When the dark sky isnt...

[Shingouzzz]

Hello !

Since many years I dream about a telescope and I wish to buy one as big as possible but I wonder if there is any table explaining which is the optimal size regarding the quality of the local sky.

I think of a table linking for example a city sky to a e.g. 10cm telescope, a village sky to a 20cm telescope and a retired place with no light to a 1000cm telescope if you are lucky enough to live in the Atacama desert...

Does anybody know if such table exists ?

Is there any simple test to know of good is the quality of the darkness ?
Like comparing the visible stars with unaided eye to a chart of the same constellation with different qualities of darkness ?

Thanks for your help !!

[Chris Peterson]

Since many years I dream about a telescope and I wish to buy one as big as possible but I wonder if there is any table explaining which is the optimal size regarding the quality of the local sky.

There is no such thing as optimal size; in terms of performance, increasing aperture will always provide viewing benefits, regardless of your sky conditions. (There is, of course, an optimal size when you consider not just optical performance, but cost and practicality as well.)

think of a table linking for example a city sky to a e.g. 10cm telescope, a village sky to a 20cm telescope and a retired place with no light to a 1000cm telescope if you are lucky enough to live in the Atacama desert...

It actually works the other way. Larger apertures provide greater benefit under light polluted skies than under dark skies. That's because larger apertures let you operate at higher magnification, and higher magnification increases contrast between astronomical objects and the sky background.

[apodman]

higher magnification increases contrast between astronomical objects and the sky background

Didn't know that, and was never conscious of it when viewing. What's the principle behind it?

[Chris Peterson]

higher magnification increases contrast between astronomical objects and the sky background

Didn't know that, and was never conscious of it when viewing. What's the principle behind it?

There are two factors. For stars, it's because increasing the magnification reduces the brightness (that is, the intensity per unit area) of the background without changing the brightness of the stars. So you can always see dimmer stars at higher magnifications. For extended objects, both the object and the background experience the same decrease in brightness with increased magnification, but your eye requires less contrast to see a large object (or larger detail) than a small one. So increased magnification allows you to see more detail under bright skies, where contrast is intrinsically lower.

[Shingouzzz]

Hello Chris !

Thank you for your explanations.
I went on Wikipedia to understand more about magnification.
If I correctly understand, the magnification depends of the lenght of the focal. so I can guess that the bigger the diameter of the primary mirror (for a newtonian telescope), the longer the focal length and the better the observation even with light pollution.

I found this page
http://www.unihedron.com/projects/darks ... /berry.htm
of the ROYAL ASTRONOMICAL SOCIETY OF CANADA.
Following the table, the light pollution level in my place is around 100 (The Milky way is brilliant overhead but cannot be seen near the horizon. Clouds have a greyish glow at the zenith and appear bright in the direction of one or more prominent city glows)
The difference of magnitude between no light pollution and a level of 100 seems to be little, from 21.7 to 21.4.

Now another beginner question: what is the best, buy a fully computer assisted telescope which automatically point in the direction of a given star even if I dont know any constellation except Ursa major and Ursa minor or is it better to first try to recognize the constellations with the naked eye ?
Or are these discussions between purists vs techie only in photography (dont use automatic programs, learn what a f-stop means, take care of the depth of field and so on...)?

[Orca]

One option for dealing with light pollution, regardless of which telescope you choose, would be the use of filters. There are filters you can get that cancel out the specific wavelengths of light that are common with halogen street lights and what not.

As for whether or not to go with computer assisted telescopes, at this point, that feature really doesn't add a lot of cost to a telescope so I'd say why not. You can get it with any type of telescope, even one with a Dobsonian mount.

At the same time it's rewarding to find objects the old fashioned way and you probably learn more in the process. Of course if you opt for a telescope that has computer assist you can always choose not to use it; then if you're having trouble finding something, just ask Hal to point you in the right direction.

[Chris Peterson]

If I correctly understand, the magnification depends of the lenght of the focal. so I can guess that the bigger the diameter of the primary mirror (for a newtonian telescope), the longer the focal length and the better the observation even with light pollution.

The magnification of a telescope is calculated as the focal length of the objective divided by the focal length of the eyepiece. This means that you can get any magnification you like from any size aperture. However, the combination of aperture and magnification determines the exit pupil, which is a factor controlling image brightness, and the aperture determines resolution. There is a good rule of thumb that the amount of useful magnification you can get from a telescope is equal to 50 times the aperture diameter in inches. So an 8" scope under good conditions can be used to about 400X.

Now another beginner question: what is the best, buy a fully computer assisted telescope which automatically point in the direction of a given star even if I dont know any constellation except Ursa major and Ursa minor or is it better to first try to recognize the constellations with the naked eye ?

The largest aperture for your money will come with a Dobsonian, which is a Newtonian telescope on a simple altitude-azimuth base. These are not driven, so you have to aim and move them yourself. You can, however, get inexpensive computerized readouts that tell you where you are and help you find objects.

Computerized goto telescopes are a great convenience, and I think that for most observers they are the best way to go. These days, few people have the time or inclination to deeply learn the sky. Also, if you are in a light polluted area, finding objects the old fashioned way (by manual star hopping) can be extremely difficult. A goto telescope under less than ideal skies makes a big difference in being able to find tricky objects.

You might want to look at something along the lines of a computerized 8" SCT, such as those made by Meade and Celestron. They are quite reasonably priced, have excellent optics, automated pointing and tracking, are easy to set up, manage, and store.

[Shingouzzz]

Hello Chris !

Thanks a lot for these very interesting explanations !

Now I have all informations I need !

Kind regards from Switzerland