The Advantages of a Rich Field Telescope (RFT)
A Rich Field telescope can be a bit of a confusing term. What exactly does that mean?
Well, it simply means that the telescope sees a lot more of the sky in one view. And this is important for beginners.
One of the most difficult things for a beginner to do is to find things in the night sky! You typically find something very prominent on a star chart then move the telescope into the area where the desired object is located. The following picture is an example of this. It shows the constellation Orion and what we want to find is the nebula where the arrow is. But, you can't easily see this nebula with the naked eye. So you point the telescope at a nearby bright star and then move the telescope toward the nebula.
But if the telescope is only looking at a small portion of the sky it can be challenging to do this. Picture this in your mind as if you are looking through a straw. This is exactly what is happening, you are looking through a straw at a very small area of sky. So it can be difficult to find things.
But, a rich field telescope is a much bigger straw. You see more of the sky in the eyepiece. The following two pictures (courtesy: ESA/Hubble) give you an idea of the view of the sky through a regular telescope and through a rich field telescope.
It is important to understand that the magnification is exactly the same for both these telescope views. You are not seeing one closer than the other. But with the rich field telescope you are seeing more of the sky. It goes beyond the "3" in the regular telescope view.
Rich Field Telescopes are often the preferred type of telescope for comet hunters and they are terrific for looking at Galaxies and Nebulae. Typically though a regular telescope will outperform when it comes to looking at the moon and planets.
Now the traditional view of a Rich Field Telescope (Typically an f/5)
How is a rich field telescope different? How is it made?
The following picture shows the light path in a telescope. The important thing to note is the focal length. In a regular telescope (the top drawing) the focal length might be 8 times the diameter of the mirror. This would be called f/8. In a rich field telescope (the lower drawing) the lens or mirror is made so the focal length is a lot shorter. It will be around 4-5 times the diameter of the mirror or an f/4 or f/5. This bend the light at a sharper angle which gives you the bigger field of view. And it also makes for a shorter telescope.
Let's take a look at some Rich Field Telescopes
A fully computerized, electronically driven telescope with outstanding 80mm rich-field optics, which can take you to any of 4,000 celestial objects with a few button pushes on its illuminated hand controller. Wow! From the first night out, any beginner can quickly access the night sky's finest gems. The multi-coated 80mm (3/1") achromatic lens is housed in a compact tube equipped with a 1.25" R&P focuser, two Kellner eyepieces, star diagonal, and a red-dot finder scope. The one-arm mount is made of cast metal for greater durability than competing GoTo scopes that use lots of plastic parts. The whole setup is incredibly portable and a joy to use.
The Explore Scientific 152mm f/4.8 is a rich field telescope (RFT), capable of showing faint objects over a wide field of view (generally two degrees or more). Explore Scientific has co-designed a Maksutov-Newtonian telescope with comet hunter and author David Levy that can help support the Sharing the Sky Foundation through the sale of this telescope. The Comet Hunter comes standard with a two-inch, 30mm focal length eyepiece with a wide 70° apparent field. This eyepiece combined with the short focal length of the telescope produce an amazing 2.86 ° true field of view with a 6.2mm exit pupil. Each Comet Hunter special edition telescope will come with a certificate signed by David Levy.
Our AstroView 120ST is a compact, rich-field refractor designed for observation of nebular clouds, star clusters, and even galaxies. Its multi-coated achromatic objective lens has a 120mm (4.7") clear aperture and has a focal length of just 600mm (f/5). Compared to our standard 120mm AstroView (f/8.3), that translates to a 66% wider field of view for any given eyepiece. The fast f/5 optical system also makes it an excellent telescope for astrophotography. The 120ST has a well-baffled optical tube to ensure good contrast. The cast-metal, 2" rack-and-pinion focuser accommodates either both 1.25" and 2" diagonals. The AstroView equatorial mount with adjustable aluminum tripod provides a sturdy platform for precision tracking. Add an optional EQ-3M electronic drive for fully automatic tracking. One-year limited warranty. Orion AstroView 120ST EQ Refractor Telescope