Using a Barlow

[Beagleboy]

S'mee again with another newbie question that'll have you all smacking hands to foreheads.....I know, I know....

Having been pretty astounded by the views of the Moon I was treated to on Monday night, I found I wanted more. So I ordered a Revelation x2.5 Barlow and a Moon filter on Wednesday morning (I can still see Monday night's Moon at the back of my retina!), and they were sitting on my doorstep when I arrived home from work yesterday. Woohoo!

Last night was beautifully clear so I excitedly setup Dobby, my Skyliner 200p in the back garden. Of course, that's when I discovered that the Moon's now well hidden behind my neighbours houses. Poop! So I didn't get a chance to play with the Moon filter, but I still thought I'd have a play about with the Barlow. I swung the scope around and sought out the Ring Nebula....because it's the easiest thing I can find at the moment and focussed in on it. I then popped the Barlow in place and refitted my 25mm eyepiece. When I tried to refocus on the Nebula I found that I was having to wind the focusser all the way in and felt like I was reaching the stops just before the background stars were in focus.

I wasn't expecting the Ring to be in sharp focus, and to be honest it looked about the same as it does in the un-barlowed 10 mm eyepiece, what's concerning me is that I didn't feel like I had enough travel in the focusser.

Am I just using the Barlow on a subject that it's not really suited to? When the Moon or planets finally make an appearance at a height where I can get an unobstructed view, will the focal point be different to that of DSO's?

I suppose I could just be using too much magnification....although the Barlow and 25mm should still be less magnification than the 10mm eyepiece on it's own, yeah?

I didn't want to be messing around with stuff in the dark in case I was doing something fundamentally stupid, so I ended up putting the Barlow away, swung the scope around towards Cassiopeia and found the Dragonfly cluster.....Wow, how beautiful is that? Absolutely mesmerising stuff.

Thanks in advance for any help, and apologies for any forehead handprints.

Craig xx

[John]

The barlow plus the 25mm eyepiece  will equal a 10mm eyepiece - so the view should be just the same as with your 10mm eyepiece although you should have a bit more eye relief. The "shorty" format barlows such as the Revelation 2.5x do require some inwards movement of the focuser to reach focus.

[laudropb]

I use the revelation Barlow with my dog and 25 mm eyepiece and have never had a problem.

[Beagleboy]

Right, so just to make sure I'm not doing anything foolish. I've got the 1.25 adaptor in place with the revelation barlow slotted as far in as it'll go then snugged up. Into this I'm slotting the x25 eyepiece, again as far as it'll go. Now to get things into focus I'm having to wind the focusser all the way in, till I'm only maybe a mm of a turn or so from the minimum stop. Is this how it works for you Laudropb? I'm still not sure if this is just normal when trying to focus through a barlow on a DSO, not even sure if I should be trying to focus on a DSO through a Barlow, or if the things are really only suited to planetary viewing, which to be honest is what I got it for in the first place!

Oh, and I'm glad your dog gets on well with it, but I'm more of a cat person ;-)

C. :-)

[John]

Barlow lenses will work just as well with those deep sky objects that benefit from more magnification (such as the ring nebula) as they do with any other type of object.

Assuming that you have not got the 2" eyepiece adaptor in place at the same time, you seem to have things in the right order.

As I said earlier, you will need to alter the focus position with the barlow in place and usually it's inwards.

Used with the 25mm eyepiece the 2.5x barlow delivers the quivalent to a 10mm eyepiece so, apart from the additional inwards focuser movement, the view of the Ring Nebula will look the same as it does with the 10mm eyepiece on it's own.

[Charic]

When you fit a Barlow of say 2x it increases the focal length of your telescope by x2, so your Skyliner goes from a normal 1200mm focal length to a 2400mm focal length, the focal ratio also changes from f/6 to f/12

Using the formula of dividing the telescopes focal length by the eyepiece focal length, you will see that normal, a 6mm EP produces 200x power. Now place the 2x Barlow in your focuser and your Skyliner is now 2400mm giving 2400/6=400x Your still using a 6mm EP, but because your effectively using a longer focal length, it produces a higher magnification from the same EP.

