3x Barlow query

[MahlerMoonMan]

Hi folks

I'm just wondering, how good optically is a 3x Barlow? Particularly the tele vue 3x? I'm thinking of barlowing my 10mm Pentax XW to give me 180x on my ED80 f7.5 .... A TV 3x Barlow is a good deal less costly than the Pentax XW 3.5mm EP! But what do you more experienced observers think: 3x Barlow + 10mm XW, or non-Barlow 3.5mm XW EP ..... Which option will be optically superior on an ED80 scope?

Many thanks,

Donaldo

[John]

Like all optical items, the quality varies and, generally, the more you pay the better the quality gets. Tele Vue barlows are about as good as they get though so I don't see any reason why the results with the 10mm XW should be unsatisfactory. Barlows do extend the eye relief of an eyepiece though so you will probably need to move the eye cup of the XW outwards a bit to find the new position - you may experience some "kidney beaning" until you find the new eye position. Another approach would be to invest a little more in a Tele Vue Powermate 2.5x. These have no effect on eye relief and really seem to just "get out of the way" leaving just the higher magnification.

Whether either approach will deliver image quality quite as good as using a shorter focal length XW would is difficult to determine as the XW's are such fine eyepieces but I reckon the results will be pretty close.

[JamesF]

Is 180x asking a bit much of the ED80 to start with?

James

[KaStern_Former_Member]

Hello Donaldo,

I do not think that it is a good Idea to push the magnification that high.

Please remember that you will be able to see the airy disc

http://en.wikipedia.org/wiki/Airy_disk

as a tiny disc when magnification is about 1mm exit pupil or slightly higher.

0.7mm exit pupil is a good number where the visible unsharpness caused by diffraction

and image scale is well balanced for observers with normal eyesight.

0.7mm exit pupil translates to 114x

Therefore I would rather recommend to get a 2x barlow.

It would give you about 120x wich is a very reasonable magnification for a 80mm scope.

John did mention some very good points.

A barlow will alter the way the rays go through the eyepiece.

The consequences will be an enlarged eye relief

and for off-axis ray bundles vignetting in the eyepiece can result.

Extended eye relief is helpful when you use short focal length simple typ eyepieces

like Ploessl, Ortho, Kellner, monocentrics and so on.

But it is not when you use eyepieces with long eye relief.

The exit pupil is moved further outward and you will have difficulties to position your eye

since the mechanics of the eyepiece will not support the eye anymore.

The Pentax XW like the XL or the Vixen LVW sipports 20mm eye relief.

The effect depends on the barlow magnification factor and the barlow "focal length".

The shorter the barlow and the stroger the added magnification the further is the exit pupil moved out.

Both the above problems are avoided by the powermates.

You may try a longish conventional 2x Barlow too.

The Televue is one example. There was a baffled 2x Barlow with 2 elements made in Japan

(discontinued as far as I know) distributed by Orion in the US wich may be useful too.

But if I were you I would rather try to get a used 5.2mm Pentax XL or 5mm Vixen LVW

than a new Barlow or Powermate.

Cheers, Karsten

[bomberbaz]

Is 180x asking a bit much of the ED80 to start with?

James

The spec from manufacturer state x160 is the highest practical magnification, that is a 3.8mm EP.

A 3.33 EP (10mm x3 barlow combo) gives you the 180 but that also take the exit pupil very low at 0.44, so i am not really sure how this is going to affect viewing as reports vary. Some saying anything below 0.6 is no good other saying as low as 0.4. Perhaps someone with more experience can help.

A 2.5 barlow as suggested by John would give you x150 and a 0.53 exit pupil, I reckon that is a good option imho.

[Naemeth]

Not sure if this is particularly helpful or not, but with my scope I can notice, especially in poor seeing, eyepieces with an 1.6mm exit pupil (8mm) losing a small amount of sharpness to the atmosphere. At a 0.5mm exit pupil, the view often loses a lot of the sharpness it had at a 1.6mm exit pupil. When the conditions are right for it, a 0.5mm exit pupil can prove useful, but don't count on using it.

I personally think the Nagler 3-6mm zoom would be far better, giving you 0.8mm - 0.4mm exit pupil, not that you will use the 0.4mm exit pupil all that much mind.

[MahlerMoonMan]

How do you work out an exit pupil again? It looks like I'll not be bothering with a 3x Barlow or a Pentax XW 3.5mm then! if the image quality is going to likely be so feint and fuzzy .... Not much point with my current ED80. As I've got a BST 8mm ED and an Orion 'Shorty' 2x Barlow, that combination will give me x150 .... Maybe that's my sensible practical limit? I don't really know very much about 'exit pupils' and what it really means but I just thought I would - when I have a really clear sky - be able to push my ED80 beyond x150 ....

So much to learn and understand! And all I really want to do is see the stars and planets!

Thanks for all the advice so far

Donaldo

[Naemeth]

How do you work out an exit pupil again? It looks like I'll not be bothering with a 3x Barlow or a Pentax XW 3.5mm then! if the image quality is going to likely be so feint and fuzzy .... Not much point with my current ED80. As I've got a BST 8mm ED and an Orion 'Shorty' 2x Barlow, that combination will give me x150 .... Maybe that's my sensible practical limit? I don't really know very much about 'exit pupils' and what it really means but I just thought I would - when I have a really clear sky - be able to push my ED80 beyond x150 ....

So much to learn and understand! And all I really want to do is see the stars and planets!

Thanks for all the advice so far

Donaldo

Exit Pupil = Eyepiece Focal Length / Telescope Focal Ratio

Or

Exit Pupil = (Aperture / Telescope Focal Length) * Eyepiece Focal Length.

Real example with both, ED80, F/7.5, FL 600mm, Aperture = 80mm, Eyepiece Focal Length = 6mm

1) Exit Pupil = 6mm / 7.5 = 0.8mm

2) Exit Pupil = (80/600) * 6 = 0.8mm

HTH

[KaStern_Former_Member]

Hello Donaldo,

exit pupil diameter = entrance pupil doiameter devided by magnification.

Your entrance pupil sice is 80mm, with your 10mm Pentax XW you get

600mm : 10mm = 60x

So the resulting exit pupil diameter is 80mm : 60x = 1,333mm

You can make an experiment:

Take your scope and aim it twoards the distant horizon, put in your eyepiece and focus it.

The step back a meter and look at the eyepiece.

You will see a brightly illuminated disc of 1,333mm diameter.

That is the area where the light exits the eyepiece.

The light is parallel light, like from a very distant object.

This light is captured by your eye and then focussed onto the retina.

There are light sensitive cells located.

The way they are packed can differ slightly from individual to individual.

" As I've got a BST 8mm ED and an Orion 'Shorty' 2x Barlow, that combination will give me x150 "

Try it. You can aim the scope towards polaris, wich does not move.

Observe it carefully. Do you see it slightly disc-like, surrounded by a faint ring?

Or do you see it as a point-like object?

Try different magnifications.

The higher the magnification you need to see a star as atiny disc the worse are your eyes.

Try different magnification when observing the moon.

The moon will take relatively high mags because it has some strong contrast at the border between light and dark.

Jupiter all in all has low contrast features.

If you push magnification too high the contrast drops too low.

For me obseving Jupiter is best with magnification of about 0.8mm to 0.7mm exit pupil.

This is all individual. The phsics behind it is true for all people.

But the physiology can differ a bit from individual to individual.

So you should try out different eyepieces and magnification

and see what is best for you.

Cheers, Karsten