Short Barlow versus long Barlow

[neil phillips]

I Just ordered a GSO 3X ED Barlow  (A bargain if you ask me)

 

And it got me thinking about focal length. Comparing to refractors, longer focal length reduces spherical aberration. And chromatic aberration with a flatter feild.

a wider sweet spot. I have owned A Celestron Ultima 2x A Celestron 3x xcel  A 3x Televue (and others) two shortys and a longun (dont start) of great quality

Out of those 3 i loved the Ultima the most. But surely the Televue design is better, ED doublet longer focal length, as mentioned above in theory, less spherical and chromatic aberration with a flatter feild and wider sweet spot. I know more elements can correct for this. But that also has its disadvantages. My thoughts at the moment are the Televue and GSO ED doublets win on design. Notice how the GSO seems to be somewhat of a clone of the TV. If the figure of the GSO lenses are up to scratch. with its long focal length and ED Doublet (less absorption and reflection than multi element) I wonder how close it can get to the TV 3X 

I will at some point be testing this GSO barlow on lunar with my revamped Orion 10" Newt so we shall see. Wonder what others think about the points raised here. Any owners of the GSO 3X thoughts on its performance

 

[Louis D]

I can't comment on the merits of the 1.25" GSO 3x, but their 2" 2x ED Barlow is quite good, especially when paired with a TV Panoptic Barlow Interface.

The two best traditional Barlows I own are the Orion long Deluxe Barlows made in Japan in the 90s.  Both the 2" and 1.25" are about 6 inches long.  The 2", though, does cause heavy field cutoff with long focal length 2" eyepieces because it' not telecentric.  However, the central area is slightly sharper than the above telecentric combination.  I've compared them against the TV 1.25" 2x, Meade 140 2x, and Parks Gold Series (Celestron Ultima) Barlows as well, and these two provide slightly sharper magnification of stars in each case.  It's not a huge difference, but it's there.  Of course, their long lengths limit their usefulness in scopes that use diagonals.

[neil phillips]

Interesting Louis.

I can see why shorty's took over, diagonals is one , newtonian overhang when imaging is another. But feel i may have a point, that shorty's may not be the best optical performers. Compared to longer designs.

Refractors too. Optically a ED 4" F11. Will be better than a ED 4" F7 especially spherical aberration. But of course that suits the lunar planetary guys only.

The deepsky guys will want faster of course. I am just talking optical aberrations. Not suitability. Those Orion barlows look very very nice. Bet they perform wonderfully

 

[Mr Spock]

I have a Meade 140. It's pretty good but I always prefer the view without.

[neil phillips]

34 minutes ago, Mr Spock said:

I have a Meade 140. It's pretty good but I always prefer the view without.

Makes sense why that would be. Always had a interest in barlows doing hi res imaging though

[Mr Spock]

Most scopes need a Barlow for planetary imaging. It's the only was to get the right image scale.

[Louis D]

I only ever use the 2" combination mentioned above so I can use my 2" collection at higher powers for funsies.