Barlow - How does yours work ?

[Charic]

There seems to be many ways to interpret how a Barlow works, and many folk  settle  one way or another, like " doubling of the power of the eyepiece"  yet the theory is much deeper, and not everyone is getting it right, or fully understands ( me possibly ) so why is the theory understood in different ways by different folk? I just want to get it right, and not just settle for "it halves the focal length of the eyepiece"  or "doubles the magnification of the eyepiece, its deeper than that so maybe some fresh views/reminders are needed?

I've stated  in the past that if you put a Barlow between the primary mirror/objective and the eyepiece, it virtually increases the effective focal length of the light path, your focal length, in turn affecting the eyepieces that precedes it.

Telescope Focal length divided by Eyepiece Focal Length gives us our magnification. So from my scope I get 200x from 1200/6mm EP.

If I now insert the 2xBarlow, its still a 6mm EP, but I now have 400x yes! therefore something has changed?  I must have a Focal length of 2400 from the insertion of a 2x Barlow to get 400x from my 6mm EP in order for the calculations to work.

Wikipedia goes along the lines of ....  " The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system.

So......How do you think your Barlow works?

[Mak the Night]

I always thought it had something to do with the distance of the lens element from the eyepiece objective. My TV 3x Barlow is 13cm long, longer than the TV 2x Barlow, yet the Barlow elements look identical.

[Dave In Vermont]

The amount of 'magnification' is determined by the distance of the lens-elements from the FP (focal-point) of the eyepiece. Hence the longer-tube for the TV-Barlow for 3X when compared to 2X TV-Barlow. Thus it does effectively act as if your 1200mm FL scope has a FL of 4500mm for the TV 3X. One could simplify this gobble-de-gook by saying a Barlow is a magnifying-glass, and this description is probably the best one to use to describe it to someone who doesn't want a lecture on optical-physics.

Now in the first picture is an Orion (USA) Shorty-Barlow 2X. In the second picture is the Vixen 8 - 24mm Zoom EP. In the third picture is the Barlow-lens screwed into the Vixen Zoom. If the Vixen Zoom were inserted into the Shorty-Barlow, the Vixen Zoom would have 2X the magnification. With just the lens-cell from the Barlow screwed into the Vixen Zoom as in picture 3, the increase is about 60% instead of 200%.

I think we all have an intuitive overview of what the Barlow is doing, but the hard part is explaining the how & why.

Dave

[Floater]

Ah, Chas, you're returning to a subject that caused me some mental turmoil a while back, with you and I jousting over the proper description of what was happening with a Barlow. I was entrenched with the lazy way of describing it as 'halving the FL of the EP', therefore doubling magnification. But the clincher that caused me to settle for 'doubling the scope's focal length' was when it was pointed out that even without an EP in the optical train (e.g. camera only) the magnification was still doubled. Argument over and, if I remember rightly, an apology from me to you for my thrawn stance ...

Ah, Dave, what are you doing? 'Thus it does effectively act as if your 1200mm FL scope has a FL of 4500 for the TV 3X.' I do hope that was a typo or I'm in danger of being put back to the funny farm ...!

Edit: just picked up on the fact that this is a spin-off from another thread where, no surprise, there's a lot of to-ing and fro-ing. I'm outta here!

[John]

Barlow lenses have a small imp living in them. They are equipped with a fantastic quality tiny telescope in there and when you look through the barlow lens the imp holds his telescope up so you get the benefit of the wonderful impish optics sandwiched between your rather mediocre human ones.

If the image is a little blurred it's because the imp is tired or a little drunk on acorn wine so he can't quite hold his little telescope steady. If the image won't come to focus at all it's because the imp is up at the wrong end of the barlow lens so you need to tap the side of the barlow lens lightly and say "this way little fellow" a few times so that he gets the message to move along the tube.

A Powermate or TeleXtender works in roughly the same way except that there are two imps involved. They do sometimes squabble though so listen carefully to the device before you install it so that you can hear what sort of mood they are in and the movement of the tube allows them to get prepared, switch the impish TV off, and clear away the tea things.

