Photographing the stars 1

An introduction to astrophotography

M31 Andrmeda Galaxy – 4 x 3 minute exposures 100mm f/5.5 Refractor


This is the first detailed blog posts / article discussing astrophotography and how to get involved. Over time, I will add more posts.

Note: I am not discussing solar photography here – that requires very specialist equipment. Do not under any circumstances attempt to point a camera or telescope at the sun – the heat and light can damage equipment and destroy eyesight.

In this post I am going to focus on talking about astrophotography (AP) and what I’ve been doing. I’m also going to provide an overview of what’s involved so that anyone reading this can pick up some ideas about how to take it forward for themselves.

This article would benefit from a few more diagrams: when I have the time I will add some more.

Astrophotography can be a complex and technical subject. There are a number of reasons for this. The basic one is the conditions under which you are making images is very different from conventional daytime photography. The main ones are: you have little light to work with due to it being dark; the targets are very dim; and you have to take unusually long exposures. There are others, but these are the primary ones.

The lack of light leads to several issues: you have to use longer exposures and you might have to boost the sensitivity of the sensor or more precisely, you amplify the signal by increasing the ISO of the sensor.

This leads to other factors: because you are pushing the capability of the sensor this introduces noise into the signal. In normal daylight photography, the signal swamps the noise and so it is not usually noticed. However, with AP the signal is generally very small so the noise can be significant.

There are ways of mitigating these things.

The second part of the equation is that the stars are not static: the earth is rotating on its axis. With conventional daylight photography this just isn’t an issue. However, if you are trying to photograph objects in the sky, it is a big issue.

Most people do not think about this, but the Earth rotates on its axis once very 24 hours, or 15 degrees per hour, which is 1 degree every 4 minutes. That may not sound like much, but the Moon is about half a degree in apparent diameter so that’s the width of the Moon every 2 minutes. There are several ways of handling this, which we will come to.

The other main factor is around what type of object you wish to photograph as different types of target have different requirements. Objects can be broken into several groups and each requires to some extent different equipment: constellations and Milky Way; sun; lunar and planetary; gaseous nebulae and wide field objects; galaxies, planetary nebulae and narrow field objects.

There are two main components to categorisation: whether your mount is driven or not, and focal length / field of view.

I’m going to take some of these and go through them – this will not be a comprehensive overview as that’s the work of a book! But I hope it will give people some ideas that they may wish to pursue.

Cameras, Lenses, and Telescopes

This section will look at your choices with regard to equipment.


You can either use an existing camera to start taking your astronomical photographs, or you can use a specialist dedicated astrocamera. The latter are highly specialised CCD/CMOS cameras which have to be attached to a computer to use. I am not going to discuss these any further in this post.

You can use the cameras within mobile phones. These can be used to take constellation and Milky Way photos. However, to do that you need to be able to control the parameters of the photo such as exposure time and ISO used. This can be done natively on some phones, or with the assistance of a suitable app such as Night Cap. However, the sensors on phones are very small, as are the lenses.

Compact cameras have limited abilities to control settings such as exposure time and in general are not suited to AP.

Bridge cameras are also limited. These have the ability to control things like exposure length but have fixed zoom lenses. These can be used to photograph the Moon as the lenses can have effective focal lengths of as much as 900mm. However, the optical designs of these lenses tend to be highly compromised e.g. high focal ratios, also sometimes they use ‘digital zoom’ (i.e. cropping) rather than optical zoom, which is of limited use.  They cannot be connected to telescopes.

You can use your existing DSLR or mirrorless camera. These have the ability to use different lenses and they can be connected to a telescope with a suitable adaptor that costs about £15.


I am going to assume a ‘full frame’ DSLR/mirrorless camera when discussing focal lengths. If you are using an APS-C sensor camera or micro fourth-thirds etc., then you have to multiply the focal length I quote by the ‘crop factor’ for your sensor, e.g. 1.5x for Nikon and 1.6x for Canon (there are other crop factors for other makes). So, a 50mm standard lens has an effective focal length of 75mm for a Nikon APS-C camera such as the D7500 (50 x 1.5).

For photographing constellations, the Milky Way, aurorae etc. then you need a wide-angle lens – from 10mm up to and including 35mm. Be aware that very wide-angle lenses like the 10mm will probably exhibit some distortion.

Lenses from 50mm up to around 200mm have increasingly narrower fields of view enabling more detailed images, or images of smaller objects. This will enable you to photograph for example the Andromeda Galaxy, M31.

There are various issues with longer focal length camera lenses including: they are not designed for night time low light photography; camera lenses, especially zooms, have complex optical trains with very many lenses – often over 20 – which degrades the image / loses light.

