GSO 3x ED Barlow

[Zeta Reticulan]

I own a 2x GSO Barlow. It is lightweight, simple, and I like its ergonomics. It performs well with my modified ST80 and a Celestron zoom. I have a 5x GSO Barlow (as Revelation Astro) as well. Although it can suffer from some chromatic aberration. It doesn’t see much use nowadays as it was replaced by a 5x TV Powermate. 

I desired a basic 3x lightweight Barlow. Among various criteria it would require good ergonomics. Basically I’d need to like the overall feel of it, it wouldn’t be so long as to poke the mirror out of the diagonal, and it would need to hold the zoom safely and securely. It would also be required to perform well in an inexpensive short tube achromat. Lastly, it had to be relatively economically priced. So, probably an impossible aspiration then?

The GSO 3x ED (2-element) Barlow at £38.40 seemed to fulfil all of these requirements. Around 125mm tall with a barrel of about 53mm including the element. I measured 22mm of clear aperture. I make the Barlow about 140 grams in weight, although admittedly my scales are not particularly accurate. The housing is nicely sturdy and constructed from aluminium. The ED element and barrel both thread out of each other. The aluminium barrel is smooth with no undercut. It is competently blackened and baffled internally. The multi-coated element contains a filter thread in its own housing. There is no compression ring although the rather large standard GSO thumb screw is reassuringly knurled. Daylight trials seemed impressive and delivered a sharp, bright and clear image. 

On the 28th of April I ventured out with the ST80 on a night of poor seeing. After viewing some open clusters at low magnifications I decided to split some doubles. I placed the 3x GSO in the 2” dielectric adapter and then put the Celestron zoom into that. This now gave me a range of exactly 50x to 150x. Initial target was α Bootis (Arcturus), easy to find as it ‘follows the Bear’. This allowed me to gain focus at around 70x so I could proceed by star-hopping to the next port of call, ε Bootis aka Izar. The orange primary star at a magnitude of 2.5 is easily seen with the naked eye. The secondary has a magnitude of 4.8. and is basically nestling just inside the first diffraction ring. I have split this double with smaller apertures and it isn’t usually particularly difficult in my experience.

However, I seemed to be having some difficulty this time. I could see a lot of what appeared to be chromatic aberration. I wondered if the poor seeing was a factor. I expect a certain amount of false colour with a short tube achromat. Although it has to be said that the ST80 doesn’t usually suffer terribly from this. At most there is only a hint of the ‘Jimi Hendrix Experience’, more often referred to as ‘purple or cyan hazing’ around the target object. This was definite and obvious colour either side of the target object, so much so that it was a hindrance to observation. At times I could actually see a rainbow-like spectrum of colour! I decided to try δ Boo as it is in the vicinity. The CA was a bit less, but still there. I decided on brighter doubles as a comparison. These included α Gem, γ Leo and later ꞵ 1 Cygni. The false colour was especially noticeable on brighter stars. Finally I tried ε Lyr (Double Double). I needed at least 150x to see all four stars in the conditions. I could still see the aberration. It wasn’t so noticeable on fainter doubles such as 24 Comae Berenices.

I compared the GSO 3x and 2x at roughly the same magnifications on a few targets. The fairly intrusive chromatic aberration was definitely being produced by the 3x Barlow. Maybe I should have expected a DIY spectrometer for a 3x Barlow under forty quid. Although my twenty quid SvBony 3x Barlow doesn’t act in this manner. I must say I am quite disappointed in the 3x GSO. It could be that my sample is a bit of a turkey, but I have since discovered a similar complaint about the same Barlow on the Internet. In all honesty I can’t recommend this Barlow lens. Even the element interface thread is an odd size and doesn't seem to fit anything else I have. I thought a relatively well made light, basic 3x Barlow with an ED element would be just what I needed. The GSO 3x unfortunately doesn’t make the grade.