FAQ

 

What's Wrong with my Image? - Frequently Asked Questions

 

Have a problem with your Liverpool Telescope observation image? Hopefully here we can let you know what's going on! 

  1. Why are my images dark?
  2. Why are my images blurry/grainy?
  3. Why are my images brighter than other observation images of the same object?
  4. Why do the stars in my images look strange/why do they have splodges in the middle?
  5. Why don't my images include the entire object?
  6. Why does my image have a bright line going straight through it? 
  7. Why does my image have a dark line going straight through it? 
  8. Why does my image have small lines or dots scattered across it?

 

1. Why are my images dark?

One of the first things you may have noticed when you have loaded your image(s) into LTImage or other .fits reading/processing software is that to start with, the images are exceptionally dark and nothing of moderate detail can be seen. This is only because the Liverpool Telescope was designed to count the number of photons hitting the detector and not to create pretty pictures on its own! All we need to do in this case, using LTImage as an example, is adjust the scaling of the image(s) in the Display tab using the right slider. This should reveal some more detail:

 

Using LTImage scaling to brighten an observation
Figure 1: Using the Display and scaling tools in LTImage to brighten an initially dark image (left) to obtain a detailed bright image (right). In this case the Fireworks Galaxy. 

 

If you are creating a 3-colour image it is slightly different in LTImage; first combine the images into a 3-colour image and then separately modify the Pixel value range for each colour/filter. You will find that reducing (oftentimes quite drastically) the maximum pixel value will bring out more detail and brighten your image.

 

3-colour pixel value modification to brighten image
Figure 2: Using the pixel scales in the 3-colour image window rather than a slider to change the 
brightness of objects in the image. 
Note: In this case the pixel values have to be changed for all of the separate colours to combine them!

 

 

2. Why are my images blurry/grainy?

There are a few factors that we have to take into account for these effects, some under our control and some that are not:

The Moon

Much like how we cannot see stars at night due to the brightness of the Sun, we get a similar effect from the Moon – with increasing intensity as it reaches a Full Moon. The Moon reflects light from the Sun down onto our atmosphere which makes the night sky generally brighter - great for getting some nice moon shots but makes faint objects and deep sky objects very hard to observe clearly. With deep sky or faint objects, increasing/decreasing the exposure time will make no notable difference as it will still collect the scattered photons coming from the Moon, thus the best time to observe such objects is when the Moon is a New Moon; when the least of it is visible and not reflecting the Sun’s light down onto our atmosphere; this is when the sky is at its darkest. Fortunately, there is an option in our Go Observing section where you can choose to observe when the Moon is ‘Up’ or ‘Down’ to maximise your chance of receiving a high quality image.

Seeing

In astronomy, seeing is the name given to the effect that makes stars ‘twinkle’. Stars (and other celestial bodies) do not actually ‘twinkle’ at all, the photons from these objects have traveled for years in a perfectly straight path but get ‘knocked around’ as soon as they reach our atmosphere. The twinkle that you see is the photons hitting and/or being scattered off the particles in the atmosphere before reaching ours eyes, and even the Liverpool Telescope. Seeing is mainly caused by atmospheric turbulence and how high your observing site is (the higher you are, the less atmosphere for the photons to travel through). Feats of engineering have been accomplished to correct for different levels of seeing happening on the night of observation due to atmospheric turbulence but it still poses its challenges. If your image is slightly blurry, it is likely that the seeing on the night of the observation was not good and the photons were scattered in the atmosphere more intensely. The best quality images come through when the atmosphere above the observing sight is at its calmest. As a calm atmosphere cannot be planned, if you are unsatisfied with your image you will have to submit a new observation request.

bad (left) and good (right) seeing example
Figure 3: Two separate obsvervations of NGC 1788 (the Foxface nebula) on two different nights. One with poor seeing (left) and one with better/good seeing (right). 
We can see that the image on the right is clearer and has more detail, where the stars and clouds on the left are slightly blurred and fuzzy. 

 

Seeing also has different effects on different filters, as they observe different wavelengths. When using RVB filters for your 3-colour observations, the B filter which is closer to the ultraviolet side of the electromagnetic spectrum suffers more from the atmospheric conditions of our atmosphere so will sometimes look slightly grainier than either R or V filters.

 

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

 

Exposure Time

Different exposure times (30s, 60s, 90s, 120s…) will collect photons from the entire field of view, not only the object being observed. The whole image will be brighter depending on the sources of light in the image and how long they are observed for!