Without a tracking mount, focal lengths above 100mm are very limited due to the short exposures that can be used. If you have a suitable tracking mount, then longer focal length lenses are usable. People do use 150-600mm lenses successfully, but at 600 you have to use a tracking mount, for reasons I will explain below.


Longer focal length lenses, especially prime lenses with a fast focal ratio, are very expensive and short focal length refractors are a better choice for astronomy.

Telescope choice is dictated by two things: focal length requirement for the type of target, optical type choice. Shorter focal lengths give wider fields of view, which suits the bigger nebulae and star clusters. Longer focal lengths have a narrow field of view and are needed for detailed images of the Moon, planets, small deep sky objects (DSO). Types of telescope fall into three broad groups: refractors, reflectors, and hybrids.

Figure 1 Large Refractor. “Great Lick 36-inch Refractor Telescope (Photo Stitch)”by www78 is licensed with CC BY-NC-SA 2.0.

Refractors are telescopes which have lenses as the primary objective. Most refractors in amateur use have objective lenses with diameters between 50mm and 150mm. Refractors are classified by the type of objective lenses and the glass used. The best for colour correction for imaging are 3 element apochromats.

Short focal length refractors with a focal ratio around f/5 to f/7 (also known as ‘rich field’) have a relatively wide field of view and are suited to larger objects like M31, the Orion Nebula (M42) etc. These are widely available at various price points.

Long focal length refractors are the classic instruments of the past with focal ratios of f/12 – f/15 being typical. These have narrow fields of view and create larger images. This makes them suited to detailed lunar imaging and planetary work.

Long focal length instruments are very large – a 100mm f/15 telescope has a tube length of at least 1.5m. They require very tall mounts. Whilst they can be used for photography, long focal length refractors tend to primarily be used for imaging. I used to have regular access to a 5” (127mm) f/15 Cooke refractor – you needed a stepladder to observe through it.

Figure 2 Newtonian Reflector.”Celestron Newtonian reflector telescope” by Elsie esq. is licensed with CC BY 2.0.

Reflectors use a curved mirror as their primary objective, usually with a diameter of 150mm upwards. Half metre class instruments (500mm / 18”) and larger are not uncommon. The Newtonian configuration is the most common form of reflector: this has the mirror at the bottom of a tube with the image reflected out of the side near the top of the tube by a secondary mirror. Thus the eyepiece or camera is near the top of the tube. Classic Newtonians had a focal ratio around f/8 but modern ‘fast’ Newtonians can be around f/4.

Large reflectors are much easier and cheaper than large refractors. For astrophotography you have to mount these on an equatorial mount although for visual work the much simpler ‘Dobsonian’ mount is popular.

You have to be careful when selecting a Newtonian for photography: check that it is designed to be used with a camera as with some it is impossible to achieve focus without physically altering the instrument. Newtonians are also fussier when it comes to routine maintenance as the optics have to be re-aligned (‘collimated’) on a regular basis. Fast instruments are prone to aberrations such as coma and require extra optics to correct the aberrations.

Figure 3 Diagram of a Schmidt Cassegrain (SCT) “File:Schmidt-Cassegrain.png” by Szőcs Tamás Tamasflex is licensed with CC BY-SA 3.0.

Hybrid telescope designs that use both mirrors and lenses exist (aka ‘catadioptric’), the commonest being: Schmidt-Cassegrain Telescopes (SCT), Maksutovs (Mak), Ritchey-Chrétien (RC). The principle advantage of these instruments is that they fold a long focal length into a much shorter tube. For example, a 200mm (8”) f/10 SCT has a focal length of 2,000mm but in a tube of around 400mm length. These are suited to imaging details on the Moon, planets, and small DSO ‘fuzzies’ such as planetary nebulae and galaxies.

One downside of these is the central obstruction which houses the secondary mirror, which does reduce contrast. Users tend to regard the increased focal lengths outweigh the loss of contrast particularly as say a 250mm f/10 refractor would cost tens of thousands of pounds and probably cost another £25k or more to mount it, let alone house it.


Figure 4 German Equatorial Mount. “File:Maksutov-Cassegrain Intes M703 mounted.jpg” by Marie-Lan Nguyen (Jastrow) is licensed with CC BY 2.5.

If you read any forums or online groups about astrophotography you will quickly realise that the universal advice is ‘get the best mount you can afford’. Why?

