2. The Telescope

2.1 Terminology and Fundamentals of Telescopes for Astrophotography

Photographers who are reading this will be familiar with much of the terminology discussed here. Telescopes are categorised by four main optical properties:

Optical designs of telescopes will be covered in detail shortly. We note however that it can be tempting to want the largest telescope money can buy but there are two things one needs to consider here. Firstly, large telescopes become impractical to carry around and set up, especially if you are by yourself or your car does not have the capacity for it. The advice "It is better to have a small telescope that gets used often than a large telescope that seldom gets used" is extremely applicable here. Secondly, as you will learn, large telescopes are actually not what you want for astrophotography. This may initially seem like an oxymoron, considering that larger aperture leads to more visual detail but actually, that applies only to observational astronomy. 

You do not care if when you look at your target through an eyepiece you see absolutely nothing, because the camera will most definitely pick up all the details after a few minutes exposure. That is the key issue at hand. Your eyes do not collect light over time to build image brightness - they simply see what is coming in. A CCD sensor on a camera however does build image brightness as light is collected over time. What in fact changes how fast your image gets brighter and brighter (revealing fainter details) is the focal ratio. Specifically, the lower the focal ratio, the faster the telescope is said to be. This term, fast, comes from the fact that you can gather the same image brightness and detail as a telescope with higher focal ratio, over a shorter period of time. 

Why is this important? Simply put, because imaging time is precious. Consider the following. Your target requires 7 minute exposures to gather enough detail to call it a good image, using a telescope of f/5 focal ratio. However, good technique states one should actually capture more than one image of the same thing in order to average them later. Let us then propose 10 images. Since you are using a DSLR, which is a colour camera, one exposure constitutes a full image. At 7 minutes per exposure, 10 exposures takes a total of 70 minutes, or 1 hour and 10 minutes. Add the time setting up and packing up and you can be done in 2 hours. Now let us do the exact same thing but with a telescope of f/10 focal ratio. This is where the numbers get interesting, and revealing. The square of focal ratio is a measure of image brightness. So, for the f/10, we calculate 10² = 100 and for the f/5, we calculate 5² = 25. As a ratio between them, 100 / 25 = 4. This means that the f/5 telescope is actually 4 times faster than the f/10 telescope. Earlier we said that we needed 7 minute exposures for our target, using the f/5 telescope. This means that we will need four times as much time with the f/10 telescope, which is 28 minutes. Multiply by 10 due to wanting 10 exposures and that is 280 minutes, or 4 hours and 40 minutes. Add in setting up and packing up time and you are looking at about 5 hours and 30 minutes to be done. 

Compare those numbers - f/5 required 2 hours of your time and f/10 required 5 hours and 30 minutes of your time, for the exact same result. Moreover, with the f/5 we were talking about 7 minute exposures but with the f/10 we were talking about 28 minute exposures. 28 minute exposures are very long exposures, even by astrophotography standards. A lot can go wrong in 28 minutes to spoil your exposure, or your tracking may not be 100% precise and minor errors will be strongly magnified after 28 minutes. A professional astrophotographer will actually use a monochrome CCD camera, not a colour CCD camera (for reasons stated in the next section). This means that in order to build a colour image, the astrophotographer must use a set of colour filters to capture the same image in Red, Green and Blue, to later combine together. There is also Luminance, to provide detail across the spectrum. This brings us back to those numbers. This astrophotographer will need 7 minute exposures with the f/5 and 10 exposures but for each filter. At 10 exposures per filter (there are four) and 7 minutes per exposure, that is actually a total time of 280 minutes, which is 4 hours and 40 minutes. This is doable in one night, but the poor astrophotographer with the f/10 telescope trying to do the same thing will need four times as much time - 1120 minutes, or 18 hours and 40 minutes. That is an unwieldy amount of time. 

The lesson here is simple - focal ratio, focal ratio, focal ratio. The key to an astrophotography telescope is focal ratio. Of course, focal length also plays a role in astrophotography, and this has to do with your field of view, or how much you appear to have magnified into your target. This is important because if your field of view is massive, then imaging a very distant galaxy (that appears tiny in the night sky) would be unfruitful. Conversely, if your field of view is really small, trying to image large targets such as the Andromeda Galaxy may be problematic as it literally does not fit in your images. Telescopes are not like camera lenses - you cannot just extend and shorten them on the spot. Focal length is fixed by optical design but there are lenses in the market, called focal extenders/reducers, that can alter this. We note however that by changing focal length optically, since aperture is fixed, you change your focal ratio and this may have undesirable effects if you are increasing the focal ratio!

