Barlow 2x or not?

[Popeye (Bert)]

I have been out of the hobby for some time and recently began to re-ignite my interest.
Many moons ago (pardon the pun,) I bought a Meade 2x Barlow to allow me flexibility with the other EPs that I bought. There has been rumours that if the Barlow is inserted into the scope first (I have an LX90) then the power is greater than 2x. Some suggest that it is then 3x.
Anyone there with sufficient knowledge to definitively inform me whether inseting the Barlow first will actually increase the power of the selected EP and if it does, by what factor?

[huckabuck]

hello popeye,
if the barlow is fitted between the diagional and the eyepiece then the magnification will be consistant with the designation of the barlow. however, if the barlow is fitted between the scope and the diagional your power will increase by 50%. therefore a 2X barlow between scope and diagional will yeild 3X. a 3X will essentially become a 4.5X. a 5X will become a 7.5X ect.

[Solanum]

As a beginner I'd be interested to know why this is. I thought a diagonal just reflected the light 90 deg, why does it have an effect on magnification?

[Popeye (Bert)]

Thanks, mate.
Now I know what to do if I want more power!
In fact it should be a boon to me as I am no longer the spring chicken that I used to be and eye relief is a problem for me. Thus I need not try to cope with a 9mm but use my 26mm EP for the same result! Brilliant!
Cheers!
Popeye

[iceman (Mike)]

Originally Posted by Solanum

As a beginner I'd be interested to know why this is. I thought a diagonal just reflected the light 90 deg, why does it have an effect on magnification?

Hi Everard.

It's because the diagonal is acting as an extension tube to the barlow. In almost all barlows, an extension tube will act as a multiplier. In some barlows (eg: 2.5x powermate), the extension tube actually decreases magnification.

Check out these 2 pages on the televue site for some nice graphs. Obviously it may not be the same for other brand barlows, but you get the idea.

http://www.televue.com/engine/page.asp?ID=41
http://www.televue.com/engine/page.asp?ID=42

[Solanum]

Ah, of course! *slaps forehead* I should have thought of that, thanks. Presumably the downside is reduced field of view (particularly on a fast scope?), and a smaller sweet spot when it comes to focussing?

[Popeye (Bert)]

G'day Solanum, I cannot answer that question as I didn't really understand the info on the links!
But the information only begs another question. At the time that I bought my Meade 2x Barlow, I also bought a couple of other EPs. One was a 40mm. I thought that this in combination with the barlow would allow me 50x power and a large field of view without the Barlow and with the Barlow get me access to a 20mm EP (so to speak, - two Eps for the price of one -) the same with the 15mm that I bought at the same time. ie 133x power without the Barlow (LX90 2000 focal length,) and effectively the use of a 7.5mm 266x power with the Barlow. However, I had a bit of trouble with the 40mm when I bought it but now I am about 9 years older (at 67,) and when I tried to look through the 40mm the other night I needed to take my eye well away from the EP to see effectively. Then I could see very little of the FOV.
What now? Do I need to toss the 40mm away?
Cheers!
Popeye