Barlow: ED or non ED

[sophiecentaur]

I am confused about the relevance of ED glass in a Barlow lens. I have been looking at the whole range of Barlow lenses and, apart from the very cheapest examples, I cannot seem to find any correlation between cost (= performance quality?) and whether or not it's claimed to have ED glass. When you consider that ED seems to be essential for a good refractor scope, how is it that the more expensive barrows don't seem to advertise use of ED. Is it to do with the focal lengths involved in a Barlow?

[Louis D]

3 hours ago, sophiecentaur said:

I am confused about the relevance of ED glass in a Barlow lens. I have been looking at the whole range of Barlow lenses and, apart from the very cheapest examples, I cannot seem to find any correlation between cost (= performance quality?) and whether or not it's claimed to have ED glass. When you consider that ED seems to be essential for a good refractor scope, how is it that the more expensive barrows don't seem to advertise use of ED. Is it to do with the focal lengths involved in a Barlow?

Read this CN thread on APO vs achro barlows.  It pretty well covers the subject.

[sophiecentaur]

On 28/02/2017 at 02:40, Louis D said:

Read this CN thread on APO vs achro barlows.  It pretty well covers the subject.

Cheers for that. One less thing to worry about, it seems (but some additional ones, after reading that thread!)

I am only concerned that I don't want to buy something that will be limiting the other pieces of my kit. My EPs are 2" so I have a bit less choice of Barlow. It looks like I should be OK by spending around £100.

[John]

Items such as the Tele Vue Powermates, the Meade TeleXtender and the Explore Scientific Focal Extender cost a bit more than conventional barlow lenses but would ensure that there is no "weak link" in your optical chain. They are all available in the 2" fitting.

[Louis D]

3 hours ago, John said:

Items such as the Tele Vue Powermates, the Meade TeleXtender and the Explore Scientific Focal Extender cost a bit more than conventional barlow lenses but would ensure that there is no "weak link" in your optical chain. They are all available in the 2" fitting.

Agreed.  If you are going to barlow 2" eyepieces, get a telecentric type as John suggests.  I created one myself by using a TV Panoptic Barlow Interface with a GSO 2" 2x ED barlow similar what is described in this CN thread.  The GSO barlow has just about the same focal length as the TV Big Barlow, so the PBI interfaces perfectly with it.  Without it, the barlow cuts off the outer field of widest field eyepieces.  Literally, it looks like a new, inky black field stop somewhat inboard of the normal one.  With the PBI, the eyepiece's original field stop is sharp and fully visible.  Without, the exit pupil becomes difficult to hold.  With it, it acts just like it did without a barlow.  Since the PBI is discontinued, and there are now affordable alternatives to the 2" 2X TV Powermate, I'd probably just get one of those alternatives today.

[rl]

I have a 5" f/10 achromatic refractor (standard crown/ flint setup) that shows moderate but not  objectional CA at high powers. Rather than using short focal length eyepieces, adding a cheap *2 barlow (crown/ flint doublet) with a longer eyepiece actually helps correct the residual CA. Putting the Barlow between the diagonal and the focuser increases the magnification over the expected *2. You have to rack out further but this isn't normally a problem on a refractor and the resultant image is definitely a lot tidier.

[timwetherell]

My favourite is the TMB 1.8x cemented ED barlow which has no detrimental effect whatsoever in my 7" refractor. But it's only available in a 1.25" so that doesn't help you out much. One thing to be aware of is that some generic 2" ED barlows have only got a clear aperture of about 35mm so there's significant vignetting with eyepieces like the nagler31. I had the same disappointment with the William Optics 2" erect image prism - can only really be used on 1.25" eyepieces which rather defeats the purpose!

[mikeDnight]

I regularly use a SW delux Barlow on my Takahashi refractor and its excellent, giving sharp, colour free views. At times, to obtain a reasonable image scale. I've even double stacked Barlows and have noticed no detrimental effects. I often observe at very high powers yet the view is always sharp and colour free. I've compared Takahashi's own Barlow and it offers no optical advantage over the SW delux. It just costs a small fortune! 

Mike