Your set up sounds ok, just ensure there is only one adaptor in the focuser tube? The is a 2" adaptor that some folk leave in situ, then they insert the second smaller adaptor! Its one or the other, not both.

I have a Moon filter somewhere, but dont use it on the bright Moon. I often leave the OTA dust cap in place, and just remove the small 2" cap. I know this reduces the aperture, but on the bright Moon, does that really matter to me........nope!

[Stub Mandrel]

Focus on a nearby star, by definition a 'nebula' has no sharp edges.

[LukeSkywatcher]

This is a puzzle. I read everything and what everyone says about the barlow is correct. Your set up (optically) seems correct also. I cant think of a good reason why your scope wont focus properly. I can only say to try instead of pushing everything in as far as they go snugly........to try leaving them pulled back (towards you) a small amount. I'm not sure really but this may bring things to focus just before you have to turn the focuser the full way round.

It might have no effect at all.  

Another thing maybe worth trying. if your revelation barlow is like mine (mine is a 1.5-2x), you may be able to remove the bottom part of the barlow to make it a 2x instead of a 2.5x. This may give you a bit more focusing space. 

[Beagleboy]

So, I pointed the telescope out my back window during daylight today and focussed on a distant pylon. Popped the Barlow in with the 25mm eyepiece and got it to focus no problem, with around 1-2mm of travel left in the focussing rack. I then popped the 10mm eyepiece in, and it focussed with a much greater range ( 5 or more mm) left in the focussing rack. So it seems my initial thought that I couldn't get the thing to focus was just me talking rubbish....sorry. Although adding the Barlow does seem to push the focal point right to nearly the very limit on the focus rack.

I'm assuming then that it is simply that the 25mm eyepiece is pushing the range limits for use with this particular Barlow?

Cheers for all the help!

[Stu]

So, I pointed the telescope out my back window during daylight today and focussed on a distant pylon. Popped the Barlow in with the 25mm eyepiece and got it to focus no problem, with around 1-2mm of travel left in the focussing rack. I then popped the 10mm eyepiece in, and it focussed with a much greater range ( 5 or more mm) left in the focussing rack. So it seems my initial thought that I couldn't get the thing to focus was just me talking rubbish....sorry. Although adding the Barlow does seem to push the focal point right to nearly the very limit on the focus rack.

I'm assuming then that it is simply that the 25mm eyepiece is pushing the range limits for use with this particular Barlow?

Cheers for all the help!

Is that 1-2mm away from being fully racked in or out?

[Stub Mandrel]

Fixing a 400mm telephoto lens recently i discovered that the difference between a 'distant pylon' and 'infinity' is not negligible, you may find that you aren't quite reaching focus with the 25mm when pointed at the sky, but at least you now have a good starting point to work from.

I've been tempted to put a few marks on my focuser tube with a sharpie for different setups to at least get me in a ballpark where I can find a decent star  to focus on without wasting time.

[Beagleboy]

It's 1-2 mm away from being fully racked in Big Sumorian.

[Stu]

It's 1-2 mm away from being fully racked in Big Sumorian.

You will probably still find (as Stub Mandrel says) that the difference in focus position between a distant pylon and a star will be enough that you can't quite get focus using the Barlow.

[Piero]

When you fit a Barlow of say 2x it increases the focal length of your telescope by x2, so your Skyliner goes from a normal 1200mm focal length to a 2400mm focal length, the focal ratio also changes from f/6 to f/12

Using the formula of dividing the telescopes focal length by the eyepiece focal length, you will see that normal, a 6mm EP produces 200x power. Now place the 2x Barlow in your focuser and your Skyliner is now 2400mm giving 2400/6=400x Your still using a 6mm EP, but because your effectively using a longer focal length, it produces a higher magnification from the same EP.

Hi Charic, 

Just a note. Although what you said is fine for calculating the magnification of an eyepiece when a barlow 2x is used, the way a barlow 2x is not exactly as you say. 

A barlow does not change the telescope focal length "transforming" the latter in a telescope with a double focal length. A 1200mm focal length telescope remains as it is, as well as the telescope secondary obstruction, if this is present, and all the other telescope features (e.g. mirror / lens shape). 

