Telescopes

An Introduction to Astrophotography

An Introduction to Astrophotography is an article that provides an introduction to the points that should be considered when starting out on astrophotography. Different set-ups are required for the solar system than for deep sky objects (DSOs). Our solar system contains bright objects such as the moon, Jupiter, Saturn, Venus and Mars that reflect sunlight. Not forgetting the sun of course (our own star). DSOs are much further away in the universe and are therefore much dimmer, for example galaxies, nebula and globular star clusters. For DSOs, more sophisticated systems are required to capture the little light that reaches us in order to produce a good image.

 

Photographing objects in our solar system

 

There are a wide range of methods, from simple to sophisticated, for imaging objects in the solar system. Below are descriptions of the main methods, the equipment you will need and some of their advantages and disadvantages. Details of how to use these products and the specific cameras available are not covered.

 

365Astronomy Digital Camera, iPhone, Smartphone Microstage Adapter for Telescopes, Spotting Scopes & Binoculars

365Astronomy Digital Camera, iPhone, Smartphone Microstage Adapter for Telescopes, Spotting Scopes & Binoculars

The simplest method is a afocal astrophotography using a compact camera. You need a telescope set up with a bright object in the eyepiece. The camera is simply held up to the eyepiece and a photograph taken. This method is typically used for imaging the moon. An advantage is that you have an instant photograph of what you have taken so can change the settings on the camera and keep taking images until you find the optimum settings. A challenge can be keeping the camera steady while taking the photo. A tripod for the camera placed next to the telescope and a camera remote control can be used to keep the camera steady.

 

Eyepiece Projection Adapter from 1.25

Eyepiece Projection Adapter from 1.25″ to T2 – Variable Lenght

Projection astrophotography uses a specifically designed eyepiece adapter to attach a DSLR camera to the eyepiece on the telescope. A DSLR is needed because the lens needs to be removed from the camera and only the camera body attached to the adapter. This is a more sophisticated system of astrophotography. The camera is held steady on the telescope so the problems with afocal astrophotography are reduced. It is advisable to use a DSLR with a live view or video function. The video function will take a number of frames per second. If, for example, a 60 second video is taken of Jupiter, specialised free software (for Windows) such as Registax or Autostakkert selects the best frames in the video and stacks them to significantly enhance the final image. Ideally a camera remote control is used. Also link the camera to a laptop computer as this allows you to see what you are imaging and makes focusing far easier.

 

Prime focus astrophotography is similar to projection but does not use an eyepiece. An specific T-ring adapter is used to attach the DSLR body directly to the back of the telescope. The telescope in effect functions as the camera lens. Videos can be taken and processed as described for the projection method above.

Celestron T-Adapter for Edge HD 8

Celestron T-Adapter for Edge HD 8

 

An advantage of the Projection method is that is can produce more detailed higher magnification images compared to Prime focus. This is excellent for the moon and planets. However, the higher magnification tends to make the image dimmer so longer exposures are required. Any atmospheric disturbance will also further distort the image at high magnification. Prime focus produces a smaller but clearer and brighter image. This can be well suited to photographing the moons of the larger planets such as Jupiter and Saturn and images of the whole moon rather than specific features.

 

Another option is to use CCD cameras. These cameras have been specifically designed with astrophotography in mind. They are smaller than the compact and DSLR cameras and are inserted into the diagonal of the telescope in the same way as an eyepiece. A USB lead connects the CCD camera to a laptop computer. Specifically designed capture software shows an image on the laptop screen of what the camera sees. Videos can be taken using controlled settings available in the capture software and processed using stacking software such as Registax as described above. There are a wide range of CCD cameras on the market and many of these cameras are designed to take a high number of frames per second, have powerful sensors and can also incorporate cooling systems to reduce background noise in the final image. This can provide better images compared to a DSLR but can present a more expensive option.  Astrophotography is also possible using a camera without a telescope. Taking photos of star constellations, the moon and star trails can be accomplished by attaching a camera to tripod and adjusting the settings for whatever image you are taking and for the weather conditions. Compact cameras, DSLR and Bridge cameras can be used. High magnification zoom lenses can also be purchased for DSLR cameras. This allows the astrophotographer to zoom in on specific objects in the night sky such as the moon or the milky way.

Celestron Skyris 445C Colour Astronomical CCD Camera

Celestron Skyris 445C Colour Astronomical CCD Camera

 

Finally, our own star, the sun, can also be imaged. However, specialised equipment incorporating strong light filters is required for solar astrophotography as the sun is obviously so bright. Only this specialised equipment should be used for imaging or viewing the sun. Standard cameras and telescopes should never be pointed at the sun. Not only will this severely damage your eyesight it will also ruin your equipment. Once these points are managed, imaging the sun is an exciting aspect of astrophotography.

