Reversed Barlow = focal reducer?

[Moonshane]

This is probably a stupid question but if I can find a double male adapter to allow me to fit a barlow lens element the wrong way round onto an eyepiece would this create a wider field/lower power/shorter focal length view? I know you can buy focal reducers but I wondered if anyone had ever tried the above for fun.

I presume there will be problems with CA and vignetting (and maybe focus point) but might be a way to reduce the numbers of eyepieces when traveling? I am thinking of e.g. the Antares 1.6x 2" barlow with a 26mm Nagler.

It's been so cloudy lately, I finally appear to be running out of things to think about.

Cheers

Shane

[nephilim]

This is probably a stupid question but if I can find a double male adapter to allow me to fit a barlow lens element the wrong way round onto an eyepiece would this create a wider field/lower power/shorter focal length view? I know you can buy focal reducers but I wondered if anyone had ever tried the above for fun.

I presume there will be problems with CA and vignetting (and maybe focus point) but might be a way to reduce the numbers of eyepieces when traveling? I am thinking of e.g. the Antares 1.6x 2" barlow with a 26mm Nagler.

It's been so cloudy lately, I finally appear to be running out of things to think about.

Cheers

Shane

An idle mind........etc etc

[Spec-Chum]

The maths would suggest so, but I imagine the focus point would end up inside the OTA.

[E621Keith]

I don't think it will work. I think a barlow acts like a concave lens and cause a divergent (reduce convergent angle) in the incoming light cone and results in an increase focal length.

A concave lens is a concave lens no matter which side you look through it, in the same way a there is no front and back side to a magnifying glass.

In reality a barlow is a more complicated doublet, but that's only for controlling CA. Turning it backwards may make the CA go crazy but I don't think it will reduce the focal length. For that you need a focal reducer that acts like a convex lens that increase the convergent angle.

[Moonshane]

cheers Keith. that makes complete sense. should have investigated the make up of the barlow a bit more. thanks all - looks like more money saved

[andrew s]

Shane - I am not sure I fully understand what you propose to try but a barlow is a has a negative focal lengths both ways round. One way has less aberations than the other which is the way it should normally be used. Thus it can't simply be used as a focal reducer.

A focal reduce is a net postive lens.

Regards Andrew

[Spec-Chum]

I don't think it will work. I think a barlow acts like a concave lens and cause a divergent (reduce convergent angle) in the incoming light cone and results in an increase focal length.

A concave lens is a concave lens no matter which side you look through it, in the same way a there is no front and back side to a magnifying glass.

In reality a barlow is a more complicated doublet, but that's only for controlling CA. Turning it backwards may make the CA go crazy but I don't think it will reduce the focal length. For that you need a focal reducer that acts like a convex lens that increase the convergent angle.

Yeah, good point.

A barlow lens is )( so theoretically symmetrical, turning it round yields )( Oops

A focal reducer would be ()

I really didn't think my first answer through, sorry Shane

[Moonshane]

Shane - I am not sure I fully understand what you propose to try but a barlow is a has a negative focal lengths both ways round. One way has less aberations than the other which is the way it should normally be used. Thus it can't simply be used as a focal reducer.

A focal reduce is a net postive lens.

Regards Andrew

you are quite right Andrew, I assumed (and we all know what that leads to) that a barlow had one concave and one convex side. I have not used one for so long that I forgot how they work.

[estwing]

looks like the purse strings are loose again Shane

[nightfisher]

Shane, i have a reducer that screws into the filter thread, your welcome to try it later this month. its 1.25 fit

[Moonshane]

looks like the purse strings are loose again Shane

nah, have to sell a few bits I don't use first! plus if I get one for maybe £50-60 used then it prevents me even thinking about a 16mm or 17mm Nagler so using my usual logic, it's saving money.

[Moonshane]

Shane, i have a reducer that screws into the filter thread, your welcome to try it later this month. its 1.25 fit

cheers Jules, I will do, thanks!

[JamesF]

A concave lens is a concave lens no matter which side you look through it, in the same way a there is no front and back side to a magnifying glass.

I can't deny that I hadn't wondered the same thing myself and this is exactly the point that decided me against the possibility. Explaining to yourself how it can be that if you reverse something that makes things look bigger in one direction then it still makes things look bigger the other way around probably gives you a better understanding of optics

James

[Cath]

I can't deny that I hadn't wondered the same thing myself and this is exactly the point that decided me against the possibility. Explaining to yourself how it can be that if you reverse something that makes things look bigger in one direction then it still makes things look bigger the other way around probably gives you a better understanding of optics

Try that with a pair of binoculars

[estwing]

nah, have to sell a few bits I don't use first! plus if I get one for maybe £50-60 used then it prevents me even thinking about a 16mm or 17mm Nagler so using my usual logic, it's saving money.

...i've used same said logic..its cost me thousands!

[alan potts]

I don't think you would ever focus it. But if you do let us know. Now can a reducer turned round become a Barlow?

Alan.