Long Focal Length Telescopes for Astrophotography

It appears everybody in astrophotography (together with me!) all the time offers the identical recommendation to newcomers: Start with a short-focal-length, wide-field telescope. The drawback with this recommendation is that it assumes that everybody needs to take pictures of galaxies, nebula, star clusters, and so on. But there’s extra within the sky than simply faint fuzzies.

The Moon and the Sun (with a correct filter), to not point out the foremost planets, are shiny as compared and do not require quick optics to picture. In truth, they require as a lot magnification as native seeing situations enable.

In addition, not like with deep-sky astrophotography with its prolonged publicity instances, a telescope setup for high-powered visible observing is commonly greatest suited for solar system imaging as properly. In truth, most lucky imaging (the approach we use for these targets) is finished with small video cameras that pop into the identical adapters we use for our eyepieces.

Schmidt-Cassegrain Telescopes

EdgeHD telescope
Long-focal-length scopes are perfect for imaging the Moon, planets, and the Sun (with an extra solar filter).
Richard S. Wright Jr.

There are a number of optics to select from for lengthy focal size–imaging. For the planets, nothing beats the worth of a big Schmidt-Cassegrain telescope (SCT). Optically talking, such a telescope has the focal size you need and greater apertures are extra inexpensive. What’s extra, these telescopes present a well-corrected picture within the middle of the sphere of view — proper the place you’re going to place your goal planet — with none further optics. A big SCT (say, 10 inches and up) will allow you to take closeups of lunar craters and sunspots (once more, with the addition of a secure solar filter).

A smaller, extra moveable SCT may higher fit your wants, although, and you may simply enhance its focal size utilizing a tele-extender. A tele-extender is an optic that will increase the focal size of your telescope, providing you with extra magnification. There are two main flavors: the time-honored Barlow and the telecentric amplifier popularized by Tele Vue in its Powermate units. These are available varied powers; 2×, 4×, even 5× and are a vital a part of your long-focal-length imaging toolkit. While it is true that it’s higher to have an extended native focal size optic, since further glass can scatter mild, fashionable optical coatings have made this concern a non-issue.

Another possibility is eyepiece projection, during which a digicam is positioned above an eyepiece to supply the extra magnification, however this system requires a customized adapter. I choose the simplicity of a devoted Powermate or a high-quality Barlow, as neither require further adapters to attach a video digicam.

Maksutov-Cassegrain Telescopes

mak-cass imaging system
A Maksutov-Cassegrain is a superb moveable lengthy focal size possibility.
Richard S. Wright Jr.

For high-resolution lunar imaging or solar imaging in white mild, an SCT can even serve you properly, however I choose the bigger, corrected-image area of a Maksutov-Cassegrain (MC). The Moon and Sun are good in comparison with the planets, and a smaller aperture makes for a extra moveable scope. An MC within the 7- to 10-inch vary is an ideal possibility for this type of work.

You may nonetheless discover that 2× tele-extender helpful for profiting from nights when the seeing is beautiful and helps your highest decision. Some telescope producers produce modified SCT designs, resembling Celestron’s Edge HD or Meade’s Advanced Coma-Free (ACF) designs, which produce a wider corrected area. But these scopes nonetheless require frequent collimation, and MCs have a tendency to remain collimated longer in my expertise.

The disadvantage of each SCT and MC designs is that they take so much longer to chill off than a Newtonian reflector, and a heat telescope will produce very blurry pictures. Look for telescopes with vents and followers to drag air via, which can speed up the cooldown, or contemplate investing in a cooling fan accent. I discovered the addition of a fan on my 9.25 Edge HD made it usable as a lot as two hours earlier in my heat Florida local weather.

High-resolution solar imaging with a long-focal-length optic turns into inexpensive with the addition of a full-aperture, white-light filter. The filter goes over the entrance of your telescope, offering nice views and pictures of sunspots. The addition of a solar continuum filter will flip your view green however supplies higher distinction permitting you to extra simply see spicules on the Suns floor.

Newtonian Telescopes

While SCTs and MCs rule the roost for lengthy focal lengths in a compact tube, a Newtonian reflector or a refractor can even serve twin functions for deep-sky and long-focal-length work by utilizing the beforehand talked about tele-extender. A big Newtonian is a finances saver, and the big aperture can drastically profit high-speed fortunate imaging — the extra mild, the higher.

Powermate on a Newtonian
A tele-extender slides proper into the focuser like an eyepiece and turns a big deep-sky instrument right into a long-focal-length energy home.
Richard S. Wright Jr.

I’ve been in a position to do surprisingly good high-resolution imaging with a 6-inch apochromatic refractor. Under steady temperatures and good seeing situations, it may possibly ship nice views of the Sun, Moon, or planets. Under distinctive seeing situations (thanks Florida), I can enhance the focal size to 4,200mm with a 4× Powermate and shoot crater closeups.

Moon close-up
The picture scale for this picture is 0.1 arcsecond per pixel. I captured this view underneath distinctive seeing situations with a 6-inch refractor pushed to a focal size of 4,200mm.
Richard S. Wright Jr.

For solar observing, a 6-inch refractor or smaller is usually advisable, as a result of in the course of the day the seeing situations don’t help a larger-aperture instrument. (The shiny Sun additionally heats up your environment, inflicting turbulence that degrades the stability of the view).

Many of the preferred telescopes marketed for visible astronomy make for poor deep-sky pictures techniques. However, compact techniques with lengthy focal lengths are wonderful for most solar system targets. Moreover, a single system that can be utilized each visually and for the type of astrophotography you have an interest in is a win-win scenario. Happy looking!

Advertisement

Leave a Comment

Your email address will not be published. Required fields are marked *

Shopping Cart