When to use a barlow?

[norm]

Hi All,

Can someone tell me when you would use a barlow? Is it simply to just double your magnification. When the magnification is doubled is the image quality diminished. I guess the latter question is dependant on the quality of the barlow and eyepeice.

Is a barlow more geared to planetary viewing or its just as versatile when veiwing DSOs ?

Lastly, what is the best barlow diameter to buy 2 inch or 1.25 or would you need both. Bit confused given 2 inch eyepieces seem more favorable.

Sorry if this has been re-hashed.

[davidpretorius]

A few situations.

I find a 2x barlow on my 12.5mm is more comfortable then viewing thru my 6.5mm. So a 2x barlow should be part of most cases.

When i image with my toucam, i put a 2.4x barlow on what is effectively 6mm to get whatever 6/2.4 is?

The big planet team use 5x barlows to get bigger images.

A 2" barlow should take a 1.25" adapter, so if you are thinking 2" ep's then go down this track

[rumples riot]

Davo not all the big planet team use a 5x. Some of us use a 2.5x.

Incidently, barlows can be used in several situations. For example, if you want to view a close double star, you can use a barlow to split the two stars. They can also be used for planetary viewing on nights of good seeing. And they can also be used to view galaxies and planetary nebulas. Their use is to primary divide the focal length of you EP's. This can mean that if you have EP's of high focal length a barlow will effectively double your EP's There is no need to get smaller EP's to cover the magnification gap.

In other words, they can be used for many purposes. They do however, require nights of good seeing to be very effective.

[davidpretorius]

Originally Posted by rumples riot

Davo not all the big planet team use a 5x. Some of us use a 2.5x.

argh yes, i keep forgetting the sct's @ f10.

Norm a scope at f5 ie newtonians (dobs are an example) with a 5x barlow is similiar to a f10 with a 2.5x

Rumples has a f10 scope and basically if he used a 5x barlow, jupiter would be bigger than the imaging screen!

[janoskiss (Steve H)]

I use my barlow when I do not have a usable EP that gives me a magnification close to what I'd get with the Barlow + existing EP.

On the downside: my Orion Shorty Plus Barlow (which is said to be pretty much as good as they get until you get into Powermates) decreases contrast, introduces some glare and stray light/reflections. The edge of the field of view seems a little bit "light polluted" (there is significant scattered light beyond the edge of field). Although the Shorty Plus is an apochromatic Barlow, it still seems to introduce some false colour on bright objects.

A Barlow increases eye relief, so long ER EPs become quite uncomfortable for me, but a barlowed short eye relief EP (like a 9mm Plossl) is more comfy barlowed.

[asimov (John)]

Yup, as Steve said. Some of my long ER EPs won't tolerate being barlowed.

[ving (David)]

if you dont have a EP suitable for planetary work (ie, 10mm or lower) you might want to barlow a 15mm.