Astrophotography requires long exposures – anything from perhaps 15 seconds up to maybe 10 minutes. The first obvious point is your mount needs to be solid, rigid, substantial. The base on which your ‘scope and/or camera rests needs to be rigid enough to resist vibration from the wind and to support the payload of camera and other kit without moving. The base used is usually either a tripod of some sort, or for more permanent settings, a pier. The second part of the equation is the mount itself.

If you are taking static photographs with a wide-angle lens, then all you need is a decent tripod to support the camera. If you want to move to taking longer exposures and using longer focal lengths, then your mount will have a drive which enables you to track the object.

Here’s why. I mentioned earlier that the Earth rotates at 1 degree every 4 minutes. Some basic maths gets us to what is known as the ‘rule of 500’. What this says is that your maximum exposure using a static camera is 500 divided by the effective focal length of the optical train in mm. So, a 50mm camera lens means: 500 / 50 = 10 seconds. This assumes a full-frame camera. If you have a cropped sensor, then you have to factor that in as well. So, a 1.5x crop means: 500 / (50 x 1.5) ~ 7 seconds.

Data is everything with AP so to do that we need longer exposures. The way we do that is to counteract the Earth’s rotation by using a tracking mount.

Equatorial Mounts

Astronomers have devised various ways of mounting a camera / telescope to follow the stars, generically these are called ‘equatorial’ mounts. The commonest form of equatorial mount that you will encounter is called a ‘German’ equatorial mount, often abbreviated to ‘GEM’.

The GEM has two axes: the Right Ascension (RA) axis and the Declination (Dec) axis. The RA axis is aligned parallel to the Earth’s axis of rotation by pointing it to the North Celestial Pole (NCP). This provides the equivalent of being able to move in longitude. The Dec axis is at right angles to the RA axis, it enables movement in the equivalent of latitude.

Because the RA axis is parallel to the Earth’s axis of rotation, once you point the camera or telescope at an object, all you have to do is ‘drive’ the RA axis and rotate it about its axis once a day. By doing that, you will track and follow the object.

This is what equatorial mounts do.

To set-up a GEM (or any equatorial mount) you have to align the RA axis to the NCP, a process called ‘polar alignment’ (PA). In the northern hemisphere we are fortunate since at the moment Polaris (alpha Ursa Minoris) is very close to the NCP. Polaris is therefore used to help with the PA process.

I’m not going into how to do that here, suffice it to say that for wide angle work with a camera, rough alignment of the RA to Polaris is adequate. For longer focal lengths ad for the ability to expose for longer, alignment is more critical and has to be more precise.

Figure 5 Camera mounted on a ‘Star Tracker’

Mounts for Cameras

There are simpler equatorial mounts designed for use with cameras and potentially small telescopes such as 60mm short-focus refractors. These are generally known as ‘star trackers’ but they are a form of GEM.

Star trackers are highly portable – they tend to fit in the hand. They can be attached to a standard tripod. Most use rechargeable batteries to power them. Generally, you attach a ball-head to the drive and attach the camera to that. Some have the option of adding a Dec axis and counterweight shaft, which enables them to carry larger lenses.

There will be a method for carrying out PA, usually with an in-built polar scope. Some have the ability to connect a separate guide camera for autoguiding.

Star trackers open up a new world of astrophotography, but depending on focal length, you are limited to 2 to 5 minutes exposure length. Autoguiding can improve matters, but most star trackers only offer adjustments in RA as the Dec axis is not powered.

Mounts for Longer Focal Lengths

The longer your focal length, or the longer you want to photograph for, the bigger and better your mount needs to be.

As a rule of thumb, to guide accurately, the weight of the entire imaging system should not exceed about 2/3 of the load capacity of the mount. So, if your GEM is rated at 15kg payload, you’re looking around 10kg for your imaging rig’s total weight.

Price essentially dictates both load capacity and capability. A decent GEM will have both RA and Dec axes driven by separate motors; there will be an in-built polar alignment scope; it will have connections for hand-controller / PC, guide camera (often called an ‘ST4 port’), and external power.

There is a lot of engineering in a decent GEM so prices are not cheap. Prices increase significantly for higher payload capacity.

A GEM will require a very substantial tripod to mount it: a Sky-Watcher HEQ5 weighs 10kg and on top of that 10kg of counterweights, plus the telescope and attachment, putting it around 30kg in total (the HEQ5 is rated to 13kg payload).

Unguided and using say a 200mm lens, a decent GEM will easily achieve 5 minute exposures. Add in a telescope and a guide camera and you’re constrained by quality of the guiding – 10 minute exposures are achievable. 

For the time being, that covers the basics of telescopes, cameras, and mounts. In later articles, I will cover how to take images, the software you need to process images, and some basics around image processing.  

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