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Barlow vs. Powermate?

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#1 MaestroMyth

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Posted 28 August 2011 - 07:58 PM

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!

#2 ischua

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Posted 28 August 2011 - 09:07 PM

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

#3 David Knisely

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Posted 29 August 2011 - 07:26 AM

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.

#4 GeneT

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Posted 29 August 2011 - 09:09 PM

It seems like in most cases, the Powermate is the way to go????

#5 David Knisely

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Posted 29 August 2011 - 11:38 PM

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.

#6 David Knisely

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Posted 29 August 2011 - 11:42 PM

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.

#7 bgavin

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Posted 20 November 2011 - 02:08 PM

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.

#8 Benach

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Posted 20 November 2011 - 05:08 PM

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.cloudynig...vais Barlow.gif
A Powermate is a telecentric system. That means that the wavefront is enlarged.
http://t1.gstatic.co...9fgVjk4dkJOX79Q

#9 simpleisbetter

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Posted 20 November 2011 - 05:46 PM

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.

#10 bgavin

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Posted 20 November 2011 - 07:17 PM

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.

#11 Benach

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Posted 21 November 2011 - 12:49 AM

bgavin: I wonder, what is the fully illuminated FOV for a 1.25" powermate?

#12 michael_m

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Posted 21 November 2011 - 01:23 AM

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?

#13 ThreeD

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Posted 21 November 2011 - 12:11 PM

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.)

#14 David Knisely

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Posted 21 November 2011 - 01:10 PM

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.

#15 johnnyha

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Posted 21 November 2011 - 03:15 PM

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.

#16 dvb

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Posted 21 November 2011 - 06:02 PM

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.

#17 GeneT

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Posted 21 November 2011 - 11:09 PM

I bought a 2X Televue Powermate; it is an excellent optic.

#18 Danzup77

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Posted 22 November 2011 - 06:22 AM

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....

#19 michael_m

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Posted 22 November 2011 - 05:45 PM

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.

#20 ThreeD

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Posted 23 November 2011 - 01:12 AM

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.

#21 michael_m

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Posted 23 November 2011 - 12:39 PM

Although I had not seen that "point 4" yet while researching, which I have now done, I did not doubt you ThreeD. The riser is more about getting the fully racked in focuser out of the inside of the UTA and also the full focuser travel for whatever I might do in the future. Thanks for your assurance from experience and your input.

#22 Startraffic

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Posted 13 November 2012 - 04:39 PM

MaestroMyth,
I have 2 Powermates, both 2" a 2x & 4x. I don't care for the views through a Barlow. They are however, pretty cheap ones. Something that no-one has mentioned, Powermates are significantly heavier, You'll need a solid focuser to carry one. I replaced a GSO I had on my SNT with a Moonlite, much better.

Clear Dark Skies
Startraffic
39.138274 -77.168898
Alt 518ft ASL

#23 Widespread

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Posted 09 February 2013 - 07:23 AM

Thank you for a very interesting discussion. Since some time has elapsed since the last post, I hope folks don't mind if I resurrect it and ask some related questions.

I recently bought a smaller refractor, and, unsurprisingly, I find that the eyepieces I had for my 8 inch SCT do not give high enough magnification for planets.

I have been thinking of upgrading my Celestron Barlow that came bundled with my SCT. Would a TeleVue 2x Barlow or PowerMate (or other product) give noticeably improved performance?

This is for a 90mm F7 Refractor. The highest magnification ocular that I have is 8.8ES82. (I also have 11mm and 14mm that could be barlowed.)

But I am not committed to the Barlow route. I guess mine is actually more of a general question regarding eyepiece strategy for this unfamiliar focal length.

Also I'm curious as to the purpose of the 2 inch Barlow or PowerMate discussed above. Intuitively, it seems like this type of amplifier would be used more for achieving high magnifications. But I'm sure I'm missing something. Any remediation of my benightedness would be greatly appreciated. Maybe people use them for the 9mm Ethos?

Cheers,
David

#24 David Knisely

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Posted 09 February 2013 - 01:54 PM

Thank you for a very interesting discussion. Since some time has elapsed since the last post, I hope folks don't mind if I resurrect it and ask some related questions.

I recently bought a smaller refractor, and, unsurprisingly, I find that the eyepieces I had for my 8 inch SCT do not give high enough magnification for planets.

I have been thinking of upgrading my Celestron Barlow that came bundled with my SCT. Would a TeleVue 2x Barlow or PowerMate (or other product) give noticeably improved performance?

This is for a 90mm F7 Refractor. The highest magnification ocular that I have is 8.8ES82. (I also have 11mm and 14mm that could be barlowed.)

But I am not committed to the Barlow route. I guess mine is actually more of a general question regarding eyepiece strategy for this unfamiliar focal length.

Also I'm curious as to the purpose of the 2 inch Barlow or PowerMate discussed above. Intuitively, it seems like this type of amplifier would be used more for achieving high magnifications. But I'm sure I'm missing something. Any remediation of my benightedness would be greatly appreciated. Maybe people use them for the 9mm Ethos?

Cheers,
David


The Barlow or Powermate would give you very good performance as good as or perhaps slightly better than the Celestron Barlow. With a Barlow, the eye relief of an eyepiece can be maintained or increased slightly which can help with some eyepieces that don't have a lot to begin with and hurt with others if the Barlow puts in too much eye relief. Powermates also maintain eye relief, but they don't increase it. As for why 2" Barlows or Powermates are used, that is for use with longer focal length 2" barrel eyepieces. The use of these is especially prevalent in the larger Dobsonians, where longer focal length wider-field eyepieces are often used and higher powers are desired. Clear skies to you.

#25 Widespread

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Posted 09 February 2013 - 03:46 PM

Thank you, David. As good or slightly better... It sounds like any improvement would be minor.

I think I understand what you are saying about 2" barlows: 2" finder EPs to locate a target, then Barlow for a better look, e.g.

Cheers,
David






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