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7, Imaging the Sun - The Basics.

How do we take a picture of the Sun when it is so bright?

You cannot capture a picture of the unfiltered Sun by pointing the camera skywards and attempting to use a very fast shutter speed to reduce the brightness. The Sun is far too bright for this to work. It’s also very dangerous as the camera lens will collect not only light but also infra-red and ultra-violet. The extreme brightness of the Sun, the infra-red and the ultra-violet will all lead to eye injury and even permanent blindness. We clearly need to use something that will protect our eyesight. 

Fortunately, there are ways in which we can take safely take images of the Sun:

PHOTOGRAPHING THE PROJECTED IMAGE

For this safe method we use a telescope to project the Sun’s image and to point the camera at the projected solar image. You then take a picture of the solar image on the projection screen, adjusting the exposure and focussing the camera carefully to get as sharp and detailed image as possible. This method works surprisingly well especially once you have practiced it a few times. The solar image might look a bit oval-shaped because you are shooting the image from one side but there are ways to overcome this. You will need a second screen to block the sunlight falling on to the projection screen (creating a shadow) around the image of the Sun itself to improve contrast between the photosphere and any sunspots.

Image of solar projection taken by Geoff Elston

PHOTOGRAPHING THE SUN THROUGH A FULL APERTURE WHITE-LIGHT FILTER

This safe method is to use a full-aperture white light solar filter securely fitted over the front of the telescope. Never attempt to use a makeshift filter always use a properly designed and certified as safe solar filter for this purpose.

You can get fairly decent images by putting the camera lens up against the telescope eyepiece (having carefully focussed the telescope beforehand) and taking a picture of the Sun through the eyepiece. You will need a low to medium magnification eyepiece that will preferably show the whole of the solar disk in the eyepiece field of view. By carefully adjusting the camera lens focus and varying the camera shutter speed you will get at least one or two good images where the whole of the solar disk is sharp and can be seen.

Image of DSLR attached to a telescope fitted with safe solar filter, taken by Geoff Elston A much better method is to use a Digital Single Lens Reflex (DLSR) camera but with its lens removed and replaced with an adaptor called a “T-mount”. This method shown here, is often called: “prime focus astrophotography”. The camera body, without its normal lens, is then coupled firmly to the telescope, turning it into a kind of telephoto lens. You will need to focus the camera using the telescope focussing knobs in the same way you would focus an eyepiece. You can use the DSLR viewfinder to focus but a better alternative is to use the “live view” option that nearly all DSLRs offer these days. Use either the X5 or X10 magnification options in live view to focus the Sun as sharply as possible. If you don’t have a camera shutter release (a simple one that plugs in to the camera body is often better as the wireless models sometimes won’t work in full sunlight). You can then use the shutter release timer option to avoid camera shake when pressing the shutter release button. The piece of card shown in the image here, placed in front of the camera body, is simply to act as a shade and helps me see the live view screen at the rear of the camera!

 

T-mount: A T-mount is a small metal ring that has a projecting tube one end (to let you put it in place of the eyepiece which should have been removed from the telescope focussing tube as it’s not needed). The other side of the T-mount has a thread that allows you to screw on to it the correct bayonet fitting for you camera.  If, for example, you have either a Canon or Nikon camera you will need to buy the appropriate bayonet fitting for Canon or Nikon to attach the whole lot together. Some have a thread to allow you to add a filter to the T-mount such as the one shown in the picture here. Image of a T-mount taken by Geoff Elston

FOCUS, THEN FOCUS AGAIN:

Most important of all is to ensure you get good focus. Spend time adjusting the focus of the telescope until you cannot get the image any sharper. No amount of processing after the image has been taken will correct poor focus. Live view can help here to help you nail the focus before you fire the shutter. The beauty of digital cameras is the ease with which we can view an image just after it has been taken and we can see if the shot is right or not.

The steadiness of the air will likely make the edge (or limb) of the Sun ripple at times and any sunspots will, at times, become slightly blurry before they become sharp again but you will get used to this. At certain times of day the Sun can appear very steady. These are periods of “steady seeing” and it is in these times you can get your best solar images. When do these periods of good seeing arise? Mostly, good seeing occurs in the early morning. Why? Well the Sun has not had a chance to heat-up the ground creating minute turbulence at ground level. We notice the turbulence because we are using a telescope which magnifies it.

Sometimes, the season helps. I’ve noticed that on some winter days we get long periods of good seeing during the morning daylight hours because the Sun, being lower in the sky (while not so good) has prevented the Sun from heating-up the ground.

Also present, though I won’t say much about it here, is the general state of the upper atmosphere. By this I mean the presence of the Jet Stream over the UK. The Jet Stream is a fast-flowing ribbon of air, high up in the atmosphere. In winter it can drift southwards covering a large part of the UK. It can spoil the steadiness of the solar disk in the same way that it affects imaging of the planets at night. 

Once in a while the Sun is so affected by poor seeing (either at ground level or high-up) that we cannot get a sharp image no matter how hard we try. These days are rare but when they occur it is better to give up than kid ourselves the fuzzy images we got are worth keeping.

