Image oddities | The Schools' Observatory

Something strange about your observation?

Images of space can sometimes look unexpected. This page explains some common features you might notice and why they appear.

Why are my images dark?

When you first load your image into FITS reading or processing software, it may look very dark, with little detail visible.

This happens because the software is designed to count photons, not to create a finished picture straight away.

To reveal more detail, adjust the scaling of the image. You can do this using the sliders in the Display menu.

Two side by side images. At the top-centre is an orange arrow pointing left to right labelled "Scaling". Left image: a black background with some very faint small dots of stars. Right image: a dark grey background with many more small dots of white stars. There is a bubble-like nebula in shades of grey and white in the centre. — Using the Display and Scaling tools to brighten a dark image and reveal detail in Messier 27

If you are creating a 3-colour image, this process is done in LTImage. First combine the images into a colour image, then adjust the pixel value range for each colour separately.

Reducing the maximum pixel value often reveals more structure and detail in the image.

Screenshot of imaging software. Most of image shows the square display window with a picture of a galaxy. The spiral arms are dusty red/orange and very faint. There is a small, bright, orange/yellow circular centre. There are many dots of small, white stars against the black background. To the right, there is a window within the software showing options such as "Pixel vales" and "Alignment". — Adjusting pixel values in a 3-colour image to improve brightness and detail

Why are my images blurry or grainy?

Image quality depends on several factors. Some are under our control, and some are not.

The Moon

Just as we cannot see stars during the day because of the Sun, moonlight can also brighten the night sky. As the Moon approaches full phase, it reflects more sunlight into Earth’s atmosphere.

This makes faint objects difficult to see. Increasing exposure time does not help, because the camera still collects scattered photons from the bright sky.

The best time to observe faint objects is near New Moon, when the sky is darkest. In Go Observing, you can choose whether the Moon should be ‘Up’ or ‘Down’.

Seeing

In astronomy, seeing describes how much Earth’s atmosphere blurs an image. Turbulence causes starlight to spread out, making objects look fuzzy.

Two side-by-side images of a very faint nebula. On the left, there are some white dots of stars against a black background, with a dark grey smudge around some stars in the middle. An orange label labelled "Better seeing" points to the image on the right. This has the same stars against a slightly lighter background of black/dark grey, and the grey smudgy cloud around the middle clouds is also lighter and more light grey in colour. — The same nebula observed on nights with poor and good seeing

Observatories are built at high locations to reduce this effect. Even so, seeing changes from night to night. If your image is blurry, the atmosphere was likely unsettled.

Seeing also affects different filters in different ways. Blue light is scattered more strongly, so blue images may look grainier than red ones.

Why are my images brighter than other observations of the same object?

This is usually caused by exposure time.

The camera collects light from the entire field of view. Longer exposures (for example, 30–120 seconds) make the whole image brighter, not just the main object.

Why do the stars in my images look strange or have bright patches?

Bright stars can sometimes appear blocky or smeared. This effect is called saturation.

Two side-by-side images. On the left, there is a cloudy nebula in the middle in shades of grey and white against a black background that has small, white dots of stars scattered throughout. The outline of a white box is placed over a part of the image, and two lines extend from the far left corners to the top right and left of the next image on the right. This shows a zoomed0in view, white a white circle outlining a bright, white star with a dark black smudged dot in its centre. — A saturated star with a dark centre and diffraction spikes

Saturation happens when too many photons hit a pixel on the camera. The excess charge spreads into nearby pixels.

A good solution is to take several shorter exposures and combine them. This brings out faint detail without saturating bright stars.

Why does my image not show the whole object?

Some objects are simply too large to fit into one image.

Telescope instruments have a fixed field of view and cannot zoom in or out. Large nebulae may need multiple images to capture them fully.

Why is there a bright line across my image?

This is usually caused by a satellite passing through the telescope’s view.

A dark grey, cloudy nebula in the middle against a black background with some small, white dots of stars. Towards the lower left, there is a diagonal white line that cuts through part of the image.

Satellites are much closer than stars or galaxies, so they appear very bright. The light you see is reflected sunlight.

Why does my image have a dark line?

This is caused by dead pixels on a CCD camera.

Two side-by-side images. Left image shows a bright sphere at the centre with a faint grey, cloudy bar cutting through in the background and the a faint grey, cloudy circle surrounding it. The rest of the image is dark grey with a few small, white dots of stars. A faint, thin, vertical, black runs from top to bottom centre-left of the image. The image on the right shows a zoomed in portion, with yellow arrows and a single red arrow pointing at the black, vertical line. — Dead pixels creating a vertical dark line in a galaxy image

When a dead pixel is read out, all pixels above it in that column are affected, creating a line through the image.

Why are there small dots or short lines scattered across my image?

These are caused by cosmic rays.

A fuzzy black/dark grey background with a few light grey spheres randomly throughout. Towards the upper left, there is a faint light grey region, as if something bright is beyond the image. In the upper right and centre-bottom, there are in total three tiny, white streaks which are circled in a red outline. — Cosmic rays appearing as small bright pixels in an image

Cosmic rays are high-energy particles that hit the detector. Some come from the Sun, while others come from distant events like supernova explosions.

Most cosmic rays are removed during processing, but a few can still appear in final images.

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