problem with 2" 2x ED barlow lens

[weega]

Hi!

I've just bought a 2" 2x ED barlow lens (brand "Telescope Service", built by GSO), I've already tried it on my dobson 15.5", f/4.3.

Mechanically it's very well built.

Optically it works very well with all my eyepieces (SP26mm, LVW13mm, PL 6.7mm).

When I say "very well" I mean I can't distinguish it from Celestron 2x ULTIMA barlow (I have it since 8 years).

It works very well both at 2x or at 1.5x, obtained using the barlow's doublets screwed on the 2"=>1.25" reducer.

But on my SWA 38mm 70°AFOV (Kunming United Optics) at 1.5x the outer field is horrible, worse than without barlow...

It can't be coma, since without barlow the outer field of the eyepiece is better...

It seems astigmatism.

Anybody has tried it a similar combination?

thanks

ciao

dan

[Astro_Baby]

I have the same Barlow (mines the SKywatcher one) but hardly ever use it. It works ok with stadard EPs but it never worked well with complex ones like Baader Hyperions (though with that said it might just have been conditions were never really good to push the mag that far because at SSP last year it seemd ok with a 5mm Hyperion).

I have heard, though I dont know how true it is that these Barlows generally tend to not work well with complex lensed EPs. As the SWA 38mm is a complex bit of glass that may be causing your problems.

[weega]

[cut]

I have heard, though I dont know how true it is that these Barlows generally tend to not work well with complex lensed EPs. As the SWA 38mm is a complex bit of glass that may be causing your problems.

mmm, 38mm SWA is an Erfle: only 5 elements in 3 groups (doublet - singlet - doublet). Moreover this barlow works very well with my vixen LVW 13mm that has 8 elements in 5 groups! I don't think the problem is in the number of lenses

Surely SWA uses "exotic" glasses to reach 70°, since the classic erfle has an AFOV of only ~60°....mumble mumble... I want a nagleeeeerrrr

ciao

dan

[great_bear]

Surely SWA uses "exotic" glasses to reach 70°, since the classic erfle has an AFOV of only ~60°

Not necessarily (although I'm not ruling it out!) Computer Aided Design has allowed designers to push the parameters quite a bit. The SWA is best described as "pseudo erfle"

Of course, none of this explains the weird problems you get with - as you say - a comparatively simple design...