How to Use the Deep Space Object Chart

How to Use the Deep Space Object Chart


The deep space object chart estimates what magnitude the visible stars should be from your location through your telescope on a moonless night.


Telescope Aperture City Suburbs Rural Dark Sky
4 inches 9 10 11 12.5
5 inches 9.5 10.5 11.5 13
6 inches 10 11 12 13.5
8 inches 10.5 11.5 12.5 14
10 inches 11 12 13 14.5


Using the Chart


Each deep space object is given a magnitude and a surface brightness score (if available). These are a measure of how dim the deep space object is. The higher the number the dimmer the object. 

Find out what magnitude you should be able to see by finding the aperture of your telescope and reading across the chart to the type of location you are observing from. If the number is higher than the magnitude or surface brightness (more on these below) then you stand a good chance of being able to see the object. 

A city sky is considered to be a magnitude 3 night sky due to light pollution. This would mean that magnitude 3 stars would be visible to the naked eye. The suburbs is  magnitude 4, rural is magnitude 5 and dark sky is magnitude 6.5. 


Limitations of the Chart


General Observing Conditions


The chart is a crude estimate as there are so many factors involved in whether you can see an object or not. Your eyes may not be sensitive enough or maximally dark adapted. Your light pollution may not reflect the average for each location given. For example, the light pollution from Las Vegas is higher than other cities and would limit the number of stars visible in the sky to about 25. The optical quality of your telescope and lenses can affect what you can see. Also, the light from the moon can affect your ability to see deep space objects.


Magnitude


The magnitude of a deep space object is calculated as the brightness a star would be if you collected all the light given off by an object and condensed it to the size of a star. Deep space objects like galaxies are larger than a star in the telescope and the light given off from them is spread over the whole galaxy. This is why many galaxies are harder to see than the magnitude would have you believe them to be. In these cases surface brightness can be a more useful measure of how dim an object actually appears. Magnitude is much more useful when observing stars.


Surface Brightness


The surface brightness of an object provides a number equivalent to magnitude for a deep space object. It is measured by taking all the light from the object and dividing it by the size of the object to give an average brightness. 

This can be a more useful measure of how dim an object will actually appear in the telescope compared to magnitude. 

You do have to be careful when using surface brightness as the number is an average of the brightness. Some deep space objects have bright centres and dim outer areas. These objects can be very easy to see in a telescope as the centre part is very bright, and this is what you observe. Many galaxies have a central bulge of stars that is much brighter than the surrounding disk of stars. 

A good example of this problem is the Orion nebula. This is a very bright and easy to observe nebula but it has a surface brightness of 11 and a magnitude of 4. The reason for the high surface brightness is because the nebula is a very large object and it is the bright centre that we see in a telescope not the very dim outer part. The picture below by NASA/ESA is the Orion nebula. Teh visible part is the very bright yellowish area in the middle but you can see with the aid of the Hubble telescope just how big the whole nebula really is.


Orion Nebula - Hubble


So when deciding whether a deep space object should be visible to you you can’t rely on either magnitude or surface brightness. The best thing to do is look at the picture and make a judgement on whether the object has bright areas that may be much easier to see than the surface brightness would have you believe.

Take a look at this for more on magnitude and surface brightness. And here is an article on improving your skills and chances of seeing deep space objects.

Have fun trying to find the deep space objects.

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