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The Telescope (OTA)

No Eyepiece Needed

The Optical Tube Assembly (OTA) is the part of the telescope that sits on top the mount. In most visual scope rigs, there is a special holder for eyepieces. But when using EAA approaches, we swap out these eyepieces for a camera. So the OTA holds the lenses or mirrors and the camera.

Any Rig Will Do

There are many different kinds of OTAs. Describing them all is beyond the scope of this EAA101. The truth is, most any good OTA will work for EAA, though we try to make some general suggestions below. Also, we encourage readers to check out the “Profiles” chapter below. In it, each EAA practitioner lists the components of his or her “rig” or kit. Because each EAA practitioner has different opinions about what might work best, there are many different kinds of lash-ups for EAA. That’s a good thing. It represents the wide and varied options from which you can choose in this amazing hobby. In this chapter, we aren’t going to “box you in” by giving opinions about what’s best. We ARE going to try to list some widely-held opinions (if such a thing exists! haha).

Aperture: It Matters

EAA is all about watching the object, appreciating it “live” in near-real-time fashion. As a result, it makes sense that EAA practitioners will want to buy a rig with the widest aperture possible. Why do we say this? Because the wider the aperture, the faster the OTA will “gather light.” That having been said, the type of scope is perhaps less important. One can enjoy EAA with a fast Newtonian reflector, a go-to Dobsonian, a standard Schmidt-Cassegrains (SCT), a Ritchey–Chrétien (RC), a Rowe-Ackermann Schmidt Astrograph (RASA), or any other style.

Focal Ratio: It Matters

Beyond sheer aperture or width, we measure the “speed” of the OTA’s light-gathering capacity by using a term called “focal ratio.” Focal ratio is defined as the ratio of the aperture of the OTA’s primary lens or mirror to its focal length. The lower the number in the focal ratio, the faster the OTA can gather light. So — objects will show up faster, in greater detail, and with greater color in an f/4 scope in a given time than they will in an f/10 scope. All things being equal, EAA practitioners will want the lowest focal ratio possible. (Again, a lower number signifies a faster ‘speed’ of light-gathering.)

The Decision Is Yours

Ultimately, the decision about which OTA to purchase will be a subjective one. It will be based on many criteria, including your budget, your desire/willingness to carry a heavy OTA (or mount it in an observatory), how wide of a field of view you prefer (size of objects), and many other considerations. If you’re just starting out and you’d just like to see options of different rigs that might work together well, this page/article can serve as a decent starting point for would-be shoppers to get an idea of the current market.

Learn More At…

(Note that none of our links are “affiliate” links. We aren’t making money off of your clicks.)

Agena Astro – “Beginners Guide to Choosing Equipment”

California Skys – “Choosing a Telescope for EAA”

OPT – “Example Astrophotography Setups”

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