The effect of a barlow is at eyepiece level, and more precisely, it moves the focal plane, which is the plane where the image is formed, ahead of a determined factor, which is the multiplier factor reported in the barlow. For instance, if this factor is 2x, the distance between the eye and the focal plane becomes half. That is why you see a magnification of 2 times with respect to the eyepiece without a barlow. 

Of course, for calculating the new magnification, you can think in terms of doubling the telescope focal length for simplicity.

Piero

[Stub Mandrel]

One interesting aspect of Barlows is that if you move the eyepiece further out, they become stronger. I have an x2 barlow that effectively becomes an ~ X2.5 when used with a camera, because of a greater distance to a the camera focal plane. Add an IR filter behind the barlow lens and it becomes a bit stronger again.

I have an M42 auto teleconverter that works this way, it had a second screw in element that pushes the lens elements forwards and also moves the main part further from the camera.

[Charic]

Hi Charic, 

Just a note. Although what you said is fine for calculating the magnification of an eyepiece when a barlow 2x is used, the way a barlow 2x is not exactly as you say. 

A barlow does not change the telescope focal length "transforming" the latter in a telescope with a double focal length. A 1200mm focal length telescope remains as it is, as well as the telescope secondary obstruction, if this is present, and all the other telescope features (e.g. mirror / lens shape). 

The effect of a barlow is at eyepiece level, and more precisely, it moves the focal plane, which is the plane where the image is formed, ahead of a determined factor, which is the multiplier factor reported in the barlow. For instance, if this factor is 2x, the distance between the eye and the focal plane becomes half. That is why you see a magnification of 2 times with respect to the eyepiece without a barlow. 

Of course, for calculating the new magnification, you can think in terms of doubling the telescope focal length for simplicity.

Piero

...........Of course, for calculating the new magnification, you can think in terms of doubling the telescope focal length for simplicity.  Why  if the Barlow does nothing to the focal length?

The Barlow lens works by reducing the convergence of the light cone heading toward the telescope eyepiece, so the work is already done?  In this way it essentially increases the focal length of your telescope, and Since magnification is determined by dividing the telescope’s focal length by the eyepiece’s focal length, you can see that by doubling the telescope’s focal length, a Barlow lens doubles the magnification of the system for a given telescope eyepiece.

Dont just think that a Barlow doubles the magnification of your eyepiece alone without affecting the telescope! 

[CraigT82]

...........Of course, for calculating the new magnification, you can think in terms of doubling the telescope focal length for simplicity. Why if the Barlow does nothing to the focal length?

The Barlow lens works by reducing the convergence of the light cone heading toward the telescope eyepiece, so the work is already done? In this way it essentially increases the focal length of your telescope, and Since magnification is determined by dividing the telescope’s focal length by the eyepiece’s focal length, you can see that by doubling the telescope’s focal length, a Barlow lens doubles the magnification of the system for a given telescope eyepiece.

Dont just think that a Barlow doubles the magnification of your eyepiece alone without affecting the telescope!

Please excuse me for being stupid but how can a Barlow have an effect on the telescope? Surely the only thing the Barlow can influence is the eyepiece?

Sent from my iPhone using Tapatalk

[Stu]

The Barlow doesn't affect the telescope at all, but it does alter the steepness of the light cone as seen reaching the eyepiece. If you project this back then it appears that it is lengthening the focal length of the telescope.

https://en.m.wikipedia.org/wiki/Barlow_lens

Piero is right in that it does not alter fundamentals such as the secondary obstruction percentage. I believe it would however reduce any astigmatism in an eyepiece because the light cone is less severe and put fewer demands on the eyepiece optics.

[John]

My understanding is that a barlow lens will reduce astigmatism where it's source is the eyepiece, as it often is. A barlow will not reduce chromatic aberration or coma though, where those are generated by the primary mirror or objective lens, as they mostly are.

[Beagleboy]

Soooooo.......