2.2 Telescope Optical Designs for Astrophotography and Inherent Problems

Newtonian Reflector

The Newtonian Reflector is the simplest type of telescope by optical design, consisting of a primary mirror at the back that reflects the light to a secondary mirror near the front (held centrally in place by four spyder vanes). This secondary mirror then reflects the light out of telescope through the focuser. 

Due to their inherently simple optical design, Newtonian Reflectors are very cheap compared to other types of telescopes. This does not detract from their optical quality or ability to produce incredible images, however. Given how these telescopes work, optically, the focal length is simply the length of the tube. The one displayed above is the Skywatcher Explorer 150PDS, a 6" aperture telescope with 750 mm focal length, making it an f/5 (excellent for astrophotography). This does mean however that long focal length Newtonian Reflectors are also long in terms of the physical tube. As an example, the Skywatcher Explorer 300PDS, which is the 12" brother of the above, has a focal length of 1500 mm (also f/5). The tube is actually just over 1.5 metres in length!

There is a disadvantage to Newtonian Reflectors of any size, however. Since the light reflected on the primary mirror needs to be reflected on to a smaller secondary mirror near the front, the primary mirror is circularly shaped. This produces images that elongate stars toward the outer edges of your images. This optical aberration is called coma. Images then appear as circularly shaped (not flat), with all the outer edge stars pointing inwards in an elongated pattern. The problem in fact gets worse as telescopes get faster (lower focal ratio), which ironically are what we want for astrophotography. Coma Correctors are commonly paired with Newtonian Reflectors and these are lenses that correct the circularly shaped image to produce flat images. 

Another disadvantage of Newtonian Reflectors is that the mirrors need to be aligned with each other (collimation). This is important so that the image forms properly. Collimation is not a difficult task but it can be confusing at the beginning. The spyder vanes that hold the secondary mirror in place as well as the secondary mirror obstruct the view from the primary mirror. This has the effect of reducing image contrast. The spyder vanes themselves cause diffraction of incoming light and produce star diffraction spikes in bright stars (some astrophotographers like this however). 

+ Cheap optical design

+ Easy to manage in terms of maintenance

+ Mirrors deep enough inside tube so that dew rarely forms, if ever

+ Easy to find low focal ratio telescopes

- Long focal length makes telescope unwieldy due to physical size

- Suffers from coma (worse as telescope is faster) requiring coma corrector

- Spyder vanes and secondary mirror obstruct view, reducing contrast and causing diffraction of light, producing star diffraction spikes in bright stars

- Collimation needed to align mirrors and keep them aligned

Maksutov-Newtonian Reflector

The Maksutov-Newtonian Reflector is a more advanced design of the simple Newtonian Reflector. The same principles apply in which a circularly shaped primary mirror in the rear reflects the light on to a smaller secondary mirror near the front. As aforementioned, this circularly shaped primary mirror leads to coma. Maksutov-Newtonian Reflectors have a lens element at the very front, which is responsible for correcting this coma as it perfectly matches the primary mirror's circular shape. 

Much the same advantages and disadvantages apply for Maksutov-Newtonian Reflectors as with Newtonian Reflectors. The design however attempts to overcome to main inherent fault of Newtonian Reflectors, which is coma. Maintenance however can be a bit harder as the tube is closed. Moreover, since there is a front lens element to the telescope, dew can easily form. 

+ No coma as front lens element matches optics to correct it but can be an issue if focal length is altered with focal extenders/reducers

+ Fairly easy to manage in terms of maintenance

+ Easy to find low focal ratio telescopes

- Long focal length makes telescope unwieldy due to physical size

- Dew can easily form as front lens element is very exposed

- Spyder vanes and secondary mirror obstruct view, reducing contrast and causing diffraction of light, producing star diffraction spikes in bright stars

- Collimation needed to align mirrors and keep them aligned

- Very specialist design and therefore not a lot of choice of telescopes in the market

Refractor

Refractors are of iconic design - a simple tube with a lens element in the front and the focuser in the rear. Light entering the telescope is refracted into focus by the lens element in the front. 

Refractors are of very pure design as they have a single lens element to focus the light and nothing more. The issue is with weight and cost. The lens element is a large piece of glass that is heavy and very expensive when apertures become large. An advantage however of having nothing obstructing the lens is higher image contrast and sharp star shapes without diffraction spikes on bright stars (some astrophotographers like these so they add them in post-processing). 

Refractors do not require collimation as there is only the single lens element at the front. As per the laws of Physics however, light of different wavelengths (i.e. colours) refract at different angles. This poses a problem as all light coming in must refract to the same point to focus properly. Some refractors are of more advanced design and employ various elements in order to properly refract all colours of light to the same focal point (these are called Apochromatic Refractors). 

