Barlow or not?

[Lorne]

Newbie so please be gentle!! I understand the concept of a Barlow Lens for multiplying the magnification, ie 20mm to 10mm in the case of a 2X etc, can anyone tell me benefits or downsides of using a Barlow on, for example, a 20mm lens instead of just using a 10mm lens.

Thanks, L.

[LukeSkywatcher]

Hi Lorne. Good question.

Here is MY take on Barlows:

The only reason i can think of to own a Barlow is to do with size of the exit pupil and eye relief.

As you increase the power of the EP (say from 20mm down to 10mm), the size of the lens in the EP you use gets smaller. By the time you get to using a 4mm EP the size of that lens is pretty SMALL.

The Barlow allows you to observe through an EP with a good exit pupil size (say 8mm or 6mm) and give you the same result as if you were looking through a 4mm or a 3mm EP...............but it is more comfortable and easier.

This all depends on the quality of the Barlow though. Some are good, some bad,some GREAT.

I have a TAL 2X barlow and it really is top notch for its price (about £30).

Some people hate using a barlow because it introduces another piece of glass between your eye and your target and that is true and understandable. However a GREAT/GOOD barlow should seem "invisible", you really should not be aware that it is there. It should have no ill effect on the overall view you see.

A bad Barlow will make images fainter and fuzzy.

I may be wrong but this is just my experience with Barlows.

My 1st barlow was a 2x Celestron that came in an EP kit that i bought.

Maybe the barlow in the kit i bought was a "Monday morning" barlow.

I am in love with my TAL 2X

[swamp thing]

Hi Lorne

As long as it is a quality Barlow there is no downside to using a Barlow.

The only foreseeable problem is with eye relief on some longer f length eyepieces.

A Barlow increases eye relief which for short and medium f length eyepieces generally makes them more comfortable.

However on longer f length eyepieces with long eye relief it can make it a bit too long.

However unless the eyepiece/s you intend using have extremely long eye relief in the first place there is usually no problems.

Best to check the eye relief of the eyepieces you are intending to Barlow.

Regards Steve

[Lorne]

Thanks for that Paul & Steve

[LukeSkywatcher]

However on longer f length eyepieces with long eye relief it can make it a bit too long.

Would that lead to "tunnel vision"?

[sixela]

No, it leads to blackouts because eye placement is more difficult.

[swamp thing]

It can move the exit pupil far enough on some eyepieces to cause vignetting. Even so It can be overcome by using a Barlow interface. which will bring the exit pupil back to an acceptable position.

Regards Steve

[Merlin66]

The exit pupil is fixed by the telescope aperture and the magnification ie exit pupil = aperture/ magnification.

So a 150mm scope at x150 would have an exit pupil of 150/150 = 1mm.

It doesn't matter how you achieve the magnification (ie barlow or not) the answer is the same.

HTH

[LukeSkywatcher]

It doesn't matter how you achieve the magnification (ie barlow or not) the answer is the same.

So if i understand this correctly there is no benefit whatsoever, there really is NO benefit to using a barlow?

They must be good for something otherwise they would not have been invented.

The view is the same but i would rather look through an 8mm EP with a 2x barlow instaead of looking through a 4mm EP without a 2x barlow.

So it must be a comfort thing.

[swamp thing]

I didn't say it altered the size i said it moved the exit pupil.

Regards Steve

[LukeSkywatcher]

I didn't say it altered the size i said it moved the exit pupil.

Regards Steve

No it doesnt alter the size..........it moves "improves" the exit pupil.

It makes observing an object more comfortable while maintaining same size....................which the brain is much happier to deal with.

[Merlin66]

The size of the exit pupil won't change ie at whatever magnification; but the eye relief will... the position of the "exit pupil" behind the eyepiece. Using a barlow allows a more comfortable view with higher magnification and obviously doubles your eyepiece/ magnification options.

[swamp thing]

I think we're all trying to say the same thing here but maybe in different ways:D

That's maybe why i only got CSE English.

[Lorne]

Thanks for all the replies, I didnt mean to stir things up so much on my first day!!

[pvaz]

They must be good for something otherwise they would not have been invented.

They are good to save money. With 1 barlow you double the amount of magnifications you got. When imaging planets you also need barlows to have a long enough focal length on the scope and get a nice FoV to frame the subject.

Long fl EPs (25mm++) sometimes get the eye relief so far up you need to keep your eye far from the rubber eye guard. This makes it hard to place it on axys with the EP and causes annoying blackouts. I got that on my 31mm EP. Had to buy a barlow interface to solve it.

[great_bear]

One other important thing: Since Barlows make the effective focal ratio of a scope slower, they can offer dramatically improved eyepiece edge-of-view performance compared to using a simple-design eyepiece on its own.

