Recent telescope purchase

Recent telescope purchase

Recently, 03/02/2008 to be exact, I purchased an Antares 105mm f/12.4 (F=1300mm) achromatic refractor made by Antares Optical (Canada). I had been interested in this scope for quite sometime especially since the time I purchased a partially built 4" scope of similar dimensions. Since, my scope was out of use due to a broken focuser and since non-standard parts were used for making the scope (PVC tubes, etc.), it was very difficult to get the scope back in working condition. What got me more interested was that the scope I was looking at and the scope that I owned, both had Vixen spec objective lenses made by the same company. I have always been pleased with the performance of my telescope and so was looking for something along similar lines. A slightly bigger telescope (say a 5" scope) would have been better however, with the long focus scopes that I am interested in, the lengths quickly become too unwieldy. A 5" scope with a similar focal ratio would be about 62" long or about 12" longer than this scope. In addition bigger scopes need even longer focal ratios to keep down chromatic aberration so something that was at lest f/13 (read 65" long) would be needed. That would have proved to be too difficult for my mount which is a Orion SV-Pro mount.

Recent years have seen rebirth of the refractor, especially the short focus scopes with a fluorite doublet or a triplet lens i.e apochromatic or APO for short. Also new to the market are the short focus refractors but with a regular 2-element achromatic lens, such as the Orion 80/100/120mm f/5 scopes. These are different from the fluorite 2-element lenses in the sense that these use the standard crown/flint lens combinations and are usually air-spaced. The Fluorite doublets on the other hand use an extra low dispersion lens that reduces the chromatic aberration even further. The APO lenses are in a complete different class than these two as the three element design combined with the extra-low dispersion elements produce a virtually color-free image. The fluorite 2-element are however costly to produce than the regular 2-element lenses while the APO lenses are way too expensive. The best part about these Fluorite and APO lenses is that the chromatic aberration can be reduced significantly even with a short focal ratio (< f/7). This is important because a shorter tube means a simpler mount will do the trick. It also makes the scope portable enough to be carried in carry on luggage on an airliner.

While the short focus refractors have seen their advantages, I am not a vary big fan of them. Although the short tubes are portable, the long focus scopes have their advantages. First and foremost is that longer focal ratios mean not just reduced aberrations, not just chromatic but also spherical and other aberrations that affect a scope. It takes a lot of work to get a short focus scope to correct for all these aberrations. Many of the cheaper scopes are not corrected very well while on the higher end, the scopes that correct (or compensate) these aberrations get too expensive. A long focus refractor can correct for many of the aberrations with even a simple crown/flint doublet and still be priced reasonably. Another advantage of these is that a higher magnification can be achieved with eyepieces that do not have extremely short focal-lengths. A 100mm diameter f/12 refractor (with 1200mm focal length) would yield 150mm magnification with a 8mm eyepiece while another refractor with similar diameter but with a focal ratio of f/5 (500mm focal length) would need need a 3.3mm eyepiece to get the same magnification. A 3.3mm eyepiece is likely to have a very short eye-relief compared to the 8mm eyepiece. Let me point out one more thing here --- while magnification is not everything, a higher magnification is needed for certain astronomical objects, mainly planets.

When I looked at the Antares Optical web-site about sometime back, they had a new product --- a 105mm (4.1") refractor which they advertised as "in the spirit of Unitron". This was in reference to some marvelous long-focus refractors made by Unitron a couple of decades ago. Unitron does not make anything similar now. This was interesting as very few manufacturers make long focus scopes. The longest focus scope in 100-120mm range that I have found from a major manufacturer is the C4 f/9.8 (102mm diameter objective and 1000mm focal length). I have long been a fan of refractors and more so of those that have focal ratios greater than f/12 or so. Looking at Antares site, they were selling the 105mm objective with three focal ratios --- f/9.5 (1000mm), f/12.4 (1300mm) and f/14.3 (1500mm). I went looking for reviews on the 1300mm and 1500mm scopes and found very few. More reviews of the 1500mm scope were found and they all pointed to negligible chromatic aberration. I looked up the dealers on Antares web-site and called a couple of them. One of these dealers, Agena Astro-products located in southern California, seemed very helpful. The owner, Manish was kind enough to answer my questions, even on a weekend. I browsed their web-site while continuing to read reviews for the scopes. My talks with the owner were mainly concerning the trade-off between the longer tube of the f/14 scope (1500mm) vs chromatic aberration of the f/12.4 (1300mm) scope. Going through the reviews and based on my talks with various people, nest day I placed order with Agena for an Antares 105mm diameter f/12.4 (1300mm focal length) scope. I think this is a good balance in the sense that the longer focal ratio (compared with some other scopes) will ensure far less aberrations than shorties (f/5 - f/7) while ensuring that the tube won't be as long as the 1500mm focal length scopes.

Now begins that waiting game. Days and days of waiting for the scope to arrive. I will keep you all posted as and when the telescope arrives.