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Old 08-10-2020, 11:06 AM
mandragara (Richard)
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Barlow drawbacks vs dedicated eyepiece?

Hi all,

First post here.

Just three basic questions about barlows and how they impact seeing.

Firstly, do barlows reduce brightness beyond the usual brightness reduction you get with magnification + the additional light absorbed by the glass of the barlow? As in, do they scatter some light outside of the FOV of the eyepiece?

Secondly, please consider the following three situations:

A) Looking through a 4.5mm eyepiece

B) Looking through a 9mm eyepiece + 2x barlow

C) Looking through a 22.5mm eyepiece + 5x barlow

My understanding is that (A) is superior, as there is less glass between your eye and the sky (and possibly other reasons depending on Q1). But how much better is (A)? Also, comparing B and C, which is superior\inferior and why?

Lastly, how do you compare barlow specifications between brands? Televue want 4x the price compared to GSO for a 2X barlow, but how do you compare the two based on specs? If a telescope cost 4x the price, there would be lots of detail as to why one costs more.

Many thanks,
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Old 08-10-2020, 11:35 AM
sunslayr (David)
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It really depends, in theory any amount of extra glass will impact the image, but in practise it's not that big of a difference compared to the reduced brightness of the magnification. A good quality barlow will be essentially invisible. In fact some eyepieces incorporate a Barlow into their design, I believe the Televue Naglers do this. The last question is a little simpler if it's the regular Televue barlow they probably claim to use better glass and multicoatings to justify the price. If instead you are talking about a Powermate, they are a little different as they have extra lenses in them to make them telecentric, that way the eye relief isn't increased along with the eyepiece magnification.
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Old 08-10-2020, 04:44 PM
Renato1 (Renato)
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Not sure how to answer your last question.

In 1.25", I have two Televue Barlows, an old Celestron Barlow, a Meade shorty Barlow and one or two GSO barlows. Plus a couple of 2" Barlows.

I could never see any difference between my Televue ones and the Celestron one.

The main problem with Barlows for me, was that I always knew I was looking through a Barlow. With a Powermate, I couldn't tell the difference.

Looking through a 4.5mm eyepiece - depends on the eyepiece. If it's a Plossl, your eye will be nearly stuck to the lens because of the very tiny eyerelief, whereas a 9mm Plossl eyepiece and Barlow will give you more eyerelief. With a 22.5mm eyepiece and 5X Barlow - if you have astigmatism, you will be able to wear eyeglasses while viewing.

But if your 4.5mm eyepiece is one of the newer ones with long eyerelief (like the inexpensive TMB type ones on Ebay), then the view is generally better than that from a 9mm Plossl in 2X Barlow - since these newer eyepieces in effective have a little matched Barlow built into them.

Remember also that if you have a refractor, an SCT or a MAK telescope, to get 2X magnification from a 2X Barlow, you have to put the Barlow into the star diagonal. And this is often a nuisance, especially if one has a heavy eyepiece in the Barlow.

If instead you put the Barlow into the refractor, SCT or MAK, and then put the diagonal into the Barlow, it is much easier to use - but you get approximately 3X magnification of the eyepiece.

I mainly used Barlows when I first started out and didn't have many eyepieces. Later I used them on short focal tube refractors, where I didn't have high enough power eyepieces. But I do now, so I rarely use them nowadays.

Last edited by Renato1; 08-10-2020 at 05:07 PM.
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Old 08-10-2020, 11:39 PM
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dannat (Daniel)

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i’m with renato- easily take the 4.5mm , then 2x barlow -wouldn’t try looking thru 5x -i presume they are just for planetary imaging
A is only slightly better, maybe B is close to 90% but it’s enough of a diff i also don’t like using barlows
more pensive barlows use better glass, more importantly have better polish-
my advice, buy middle of the road, cheap ones can b port, high priced ones are only a few percent better, try finding an old good one, tv, celeste on ultima, baader 2.25x are good -sml and not easily seen thru the eyepiece
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barlow, brightness, eyepiece, lens, magnification

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