Powermate - Barlow

[+RonC]

Can someone explain the difference between a Powermate and a Barlow lens and there applications please?

 

Cheers

Ron 

 

(Off to work now, so will not be responding until tonight!)

[dawson]

You need an overdraft for one, but not for the other

 

Sorry!

 

James

[Tibbz2]

I think the main thing is that Powermates have far better image quality than barlows, which is probably expected of TV tbh!

[Doc]

A Barlow is a 2 or 3 element "negative" lens set that diverges the light entering it. This increases the magnification that an eyepiece will deliver, but with some wider-field eyepieces, the Barlow can diverge the light enough that some of it actually misses the field lens of the eyepiece. This can produce some vignetting in the outer parts of the field.

 

A Powermate also increases the magnification, but uses two sets of lenses to do that: a smaller two-element doublet negative lens set, and a somewhat larger two element positive doublet lens set some distance behind the negative doublet. The larger positive lens set reconverges the light from the negative lens to some degree and thus avoids the vignetting problem. It produces a light cone which more faithfully re-creates the light cone from a telescope with an intrinsically-longer focal length, rather than only approximately doing it like a Barlow does.  

 

Barlows usually require some inward focus travel to work, which can be a problem with telescopes that do not have enough focus travel. The Powermate by contrast requires only a small amount of outward focuser travel in order to work, so with some scopes, it can achieve focus when a Barlow would not.

[catman161]

Hey Ron,

This is from TV's website but is a source of info if you haven't read it yet.

http://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_dev

[dawson]

Ron

I asked a similar question in a round-a-bout ginger way a few months ago. Here is the thread:

http://www.eastmidlandsstargazers.org.uk/topic/4257-barlows-and-focal-length-and-f/?fromsearch=1

James

[T A WOW]

I believe that with a power mate it doesn't matter how far away you put the EP you still get the same stated magnification, but with a barlow the mag will change if you move the EP closer or further away from the lenses.

[Brantuk]

On a practical note - in use the PM is virtually invisible - it just does the job of magnifying with no apparent effects noticed by the eye across the fov. Whereas you will notice some distortions with a barlow - especially at the field edges (depending on eyepiece in use). Otherwise it's pretty much as Doc says above.

If we're at a meeting together Ron I have both so you can compare in your scope.

[+RonC]

Thanks all, will read the threads and maybe see you Kim at some point!