Archive for August 14th, 2012


What aperture costs

August 14, 2012

Just for the heck of it, I decided to find out which telescopes are the best deals in terms of light grasp. My comparison group consists of widely available commercial dobsonian reflectors ranging from 3″ to 16″ in aperture. Sometimes I included two models at the same aperture to show the effect of differing features, like the finder scope and better eyepieces on the Orion Funscope versus the Celestron Firstscope, or the collapsing truss-tube design on the Meade Lightbridge scopes. Prices are street, not list, as of this writing. Each entry follows this layout:

Brand Model – mirror diameter (mm) – mirror area (in^2) – price – cost per in^2

Celestron Firstscope – 76mm – 7.065 in^2 – $38 – $5.38/in^2

Orion Funscope – 76mm – 7.065 in^2 – $50 – $7.08/in^2

Orion SkyScanner – 100mm – 12.56 in^2 – $100 – $7.96/in^2

Orion StarBlast 4.5 – 114mm – 15.89 in^2 – $200 – $12.59/in^2

Bushnell Ares 5 – 127mm – 19.63 in^2 – $165 – $8.41/in^2

Orion StarBlast 6 – 150mm – 28.26 in^2 – $280 – $9.91/in^2

Orion XT6 – 150mm – 28.26 in^2 – $280 – $9.91/in^2

Orion XT8 – 200mm – 50.24 in^2 – $350 – $6.97/in^2

Orion XT10 – 250mm – 78.5 in^2 – $580 – $7.39/in^2

Meade 10″ Lightbridge – 250mm – 78.5 in^2 – $700 – $8.92/in^2

Meade 12″ Lightbridge – 300mm – 113 in^2 – $1000 – $8.85/in^2

Orion XT12i – 300mm – 113 in^2 – $1100 – $9.73/in^2

Orion XX14i – 350mm – 154 in^2 – $1700 – $11.04/in^2

Meade 16″ Lightbridge – 400mm – 200 in^2 – $2000 – $10.00/in^2

Interestingly, there are two low points where the price dips below eight bucks per square inch: at the low end, with the 3″ scopes, and in the middle, with the 8″ and 10″ solid-tube dobs. It’s also interesting to note that the StarBlast 4.5, which is an extremely popular scope, is the most expensive in this group in terms of cost per square inch.

I only included dobs because everything else is more expensive–tripod-mounted Newtonians, catadioptrics, and refractors alike. Here are some comparative costs for beginner instruments of those other designs; for fairness, I only picked models with mounts included.

Orion SpaceProbe 3 Alt-Az (reflector) – 76mm – 7.065 in^2 – $100 – $14.15/in^2

Orion GoScope 80 (refractor) – 80mm – 7.74 in^2 – $100 – $12.92/in^2

Orion StarMax 90 Tabletop (catadioptric) – 90mm – 9.85 in^2 – $200 – $20.30/in^2

The difference here is that there is no mid-aperture dip as there is for the dobs, or if there is, it doesn’t bring the price per square inch near the $10 mark, let alone under it. Indeed, one would be hard-pressed to find a new, high quality 4″ achromatic refractor for less than $300-400, which is up in the neighborhood of $30/in^2. Similar prices obtain for mounted 5″ Maks and 8″ SCTs. That’s why I didn’t bother to account for the effect of the central obstructions in calculating the costs of the reflectors and catadioptrics; even what seem to be large secondary obstructions are usually less than 10% of the total collecting area, and dobs cost anywhere from a half to a fifth as much as other popular, “everyman’s” scopes in terms of collecting area.

Now,  I’m not saying that dobs are objectively superior to other scope designs. They’re cheaper. We all knew that, I’ve just quantified it, snapshot style, using currently available models and prices. I did it because I had a gut feeling that 8″ and 10″ solid-tube dobs were in sort of a sweet spot, price-wise, and that actual costs (per square inch of collecting area) rose a bit on either side. And they do.

Finally, these numbers put some classic deals into perspective. When the SkyWatcher 130N was on sale for $100, its cost was just a hair over five bucks per square inch–better than any of the models listed above by a considerable margin. The SkyWatcher 70AR was at one point selling for $36 shipped, or just a hair rover six bucks–a bit more expensive, per square inch, than the Celestron Firstscope, but for a much more capable instrument. I think the  only refractor deal in history that has ever equalled that was the Galileoscope. With 50mm of aperture and an introductory price of $15, it was originally selling for $4.78/in^2, but that was without a mount. Similarly, the value of buying used is now apparent–when I got my XT10 for $350, that was a cost per square inch of $4.46, much less than any of the new scopes, even the cheapies.

What should you buy? That’s a more complicated question, and it can only be answered by reference to your situation and your observing goals. I own a mid-sized Mak because sometimes it’s nice to have 5″ of light grasp in a package a foot long, and I bought it new because sometimes it’s nice to have something fresh off the line. But if you’re interested in deep-sky work, where every photon counts, and you’re on a budget, then you might find the numbers above useful in considering which scope will give you the most bang for your buck, or for evaluating future scopes.

Why you want as much aperture as you can get is the subject of the next post–and for a contrary view, see this post.