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Siebert Modular Barlow on a Small Focal Length Scope For Higher Powers

Posted by victor bradford   06/02/2008 07:00AM

The TV-60 has helped redefine the term, "portable quality scope," and is an excellent example of the choices available to those who want a good light scope they can carry in a backpack or airline bag. Unfortunately, the small focal lengths of such scopes also help redefine the term, "adequacy of one's eyepiece collection," because an eyepiece which gives a good high power view in another scope may only give a medium power view in a small focal length scope. That's too bad, because the TV-60's excellent optics cry out for higher powers.

One good option is to buy a quality zoom eyepiece in a smaller focal length, but this option is expensive and such eyepieces have fairly limited fields of view. Another good option is a Powermate-type Barlow, but this is also expensive and you are limited to one magnification. Yet another good option is to buy more small focal length eyepieces, but such eyepieces are often expensive, and carrying too many will negate the portability of a small scope.

All this makes a Barlow sound ideal, but alas, the standard Barlow will not allow my TV-60 to focus. Having had good results with Siebert Optics eyepieces, I visited their website to investigate small focal length eyepieces and became intrigued with their section on Barlows. I called the company and was informed that their newly-developed modular Barlow might indeed work, and decided it was worth a try. This Barlow could allow three magnification selections (1.5, 2, and 2.5), and used a modular configuration rather than a zoom.

Sounded great, but how did it test? I used the modular Barlow at medium and high powers on Alpha Gem, the moon (in particular Vallis Schroteri), and Saturn. I also viewed three terrestrial objects with high contrast at near and far distances and low and high lighting conditions. I used a versatile 8-24 zoom eyepiece to quickly give equivalent magnifications, which allowed me to test the effect of the Barlow rather than the eyepiece (although the zoom, like individual eyepieces, might give different optical quality at different focal lengths). I then used an RKE, a Nagler, a good Plossl, a cheap Plossl, a TMB, a Brandon, and a Siebert starsplitter 5.9mm. Seeing was about a Pickering 7 or 8, and the daylight testing was with low atmospheric turbulence. I am a purely visual astronomer, with perhaps advanced beginner status (I have been observing for over 20 years), so your standards may differ.

I am pleased to report that the Barlow gave excellent performance and allowed the TV-60 to successfully venture into higher powers (at least 150X). The optical performance at this power gave especially good results with Alpha Gem and the moon and on terrestrial views with adequate lighting. It seemed like I was getting a new high-power scope! I found few disadvantages, common to any Barlow, which were far outweighed by the benefits. Given the limitations of terrestrial views (in particular, lighting and atmospheric turbulence), the modular zoom and the 8-24 eyepiece may be all I need to carry (unless I want more field of view), although for night observing I may purchase another eyepiece to allow about 200 power. Notably, the customer service and creativity of Siebert Optics was excellent, which illustrates the advantages of relying on a dedicated fellow astronomer who designs his own products (I have no financial interest in the company and paid market price for the Barlow). I did not try this combination with any of my other scopes, but see no reason it could not work with them too.

--IN SUMMARY, the Siebert Modular Barlow is worth your consideration, especially if you have a small focal length portable scope like the TV-60 and want to take advantage of its optical excellence at higher powers. It seemed more like getting a new telescope, rather than just a new eyepiece.

--Hope this helps you enjoy your small scope, or that it gives you another reason to consider one.