MaestroMyth
sage
Reged: 02/16/06
Loc: CT
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Barlow vs. Powermate?
#4774561 - 08/28/11 08:58 PM
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Hi all,
Going to upgrade a 'free' Barlow that came with my scope.
I'm looking at the TV line and I don't understand the difference between their Barlow and Powermate. 2x, 3x, 4x, etc.
I use an 8 inch SCT for visual and astrophotography.
If not a TV offering, can anyone recommend a Barlow that would be an upgrade over my standard Parks Barlow?
Thanks in advance!
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ischua
Under the Radar
Reged: 07/17/08
Loc: Below Snow Plop, N Y
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Re: Barlow vs. Powermate?
[Re: MaestroMyth]
#4774663 - 08/28/11 10:07 PM
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A Powermate is a corrected barlow no aberrations.
You can unscrew the 2" eyepiece housing and screw on a T-ring adapter for attachment of CCD or camera
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David Knisely
Postmaster
Reged: 04/19/04
Loc: southeastern Nebraska
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Re: Barlow vs. Powermate?
[Re: MaestroMyth]
#4775241 - 08/29/11 08:26 AM
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Quote:
Hi all,
Going to upgrade a 'free' Barlow that came with my scope.
I'm looking at the TV line and I don't understand the difference between their Barlow and Powermate. 2x, 3x, 4x, etc.
I use an 8 inch SCT for visual and astrophotography.
If not a TV offering, can anyone recommend a Barlow that would be an upgrade over my standard Parks Barlow?
Thanks in advance!
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. This can be important for some solar H-alpha filters that require very long true f/ratios in excess of f/30. A Barlow will also increase the eye relief of most eyepieces, which may or may not be a beneficial thing. If it extends the eye relief too much, it can be hard to get or keep your eye in the right location to catch all the light. The Powermate on the other hand does not increase the eye relief of the eyepiece significantly. 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. Clear skies to you.
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GeneT
Ely Kid
Reged: 11/07/08
Loc: South Texas
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Re: Barlow vs. Powermate?
[Re: David Knisely]
#4776786 - 08/29/11 10:09 PM
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It seems like in most cases, the Powermate is the way to go????
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David Knisely
Postmaster
Reged: 04/19/04
Loc: southeastern Nebraska
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Re: Barlow vs. Powermate?
[Re: GeneT]
#4777133 - 08/30/11 12:38 AM
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It seems like in most cases, the Powermate is the way to go????
It can be a good choice, although a good Barlow like the Tele Vue 2" 2x model can also do the job. I do like the Powemates, as with my Newtonian before I shortened the trusses, the Barlow would not come to focus due to insufficient inward focus travel. My 2.5x Powermate avoided that problem and gave me excellent performance. Now that I have shortened the trusses, I am now looking around for a 2" barrel "image amplifier" which may turn out to be a Powermate or a Tele Vue Barlow. Clear skies to you.
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David Knisely
Postmaster
Reged: 04/19/04
Loc: southeastern Nebraska
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Re: Barlow vs. Powermate?
[Re: ischua]
#4777141 - 08/30/11 12:42 AM
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A Powermate is a corrected barlow no aberrations.
You can unscrew the 2" eyepiece housing and screw on a T-ring adapter for attachment of CCD or camera
Well, there is nothing really wrong with a Barlow, so Powermate isn't really a "aberration corrected" Barlow. The only thing the Powermate corrects for is mainly that divergent light which can induce the outer field vignetting in some wider-field eyepieces. Most decent Barlows do not introduce much in the way of significant aberrations, as they tend to be well-corrected. Indeed, some Barlows can also be screwed onto eyepieces or camera adapters. Clear skies to you.
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bgavin
scholastic sledgehammer
Reged: 01/25/11
Loc: 38°41' x 121°13'
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Re: Barlow vs. Powermate?
[Re: David Knisely]
#4926233 - 11/20/11 02:08 PM
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Q: Does either Barlow or Powermate use only the very center of the scope's image circle?
My question is framed for refractors and field flatteners.
If only the center is used, this should be much more flat than the far corners.
I'm trying to determine if a field flattener is required when using a Powermate for prime focus imaging.
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Benach
Pooh-Bah
Reged: 01/24/08
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Re: Barlow vs. Powermate?
[Re: ischua]
#4926507 - 11/20/11 05:08 PM
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A Powermate is a corrected barlow no aberrations.
You can unscrew the 2" eyepiece housing and screw on a T-ring adapter for attachment of CCD or camera
Uhm, no, you're mistaken here. They're two totally different thingies. A Barlow is a negative group that puts the focus further away:
http://www.cloudynights.com/ubbthreads/attachments/1442094-Bravais%20Barlow.gif
A Powermate is a telecentric system. That means that the wavefront is enlarged.
http://t1.gstatic.com/images?q=tbn:ANd9GcTtgPux0-RLe13wKDPv3xLCeCX1lsvDHQNQ2vB7nFAEl9fgVjk4dkJOX79Q
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simpleisbetter
Carpal Tunnel
Reged: 04/18/11
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Re: Barlow vs. Powermate?
[Re: Benach]
#4926561 - 11/20/11 05:46 PM
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MaestroMyth,
Benach's actually correct in his description, they are two different things. TV's website actually does a good job describing the differences. But Ischua's observation about being able to attach a camera directly to a Powermate (using the appropriate TV Powermate adapter) is also right, and a plus when it comes to planetary imaging (and that's about the extent of my imaging knowledge...).
Since your stated goal is visual and some AP with your C8, I'd recommend the 2x Powermate.
