barlow and focal length

[tms48]

Is it correct that a barlow works by changing the focal length of your telescope rather than just magnifying the view, and if so does that mean my f5 becomes an f10. So using a 2x barlow I would be able to use wider view eyepieces, that are normally recommended as being better on slow telescopes.

[Moonshane]

what an excellent question. instinctively I'd say the answer is no but I cannot say why. I don't think that the way a barlow works is as simple as e.g. doubling focal length. one issue with barlows possibly is that they also double the eye relief which can cause positioning problems with longer focal length eyepieces.

[great_bear]

Is it correct that a barlow works by changing the focal length of your telescope rather than just magnifying the view, and if so does that mean my f5 becomes an f10.

That is indeed correct.

So using a 2x barlow I would be able to use wider view eyepieces, that are normally recommended as being better on slow telescopes.

Also completely correct.

I don't think that the way a barlow works is as simple as e.g. doubling focal length.

It is, indeed, that simple

one issue with barlows possibly is that they also double the eye relief

Not quite - the change in eye relief is proportional to the focal length of the Barlow. So a short-style barlow affects eye relief more dramatically than a long Barlow does - even if both are 2x Barlows.

[great_bear]

Some other notes:

• Although a Barlow changes effective focal length it won't give you the all of the performance benefits of a longer-length scope. i.e. dropping a 1.6x Barlow into a Sky-Watcher 150P won't make it perform as well as a 150PL - although you will get identically-sized magnifications from the eyepieces.
  
• What happens with a Powermate? Tele Vue Powermates (e.g. 2.5x) restore the angle of the light cone, so is the F5 scope now an F12.5 or an F5 again? I don't know the answer to this...
  

[tms48]

Thanks Ive ordered a 9mm wider angle to replace the standard 10mm so hopefully it will be ok.

[Moonshane]

That is indeed correct.

Also completely correct.

It is, indeed, that simple

Not quite - the change in eye relief is proportional to the focal length of the Barlow. So a short-style barlow affects eye relief more dramatically than a long Barlow does - even if both are 2x Barlows.

cheers Jeremy - I was not aware of the short/long relationship of barlows; my excuse is that I never use them so don't understand them

my other comment (being not so simple) was based on a post by another member (cannot find it despite searching) which from memory suggested there was more to it even though the effect was the same.

[swamp thing]

Hi

It seems to me that a Barlow alters the light cone from a steep angled say f/4.5 primary by use of diverging lenses to lessen the angle so the light cone appears as it would with a f/9.

However because it only alters the angle it doesn't correct any spherical or chromatic aberration.

Therefore if your primary aint good it won't help the image, I think.

Regards Steve

[sixela]

Is it correct that a barlow works by changing the focal length of your telescope rather than just magnifying the view,

It's the same thing. The effective focal length relates the entry angle for a light bundle and where it maps on the focal plane, so it directly (together with the focal length of an eyepiece) determines magnification.

and if so does that mean my f5 becomes an f10. So using a 2x barlow I would be able to use wider view eyepieces, that are normally recommended as being better on slow telescopes.

Yes. The catch is that 2" wide field eyepieces are usually especially recommended for the lowest power eyepieces, and a barlow by definition won't save you there (you can't replace a 31mm ES 82° eyepiece with a 4" barlow and a 4" 62mm 82° eyepiece).

[sixela]

Is it correct that a barlow works by changing the focal length of your telescope rather than just magnifying the view,

It's the same thing. The effective focal length relates the entry angle for a light bundle and where it maps on the focal plane, so it directly (together with the focal length of an eyepiece) determines magnification.

and if so does that mean my f5 becomes an f10. So using a 2x barlow I would be able to use wider view eyepieces, that are normally recommended as being better on slow telescopes.

Yes. The catch is that 2" wide field eyepieces are usually especially recommended for the lowest power eyepieces, and a barlow by definition won't save you there (you can't replace a 31mm ES 82° eyepiece with a 4" barlow and a 4" 62mm 82° eyepiece).

As great_bear also correctly points out, a barlow also won't correct problems present at the original focal plane of a scope indirectly related to f/ratio on a given design (a Newtonian f/5 scope with a barlow still has the coma of an f/5 Newtonian, an achromatic refractor short tube with a barlow still has the chromatic aberration of a short tube, etc.).

[great_bear]

What happens with a Powermate? Tele Vue Powermates (e.g. 2.5x) restore the angle of the light cone, so is the F5 scope now an F12.5 or an F5 again? I don't know the answer to this...

An answer to this question has been provided by Mike Hosea <here>