Author Topic: Barlow vs. PowerMate  (Read 268 times)

propdiagairil

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Barlow vs. PowerMate
« on: December 29, 2017, 02:54:13 AM »
I've heard on more than one occasion that a Televue Powermate is NOT a Barlow.

I have a 2x Powermate as well as an old Meade 2x Barlow. They both double my focal length, more or less...

Without discussing quality, how are these devices different enough to not share a name?

Could someone enlighten me?

thanks,

Dave



ecapwaiwa

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Re: Barlow vs. PowerMate
« Reply #1 on: January 01, 2018, 09:35:09 AM »
There's more than one way to skin a cat.

revekosque

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Re: Barlow vs. PowerMate
« Reply #2 on: January 08, 2018, 05:55:25 AM »
TeleVue has their explaination on the differences here http://www.televue.c...id=53&Tab=_back

Bjorn

housletica

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Re: Barlow vs. PowerMate
« Reply #3 on: January 08, 2018, 12:07:26 PM »



John Abreu

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Re: Barlow vs. PowerMate
« Reply #4 on: January 09, 2018, 08:22:16 PM »
Note also that the ES Focal Extender, and Meade TeleXtender are both telecentric designs, like the Powermate.

safrioheartli

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Re: Barlow vs. PowerMate
« Reply #5 on: January 11, 2018, 02:04:13 PM »
The TV literature says the main difference is that a regular Barlow can push the
eye relief of an eyepiece out, especially longer fl eyepieces.

Using projection (for focus against a distant wall, through the scope),
and a 2x shorty (shorties are supoosed to have a stronger effect),
I can see:

A 30mm Plossl has its eye relief shifted from about 20mm to about 30mm..
A 16mm Plossl has its eye relief pushed from ~10mm to ~14mm.
The 10mm Plossl goes from 6mm ER to maybe 8mm. Not much...but a good thing.

Since I usually pick a shorter FL as a stepping-off point for a BArlow,
I actually welcome the eye relief push in this case. I prefer the Barlow.

If you are using a long-FL eyepiece and Barlowing,
it's nice to not have your eye location pushed way back.

It's kind of a special niche. For very big long scopes,
some people have very long-FL eyepieces, though,
and they might want to keep the field qualites and push the power
without their eye ending up way far away.
For them, a shorter FL eyepiece will usually do well, though...

It's worth considering what you are multiplying.
In my case it was an offer to "protect my home from tigers"
(save me from a problem I don't have)...

Matthew Calhoun

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Re: Barlow vs. PowerMate
« Reply #6 on: January 11, 2018, 02:14:54 PM »
Another thing to watch out for with true Barlows is the fact that their magnification factor (M), varies depending on the eyepiece used with it. The so-called 2x Barlow you buy may have an M of 2 for one eyepiece and an M of 2.2 or 1.8 with another. It all depends on the relative positions of the Barlow lens element and the field stop in the eyepiece. Shorty Barlows are more susceptible to this phenomenon than longer ones.

M = 1 - L/f<sub>b</sub>
Where M is magnification factor.
L is distance from Barlow lens element to eyepiece field stop.
f<sub>b</sub> is the Barlow's focal length (a negative value)

Regretably, manufacturers don't provide a spec for f<sub>b</sub> for their Barlows.

vichanettgrif

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Re: Barlow vs. PowerMate
« Reply #7 on: January 13, 2018, 02:51:06 AM »
I compared a VIP Barlow with a 2x Siebert OCA (telecentric) this night. Combined with a RKE 8mm and a ES 80mm apo : Siebert gave sharper image edge to edge.
I cannot say if another combination is better on VIP or Siebert : may depend on the EP (field curvaure).
From 1.3x to 2.0x my Siebert OCA's statisfy me (2.7x is nice but transmission fall a bit : 6 lens total). Perhaps other optics from him could match other brand product as well.

flasattecof

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Re: Barlow vs. PowerMate
« Reply #8 on: January 31, 2018, 12:44:44 PM »
TeleVue's explanation of the differences is a good one. My simple explanation is that after owning and using both A TeleVue Barlow and PowerMate, I preferred the view in a PowerMate when used with a variety of focal length eyepieces. So, I sold the Barlow. However, someone considering a PowerMate vs. a Barlow will be happy with either. Not to clutter up the discussion, however, I weaned myself away from my PowerMate because I prefer just using an eyepiece. I enlarged my spectrum of focal lengths rather than try and stretch a few eyepieces into more by using either a Barlow or PowerMate.

dustsungline

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Re: Barlow vs. PowerMate
« Reply #9 on: February 02, 2018, 08:59:32 PM »
PowerMate is heavier, has little to no change in magnification with distance from the lenses, doesn't change the eyepiece's eye relief,
is less likely to vignette an eyepiece with a large field stop, and will induce fewer aberrations in most eyepieces.

A Barlow is lighter, might be usable at different magnifications by changing eyepiece-to-lens distance, adds eye relief to eyepieces with short eye relief,
and works fine to turn a mid-power eyepiece into a high power one.

In this era of long eye relief eyepieces down to 2.5mm focal lengths, and planetary eyepieces down to 1.6mm, the need for Barlows or PowerMates is diminishing.

exvermabo

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Re: Barlow vs. PowerMate
« Reply #10 on: February 09, 2018, 10:24:41 AM »
I would use a regular barlow for shorter focal length eps, if you tend to barlow longer f.l. eyepieces I would go with telecentric, otherwise the eye relief can become too much. Eyeguards normally in the correct place are now in the wrong place which can be trouble.  Regular barlow is great for shorter ep's like orthos that can use a boost in eye relief.

btw Astro-Physics does tell you the focal length of their 2" barlow, the prior "Barcon" version was -127mm:

&gt;&gt;The Barlow focal length is -102 mm which is longer than conventional types