Long Focal Length Telescopes for Astrophotography – Sky & Telescope

It seems everyone in astrophotography (including me!) always gives the same advice to newcomers: Start with a short-focal-length, wide-field telescope. The problem with this advice is that it assumes that everyone wants to take photos of galaxies, nebula, star clusters, etc. But there’s more in the sky than just faint fuzzies.

The Moon and the Sun (with a proper filter), not to mention the major planets, are bright in comparison and don’t require fast optics to image. In fact, they require as much magnification as local seeing conditions allow.

In addition, unlike with deep-sky astrophotography with its lengthy exposure times, a telescope setup for high-powered visual observing is often best suited for solar system imaging as well. In fact, most lucky imaging (the technique we use for these targets) is done with small video cameras that pop into the same adapters we use for our eyepieces.

Schmidt-Cassegrain Telescopes

There are a few optics to choose from for long focal length–imaging. For the planets, nothing beats the value of a large Schmidt-Cassegrain telescope (SCT). Optically speaking, such a telescope has the focal length you want and bigger apertures are more affordable. What’s more, these telescopes provide a well-corrected image in the center of the field of view — right where you are going to place your target planet — without any additional optics. A large SCT (say, 10 inches and up) will enable you to take closeups of lunar craters and sunspots (again, with the addition of a safe solar filter).

A smaller, more portable SCT might better suit your needs, though, and you can easily increase its focal length using a tele-extender. A tele-extender is an optic that increases the focal length of your telescope, giving you more magnification. There are two primary flavors: the time-honored Barlow and the telecentric amplifier popularized by Tele Vue in its Powermate devices. These come in various powers; 2×, 4×, even 5× and are an essential part of your long-focal-length imaging toolkit. While it’s true that it’s better to have a longer native focal length optic, since additional glass can scatter light, modern optical coatings have made this concern a non-issue.

Another option is eyepiece projection, in which a camera is placed above an eyepiece to provide the additional magnification, but this technique requires a custom adapter. I prefer the simplicity of a dedicated Powermate or a high-quality Barlow, as neither require additional adapters to connect a video camera.

Maksutov-Cassegrain Telescopes

For high-resolution lunar imaging or solar imaging in white light, an SCT can also serve you well, but I prefer the larger, corrected-image field of a Maksutov-Cassegrain (MC). The Moon and Sun are brilliant compared to the planets, and a smaller aperture makes for a more portable scope. An MC in the 7- to 10-inch range is a perfect option for this kind of work.

You might still find that 2× tele-extender useful for taking advantage of nights when the seeing is exquisite and supports your highest resolution. Some telescope manufacturers produce modified SCT designs, such as Celestron’s Edge HD or Meade’s Advanced Coma-Free (ACF) designs, which produce a wider corrected field. But these scopes still require frequent collimation, and MCs tend to stay collimated longer in my experience.

The drawback of both SCT and MC designs is that they take a lot longer to cool off than a Newtonian reflector, and a warm telescope will produce very blurry images. Look for telescopes with vents and fans to pull air through, which will accelerate the cooldown, or consider investing in a cooling fan accessory. I found the addition of a fan on my 9.25 Edge HD made it usable as much as two hours earlier in my warm Florida climate.

High-resolution solar imaging with a long-focal-length optic becomes affordable with the addition of a full-aperture, white-light filter. The filter goes over the front of your telescope, providing great views and images of sunspots. The addition of a solar continuum filter will turn your view green but provides better contrast allowing you to more easily see spicules on the Suns surface.

Newtonian Telescopes

While SCTs and MCs rule the roost for long focal lengths in a compact tube, a Newtonian reflector or a refractor can also serve dual purposes for deep-sky and long-focal-length work by using the previously mentioned tele-extender. A large Newtonian is a budget saver, and the large aperture can greatly benefit high-speed lucky imaging — the more light, the better.

I’ve been able to do surprisingly good high-resolution imaging with a 6-inch apochromatic refractor. Under stable temperatures and good seeing conditions, it can deliver great views of the Sun, Moon, or planets. Under exceptional seeing conditions (thank you Florida), I can increase the focal length to 4,200mm with a 4× Powermate and shoot crater closeups.

For solar observing, a 6-inch refractor or smaller is typically recommended, because during the day the seeing conditions don’t support a larger-aperture instrument. (The bright Sun also heats up your surroundings, causing turbulence that degrades the steadiness of the view).

Many of the most popular telescopes marketed for visual astronomy make for poor deep-sky photography systems. However, compact systems with long focal lengths are excellent for most solar system targets. Moreover, a single system that can be used both visually and for the kind of astrophotography you are interested in is a win-win situation. Happy hunting!

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