Saturday, April 26, 2014

Celestron 70az: What can you expect to see?

When someone buys a new telescope his first question is what i can see with it. The second is what is the highest zoom i can achieve. Although the maximum zoom can be an attractive measurement when buying a new telescope, the buyer should focus on the aperture size: how much light i can gather? This is the main difference from theoretical to practical highest zoom.

So, what can you expect to see with this telescope?

During the tests i did, the average seeing conditions with medium-low light pollution. These two factors can really improve your experience or.. to destroy it. In my place i would categorize the light pollution in class 5, based on Bortle 's scale. I also tried to improve the lenses by blackening the edges, as i described in earlier post.

A quick note here before discussing my personal views on the topic. Planets and stars have different brightness that we call it apparent magnitude. Lower the number is, brighter the stellar object is. The practical limit of the telescope is around 10 as it is expected by a 70mm telescope. Here is a list of famous stars and planets:

Apparent Magnitude of celestial objects Source: ESA
Apparent Magnitude of celestial objects Source: ESA

To calculate the achieving magnification of the telescope we use this equation: 

magnification = focal length telescope / focal length eyepiece

In our case the telescope has 900mm length, thus we always divide this number by the length of the eyepiece. Here is a table describing the achieved magnification (zoom) in combination with a barlow and some comments from personal experience. 

Tube Length (mm)
Eyepiece
Barlow ( 1X – 2X)
Zoom
Comments
900
32
1
28,13
Wide view, best for star gazing
900
32
2
56,25
Good for star gazing and big planets
900
20
1
45
Wide view, good for star gazing.
900
20
2
90
Lot of light. Stars and planets.
900
15
1
60
Good for star gazing, less light than 20mm
900
15
2
120
Loosing a lot in contrast, but acceptable
900
10
1
90
Less light, but acceptable for stars
900
10
2
180
Telescope limit, bright planets only
900
6
1
150
Telescope limit, bright planets only
900
6
2
300
Unacceptable, even during the day

In short: if you want to use the telescope for star gazing you should use magnification up to 100. For planets, you can go up to 180x, but you are going to loose a lot in terms of brightness. Anything more is unacceptable! In the future, i will make calculations what are you loosing in terms of brightness.