-element (achromatic) ones. The best ones in his tests were the Celestron Ultima and its twins the Orion Shorty Plus and Antares Ultima, and Tele Vue's various Barlows, especially their Powermate series. "27mm clear aperture" refers to the diameter of the side of the Barlow lens towards the telescope objective and how much light it admits.