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Ritzycat

What exactly is "aperture"?

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Hello! When I was looking at telescopes a few years ago, I never really had this question answered. I guess my scope was sort of an impulsive buy, but luckily it was pretty good impulsive buy for a first timer. I know the length of aperture is the width of the scope, at least I think, but Is Aperture more a measure of magnification or "light collection? Do higher aperture scopes simply have more useful magnification or do they collect more light, as in, do faint objects appear brighter?

Any help is greatly appreciated - just a little curiosity!

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Aperture is as you said, the width of the scope. The wider the aperture, the more light it collects. Magnification is figured out by dividing the focal length of the scope by the length of the eyepiece you're using. For example a scope with a focal length of 750mm and an eyepiece of 25mm gives you a magnification of 30. 

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Aperture determines how much light the telescope scoops up.

The size of the image that is then formed determines how bright a diffuse object like a nebula or galaxy will appear.

The more magnification, the larger the image and the more the collected light is spread out... and therefore, the dimmer it appears.

Doesn't really apply to stars, which are point sources and thus the image should in theory stay as a point and not get larger with increased magnification.

It can all be very confusing!

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The aperture of a scope is the diameter of the primary mirror or objective lens. The scope tubes are usually a bit larger but it's the diameter of the optical elements that counts.

The larger the aperture, the more light that the scope collects and light collection is very important in astronomy as most often the things we are tying to view are faint and very far off.

In terms of light collecting area, a 150mm mirror or lens collects 4x as much light as a 75mm one and has more performance potential than the smaller one because of this.

There are a number of other factors that come into play which affect the ultimate performance that a scope gives including seeing conditions, optical quality, observer experience etc but aperture is significant especially when the intended targets are faint such as deep sky objects.

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A scopes aperture also affects the resolution or detail which can be seen although this to is conditional on seeing conditions  :smiley:

   

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Edited by Laurie61

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What would you guys say is the biggest "bottleneck" for those looking at DSOs? Aperture, light pollution, other viewing conditions, lens quality etc. As in, which of these factors is most important?

Also another thing - would it be happier viewing conditions under the sky in the Sahara with 4" in scope, or in a relatively suburban area with a 8" scope?

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Light pollution - a 4" scope in a dark sky will see more than an 8" in an urban sky.. But as always with DSO, the more aperture the better.

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Then comes aperture fever, you get a larger scope, see a little more, want to see even more so get an even larger scope, and so the cycle continues 

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Then comes aperture fever, you get a larger scope, see a little more, want to see even more so get an even larger scope, and so the cycle continues 

...until you discover astrophotography!   :laugh:

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...until you discover astrophotography!   :laugh:

nah been there done that had the ROR and CCDS its pants compared to going to a truly dark site and having millions of years old photons raging into your retinas!!!!!!! :icon_eek:  

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nah been there done that had the ROR and CCDS its pants compared to going to a truly dark site and having millions of years old photons raging into your retinas!!!!!!! :icon_eek:  

Sorry to dissapoint mate but apparently the photons that are hitting your eye are newly created by the atoms in your mirror stimulated by the photons of starlight etc and have travelled just the focal length of your scope. It's a nice thought though :smiley:

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Sorry to dissapoint mate but apparently the photons that are hitting your eye are newly created by the atoms in your mirror stimulated by the photons of starlight etc and have travelled just the focal length of your scope. It's a nice thought though :smiley:

:angry4: its lies all lies!!! lol ta for that john i may just throw the towel in after that comment.

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 In astro, the APERTURE or the maximum unobstructed diameter of the objective is king. Just for the record, if things are not confusing enough, non-astrophotographers also use the term aperture but they mean something quite different. Those photographers are referring to the focal ratio (f/number) of an optical system. Therefore a 300mm f/5 telescope (1500mm focal length) would probably be interpreted as a 300mm focal length system with a clear aperture of 300/5 or 60mm. This is an incorrect meaning of aperture which unfortunately has now become the norm. After all an aperture is really just an opening and has physical dimensions. F/numbers have no dimensions, they merely reflect the ratio between the focal length and the aperture.  Try explaining that to your photographer friends.

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Aperture can establish as soon as your eyes have accomplished becoming fully dark adapted, therefore achieving heightened sensitivity of the rods and cones.

Edited by scarp15

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.........you may have heard the term " light bucket "?  The bigger the bucket, the more  there is available?

If you're collecting rainfall to water your garden with a bucket, it will take some time to fill, depending on the magnitude of the rain fall, but given the same magnitude, a bigger bucket will collect more rain, as will the roof of your house, capturing the rain, flowing through the gutters and pipes to a water butt. The more surface area - opening you have, the more you can capture.

Your Aperture works in the same way. For visual astronomy, the bigger the aperture, the fainter the objects it will reveal to your eyes under the right seeing conditions.

Brightness and low power is the way to go for  DSO visual observations, as you go higher in magnification, the field of view reduces, so does the brightness of the target  that you're hoping to view, which  is probably invisible to the human eye in the first place. 

Higher magnifications are best for Planetary targets, using any  EP that frames the subject. 

Edited by Charic

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Sorry to dissapoint mate but apparently the photons that are hitting your eye are newly created by the atoms in your mirror stimulated by the photons of starlight etc and have travelled just the focal length of your scope. It's a nice thought though :smiley:

:grin:

Next you'll be telling us there's no such thing as Father Christmas! 

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to me bigger aperture =

more resolution = more detail

more light gathering

more planetary moons

more wows

more size

more weight

more cost

more options

more effort to set up

more critical collimation

less true field of view

more cooling time

I agree though that a darker sky is more important than more aperture but more aperture is generally a good thing.

I have several scopes as I firmly believe that no single scope covers all observer moods/condition, observing conditions (local and nationally), targets, and experience.

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Coming back to the OP's original question, re aperture and limiting magnification...

As above, aperture is the diameter of the primary light collecting optic - the mirror or the objective lens. Light collecting ability is proportional to the area of the primary / objective, or the square of the diameter. Hence the comment that doubling the diameter gives 4x the light collecting ability.

Limiting magnification is the maximum realistic magnification you can get from a telescope. This is proportional to the light gathers and is approximated at 50-60 times aperture (not sure what aperture size this approximation stops holding - I suspect the very large dobs can go higher than this ad the area is so much greater - Moonshane and others will be able to advise).

This all means that the aperture is distinct from the limiting mag, but had s direct impact on it.

HTH,

James

Edited by jamespels

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Aperture can only extend useable magnification so far and is limited by atmospherics in the real world.

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