TAKING THE IMAGE AND WHAT HAPPEN AFTERWARDS:

Camera settings: If you set the camera sensitivity setting to either ISO100 or ISO400 then you should get a good image by using a shutter speed of about 1/400th second or 1/4000th second. There is no hard and fast rule here, just try different shutter speeds until you get a good image of the solar disk. If you overexpose your image you will get a bright white disk with no sunspots visible as they will have all been bleached-out. If you under-expose the image the solar disk will barely show up.

Avoid the auto-exposure option on most cameras as it gets it wrong by attempting to expose for both the bright solar disk and the surrounding black sky (the solar filter will render the blue daylight sky nearly black). A more advanced option for controlling exposure is to use a camera’s histogram option to get a good balanced exposure. You will need to look at your camera’s operating booklet to activate this option if you have never used this before.

Image formats: Most DSLRs will let you set the image format. The default will often be JPEG. This means that when you take the image it will appear in that format and download to your computer that way too.

For solar I would recommend you try using RAW or TIFF if you can as they allow you to process the images without any nasty artefacts appearing. After you have processed the images you can convert the final images into JPEG for emailing

JPEG is a very common format. The camera does nearly all the processing for you and the images don’t take up much hard disk space as the images are often compressed. However, trying to process JPEG images will often lead to them degrading rapidly. I only use this format for quick shots of the Sun where I don’t want to do much in the way of processing afterwards.

Telephoto lenses: If you have a DSLR camera with a telephoto lens, you will need a full-aperture solar filter that can be screwed on to the front of the lens to make it safe to photograph the Sun. Never attempt to use ordinary photographic filters either singly or in combination in an attempt to dim the Sun. No ordinary photographic filters will render the Sun safe to look at meaning that you are potentially putting your eyesight at risk by attempting this. 

To be able to see the solar disk and any sunspots visible you will need a telephoto lens of at least 500mm. Any less than this and the Sun’s disk is far too small to make out any real detail in the sunspots, although it might be possible to zoom and crop the image during processing to remove most of the surrounding sky and worth trying. 

Telephoto lenses (with a safe solar filter fitted to the front) are great for capturing partial or annular solar eclipses. They can also be used for the partial phases of a total solar eclipse. You do, however, need to be in just the right geographical location to see these true wonders of nature!

Webcams and planetary imaging cameras: If you have a webcam, or planetary imaging camera, you can use these to image the Sun. You will still need a telescope and a full-aperture solar filter to make it safe to look and image the Sun. You will also need a laptop or computer to operate the webcam or imaging camera. It will still be necessary to focus the webcam carefully (in the same way you would an eyepiece or DSLR) by looking at the computer screen and you will also need to adjust the shutter speed and gain of the camera to get a good exposed image.

Where this type of imaging device scores highly is in its ability to take short videos of  hundreds of frames and then by using software such as Registax or AutoStakkert! (both are freely available on the Web) to allow you to quickly and easily sort, align and stack the best (by this, I mean the sharpest) frames into a composite single image. This single composite image can then be sharpened and processed further to bring out any detail in the image. When you have observed the Sun frequently you will have noticed that solar disk is not always sharply defined and steady. If we attempt to image the Sun using single images (as with the DSLR mentioned above) we might be able to capture an image of the Sun when the disk is nice and steady. We might not be so lucky and get a blurred image. This is where the high frame rate a webcam or imaging camera can come into its own It can record both sharp and less sharp images of the Sun on a vast number of frames in a short video sequence. It’s the “magic” of Registax or AutoStakkert! that allows us to easily sort the frames to reject the less sharp frames and keeping the best for our final composite single image. This is not to say that DSLRs cannot get good images of the Sun, they can and do, but you are more likely to record finer detail with the video and stacking method. The software will also allow you to draw out detail that is not always visible in the unprocessed video frames by using the built-in sharpening filters.

Processing the images: Whether you use a DSLR, webcam, or even a relatively high-end imaging camera, you will still probably need (and want) to process your images to show them at their best. PhotoShop, if you have it, is great but it is very expensive. The alternatives are: PaintShop Pro or GIMP 2, which is a free download from the Web. Either of these imaging software packages can help you create great images of the Sun. It will take a while to learn how to use them successfully but it is worth the effort. Don’t overdo the processing. If you start to notice artefacts appearing in your images than you have gone too far. You want to try to show the Sun as it would appear to you if you were observing it visually.

Processing might be to adjust the colour or tweak the brightness of contrast the final image. The Sun is really a yellow star but don’t feel that you must always colour your images so they appear yellow as a white solar disk is also acceptable. It is far more important to process the image to bring out the detail of any sunspots visible without overdoing it and introducing image artefacts.

Once you have finished processing your images, always label them clearly as this will pay dividends in the future when you look back at your images of the Sun and you know when you took them. You will never remember when an image was taken unless it is highly unusual or shows a rare event.

In the image filename always use the format: Year, Month, Day, then the time in UT (never BST), then the ISO and the shutter speed. If possible, in the image itself try to show which way is north and east as this helps to show the orientation of the Sun when the image was taken. If you send any images to me please include your name on the image as it really helps me identify who took the image!

This is by no means a full guide to solar imaging but I hope it is enough to at least get you started. A second short guide on using a webcam (with a telescope and full aperture solar filter) to make solar images will follow soon.

If you want further help please email me.