+ Increased contrast and no star diffraction spikes in bright stars due to lack of front obstructions

+ Extremely simple to manage with zero maintenance required

+ No real need to collimate

+ Easy to find low focal ratio telescopes

- Design is long, heavy and expensive and becomes prohibitively so at apertures of 5" and larger

- Some may still need focal reducers for optimum performance, which tend to come paired or offered as an accessory

- Dew can easily form as front lens element is fairly exposed (refractors have integrated dew shields, which help)

Schmidt-Cassegrain

Schmidt-Cassegrain telescopes are very popular as they have inherently long focal lengths and are therefore very useful for planetary astrophotography. Light enters the front through a coma corrector plate (similar to a Maksutov-Newtonian Reflecor) and is reflected from the mirror at the rear on to a secondary mirror that acts as an image flattener, reflecting the light to the rear and out the focuser. 

The Schmidt-Cassegrain design of telescopes is of closed tube design, but effectively uses the length of the tube three times to focus the light. This leads to a much longer focal length than is inherent to the physical tube, making for a compact telescope capable of a lot of magnification. Unfortunately for deep space astrophotography, this does imply a high focal ratio and f/10 are commonplace. 

As with other reflector kind of telescopes, the secondary mirror presents an obstruction that reduces image contrast. Additionally, since the tube is closed and the coma corrector plate is at the very front and exposed, dew can easily form. Maintenance is easy as the the tube need not be opened and thus dust is not introduced and can be easily cleaned from the front coma corrector plate. Collimation is not needed often but is not as easy to achieve as with simple Newtonian Reflectors. Collimation involves simply tilting the primary mirror at the back to correctly match the secondary mirror at the front. 

+ Compact design due to multiple reflections makes for long focal length with short physical tube

+ No coma by inherent design, but can be an issue if altering focal length with focal extenders/reducers

+ Easy to manage with little maintenance

- Design makes for high focal ratios with f/10 being commonplace, needing a focal reducer

- Focal reduction to reduce focal ratio can also introduce coma

- Dew can easily form as front coma corrector plate is very exposed

- Secondary mirror presents an obstruction, reducing contrast

- Collimation can be more difficult than with Newtonian Reflectors

Ritchey-Chretien

Though not heard of often, Ritchey-Chretien telescopes are actually extremely widely used in professional installations. In fact, the Hubble Space Telescope is of Ritchey-Chretien design. These telescopes are open tube telescopes similar in design to Schmidt-Cassegrain telescopes but remove coma by hyperbolic shaped mirrors. 

Since there are no lens elements at the front and tube is open, there is little chance of dew forming on the mirrors, but it does open the tube to dust falling in. The hyperbolic shaped mirrors remove coma almost entirely and this is effective even when changing the focal length of the telescope with focal extenders/reducers. Since the telescope retains its Cassegrain design, the focal length achieved is much longer than the physical tube size. Since these telescopes are designed for astrophotography however, they are designed with lower focal ratios than Schmidt-Cassegrain telescopes. 

As with Schmidt-Cassegrain telescopes however, collimation is not often necessary but can be more difficult to achieve than with Newtonian Reflectors. It is however necessary to ensure that the primary mirror at the rear is tilted properly toward the secondary mirror at the front. Additionally, the secondary mirror and spyder vanes that hold it in place present an obstruction that reduce contrast and also produce star diffraction spikes in bright stars. 

+ Compact design due to multiple reflections makes for long focal length with short physical tube

+ No coma by inherent design, even when changing focal length with focal extenders/reducers

+ Easy to manage with little maintenance

+ Design makes for lower focal ratios than Schmidt-Cassegrain telescopes

+ Mirrors deep enough inside tube so that dew rarely forms, if ever

- Collimation can be more difficult than with Newtonian Reflectors

- Spyder vanes and secondary mirror obstruct view, reducing contrast and causing diffraction of light, producing star diffraction spikes in bright stars

- Focal ratios still too high for serious astrophotography, requiring a quality focal reducer

2.3 Effects of Focal Length on Field of View

In sub-section 2.1, we covered the fundamentals of astrophotography with a telescope, including the four main factors to consider when choosing a telescope. In particular, aperture was deemed and explained as less important than focal ratio when it comes to astrophotography. Though focal ratio is extremely important to consider, the focal length was mentioned as having some importance when it came to the level of magnification and therefore the field of view. This sub-section is dedicated to covering the details of this concept. 

The field of view of an optical system depends on two things - the telescope and the camera. The telescope plays its role through its focal length. The camera plays its role through the size of the CCD sensor and the size of each pixel within the CCD sensor. In order to explore how the telescope affects field of view, we will fix the camera being used and simply change the telescope's focal length, using some real-life examples against the night sky. 