 

4. Why do the stars in my images look strange/why do they have splodges in the middle?

 

If you have taken an observation and changed the scaling of your image so that you can now see all of the detail, you may have stars or bright regions that have blocky/smudged centers of colour that ‘bleed’ out onto the image. This is called saturation and is most common when trying to observe a faint object, like a nebula, whilst there are also bright stars in the field of view. When observing with a high exposure time (for example, 120s with the Liverpool Telescope using the NSO) then the detector can collect more photons from the dim areas of an object and thus more detail, but, some stars in the field of view can be much brighter than anything else in the field of view and the detector will also collect photons from these stars for 120 seconds. This saturates the pixels that collect the light from the stars; there are simply too many photons for the detector cell to hold and the charge bleeds into the surrounding cells (see our page on CCDs, how they collect photons and then digitize the signal). The best way to observe an object with these attributes (a faint object but with particularly bright regions/objects in shot) is to take multiple low exposure observations and then stack them together – this will make objects such as stars brighter but it will not saturate them.

 

four different exposure times for the bubble nebula, showing the saturation of the central star
a) Bubble Nebula observed with a 30s exposure. Central star saturation is minimal.
b) 60s exposure. Central star saturation more intense. 
c) 90s exposure. Central star saturation grown again.
d) 120s exposure. Central star very saturated; pixels have bled onto regions outside of the star on the image. 

 

 

5. Why don't my images include the entire object?

You may notice that when observing an object such as the Rosette Nebula, or the Fishhead Nebula, that you cannot see the entire thing, but a small region of the object. This is because some objects are so large in the Liverpool Telescope’s field of view that the entire object cannot fit into a single frame. The Liverpool Telescope does not have the ability to zoom in and out of objects, but has a set field of view on the night sky!

 

 

6. Why does my image have a bright line going straight through it?

You may have encountered an image that looks like this whilst using telescope observations: 

As we can see, there is a very bright straight line cutting across the image in the bottom-left. You may have guessed it already, but it is in fact a satellite! Periodically, satellites will just happen to cut across the Liverpool Telescope's field of view whilst it is in the short process of observing a specific target. If you imagine the chances of this happening then they are indeed quite slim, so they only appear on very few images observed but unfortunately there is no option to remove the line from the image, only to request another observation! 

You may be wondering why the line is so bright compared to the actual stars in the image: because the satellite is so much closer than the objects we observe (a matter of a few thousand kilometres away rather than light-years) we capture the very small amount of light that it reflects from the Sun, Moon or light diffracted by Earth's atmosphere that hits it. This and taking into account the exposure times of the telescope, the images do collect rather a lot of the light bouncing off! 

 

6. Why does my image have a dark line going straight through it?

You may have also come across an image which has a similar problem to this:

a) Zoomed-out image of a galaxy with a line of dead pixels.

 

b) Zoomed-in image of a), yellow arrows point out the column of dead pixels,
the red arrow points out the dead pixel which makes the above
pixels information void.

 

Sometimes on your observation, a vertical dark line may appear. These are called dead pixels and are specific to how a CCD camera (the camera used on the back of a telescope to capture the incoming photons) and how they read out their data. The lines can vary in length due to how far up the CCD grid the bad pixel lies. As a CCD reads out in vertical lines, a 'dead' pixel when read out will make the data void for the rest of the column that gets read out afterwards, which is why the line then carries up to the top of the image. 

 

7. Why does my image have small lines or dots scattered across it?

 

Sometimes, you may encounter small lines (usually a couple of pixels worth) scattered across your image. They can be white (on a single-colour image) or coloured depending on the filter. The white version will look something like this, or larger: 

 

Zoomed-in image, the red circles point out cosmic rays that appear as random white pixels.

 

If you have a coloured image, these will just appear as coloured pixels/lines. The reason for this is that tiny particles called cosmic rays are constantly finding their way through the atmosphere and hitting the telescope detector. Cosmic rays can come from inside our Solar System or from outside, the lower energy one's originating from the Sun and the high-energy one's originating from high-energy events outside of the Solar System (supernovae, black hole jets). The particles travel close to that of the speed of light. Stopping cosmic rays from hitting the detector is currently unavoidable, but there are post-observation processes to even out the image which the NSO usually incorporate for observations which get back to you....but the occasional few slip through the net!