How to select your barlow power

[McMoon]

As in the title but what are the limiting factors.

Scenario 1. I have a Skymax 127, my EP's are 8mm, 10mm, 18mm, 25mm and 32mm and I have a 2x barlow. It may make some of my EP's obsolete but is there a time a different power barlow would suit?

Scenario 2. Equipped as above but this time I have my webcam in place. Given the 127mm/5" aperture I have, is a 2x barlow my best choice or can I up the power and where would I see the limits?

In # 1 I expect the 2x I already have would be the best to run with but I am unsure what would be best in #2.

Thanks all.

Andy

[John]

With scenario 1, at least 2 of the focal lengths that a 2x barlow would deliver (4mm and 5mm) would not really be any use with the 127mm mak - too much power. In fact I don't see it delivering much at all apart from a 12.5mm where you have a slight gap in your range. A 1.6x barlow might make more sense but I don't know of one in the 1.25" fitting. A 6mm eyepiece giving 250x for really good seeing conditions might make sense as an alternative.

I'm not an imager but I believe that the 2x or even a 3x barlow would be useful there for planetary imaging with a webcam, to get the image scale up to a more reasonable size.

[LukeSkywatcher]

For visual use with your collection of EP's, i would not bother with a 2x barlow, but you have one so thats that. For imaging with a web cam............and your 127mm scope, i would say 2x barlow is about as far as you can push it.....................maybe 3x...................but that would be asking a lot.

[E621Keith]

Based on your scope and current eyepiece collection, I wouldn't bother with a barlow unless you want to increase eye relief.

There are a few scenario where a barlow is useful for visual use

1. The most common scenario: You don't have a full spreads of eyepiece. A barlow can be used to fill in the shorter or missing focal length

2. You have multiple scopes. You can use a barlow to make your eyepiece collection go further.

3. You are using a binoviewer and need to extend the back focus

4. You have a zoom eyepiece and want higher magnification

5. Using barlow for its secondary effects such as increasing eye relief or cleaning up the edges (by increasing the f ratio)

2x barlow is the most common, followed by 3x and 1.6x. However, your eyepiece collection is quite complete so you won't need a barlow unless you wanted the secondary effects.

[McMoon]

Thanks all. I realise I have quite a range of EP's and the barlow is really there for the secondary effects, eye relief.

As for the imaging, what mames my limit 2x? Is it the aperture, optical quality or the fact that I have a webcam to use for imaging?

Sorry, probably all sounds like lame Q's to some of you but I am sure there is a reason.

[Pig]

How does the difference with the math for selecting a Barlow differ form that of normal magnifcation power eyepiece ?

EG Is using an 8mm eyepiece with a 2 X Barlow the same as 4mm eyepiece on it own ?

Are there other considerations ? IE speed of the scope / aperture

[E621Keith]

How does the difference with the math for selecting a Barlow differ form that of normal magnifcation power eyepiece ?

EG Is using an 8mm eyepiece with a 2 X Barlow the same as 4mm eyepiece on it own ?

Are there other considerations ? IE speed of the scope / aperture

A barlow can increase the eye relief of eyepiece. A 8mm plossl with 2x barlow will have a longer eye relief than the 8mm alone, where as a 4 mm plossl will have a lot less.

[Pig]

Thank you Keith

[rwilkey]

Hi Andy, Barlows with long focal length scopes such as Maks and SCT's are not great really as they often push the magnification too far, better to get magnification via the eyepiece choice in my view. However, the rules differ for imaging, of which I have no experience.

[McMoon]

Hi Andy, Barlows with long focal length scopes such as Maks and SCT's are not great really as they often push the magnification too far, better to get magnification via the eyepiece choice in my view. However, the rules differ for imaging, of which I have no experience.

Funny you should say that. I was at a Stargazing live event the other evening and there was a guy there with an SCT who said he would opt for NO barlow and drive the magnification by use of the EP's alone. Then along came someone to peer through his scope and I never got the opportunity to ask his why as we left soon after.

[rwilkey]

I think it is because the quality of the image is more important to stargazers, it certainly is to me, and i would rather see a small sharp image rather than an enlarged feinter one. The other thing is, is that a planet will always look like a pea in a telescope, no matter what type of telescope or eyepiece you use, at least, that is my experience.