TeleVue Barlow question

[NGC 1502]

 

Hi everyone.   TeleVue makes two 1.25” fitting Barlows, a 2x and a 3x.   They are very different in physical length, the 3x being much longer.   Does anyone know if they are optically the same, and if an extension tube were added to the shorter 2x version, the effect would be the same as using the 3x ?

I know that actual magnification can vary according to the eyepiece to Barlow lens distance, or if when using a Barlow with a refractor the magnification changes if it’s used before or after the diagonal.

But does anyone know the answer to my specific question above ?  I know I could phone TV and get an honest answer, but just wondering if anyone on here knows - optically the same, just a different length ?

Thanks in advance, Ed.

[pregulla]

I don't know, but I would guess they use different lenses. TV claims they are parfocal, from the pictures it looks like the nose-piece is the same length in both. When using the same lens for higher magnification it needs to be moved further in, so the nose-piece would be longer if 3x was the same as 2x.  Stronger negative lens would make the cone steeper and only extend the distance from the lens to the eyepiece. Based on their pictures only that what seems to be happening.

Full disclosure: I am not an expert and it's just a speculation.

[NGC 1502]

 

Many thanks for that.........?..........happy to hear from anyone else.

Ed.

[Alan White]

I see what you are saying Ed and tend to agree, it does look to be the EP end that is extended,
not the nose piece end.
Suggesting that distance increase equates to higher magnification.
Indeed this is the case with a barlow, pull the EP back up or out a bit and magnification looks to increase.

I think you should ring TV and ask Al or David what the case is.
 

[CraigT82]

Have just been wondering this myself. TV produce a chart that shows the 2x barlow produces 3x mag with and extra 6.5cm of extension. If the 3x barlow is 6.5cm longer than the 2x then I'd say that the barlow lens element itself is the same one in both. 

[Alan White]

Time to get measuring I think.

 

[Stardust]

I have both, the 3x is 3 cms longer. Looking through them they optically not the same