The above was imparted to me in confidence by Al Nagler himself and confirmed by Terry Pratchett so it's got to be accurate

I hope that has cleared things up. Next week: The Collimation Gremlins.

[JamesF]

[JamesF]

The way I think of it is this:

From the primary lens/mirror you have a cone of light that reaches the focal plane.  "Fast" telescopes have a short fat cone, whereas "slow" telescopes have a tall thin cone.  That is, the faster the telescope the larger the angle the sides of the cone make with the optical axis.  All telescopes of the same focal ratio have light cones that form the same angle with the optical axis.

A barlow "bends" the light away from the optical axis, making the cone taller and thinner and reducing the angle to the optical axis, thereby changing the focal ratio and the effective focal length.

The effective focal length of an optical system however is not just dependent on the optical components themselves, but also the spacing between them.  This is why it is possible to put a barlow labelled as "2x" together with an extension and then the eyepiece to give a 3x increase in effective focal length when the image is in focus, because in such an arrangement the distances between each of the optical components changes when compared with the configuration it was designed for.

James

[JamesF]

I always thought it had something to do with the distance of the lens element from the eyepiece objective. My TV 3x Barlow is 13cm long, longer than the TV 2x Barlow, yet the Barlow elements look identical.

I have often wondered if the TV 3x barlow is actually the same as the 2x, using the greater distance between the focal plane and the barlow lens group to achieve the increase in power.

James

[Mak the Night]

I have often wondered if the TV 3x barlow is actually the same as the 2x, using the greater distance between the focal plane and the barlow lens group to achieve the increase in power.

James

Yeah, that's what I thought. All I need now is someone to explain shorty Barlows to me lol!

[Mak the Night]

Barlow lenses have a small imp living in them. They are equipped with a fantastic quality tiny telescope in there and when you look through the barlow lens the imp holds his telescope up so you get the benefit of the wonderful impish optics sandwiched between your rather mediocre human ones.

If the image is a little blurred it's because the imp is tired or a little drunk on acorn wine so he can't quite hold his little telescope steady. If the image won't come to focus at all it's because the imp is up at the wrong end of the barlow lens so you need to tap the side of the barlow lens lightly and say "this way little fellow" a few times so that he gets the message to move along the tube.

A Powermate or TeleXtender works in roughly the same way except that there are two imps involved. They do sometimes squabble though so listen carefully to the device before you install it so that you can hear what sort of mood they are in and the movement of the tube allows them to get prepared, switch the impish TV off, and clear away the tea things.

The above was imparted to me in confidence by Al Nagler himself and confirmed by Terry Pratchett so it's got to be accurate

I hope that has cleared things up. Next week: The Collimation Gremlins.

Soooo ... all I need to do is appease the Barlow imps and remember not to feed the Collimation Gremlins after midnight, right? 

[Charic]

..........the end result is the same when using a Barlow, its how we get there,  which makes for an interesting thread.

Gordon, the subject was raised, and rather than continue on someone else's thread it was right  to start another, and with so many ideas floating around, I just thought I`d open up again and see  how some of the good folk here understand how  it works. 

Not sure what Johns drinking    but  John does know his subject, and as the Guru for many folk here, I'm going to have to change my outlook as to how the Barlow works.

Now,  how many  imps was that,  and at which end? ..........this is gonna mess my afternoon up?

[JamesF]

Yeah, that's what I thought. All I need now is someone to explain shorty Barlows to me lol!

I have no idea how shorty barlows work, but it strikes me that you could use a combination of lenses to first shorten the effective focal ratio, bringing the focal plane forwards followed by a divergent lens (or lenses) that achieve the required angle to the optical axis.  Compared with a standard barlow that might well allow the focal plane to be closer to the lens group meaning that the barlow body could be shorter.

James

[Stu]

Barlow lenses have a small imp living in them. They are equipped with a fantastic quality tiny telescope in there and when you look through the barlow lens the imp holds his telescope up so you get the benefit of the wonderful impish optics sandwiched between your rather mediocre human ones.

