Barlows... Double or Single Lens

[star_lord]

My amateur mind tells me a single lens would be better, as there is less between the scope and the eye...

However, I have no idea.

Can anyone help?

[ronin]

Usally 2 bits are better they compensate for colour more, often the "single" lens will be a glued doublet as per an achromat lens. Usually defined as "2 elements in 1 group".

What they are trying to avoid is adding significant CA to the final image.

They take care of the additional light loss by the coatings - well they should.

[michael.h.f.wilkinson]

Any single lens Barlow should be avoided like the plague. These are generally cheap, plastic-lens, flimsily built horrors that give decent Barlows a bad name. At least, all single lens Barlows it has been my profound lack of pleasure encountering fit in that description. My TV PowerMate and Meade TeleXtenders (not true Barlow technically) are 4 lens affairs (4 elements in 2 groups) and they take any single lens and most two lens Barlows to the cleaners. They just don't seem to be in the light path, except that they produce higher magnification. Modern coatings mean that the number of groups in a design (which dictates the number of glass-air interfaces) is much less of an issue than it was before coatings were available.

[rwilkey]

As Michael & Ronin say, a doublet is usually used to help correct CA.  There is another school of thought that more elements (lenses) diminishes the view, I do not subscribe to this as many fine views are obtained with multiple well-corrected lenses.  The comment 'Putting more glass in the optical path' is rubbish to my way of thinking and is not born out by experience.

[John]

As said a doublet should be the minimum quality selected.

With a quality barlow or telextender modern glass and coating technologies mean that additional lens elements have little or no discernable negative impact on image quality wheras the abberrations introduced by a singlet barlow lens are pretty awful

[star_lord]

Aweomse. Cheers guys. A double it is!

[knobby]

Aweomse. Cheers guys. A double it is!

Now for the next choice ... Which one

[Tinker1947]

Now for the next choice ... Which one

Maybe these....

http://www.firstlightoptics.com/tal-eyepieces.html

[Jim Steele]

http://www.firstlightoptics.com/tal-eyepieces.html

Tal barlows are very good but a little hard to come by these days since I believe the UK importer has stopped importing from Tal. Maybe this will change with the roubles recent troubles.

[ScottS]

Here's another option for you with a 3 element design. Works great with my setup.

[John]

Of the less expensive ones, I think the Revelation 2.5x, as linked to above by Scott, is one of the best I've used.

[ScottS]

Of the less expensive ones, I think the Revelation 2.5x, as linked to above by Scott, is one of the best I've used.

I might add that there is some debate as to whether it is a true 2.5x though. I find it works well though and helps with laser collimation as a bonus!

[John]

I might add that there is some debate as to whether it is a true 2.5x though. I find it works well though and helps with laser collimation as a bonus!

Yes, I've seen those discussions. Maybe more like 2.2x perhaps ?.

[SpaceWalker]

Yes, I've seen those discussions. Maybe more like 2.2x perhaps ?.

Really? Interesting. There was a discussion on CN where Bill Paolini clamed the 2.25x Baader Q works like 2.5x. So, the 2.5x Barlow works as a 2.2x, and the 2.2x like 2.5x...funny, you never know with these telenegative Barlows...my 2x Ultima is actually 2.2x   .

[Alfian]

I have the Rev. 2.5x too and it is good, solidly made too. Likewise I've also seen comments about it being less than 2.5x - keep meaning to test it.

[nicks90]

I've been using a 2 inch Antares x1.6 Barlow with my panaview eps.

Lurrrrvely.