Stacking Barlows?

[maw lod qan]

So, I understand the principle of a 2x barlow takes a 20mm eyepiece and basically makes a 10 out of it.

What  does stacking barlows together do?

I have a good 3x and a cheap 2x. I can see the change with the two  combined but was wondering how the actual figures work out.

Edited July 24, 2020 by maw lod qan

[AstroMuni]

The magnification would be 3 x 2 x mag of using the 20mm. So if the original 20mm was giving a magnification of (say) 50x then with these 2 barlows added you would get 50 x 6 = 300x

[vlaiv]

I don't really know, but my gut feeling is that you need to consider each barlow focal length and their separation.

Then apply this formula to get total focal length of combined lens:

where f1 and f2 are respective focal lengths and d is distance between them.

And finally to get magnification factor of combined barlow, use barlow magnification formula:

M = 1 + X/F

Where X is focal plane distance and F is focal length of combined barlows (result of above formula).

[AstroMuni]

19 minutes ago, vlaiv said:

I don't really know, but my gut feeling is that you need to consider each barlow focal length and their separation.

That would be the underlying formula, but as a rough guide I guess the multiplication of mag factors for each barlow would suffice.

[vlaiv]

5 minutes ago, AstroMuni said:

That would be the underlying formula, but as a rough guide I guess the multiplication of mag factors for each barlow would suffice.

I actually used combination of two barlows to do some planetary imaging and results are not what you would expect.

It is easier to use single barlow to obtain needed magnification by using extension tubes. As above formula says - magnification of a barlow depends on eyepiece distance to barlow lens - increase distance - increase magnification.

[maw lod qan]

Thanks for the replies.

[AstroMuni]

13 minutes ago, vlaiv said:

magnification of a barlow depends on eyepiece distance to barlow lens - increase distance - increase magnification.

This would work upto a point. i.e. you cant increase the distance beyond a certain point to get more magnifcation. 🙂

[Louis D]

7 hours ago, AstroMuni said:

This would work upto a point. i.e. you cant increase the distance beyond a certain point to get more magnifcation. 🙂

And that distance is the focal length of the barlow, IIRC.

[Merlin66]

You can also stack TV Powermates.....worked for me.

 

[vlaiv]

9 hours ago, AstroMuni said:

This would work upto a point. i.e. you cant increase the distance beyond a certain point to get more magnifcation. 🙂

 

1 hour ago, Louis D said:

And that distance is the focal length of the barlow, IIRC.

I'm not quite certain you got that right.

Here is diagram of edge case scenario - when barlow is put its own focal length in front of focal point of a telescope:

Since barlow is diverging element if you place it it's own focal length in front of focal plane of telescope it will create collimated beam - you won't be able to focus it any more. Placing barlow further away from focal plane will result in diverging light rays.

On the other hand if you place  barlow closer to focal plane - rays will still converge - but how close, that depends on how close barlow lens is to focal plane.

This means that you can achieve as much magnification as you like, provided that you can place barlow at most its own focal length in front of focal plane of telescope (if barlow has long focal length you might have issues with newtonian scopes as it might need to be placed inside focuser tube or even in the same place as secondary mirror) and if you provide enough extensions for eyepiece on the other side.

Placing barlow at its own focal length from the eyepiece (not focal plane) - makes barlow work at x2 magnification.

In any case, barlow will have least optical aberrations when working at distance that gives specified magnification.

[Merlin66]

19 minutes ago, vlaiv said:

Placing barlow at its own focal length from the eyepiece (not focal plane) - makes barlow work at x2 magnification.

Vlaiv,

can you explain this???

[vlaiv]

1 hour ago, Merlin66 said:

Vlaiv,

can you explain this???

According to barlow formula:

M = 1 + X/F

Where F is focal length of barlow and X is distance between barlow element and "eyepiece" (here we don't actually count eyepiece but point that eyepiece considers it's operating focal point).

If X and F are equal - or barlow is placed its focal length away from eyepiece we have:

M = 1 + F/F = 1 + 1 = 2

Magnification is x2

Diagram of this configuration would be something like this:

Blue rays are scope incoming rays - and if there was no barlow in position those would be ending in scope focus point (red dotted lines), but since we have a barlow - that is closer to scope focal point than its own focus length - it diverges incoming rays (blue dotted lines) and those fall on focal plane of eyepiece - which acts as if focal point of scope has been moved outwards.

Magnification of barlow is given with above formula and if you take x2 magnification (eyepiece - barlow distance equals barlow focal length) then barlow will be some distance away from focal point of telescope - I think it depends on focal length of barlow and F/ratio of scope beam (possibly even focal length of scope?).

 

[Louis D]

All I can figure is I was misremembering something about this.  Based on the formula, all I can figure is if the barlow focal length is extended enough, you end up with parallel rays and you've created a relay lens that you terminate with a focal reducer/condenser lens.

[vlaiv]

9 minutes ago, Louis D said:

All I can figure is I was misremembering something about this.  Based on the formula, all I can figure is if the barlow focal length is extended enough, you end up with parallel rays and you've created a relay lens that you terminate with a focal reducer/condenser lens.

You can't really change focal length of a barlow lens, but you are right - if you place barlow lens so it is it's own focal length away from focal plane of telescope - irrespective of any other lenses that come after - you have created a collimated beam of light because in that case barlow lens diverges incoming rays in exact same extent as scope converges them and they end up being parallel again.

This case could be considered as infinite magnification because parallel rays converge in infinity ... (abstract math stuff, I know ).

[Louis D]

10 hours ago, vlaiv said:

You can't really change focal length of a barlow lens, but you are right - if you place barlow lens so it is it's own focal length away from focal plane of telescope - irrespective of any other lenses that come after - you have created a collimated beam of light because in that case barlow lens diverges incoming rays in exact same extent as scope converges them and they end up being parallel again.

This case could be considered as infinite magnification because parallel rays converge in infinity ... (abstract math stuff, I know ).

That has to be what I misremembered.  Place the barlow lens its own focal length away from the focal plane of the telescope, not the eyepiece the barlow's focal length away from the barlow lens.  My mind is getting to be like Swiss cheese.  Being confined at home 24/7 isn't helping matters.

[Don Pensack]

And if a positive lens follows the negative lens, the rays can be made parallel as they exit the lens.

Yet, the increase in image scale will still occur.

This is how the telecentric barlow works:

https://www.cloudynights.com/topic/712460-barlow-lens-gives-two-possible-focus-points/?p=10263173