By: Vlad Fedosov
A barlow lens is quite a simple astro accessory, but also one of the most valuable additions to the amateur astronomers tool case. What is a barlow, and why can I claim that its one of the most valuable accessories you can buy? Simply put a barlow is a lens that you but in between your eyepiece(or camera) and telescope that will multiply the magnification by a set factor(usually from 1.8-5x). What this means is that with the addition of just one accessory you effectively double your eyepiece focal length/magnification collection! I had the opportunity to test some of the best barlows on the market form TeleVue including a TeleVue 2x, 3x barlows and 5x Powermate. These where all 1.25X models with the newer compression ring eyepiece holder. Lets take a look and see how they perform under the stars!
I did this comparison from my mag 4 backyard on an early winter night here in Vancouver, WA. We had a rare clear night and I was antsy to get out and do some observing after weeks of clouds and rain! The seeing was ok this night, and the transparency was about average. The scope used was my William Optics FLT-110(TEC oil spaced f/7 715mm triplet). In order to make this as fair of a comparison as possible I used my trusty Badder 24-8mm Zoom so that I could get all the barlowed view to the same magnification. This is also one of my favorite and most used eyepieces so I’m quite familiar with its performance un-barlowed.
The first target for the night was Venus. This was the only planet visible at this time of year and also as you might guess the first thing visible in the twilight sky. The first look I had of the planet was with a non borlowed view so that I can get a sense of what the “prime” image looks like. What is the prime image that I’m referring to? Well for the purpose of this analysis we can assume that the prime image is the absolute best image you are going to get with the scope and eyepiece combo being used given the conditions that this night affords. Adding a barlow will of course make the image larger but besides that all it can do is add harm and at best leave the image 100% unaltered. At the 8mm setting the FLT-110 produced a mere 89x. Venus looked quite nice with a cool looking ~3/4 phase globe. No surface features where noted besides the phase.
The first TeleVue unit that I decided to try out was the 5x Powermate as I knew that this would be my reference magnification that I would be able to achieve with the other two barlows. I set the zoom to 24mm which produced 149x with the 5x multiplication factor. The view was certainly larger and the air turbulence was more apparent. Secondary color was also quite a bit more apparent. While the FLT-110 is a triplet APO it does show a bit of secondary color on really bright objects like this. Also the planet was low to the horizon adding some atmospheric refraction adding to the unwanted color. Otherwise the image appeared nice with nothing just a bit more light scatter.
Next up was the 3x barlow. After putting it into the diagonal I set the zoom eyepiece to about 14mm to get the same 149x that was produced by the 5x Powermate. Fairly quickly it was apparent that the view with the 3x barlow was a tiny bit crisper then with the 5x Powermate! Light scatter was kind of hard to judge as the planet is just so bright and low to the horizon. Secondary color also seemed like it was more under control. It was still there but I would estimate about 25% less intense. A very positive result. After studying the planet for a while, I proceeded to the last contender, the 2x unit. Setting the zoom eyepiece to ~9.5mm I was greeted with a similar view to what was just observed with the 3X barlow. I really could not pick apart much of a difference between the 2x and 3x barlows no matter how hard I tried. This is not too much of a surprise as the 2x and 3x barlows might use the same optics. How so? Well you see with a barlow the greater the distance there is between the barlow optic and your eyepiece the greater the magnification. The tube length on the 3x model is certainly longer than the 2x model… This is in fact more or less how a zoom eyepiece works as well!
Having stared at Venus for what was certainly the longest period of time I ever have I moved on to one of my favorite sights in the night sky; the Double Double in Lyra. For those not familiar with this object it is a very widely spaced easy binocular double star(208.2 arcsec) that the two components are themselves double stars that are moderately difficult to split with a small telescope(2.3 & 2.4 arcsec). If you have not seen this one, you really owe it to yourself to check it out! Depending on how sharp of a scope you have and how good the seeing conditions are you can split all four components at a very low power. I have personally slip it as low as 60x in this FLT-110! Tonight the seeing was not the greatest so I started with the prime unbarlowed view at 89x. All 4 of the components where quite easily evident. At this point I knew that at the test power of 149x that I was using for this comparison it would be easy work to split all 4 components. After trying the three TeleVue units I was not disappointed. All produced a nice split! The only notes that I made is that the 5x Powermate produced a very, very dim purple halo(indicating that it added to the chromatic aberration of the refractor) around the stars with a noticeable amount of light scatter. The Airy disk was also not quite as sharply defined as in the 3x barlow. The view in the 2x and 3x barlows where again almost identical but in a way I did prefer the 2x view on this object. It was likely not a function of the barlows themselves but rather of the zoom eyepieces setting. I finished up the night by observing some more double stars and did not make any additional notes on the performance differences of these units.
In case you are wondering whether you should invest money into a barlow or simply buy more eyepieces, this is a most interesting question and one that I have struggled with in my early astro days. Overall the largest benefit of getting a barlow was stated in the intro. You effectively double your eyepiece collection! So with a little planning all you really need is a 2-3 eyepieces that will cover all your observing needs. The other very nice thing if you have a short focal length telescope(such as an APO) is that you can use a longer focal length eyepiece with a barlow and retain the longer eye relief. This is in fact how most long eye relief eyepieces are made, they simply have a built in barlow. Lastly if you are ever consider getting into imaging the planets you will almost certainly need a barlow anyhow.
In conclusion witch unit would a recomend? Well to me with any barlow or reducer it comes down to how close is the view to the prime view that you saw before introducing the barlow unit. In this case the 2x and 3x TeleVue units produced an image that was in the range of 90-95% as good as the prime view. The 5x Powermate is also a nice unit and if it where not for the direct comparison to the other two units I would have been quite happy with its performance. Overall I would rare it closer to 85-90% as good of a view as prime. Are these better then the entry level offerings that cost only a fraction of the cost? YES! As with any TeleVue product its hard to find too many faults with them. When you buy the best there is not too much to have reservations about!
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