If the image is a little blurred it's because the imp is tired or a little drunk on acorn wine so he can't quite hold his little telescope steady. If the image won't come to focus at all it's because the imp is up at the wrong end of the barlow lens so you need to tap the side of the barlow lens lightly and say "this way little fellow" a few times so that he gets the message to move along the tube.

A Powermate or TeleXtender works in roughly the same way except that there are two imps involved. They do sometimes squabble though so listen carefully to the device before you install it so that you can hear what sort of mood they are in and the movement of the tube allows them to get prepared, switch the impish TV off, and clear away the tea things.

The above was imparted to me in confidence by Al Nagler himself and confirmed by Terry Pratchett so it's got to be accurate

I hope that has cleared things up. Next week: The Collimation Gremlins.

Brilliant

[Mak the Night]

I have no idea how shorty barlows work, but it strikes me that you could use a combination of lenses to first shorten the effective focal ratio, bringing the focal plane forwards followed by a divergent lens (or lenses) that achieve the required angle to the optical axis.  Compared with a standard barlow that might well allow the focal plane to be closer to the lens group meaning that the barlow body could be shorter.

James

That does make sense, I think, my knowledge of optics is sadly limited lol, but shorties usually just have one lens element. I know Al Nagler doesn't like shorties much. I have noticed some vignetting with a shorty and a 32mm Plossl, although it could just be my eyes. As I usually don't wear eye glasses I sometimes find long eye relief a little uncomfortable. I believe TeleVue have an eye guard extender for this.

A Celestron 'shorty' with element removed.

[Stu]

I have no idea how shorty barlows work, but it strikes me that you could use a combination of lenses to first shorten the effective focal ratio, bringing the focal plane forwards followed by a divergent lens (or lenses) that achieve the required angle to the optical axis. Compared with a standard barlow that might well allow the focal plane to be closer to the lens group meaning that the barlow body could be shorter.

James

Could it not be that a shorty Barlow simply has a stronger effect by using different optics so is able to achieve the same magnification in a shorter distance?

[Stu]

The way I think of it is this:

From the primary lens/mirror you have a cone of light that reaches the focal plane. "Fast" telescopes have a short fat cone, whereas "slow" telescopes have a tall thin cone. That is, the faster the telescope the larger the angle the sides of the cone make with the optical axis. All telescopes of the same focal ratio have light cones that form the same angle with the optical axis.

A barlow "bends" the light away from the optical axis, making the cone taller and thinner and reducing the angle to the optical axis, thereby changing the focal ratio and the effective focal length.

The effective focal length of an optical system however is not just dependent on the optical components themselves, but also the spacing between them. This is why it is possible to put a barlow labelled as "2x" together with an extension and then the eyepiece to give a 3x increase in effective focal length when the image is in focus, because in such an arrangement the distances between each of the optical components changes when compared with the configuration it was designed for.

James

This is basically my understanding of it too.

The only other thing to add (as mentioned in the other thread) is that the increase in focal length does not change the other characteristics of the scope such as CA and coma, but it can reduce astigmatism caused by the eyepiece.

[Mak the Night]

Could it not be that a shorty Barlow simply has a stronger effect by using different optics so is able to achieve the same magnification in a shorter distance?

That'd be my guess. It could also explain why there are sometimes vignetting problems associated with shorties.

[JamesF]

Could it not be that a shorty Barlow simply has a stronger effect by using different optics so is able to achieve the same magnification in a shorter distance?

I'm not sure.  I think that for the same power you have to leave the light cone at the same angle regardless of how long the body of the barlow is.  If you change the lens so it refracts the light more then I don't think that will happen.  It just struck me that if there was some sort of reducer lens before the divergent lens then you'd be starting from a smaller diameter part of the light cone, so the focal plane would move inwards and the body could be shorter.

I honestly have no idea how it's done though, so I certainly won't claim your suggestion is wrong.

