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Newbie but not newbie: 2" Barlow lens....why not?


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I am new here at the longe. My telescope is an ES ED 127mm F/7.5 Apochromatic triplet (fl=952mm). Had first light Friday.  This is my second scope as I spent about 10 years with an 8-inch schmidt-cassegrain, and then about 15 years with nothing, now this.

I have a rather low quality 2x barlow and am considering replacing it with a 2" 2.5 x Barlow (which will also accept one and a quarter inch eyepieces with included adapter). My diagonal is 2". Right now I have a 2" Meade series 5000 20mm widefied, 9mm Celestron X-el LX and a cheapo 6mm Plossl.

Now, the 2" barlows are really not much more than the 1-1/4" ones. I have only found 1 lens in the 2" 2.5X style.

The question I suppose is : Would it be decadent to replace a 2x 1-1/4" Barlow with a 2 inch It seems like it would be brighter and perhaps have less the distortion? 

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A 2" Barlow won't lessen the distortion you mention, this can only be gotten ridden of by using a high quality Barlow. If you're suffering dim and/or vignetting images - buy top quality. A top-end Barlow is one which effectively disappears in the optical-pathway - only introducing higher power views without dimming or distorting. I rarely suggest TeleVue® to people due to it's high price, but for a Barlow I make an exception. Luckily a TV-Barlow isn't a great leap price-wise. And it will be the last Barlow you'll ever buy, as there is no going-up from the top.

Don't get me wrong here - there are some very good 2" Barlows out there. I recently got a GSO 2" 2X one, and I'm quite impressed by the image quality it is giving me. But it's still no match for my TeleVue 2X & 3X 1.25" models. You might also wish to look into the PowerMate series. While not true Barlows, they are excellent in adding magnification - without even appearing to be there by causing any dimming or distortion.

Hope this helps -


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Barlows vary a lot depending on the design and quality of manufacture so it's hard to say if a 2" would be better or worse - just depends which one you got. The advantage is that should you go for a 2" it will work with any additional 2" eyepieces you get in the future. The disadvantage is that for the same quality, the extra glass will cost a bit more. Everyone has their favourites (mine is the TMB 1.8x 1.25") but given that you've got some top quality glass up front, I'd be tempted to invest a bit more in a good barlow at the back :)

Odd though it sounds, it may also depend on your mount. Why? Well because if it's not tracked then you may find observing at high magnification much easier with wider angle eyepieces which tend to be 2" so a 2" barlow would be handy. If the mound tracks then a narrow field is fine a lot of the time. I use eyepieces with a 30° FoV on planets but that would be horrible if the mound didn't track.


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Just keep in mind that you'll need a Televue Panoptic Barlow Inteface to avoid vignetting with wide field 2" eyepieces longer than 22mm.  Unfortunately, they haven't been available for sale for years.  Alternatively, pick up a telecentric "barlow" like a Powermate.  It's not really a barlow at that point, though.

For shorter eyepieces, I get cleaner views with my 18 year old Japanese made Orion 2" 2X Deluxe barlow than I get with my TV 1.25" 2X barlow.  Unfortunately, it has too long of a focal length to work properly with the TV PBI.  However, my GSO 2" 2X ED works perfectly with it due to it having almost exactly the same focal length as the TV Big Barlow.  Together, I spent less than $110 on the two and achieve almost Powermate levels of performance with long focal length eyepieces.

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