 

Lunt LS60THa/B600C 60mm H-Alpha Telescope, Crayford Focuser

Lunt LS60THa/B600C 60mm H-Alpha Telescope, Crayford Focuser

Refractor telescopes can be used for solar projection. This is a simple method of solar astrophotography whereby the telescope is pointed at the sun and the image of the sun is projected onto a piece of white card placed behind the eyepiece. A camera can then be used to take a photograph of the projected image. A Herschel wedge (prism) can also be mounted onto the back of a refractor telescope with a standard eyepiece placed into the wedge. This filters out a high percentage of the light and heat allowing the sun to be imaged through the telescope using the methods described above for the moon and planets. The quality of image will depend upon the telescope and camera. Another option is to place a white light solar filter over the end of the telescope. This filters out a high percentage of the sunlight also allowing the sun to be imaged. However, this does mean that some wavelengths of light are filtered out which reduces the quality of the final image compared to using a herschel wedge. For the best images of the sun solar telescopes can be used. They incorporate specialised filters, for example hydrogen-alpha, to enhance wavelengths of light to specifically highlight the sun’s features. They also use the video and frame stacking method described above. However, solar telescopes tend to be expensive with a basic package typically being in excess of £500, so budget can be a limiting factor.

 

Photographing deep sky objects (DSOs)

 

It is difficult to talk about imaging DSOs without mentioning telescope mounts. The stars and other celestial objects appear to move across the sky due the earth’s rotation. To obtain a long exposure of a dim DSO the telescope needs to track the object as it moves across the sky to keep it in the eyepiece over a long period of time. This is the role of the mount. Below is a description of the types of mount that can be used for DSO astrophotography, camera options and the use of autoguiders.

 

Celestron Advanced VX Telescope Mount

Celestron Advanced VX Telescope Mount

Fork mounts can be used to take images of the solar system and DSOs. They either have 1 or 2 arms. One-armed fork mounts are best suited to imaging the solar system. A two-armed fork mount with a wedge to tilt the telescope to the degree of latitude of a specific location can be used to take long exposures of DSOs. Many telescopes invert the image. A de-rotator can be used so that an image is taken of how the object actually appears in the night sky. However, this is still limited to minutes of exposure rather than hours that are required for high quality images of DSOs far away in the Universe. Equatorial mounts can provide longer exposures. They firstly need to aligned to the celestial pole (very near the north star (polaris)). The equatorial mount (with a suitable telescope attached) then accurately tracks the object to be imaged as it moves across the sky using the celestial pole as a reference point and staying parallel with the earth’s axis of rotation.

 

A DSLR camera can be used for imaging DSOs. CCD cameras specifically designed for imaging DSOs are also available. CCD cameras incorporating cooling devices are especially helpful for long exposure astrophotography to reduce background noise that will effect the quality of the final image. DSO images are normally processed by staking images. The images (exposed for perhaps 1-5 minutes) are stacked using software such as Deep Sky Stacker or Maxim. This is different to stacking video frames using Registax or Autostakkert for the moon and planets. Projection and Prime Focus techniques can also be used for imaging DSOs. However, a longer back focus is often required requiring an adapter to extend the DSLR further away from the telescope.

 

Autoguiding can improve imaging during long exposures. An autoguider is a camera that uses a bright object (such as a star) near to the object being imaged to keep the object centred in the field of view. Otherwise it may drift causing a blurred final image. There are different types of autoguider. Some autoguiders can be attached to a finderscope which is attached to the main telescope itself. Other, so-called off axis autoguiders, are attached to the telescope and divert some of the light coming into the telescope off to a separate autoguider.  Finally, some CCD cameras have a separate autoguiding sensor next to the main imaging sensor. Autoguiders can be linked to a laptop computer and specialist software used to set up autoguiding.

Celestron NexGuide Autoguider

Celestron NexGuide Autoguider

 

What system to use can often depend upon your budget, telescope, mount and imaging equipment. Thousands of pounds can be spent on purchasing equipment for DSO astrophotography. However, this doesn’t mean you can’t get some really good images with simpler and far less expensive equipment. A good equatorial mount will provide a series of short exposures of a few minutes without the use of autoguiding.  Many astronomers use a DSLR or CCD camera to take a large number of short exposures and process them. This can be suitable for brighter DSOs such as the andromeda galaxy (Messier 31) and bright globular clusters (Messier 13).

 

Summary

 

Astrophotography is a very rewarding hobby but does take time and dedication to master. Once you are familiar with the use of your telescope and mount set-up, it is a good idea to start with imaging the solar system. The moon is a good starting point as it is large, bright and easy to find. What you learn can then take you onto imaging the larger planets; Jupiter, Saturn, Venus and Mars. You will then be in a good position to start DSO astrophotography. Depending upon your set-up you may choose to invest in a different mount or camera at this stage.

 

Zoltan at 365 Astronomy can be contacted for friendly advice about equipment, techniques and for support required after you purchase.

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Posted on April 6th, 2015.