Barlows 2x/3x/5x?! - Highest Practical Magnifications

[Brave Sir Robin]

Hello,

I'm considering getting a Skywatcher 200p as my next scope.

This is an f/5 scope compared to my existing f/8 scope and obviously planetary images will appear smaller, so I was thinking of investing in a decent barlow lens to give me some flexibility.

Now I know the power of the barlow will change the f ratio of the scope 2x barlow will change an f/5 to an f/10 and this will effect the apparent size of targets in the EP.

My questions are these:

I am aware that the highest practical magnification for a scope is roughly twice the diameter in mm - so woudl be 400x in the skywatcher.

1. Does a Barlow lens change the highest practical magnification, or merely change which eyepiece is required to acheive it?
2. What are the considerations to bear in mind with a 2x 3x or even 5x Barlow lens. As in all things astronomy, there MUST be a trade off. What is it?

I look forward to being enlightened....!

[John]

The highest practical magnfication limits appy whether using a barlow or not. Don't be misled though, 400x is not practical 99.9% of the time. 200x - 250x are more practical max magnfications in an 8" dob. Even with my 12" I find 250x - 300x as much as is worth using most of the time.

Scopes are not about maxing the magnification. Their main function is to gather as much light as possible. Much viewing, especially of deep sky objects, is done at low to medium magnfications so 30x - 80x or thereabouts.

[rwilkey]

Hi Brave Sir Robin, 2x is the normal mag for visual work, 3x and 5x is more for astrophotography applications.  But I agree with John above, you cannot achieve the theoretical max in any scope, and as John hints, much stargazing is done at low power, I generally or most often use 30x, which seems very low (with an ES82 30mm) but what you see in the sky is phenominal at that value.  Different targets are better at different mags, Jupiter for example is great at 150x, whereas Saturn is good at around 200x or a bit more. 

[Floater]

Uh oh! As is often the casel, I need help here, please.

I don't understand how the Barlow will change the focal ratio. Surely it's the simulated 'focal length' that is affected.

i.e. My scope has an aperture of 150 with a focal length of 1200 = f8. A 10mm eyepiece gives me a magnification of 120x. A 2x Barlow with the 10mm EP simulates a 5mm EP, which equals a mag of 240x. But it's still an f8 focal ratio, surely?

Robin, sir, if I have failed to understand your teaching, please forgive?

[Charic]

when you add a Barlow,  say a 2x,   you  effectively double the focal length of the telescope. If the Focal length, doubled, is divided by the aperture, the focal ratio changes too.

F-1200 / 200 = f/6

with a 2x Barlow, Focal length (on my telescope is now 2400)  

F-2400 / 200 = f/12

can even get f/24 and more on a Skyliner?

[John]

I think the effective focal ratio of the optical system changes too as Charic says. You can get focal reducers as well, eg: for schmidt-cassegrain scopes and these reduce the effective focal length with a corresponding reduction in the focal ratio, usually from F/10 to F/6.3 or even F/3.3.

[Floater]

Thanks for that, guys, but I'm not handling it well.

Charic, you agree that the Barlow, effectively, doubles the focal length of the scope. Surely that is what has an affect on visual observing and not a theoretical doubling of focal ratio? Unless that plays a part in the exit pupil?

Please understand I'm challenging no-one and nothing here, just trying to cement what I thought I had learned!

[RikM]

Focal ratio is unrelated to magnification. The effective magnification depends on the focal lengths of the eyepiece and telescope.e.g. My Pronto is 450mm f6.4 a 10mm eyepiece gives 45x magnification. My 250PX is f/4.7 but has a focal length of 1200mm. The same 10mm eyepiece gives 120x magnification. Nothing to do with focal ratio.

[Charic]

..no worries matey. 

If you insert a Barlow, you magnify the image. This involves moving the focal plane?  A different telescope with a longer focal length does the same,  it  too can give a higher magnification ( and depends on eyepiece).

So that you don't have to get a longer OTA, you can add a Barlow. This effectively doubles the telescopes focal length.

(This is what a 'Bird-Jones' type telescope achieves. by placing a corrector/Barlow lens at the base of the focuser tube, to make that tiny tube feel bigger than it really is?)

To get your focal ratio, you divide the telescopes focal ratio and the aperture.

If you Barlow, you  change the focal length of the telescope. If the focal length doubles, divided by the same aperture, the focal ratio has to double too.