The camera we will use for these examples is the Canon EOS 650D DSLR - a particularly good DSLR for astrophotography. Dedicated astronomical CCD cameras can be used all the same, but it is more common for astrophotographers to get into astrophotography through a telescope by starting with a DSLR camera. The Canon EOS 650D has a CCD sensor size of 22.3 x 14.9 mm, with resolution 5184 x 3456 pixels and a pixel size of 4.3 µm. These details were added to Stellarium for simulation of field of view:

We now add a telescope. The Skywatcher Explorer 150PDS is a good example, with a 6" aperture, a focal length of 750 mm and therefore a focal ratio f/5:

We can now enable the ocular in Stellarium to get an idea of the field of view presented by this optical system - the Skywatcher Explorer 150PDS telescope and the Canon EOS 650D DSLR camera. The red rectangle shown below on the night sky, next to the Cassiopeia constellation, is the field of view:

On the second screenshot above, we have panned up slightly and zoomed in on the Heart Nebula. The Skywatcher Explorer 150PDS with its 750 mm focal length seems to just about suffice to encompass the Heart Nebula, but not entirely. Let us now reduce the focal length of the telescope to a shorter 500 mm. 

Clearly reducing the focal length has increased the field of view and now the view encompasses the entire Heart Nebula quite well. Conversely, if we increase the focal length to 1500 mm:

Quite clearly, increasing the focal length has decreased the field of view and now the view is extremely tight and encompasses only the core of the Heart Nebula. Overall we can summarise the effects of focal length on field of view as:

We note of course that changing focal length changes focal ratio, as focal ratio is just focal length divided by aperture. For example, if you have a telescope with a short focal length and you wish to zoom in more, you would think a focal extender is a good idea. Let us say your telescope has a 6" aperture (150 mm), a 750 mm focal length and is an f/5 (perfect for astrophotography) but you wish to tighten the field of view and therefore use a 1.33x focal extender to make this 1000 mm. Since the aperture is fixed at 150 mm but your focal length is now 1000 mm, your new focal ratio becomes 1000 / 150 = 6.67 (or f/6.67). Though this is still good for astrophotography, your imaging is now slower. By how much can be estimated by squaring f/6.67 and squaring f/5, then finding the ratio between them, i.e. 6.67² = 44.4889 and 5² = 25 so 44.4889 / 25 = 1.78 (rounded off). In effect, your focal-extended telescope is now 78% slower than previously. In other words, if an exposure took you 5 minutes to capture a certain level of detail, it should now take you roughly 8.9 minutes. This may not seem like a big deal but take into account you may be capturing a lot of exposures and this extra time stacks up. 

Conversely, you may have this telescope (6" aperture, 750 mm focal length and therefore f/5 focal ratio) but wish to capture a larger portion of deep space. For this you use a focal reducer (very common in astrophotography, mainly to reduce the focal ratio and make the telescope faster). Let us say you use a 0.67x focal reducer to make the focal length 500 mm. This again changes the focal ratio accordingly, to 500 / 150 = 3.33 (or f/3.33). The telescope has now become faster. By how much can again be calculated, i.e. 3.33² = 11.1111 and 5² = 25 so 25 / 11.1111 = 2.25. This means your telescope is now over twice as fast (225% faster). An exposure that used to take you 5 minutes at f/5 would now take you roughly 2.2 minutes. Again, this really does make a tremendous difference when it comes to capturing a lot of exposures. Of course, one could say that if 5 minutes was ok, then why not capture 5 minute exposures at f/3.33 as well? A good idea, actually, as you will be capturing more faint detail and making the image brighter than using the same exposure time but at f/5. 

An important thing to note however, is that depending on the telescope optical design, changing focal ratio may not be ideal (also dependent on focal extender/reducer being used). For example, in a Newtonian Reflector, the lower the focal ratio, the more pronounced coma becomes and the more demands are placed on the coma corrector. These demands may exceed capabilities and yield undesirable results. This is why, generally, a telescope is bought with its initial specifications in mind and how well it performs with a specific focal reducer and perhaps coma corrector. The subject matter is not as simple as a camera lens on a DSLR, where though of greatly varying optical quality, can generally produce nice results at various magnifications (particularly since in terrestrial photography, the subject is generally well illuminated and readily visible). 

2.4 Choosing the Right Telescope

Choosing the right telescope for your own astrophotography is not an easy task as there is a lot of choice out there, of varying optical designs. Hopefully after having read the above, you will have a good idea of what optical design you would prefer and what kind of focal ratio you are looking for. One can however come up with various recommendations:

With the above information, one should be able to make a more informed decision of the telescope to buy. The following are recommendations to date (24 / 12 / 2013) of telescopes for astrophotography. These vary in price and capabilities but meet all four points above in a varying fashion. These are only some as there are of course more in the market, particularly other markets (e.g. the USA's).