Shortening a Barlow?

[Alfian]

I have an Antares 3x Barlow which I had put up for sale but still sits unused. Over time I have used a Barlow less, preferring to use stand alone EPs. However recently I've wanted to push the magnification a little and felt it would be handy to have something like a 1.75-2 x Barlow.  I know on some Barlows there is a screw off nose/lens section that can be attached to an EP using the filter threaded section,  so I am assuming that the length of the Barlow tube can be adjusted to change the power of the Barlow.  This has led me to speculate whether I could cut the Antares 3x down and re-drill/tap a new locking thumbscrew position to give a lower power. How much I would have to cut off is the thing.  Has anyone tried this and if so , any tips?

 

[CraigT82]

You could try going the other way first... add in say. a 50mm extension, between barlow and EP and see how much extra mag that gives you, that might then give you an idea of how much to take off?

[John]

What you need to change is the distance between the barlow lens element and the lenses in the eyepiece. It is that distance plus the focal length of the barlow lens that determines the degree of amplification. There is maths that can work it out but you need to know the focal length of the barlow lens. If you are happy to experiment then it's the top section of the barlow that will need to be shortened I think. This will bring the eyepiece lenses closer to the barlow lens and should reduce the amplification.

I hope I've got that right !

 

 

[Alfian]

Hi John, yes thats pretty much what I thought. Fortunately the bottom section screws out which means that any alteration to the top doesn't put the barlow lens itself at any risk. Unfortunately the threads on the barlow lens are not the same as the 1.25" EP filter threads, that would have been useful. It might work out if I go ahead, a bit trial and error unless someone can give some formula as you suggest.

[YKSE]

The formula for a barlow is quite straight forwards:

M=1 - d/f

Where 

M is the barlow factor, d is the distance between the middle of barlow doublet to the top the barlow housing, and f is the focal length of the barlow (a negative value). The implications is, the nominal barlow effect (3x in your case), works for eyepieces with  field stop exact at its shoulder, equals to on the top of barlow housing.

so for your barlow, it goes like this

3=1 - d/f

we get f=-d/2

If you want to, say get a 1.5x barlow, the new distance dn should meet

1.5=1 - dn/f

i.e.

dn = (-0.5)*f =(-0.5 )*(-d/2) =d/4

[Alfian]

31 minutes ago, YKSE said:

The formula for a barlow is quite straight forwards:

M=1 - d/f

Where 

M is the barlow factor, d is the distance between the middle of barlow doublet to the top the barlow housing, and f is the focal length of the barlow (a negative value). The implications is, the nominal barlow effect (3x in your case), works for eyepieces with  field stop exact at its shoulder, equals to on the top of barlow housing.

so for your barlow, it goes like this

3=1 - d/f

we get f=-d/2

If you want to, say get a 1.5x barlow, the new distance dn should meet

1.5=1 - dn/f

i.e.

dn = (-0.5)*f =(-0.5 )*(-d/2) =d/4

Hi, that is exactly what I need! Thank you very much. I'll be getting my maths head on and giving it a go. Assuming it all works out I,ll report back on the result.

[TheLookingGlass]

I've done this using a 2 inch GSO ED Barlow. 

What I do, if I want a bit less magnification, is unscrew the barlow element from the 2 inch GSO Barlow and add it to a 35mm Extension tube and it gives me close to 1.9x magnification with my eyepieces. I've drift timed my eyepieces with the 2 inch GSO and it is closer to a 2.2x barlow than the 2x it is marked at. 

Sometimes I screw the barlow element right to my eyepieces, (which all have Antares twist lock adapters), giving me roughly 1.5x magnification, although when I do this last bit using the 28mm ES 68, the edges vignette because the field stop clips the light cone IIRC.