Folk often say Buy 3 EP's and a Barlow, that completes your set! ie an 8, 12  & 32mm . If you Barlow those EP's you will get the equivalent of a 4, 6 & 16mm EP. The Barlow adjusts the focal point, with its optics, by virtually  changing the focal length of the telescope by whatever factor the Barlow is, ie 2x or 5x ect. making everything downstream of the Barlow to behave accordingly.

If someone tells you to buy a Barlow, as this will double your eyepiece collection, the Barlow is  only doubling the focal length of the telescope. It  plays no part with the eyepiece directly, as the barlow is upstream in the optical train.

By using the new figures, will give you the magnifications, so again on my telescope  of F-1200 D-200 using an 8mm EP I get  150x magnification. Now add my Barlow, my focal length is now F-2400  The new sum for the same 8mm EP is F-2400 / 200 = 300x magnification, So look at it as either you now have TWO EPS using a Barlow from one 8mm giving two mags of 150x & 300x. Or just consider that the Barlow has doubled my focal length. The results are the same.

But the fact is the Barlow  will change the telescopes focal length by the factor printed on that Barlow. The rest is as a result of fitting that barlow . Phew!

[JamesF]

I like to think of it this way...

One can imagine that there's a cone of light produced by the objective that comes to focus at the eyepiece.  In a fast scope this cone is fat and short.  In a slow scope it's long and thin.  The focal length of the telescope is the height of the cone, the tip being at the focal plane and the base being at the objective.

A barlow uses additional optical elements to lengthen the light cone, effectively by bending it away slightly from the optical axis of the OTA.  By doing this it increases the effective focal length of the entire system (because the "bent" cone is now longer overall) and as a side-effect makes the image larger at the focal plane.  Because the aperture remains the same, the effective focal ratio also changes.

This is a bit simplistic, because optics is a little more complicated than that (think of a 127 Mak, where the effective focal length can be 1500mm without the light travelling anywhere near that far even if it is "folded over" on itself), but I think it conveys the general idea in a not-too-inaccurate fashion.

James

[Floater]

I know that focal length divided by aperture equals focal ratio. My point is that the 2x Barlow doubles the magnification provided by a particular eyepiece in a particular telescope.

Perhaps it's a case of saying the same thing in a different way - but with the same eyepiece in the same telescope, the Barlow doubles magnification, and only theoretically doubles focal length. An f8 scope is still an f8 scope; just the eyepiece focal length has been, effectively, halved with the Barlow in the 'train'.

[JamesF]

Perhaps it's a case of saying the same thing in a different way - but with the same eyepiece in the same telescope, the Barlow doubles magnification, and only theoretically doubles focal length. An f8 scope is still an f8 scope; just the eyepiece focal length has been, effectively, halved with the Barlow in the 'train'.

It genuinely changes the focal length, but of the entire optical system, not the telescope on its own.  In fact it's probably better to say that adding a barlow creates a new optical system with a new focal length dependent on the barlow multiplier and the original optical system to which it has been added.  When push comes to shove, we're not really worried about the characteristics of the OTA, but the characteristics of the optical system as a whole.

James

[John]

When you use a barlow lens or focal extender the primary mirror or objective lens has not changed it's figure, agreed. But there is sometimes more to the focal ratio and focal length of a scope than just that of the primary mirror or objective lens.

Edit: I think James is saying the same thing.

[Charic]

I know that focal length divided by aperture equals focal ratio. My point is that the 2x Barlow doubles the magnification provided by a particular eyepiece in a particular telescope.

Perhaps it's a case of saying the same thing in a different way - but with the same eyepiece in the same telescope, the Barlow doubles magnification, and only theoretically doubles focal length. An f8 scope is still an f8 scope; just the eyepiece focal length has been, effectively, halved with the Barlow in the 'train'.

Your right. Thats how we understand it. A Barlow can turn one lens into two, ie doubles the magnification. 

But in reality, only the telescopes focal length is actually changed. When you insert an 8mm EP. its always going to be an 8mm eyepiece. FULL STOP

But that 8mm is 150x on my telescope

Its the same 8mm ,  but now its 300x?

The EP hasn't changed, it's still an 8mm, don't forget that, The Barlow has done nothing to the eyepiece, its still an 8mm EP.  Nothings simple or easy to fathom sometimes, even I've been caught out on certain issues.  That 8mm will always be 8mm. Check this out, put your 2x Barlow in the focuser, now look at the print on the EP.........still says 8mm? Thats the way it is   Your telescope has changed! and thats how you should view(excuse the pun) the workings of a Barlow.

It will become apparent, you just gotta think the other way, but until it sinks in, there's  still doubt. But we can't all be wrong.....can we!