This can make a considerable improvement to eyepiece astigmatism, which is often often misidentified as "coma".

[LukeSkywatcher]

I think we're all trying to say the same thing here but maybe in different ways:D

That's maybe why i only got CSE English.

I think we are.

Im pretty sure i am confusing exit pupil with eye relief.

[LukeSkywatcher]

They are good to save money. With 1 barlow you double the amount of magnifications you got. When imaging planets you also need barlows to have a long enough focal length on the scope and get a nice FoV to frame the subject.

Yes they are good at saving you money also. A single Barlow can cost you half of what you would pay for 2 EP's (say a 20mm and a 10mm) and deliver the same result.

I did think of posting this benefit but decided against it.

[great_bear]

Im pretty sure i am confusing exit pupil with eye relief.

Yup

• Eye Relief: How far from the eyepiece your eye needs to be to collect light-beams from all parts of the image.
  
• Exit Pupil: The width of those light beams.
  

[Moonshane]

The other thing to consider with barlows is that it's another link in the chain when fiddling in the dark.

Also, they tend to make the eyepiece stick out a little far for my liking.

These are the main reasons I sold mine.

They can be used for the barlowed laser method of collimating the primary if you have a newtonian reflector.

You can usually unscrew the lens element and screw it in like a filter - this usually (with a 2x barlow) gives 1.5x increase in magnification. If you use a diagonal, the usual place to insert a barlow is between the diagonal and the eyepiece. Inserting it between the focuser and the diagonal effectively makes it a 3x barlow. Using these methods you can assess your proposed next eyepiece purchase and see if the magnification it would give will suit your kit. Eg. If you have a 15mm eyepiece and are thinking about a 5mm, 7.5mm or 10mm eyepiece next, you can get all of these with a diagonal and 2x barlow using the above methods.

Personally though I'd prefer another couple of eyepieces to a barlow.

Hope this gives a few more pros and cons

[LukeSkywatcher]

Long fl EPs (25mm++) sometimes get the eye relief so far up you need to keep your eye far from the rubber eye guard. This makes it hard to place it on axys with the EP and causes annoying blackouts.

Paulo, i am GLAD you brought this up now. I have experienced this with my 32mm EP (Celestron Plossl).

However, today i took ownership of an 8mm Vixen NPL EP and during a daylight test of the EP i found the same problem.

If i put my eye right up to the rubber eye guard i find that if i move my head/eye that i get the same annoying "blackout" as i get with the 32mm.

The eye relief on the 8mm Vixen is said to be 4.5mm (i think). I have used other 8mm EP's before and never suffered the "blackout".

Do you have ANY idea why the 8mm Vixen is showing this "blackout"........when other 8mm (or 6mm,4mm) EP's i own dont show it?

Maybe it wont be visible at night?

[LukeSkywatcher]

Yup

• Eye Relief: How far from the eyepiece your eye needs to be to collect light-beams from all parts of the image.
  
• Exit Pupil: The width of those light beams.
  

Ok that explains things. Thanks.

[great_bear]

Do you have ANY idea why the 8mm Vixen is showing this "blackout"........when other 8mm (or 6mm,4mm) EP's i own dont show it?

Maybe it wont be visible at night?

This is caused by one of two things:

• During daylight (as you know) your pupils contract, making it harder to hold your eye in the "sweet spot" when dealing with a small exit-pupil from the eyepiece.
  
• The eyepiece may be suffering from "Spherical Abberation Of The Exit Pupil"
  

Either way, test it at night before worrying

[LukeSkywatcher]

This is caused by one of two things:

• During daylight (as you know) your pupils contract, making it harder to hold your eye in the "sweet spot" when dealing with a small exit-pupil from the eyepiece.
  
• The eyepiece may be suffering from "Spherical Abberation Of The Exit Pupil"
  

Either way, test it at night before worrying

Thanks Jeremy. I wont worry about it until i do a dark test on it.

I'm pretty confident that it will perform as well as my other Vixen EP's. It was just something i noticed today during a daylight test. My other Vixen EP's are 25mm and 30mm and didnt show this during daylight testing.

But 25-30mm down to 8mm is a BIG step.

[pvaz]

Yes they are good at saving you money also. A single Barlow can cost you half of what you would pay for 2 EP's (say a 20mm and a 10mm) and deliver the same result.

I did think of posting this benefit but decided against it.

Well it depends on what EPs you're talking about. With a 2" Antares 1.6x barlow (for about 75£) I got 2 new focal lengths available (13 and 6mm) with my 2 Ethos. If I ware to buy a 13mm and a 6mm Ethos instead it would cost me 800£+ instead of 75£...