Does it sound right that when fitting the barlow and using my 25mm eyepiece that I'm right on the limit of the minimum focal adjustment of my Skyliner 200p. The last few posts weren't quite clear on that matter....

:-)

[Stu]

My understanding is that a barlow lens will reduce astigmatism where it's source is the eyepiece, as it often is. A barlow will not reduce chromatic aberration or coma though, where those are generated by the primary mirror or objective lens, as they mostly are.

Yep, agreed

[Stu]

Soooooo.......

Does it sound right that when fitting the barlow and using my 25mm eyepiece that I'm right on the limit of the minimum focal adjustment of my Skyliner 200p. The last few posts weren't quite clear on that matter....

:-)

Apologies if this drifted a little Beagleboy.

I've not used your particular scope but I'm sure someone will be along soon who has

[Charic]

The Barlow doesn't affect the telescope at all, but it does alter the steepness of the light cone as seen reaching the eyepiece. If you project this back then it appears that it is lengthening the focal length of the telescope.

https://en.m.wikipedia.org/wiki/Barlow_lens

Piero is right in that it does not alter fundamentals such as the secondary obstruction percentage. I believe it would however reduce any astigmatism in an eyepiece because the light cone is less severe and put fewer demands on the eyepiece optics.

.....your link provides this from the first sentence, " 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".......it goes on.

There seems to be many ways to interpret how a Barlow works, and most just settle for the doubling of the power of the eyepiece, yet the theory is deeper, and not everyone is getting it right, or fully understands the theory! and possibly me still  I just want to get it right, but it 's not the first time this topic has risen?( but until proven that I`m totally  wrong, I`ll be sticking to this theory, but I can be taught!) However, if you put a Barlow between the primary mirror/objective and the eyepiece, it increases the effective focal length of the light path, you have already said that,  the steepness of the light cone has been altered.

Therefore, I say, without even putting an eyepiece in the barlow, you have affected the focal length somewhat by changing the  steepness of the light cone,  not physically by another 1200mm by  using a 2x Barlow, but virtually,  so the telescopes physics must have changed somewhat?

Focal length divided by Fe 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, I must have a Focal length of 2400  to get 400x from a 6mm EP, a doubling of the focal length  by a factor of 2x produced from the Barlow? 

 Will start a new thread!

[Stu]

.....your link provides this from the first sentence, " 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".......it goes on.

There seems to be many ways to interpret how a Barlow works, and most just settle for the doubling of the power of the eyepiece, yet the theory is deeper, and not everyone is getting it right, or fully understands the theory! and possibly me still I just want to get it right, but it 's not the first time this topic has risen?( but until proven that I`m totally wrong, I`ll be sticking to this theory, but I can be taught!) However, if you put a Barlow between the primary mirror/objective and the eyepiece, it increases the effective focal length of the light path, you have already said that, the steepness of the light cone has been altered.

Therefore, I say, without even putting an eyepiece in the barlow, you have affected the focal length somewhat by changing the steepness of the light cone, not physically by another 1200mm by using a 2x Barlow, but virtually, so the telescopes physics must have changed somewhat?

Focal length divided by Fe 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, I must have a Focal length of 2400 to get 400x from a 6mm EP, a doubling of the focal length by a factor of 2x produced from the Barlow?

Will start a new thread!

Charic, I was basically agreeing with you!

There is no simple answer, for instance as we have said adding a Barlow doesn't suddenly get rid of CA and coma which it would do if it truly changed the focal length of the scope.

What it does do is make the scope appear to have a longer focal length as seen by the eyepiece in terms of the steepness of the light cone.

I think a new thread is a good idea!

[Stub Mandrel]

Sounds like counting angels on pinheads!

I would say that, as the barlow goes between the primary and the point of focus, it acts to effectively increase the focal length of the primary. To have an effect on the eyepiece it would need to be between the eyepiece and the focal point, in which case it would increase the focal length of the eyepiece and reduce the magnification.

Soooooo.......

Does it sound right that when fitting the barlow and using my 25mm eyepiece that I'm right on the limit of the minimum focal adjustment of my Skyliner 200p. The last few posts weren't quite clear on that matter....

:-)

Yes.