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bgavin
scholastic sledgehammer
Reged: 01/25/11
Loc: 38°41' x 121°13'
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Re: Barlow vs. Powermate?
[Re: simpleisbetter]
#4926731 - 11/20/11 07:17 PM
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I have both 2.5x and 5.0x Powermate in 1.25".
Both have the barrels unscrewed, and the optics screwed into a Televue T2 ring (male) for Powermate.
These mate with a standard T-ring on my Nikon.
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Benach
Pooh-Bah
Reged: 01/24/08
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Re: Barlow vs. Powermate?
[Re: bgavin]
#4927158 - 11/21/11 12:49 AM
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bgavin: I wonder, what is the fully illuminated FOV for a 1.25" powermate?
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michael_m
sage
Reged: 11/16/10
Loc: Mount Vernon, Wa
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Re: Barlow vs. Powermate?
[Re: Benach]
#4927180 - 11/21/11 01:23 AM
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Mr. David Knisely,
When you say that the PowerMate only needs a small amount of outward focuser travel to work, how much are we talking about? For example, my 8mm Ethos, my farthest outward focusing lens, leaves me with approximately 1/4" of remaining outward focus. Is that enough to use it on the 8mm Ethos?
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ThreeD
scholastic sledgehammer
Reged: 12/23/08
Loc: Sacramento suburbs
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Re: Barlow vs. Powermate?
[Re: michael_m]
#4927715 - 11/21/11 12:11 PM
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It's so close that it's practically parfocal -- no where near 1/4". (At least for the 2.5x and I from what I've read I believe it is true for all of them.)
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David Knisely
Postmaster
Reged: 04/19/04
Loc: southeastern Nebraska
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Re: Barlow vs. Powermate?
[Re: michael_m]
#4927815 - 11/21/11 01:10 PM
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Quote:
Mr. David Knisely,
When you say that the PowerMate only needs a small amount of outward focuser travel to work, how much are we talking about? For example, my 8mm Ethos, my farthest outward focusing lens, leaves me with approximately 1/4" of remaining outward focus. Is that enough to use it on the 8mm Ethos?
I generally have to use around 1/4 inch outward focus for my 2.5x Powermate. However, I can slide the Powermate itself outward and lock it in place with my focuser set screw or use an extension tube if I need to. Clear skies to you.
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johnnyha
Postmaster
Reged: 11/12/06
Loc: Sherman Oaks, CA
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Re: Barlow vs. Powermate?
[Re: David Knisely]
#4928054 - 11/21/11 03:15 PM
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It is crazy how much outward focus is required for the 8mm Ethos, one reason I switched back to using my 7mm Pentax XW. Probably over an inch more than any of my other EPs including 13 and 10 Ethoi. It's the kind of eyepiece that can require you to add an extension tube to reach focus.
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dvb
different Syndrome.
Reged: 06/18/05
Loc: Vancouver, Canada
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Re: Barlow vs. Powermate?
[Re: johnnyha]
#4928301 - 11/21/11 06:02 PM
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Sounds like a Barlow might be good choice for eyepieces like Plossls and Orthos with shorter eye relief, to allow more comfortable eye relief when reaching into the higher powers.
Sounds like the Powermate would be a good choice at the lower focal lengths, with wider field eyepieces.
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GeneT
Ely Kid
Reged: 11/07/08
Loc: South Texas
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Re: Barlow vs. Powermate?
[Re: MaestroMyth]
#4928758 - 11/21/11 11:09 PM
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I bought a 2X Televue Powermate; it is an excellent optic.
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Danzup77
professor emeritus
Reged: 09/12/11
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Re: Barlow vs. Powermate?
[Re: GeneT]
#4929027 - 11/22/11 06:22 AM
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I have a 2X and a 4X powermate and LOVE em....well I only use the 2X. I actually got the 4X as kind of an early christmas present although I have no idea how I will use it....lol. Guess I just need a 30-something mm piece now to give it some use lol.....
Honestly if it wasnt for these folks here I wouldnt know 10% of what I know now....
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michael_m
sage
Reged: 11/16/10
Loc: Mount Vernon, Wa
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Re: Barlow vs. Powermate?
[Re: Danzup77]
#4929965 - 11/22/11 05:45 PM
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Another solution that can work, depending on your circumstances, is to put a "riser" on the body of the focuser. In my case the circumstances match well.
I have the Starlite Feathertouch. My focuser protrudes into the UTA about 5/8" when fully racked inwards. So I loose that amount of focuser travel. I should have mounted the focuser board outwards more when I built the UTA. Rookie mistake. So I just bought the R20-0.65" riser and now will have all the outwards focus I could ever need while not protruding inside the UTA when racked all the way in.
It's a good solution, especially since I don't have the mirror yet and the truss pole length has not been set up to my most inwards focusing lens. I know I'll be using a PowerMate 2X when conditions allow. True, I may not have needed to add the riser, at least according to ThreeD, but now I will have the full focuser travel available and I know the 8mm Ethos, or even a 6mm future Ethos, will come to focus no matter what while using the PowerMate.
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ThreeD
scholastic sledgehammer
Reged: 12/23/08
Loc: Sacramento suburbs
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Re: Barlow vs. Powermate?
[Re: michael_m]
#4930564 - 11/23/11 01:12 AM
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I know I'll be using a PowerMate 2X when conditions allow. True, I may not have needed to add the riser, at least according to ThreeD, but now I will have the full focuser travel available and I know the 8mm Ethos, or even a 6mm future Ethos, will come to focus no matter what while using the PowerMate.
I can only speak to the 2.5x from personal experience but note that TV says they are essentially parfocal in point 4.
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