What "Bad" effects does a barlow have for visual Observation?

[davewaldo]

Hi Guys, I'm curios what "bad" effects a barlow has.

I am contemplating buying a Orion Deluxe 2" Barlow which seems to be a good unit.

I know using a 2X barlow will double my focal length :) and 1/4 the amount of light available to view:( . I know this effectivly loses 2 stops of light if I were to be imaging anything.
However what I don't know is how this effect the view for visual work. I don't see it being a problem on the moon or bright planets however would it effect the visablility of faint stars in clusters or faint details of nebulas?

I would imagine it would effect these things or does our eye compensate for the difference making it less noticable?

The reason I ask is that I will soon be purchasing some EPs and I'm trying to figure out the best combo.

Thanks Everyone!

[iceman]

Good barlows won't be noticeable visually, bad ones will.

The Orion seems ok by all reports. The best of the bunch is the 2.5x powermate, but it is considerably more expensive than most.

[janoskiss]

That's not really a "bad effect": it is the effect; you have a fixed amount of light and you're deciding how to distribute it across the focal plane. The more magnification the more thinly the light is spread.


Again with visual the brightness goes down as magnification goes up (inverse square). Whether you are using a barlow to get there or a shorter FL EP, the effect is practically the same.

I find that with barlows the biggest issue is stray light and/or scatter. The Orion Deluxe 2" (aka UO 2") is a great barlow and the internal baffling works very well at minimising scatter and unwanted reflections. The only downside is that it is big, bit heavy, and needs to sit a fair way out of the focuser. (See Dave's review)


Like I already indicated, this is not really a question of using a barlow or not. It's more a question of what magnification for what objects. Two or three widefield EPs around the ~10mm (2mm exit pupil) mark has DSOs pretty well covered IMO. Powers much higher (exit pupil ~1mm or FL about 5mm and less) will have limited use on DSOs: just too dim.

[Starkler]

Which is why buying a mediocre barlow is a bad idea. It will make any eyepiece you place in it perform poorly.

[Don Pensack]

If there is a problem with a barlow it is usually one of the following:
1)Barlows increase the eye relief of eyepieces used in them. This is not a problem for most mid and high power eyepieces, but could be a problem for a low power eyepiece if the low power eyepiece already has so much eye relief it's difficult to hold your head steady enough to stay in its exit pupil at the long distance behind it.
2) Barlows often vignette lateral rays in an eyepiece that already uses the majority of the inside diameter of the barrel to field the light rays for that eyepiece. This most often occurs when barlowing low power eyepieces with field stops nearly the diameter of the barrel. This vignetting does not occur with mid- and high-power eyepieces. It is visible as a darkening of the edge of the field.
3) Some cheap barlows aren't adequately baffled and scatter light in their own barrels. Short barlows are not prone to this, but due to the deeper curves on their lenses, vignette more with long focal length eyepieces.

Despite the above, you should get one. The Orion Shorty Plus is excellent, as is the standard TeleVue.
Look for multi-coating on the lenses, and if 2", look for a free 1-1/4" adapter.
GSO makes an excellent 2" barlow.

[mickoking]

As a visual observer I dont use a barlow, I personally don't rate them.

[74tuc]

Good write-up on Barlows here:

http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/BarlowLens.pdf

[74tuc]

Good one Don,

Your points 1,2, & 3 ... excellent.

Jerry.

[Rez]

Does anyone use the Bintel Barlow 2x ED (2") ?

I'd be interested to know how people have found it ?

Rez

[janoskiss]

Just get the non-ED one. It works exactly the same and it's cheaper. It's a good barlow for the money, i.e., if you want to stay < $100.

[Rez]

The Bintel Barlow 2x ED (2") is only $99, is there any particular reason I don't want the ED one ?

[janoskiss]

ED glass in the barlow is a marketing hype. The purpose of ED glass is to reduce false colour. I used a couple of non-ED GSO barlows last w/e and looked for false colour at mid to high powers and saw none. So my advice is save a your money and get the non-ED version.

Or better, for $99 + postage you can also get the Japanese made Orion Deluxe 1.25" from Sirius Optics in QLD (well probably you can, ring them and ask), which I have no doubt is better than either GS barlow (based on my experience with the 2" Deluxe). These barlows are FMC and internally baffled which makes a lot of difference on bright objects like planets. So unless you specifically want a 2" barlow, I'd recommend the 1.25" Deluxe.