James

[JamesF]

That does make sense, I think, my knowledge of optics is sadly limited lol, but shorties usually just have one lens element.

It may be that it appears to be one element but is actually several elements combined (perhaps even cemented together).  I have one of the well-regarded Celestron Ultima barlows as well as the GSO/Revelation 2.5x, both of which are quite short.  I'm somewhat loathe to take them apart to find out in detail how they're put together though

James

[John]

Setting aside, for a moment, the Imp concept (which I totally endorse of course), the optical elements of a barlow lens have a focal length in the same way that an eyepiece does and these focal lengths will vary depending on the characteristics of the optical elements eg: lens curvature, spacing etc. I believe "shorty" barlows, apart from being more cramped for the Imp, use a shorter focal length optical element. It's always 2 elements, and sometimes 3 or more. Sometimes air spaced and sometimes cemented.

I think that it's a combination of the focal length of the optical element and the physical length of the tube they are housed in (which affects the spacing between the barlow optics and the eyepiece optics) that deliver the amplification factor. Change the spacing and you get a different amplification factor.

The Imps are angry with me now, I can tell .....

[Mak the Night]

It may be that it appears to be one element but is actually several elements combined (perhaps even cemented together).  I have one of the well-regarded Celestron Ultima barlows as well as the GSO/Revelation 2.5x, both of which are quite short.  I'm somewhat loathe to take them apart to find out in detail how they're put together though

James

Yes, I know the element is made up of multiple lenses, but on many Barlows (usually the more expensive ones) the entire element designed to be able to be removed to facilitate being screwed directly into an eyepiece. I do this with a 10mm Celestron Luminos to effectively give it into a 6.7mm f/l. In this way I can increase its magnification on my Mak from 130x to 208x as it will increase the magnification by approx 1.6x. In this way I don't exceed the 241x recommended resolution that I would get with the 10mm Luminos 'Barlowed' to 260x.

Plus it saves me buying the 7mm Luminos as well! As I am running out of room in my EP box lol.

[JamesF]

Ah, yes, what you're doing there is basically the same as adding an extension to a 2x barlow to make it a 3x barlow, but in reverse.  You change the spacing of the optical groups in such a way that the barlow doesn't achieve it's "design power", but something a bit lower.

James

[JamesF]

The only other thing to add (as mentioned in the other thread) is that the increase in focal length does not change the other characteristics of the scope such as CA and coma, but it can reduce astigmatism caused by the eyepiece.

Indeed.  A well-corrected scope should produce identical light cones regardless of the frequency of the incoming light.  That doesn't happen when you get CA.  The light cone at the blue end of the spectrum is different from that of the red end.  A well-corrected barlow must treat all colours the same, so it might conceivably even act to make the apparent CA more obvious because extending the effective focal length may exaggerate the difference between the light cones.

James

[Peter Drew]

I think JamesF and John have the Barlow well nailed which has saved me a lot of one finger typing. My similar understanding(?) is that the power of Barlows can be increased either by "stronger" optical elements or a longer separation between the eyepiece and the Barlow. The further into the focal plane the Barlow is inserted the greater the final magnification, this is how zoom eyepieces work. An easy way to check the focal length of a negative lens is to draw a circle twice the size of the lens, shine the Sun through it and when the image fits the circle the focal length is the measurement between the lens and the drawn circle.  

[Mak the Night]

Ah, yes, what you're doing there is basically the same as adding an extension to a 2x barlow to make it a 3x barlow, but in reverse.  You change the spacing of the optical groups in such a way that the barlow doesn't achieve it's "design power", but something a bit lower.

James

It's surprisingly useful if you want to limit magnification for any reason. Saturn looks pretty good at 260x with my Mak but it is a bit grainy, so pulling back to 208x is a nice compromise.  I live in the greenbelt, so LP isn't a huge problem, but sometimes conditions warrant a variety of magnifications. The only thing that concerns me is the length it adds to the eyepiece when placed into a diagonal. That Luminos is 3/4 of a pound!