Please explain Barlows

[seer]

Could someone please explain how Barlows differ in use between a refractor and reflector? What works for one scope doesn't work for another. I know there are Shorty Barlows and longer ones.

[notFritzArgelander]

Shorty barlows are more convenient used with a diagonal. Apart from that, avoid low quality units that degrade the image.

[Don Quixote]

This is a good question Seer.
My comments are restricted to my experience as a visual observer. I have not used the barlow for AP.

My experience has been that they pretty much work the same way and give additional magnification according to the x-factor of the barlow lens. They are useful to maintain eye relief in a longer focal length eyepiece while augmenting the power by the factor of the barlow. I believe the exit pupil is also reduced.
There may be some change in the point of focus in the optical train while using the barlow, but I am not certain of this.
My input may not add much, but here it is for what it is worth.
Hopefully more experience observers will chime in.

I am also interested to learn more about this.

[Don Quixote]

Shorty barlows are more convenient used with a diagonal. Apart from that, avoid low quality units that degrade the image.

Thank you nFA. :-)

[seer]

Last night I tried a 2x Barlow that I got with one of my Meade refractors in my 76mm reflector and it would not work. The 76mm reflector came with a real junky 3x Barlow that overpowers almost everything.

[Don Quixote]

Last night I tried a 2x Barlow that I got with one of my Meade refractors in my 76mm reflector and it would not work. The 76mm reflector came with a real junky 3x Barlow that overpowers almost everything.

It will not come into focus then ?

[seer]

Last night I tried a 2x Barlow that I got with one of my Meade refractors in my 76mm reflector and it would not work. The 76mm reflector came with a real junky 3x Barlow that overpowers almost everything.

It will not come into focus then ?

I guess that might be the way to put it. It's kind like what you would see if you were trying to collimate.

[StarBru]

I occasionally use a 2x Meade Shorty barlow or a 2.5x Televue Barlow while observing with a refractor, reflector, and a SCT. As long as I don't exceed the maximum magnification capacity of each scope and allowing for seeing conditions, I can usually get a better view visually. That being said, I have read that using a Barlow results in loss of brightness. I have also read that a 2x Barlow will, as an example, essentially make an f/5 scope into an f/10 scope and will improve the view through the eyepiece. I do not know whether that is because the abberations are not as noticeable of the eyepiece or of the scope. I don't know for sure, I'm certainly no expert, especially of the technical side of it. Someone please correct me if I have mis-stated anything! I just know I have no problem slipping in a Barlow when I need more magnification and I don't have the eyepiece to give me the magnification needed. Each time I use a Barlow, I have to refocus. I hope this helps.

I also have a 2nd 2x Barlow that is a little longer than my Shorty Barlow and when using it, it has on occasion not come into focus but I can't remember which scope it happens with. I just grab a different Barlow when that happens.

I don't dabble in AP, so I can't help you there.

[Don Quixote]

All other things being in proper order, my guess is that the addition of that barlow into the optical train has pushed the focus point for that combination beyond the capability of the focuser. I wonder if a "shorty" barlow would do the trick.

Have you tried your set up in daylight on a distant object to see if you can get focus with the barlow and lens combination?

[Richard]

For me they seem to work the same in all scopes I have a 2x shorty but once one has plenty eyepieces they tend to become spares just like my Plossel eyepieces used perhaps one a year, but great when I started , just make sure its at least a doublet , some scopes come with a barlow just so that they can advertise great magnification those are rubbish
Ok Just read the OP post again I think F5 or lower will work well irrespective of scope design

[Lowjiber]

http://astunit.com/astunit_tutorial.php?topic=barlow

Clear Skies

[helicon]

I actually prefer not to use barlows as I find the images they produce a bit unsatisfactory, even though I have a couple in my kit. Instead I use 10, 8, and 6mm eyepieces to accomplish the same task of magnification. In your situation I would choose a shorty barlow and then you can experiment and see if the scope will come to focus. As you mention, collimation of the primary might be an issue. If it's off the effect will only be magnified resulting in a poor view.

[Gulf Coast Guy]

Barlows alter the effective Focal Length of the OTA with which they are used. by the factor stated on the barlow All the other calculations; Power, Focal ratio, Exit pupil, etc. remain the same.For example with a 2x barlow a480mm Focal length Refactor is treated as if it were a960mm in all calculations. The longer eye relief of lower FL eyepieces is retained.

[Gulf Coast Guy]

n the example above if the objective were 80mm The Focal ratio without the Barlow would be f6 with the Barlow f12, with most if not all the up and down side of each.

[TCampbell]

It wasn't specifically asked ... but I thought I'd bring up in case anyone is thinking of using a Barlow with a camera on the scope.

With most lenses, the back-focus distance is important because light is projected in a cone-shape (they are optimized for a specific back-focus distance). In a telecentric Barlow (such as a TeleVue Powermate), the light exits traveling in parallel paths. This means the distance from the telecentric to the image plane can vary without affecting image size or quality.

A telecentric (such as a Powermate) is more properly called an 'image-space telecentric' because the light travels in parallel paths on the outbound side (the side projecting the image onto a camera sensor.). There is such a thing as an object-space telecentric where the light is parallel on the inbound side (the side where the object being imaged is located). Object-space telecentric are probably not applicable to astronomy.

In particular ... image-space telecentrics are great for planetary imaging, where the planet is typically "barlowed up" to some fairly high focal ratio (such as f/25-f/30)