Many would argue that the refracting telescope also known as a refractor is one of the finest possible telescope designs, it is well known for offering high contrast views and sharp optics. However, the truth is as is often the case it is far more complicated than it might appear to be at first. The refracting telescope design, also known as a refractor, has many advantages:
- No central obstruction, leading to high contrast views. The aperture is 100% clear and there is no secondary mirror in the way to block some of the light from coming in. The lack of central obstruction also reduces diffraction artifacts such as diffraction spikes, this usually leads to tighter looking stars which are more pleasing to look at and photograph
- Best resolving power per inch of aperture as compared to other optical designs, due to the lack of any obstruction, you could see finer details in in a refractor compared to other optical designs of the same size and focal length.
- A refractor rarely if ever needs collimation, normally it is adjusted once at factory and doesn’t need alignment again. Collimation is the process of aligning all optical surfaces (mirrors and lenses) to get achieve the maximum performance of the telescope.
- A refractor is easy and intuitive to use, when people imagine a telescope, they usually imagine a refracting telescope. When person seas a refractor, the immediately know which end to look in from and which end to point at the sky.
- Easy to manage condensation due to sealed optical design. Condensation is when dew forms on surfaces that are exposed to the sky, just like when your car is wet early in the morning. Many other telescope designs have exposed mirrors deep inside the telescope that can form condensation.
On the other hand, refractors have many disadvantages:
- The refracting telescopes tend to be very long, so they tend to be harder to look through when looking straight up and will need a much taller tripod. Also, this longer telescope design generally needs a sturdier telescope mount to handle the weight. So for example a 1000mm focal length refractor will be in total 1m long. while a 2000mm focal length SCT telescope will be less than 500mm long
- A good refractor is expensive to make. a cheap refractor usually suffers from chromatic aberrations, an un-flat field of view, vignetting, tube flexture etc… to improve performance, normally the manufacturer has to use more expensive glass to enhance optical performance, to improve image distortion more lenses have to be added as many as 4 in some designs further driving up the cost. Making large lenses is expensive a good quality 5″ refractor could easily cost US $ 7000 while a top end 8″ SCT can be had for US $ 1400
- Refractors can be very heavy for the same aperture when compared to mirror based optical designs.
So, who would I recommend a refractor to? I will give recommendations for both visual and astrophotography use. If you are getting started in astrophotography, I highly recommend you get a compact refracting telescope of 50 mm to 80 mm aperture as this will give you a very easy starting point to learn all the skills needed to get outstanding results. If you are and advanced astrophotographer, wide field instrument such as the William Optics Redcat is very attractive for creating mosaics. For visual use refracting telescopes of 80 to 100mm aperture tend to be easy to lift and transport and offer superbly sharp images and would be of particular interest for people who like to observe double stars, lunar observing globular clusters. The image quality of a good refractor will be a pleasure to use in these situations. If you have no objections to size and your budget is not restricted, going for a 132mm or even a 150mm refractor will give you endless pleasure in use. Such an instrument would give superb planetary views and if it is of good optical design will make for an amazing astrophotography instrument. This article is just a high-level overview, I am sure I have missed some use case scenarios where refracting telescopes could be suitable or unsuitable.