[Floater]

Okay, guys, I'm cool on this.

The telescope focal length is doubled or the eyepiece focal length is halved. The effect is the same and we both see the same end result - but think of how we got there in different ways.

Result.

Peace.

[Charic]

Sorted............Lol   Thats the fun of forums, especially this one. 

Finally Floater, why have we still got  total pants  weather up here, I've not seen a clear night in ages, and I've got my new 6mm WO SPL to test?

[Floater]

Sorry, Charic, but having read your edit I have to add this:

With the eyepiece in, it still says fr8 on the side of my scope ...l

Wish I could be more sure of the weather. :-)

[JamesF]

With the eyepiece in, it still says fr8 on the side of my scope ...l

It does, but that refers only to the objective lens.  Once you start adding additional optical components the game changes entirely.

For me, thinking of the barlow as changing the focal length of the eyepiece doesn't really make good sense.  I can put a camera which has no focal length into an OTA.  If I then add a 2x barlow it can't change the focal length of the camera, but the image still gets bigger.  It makes more sense to think of it as a component of the optical system into which you're putting the eyepiece (or camera).

James

[Floater]

Thanks James, Robin, Rik, John and Charic.

I'm done. Over and out.

Now, what was that about the weather ...?

:-)

[Charic]

.......back to the OP.

If you buy a Newtonian, Take its Aperture as your practical  magnification limit.  So on my 200P Skyliner F-1200 D-200 I see 200x as my maximum.......But hold on! I often use 300x on the Moon, but as the Moon is so bright and large and near, its not a problem. (in fact the problems for some folk with their Dobs is the tracking speed required at higher magnification)

Now take the  Skyliner to the vacuum of space, and 400x could be achieved, as there is no weather or atmospheric conditions to look through. This is the deciding limit on your telescopes ability.

If you fit a Barlow, doubling your telescopes focal length, you will reach your limits far quicker than you expect, due to the restrictions of the atmosphere.

In answer to your question 1, magnification is the result of telescopes focal length divided by eyepiece focal length. As I said to Floater, the 8mm still remains an 8mm, whether you fit a Barlow or not. Its just that the same 8mm will have a much higher magnification of 300x with a Barlow fitted, due to the increased focal length. Its up to you after that, to decide how much magnification you need or can achieve under your conditions, and EP dependent.

Any further magnification, obviously brings you nearer to the telescopes limits much quicker, and as you go higher in magnification, the field of view reduces, and the image gets darker, but sometimes with better contrast.

You don't need a 5x Barlow if you have a 2x & 3x Barlow, as these can be stacked also, but you won't get exactly 5x from 3&2?

lastly a bigger telescope will provide a larger image scale. If I were to win the lottery, I would buy me a 12" f/5 solid tube Skyliner and be done with it. I'm f/6 now, and don't fancy the issues associated with the 10" f/4.8?

[Floater]

I return to this only to apologise, Brave Sir Robin, for having stolen your thread - or at the least, having frayed it!

I'm feeling guilty.

Also, thanks to the responders who took the trouble to clarify the situation. It was James's post about introducing a camera to a barlowed scope which made me realise that my 'method' of halving the EP's focal length was intellectually slovenly.

Hope the to-and-fro didn't spoil it for you, Brave Sir Robin. :-)

Clear Skies.

[Charic]

Sorry, Charic, but having read your edit I have to add this:

With the eyepiece in, it still says fr8 on the side of my scope ...l

Wish I could be more sure of the weather. :-)

.......I was being a little facetious in  thread #14. Just trying to reiterate that the Barlow only changes the optical system, and the end result is a different magnification for the same eyepiece fitted. Nothing actually changes on the eyepiece or telescope in regards to print/text or otherwise? 

Many folk will still say that a Barlow halves the focal length of the eyepiece,increasing magnification, and sometimes bettering the eye relief, its  a solution for having 'two' eyepieces for every one, if fitted to a Barlow.

That's the popular/easy  way of thinking!

[Brave Sir Robin]

Thank you all for your posts, sorry I've been a bit quiet, I was out of the country.

Brave sir Robin ran away. Bravely ran away-away.

[Brave Sir Robin]

All really informative, thanks guys! And no floater, no problem :-)

Looks like I'll be gettin' me a Barlow then.

Now...... Which brand should I get......?

*lights blue touch paper and runs*

Brave sir Robin ran away. Bravely ran away-away.

[Charic]

Sir Robin, I have one of these http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-deluxe-2x-barlow.html

Check  the small print, several options available from the one Barlow?