Newbie Qn: About focal length and Aperture

[weixing]

Hi,
If both scope have the same magnification, the one with the larger aperture will show brighter image(assume both are same type of telescope).

A telescope with longer focal length generally produce better image than a telescope with shorter focal length, especially refractor(of course there will be some short focal length refractor that can generally very good image at high magnification, but it will be very expensive).

[Quasar]

Hmmmm so i think for this discussion, in summary:

Aperture:(Or diameter of telescope) Affects brightness of planets - The larger the aperture, the brighter the image will be in general. Measured in both inches and mm/cm

Focal length, f/l (of telescope): When applied with the focal length of the eyepiece, will give you magnification of the image. Measured in mm usually.

Focal Ratio (F/Ratio): Called the "speed" of the telescope. Derived from dividing focal length by the aperture. (Using the same units) so a 80mm aperture telescope with a 800mm focal length will have an f/ratio of 10.
Generally, the "faster" a scope, the wider the field of view (fov). "Faster" scopes are usually used for viewing deep sky objects since they are usually more spread out compared to planets, which fare better with "slower" optics.

So the 3 have a relationship like

focal length = focal ratio
Aperture

So you can figure out what u'll need in general using that.

Then using the focal length, you can get the magnification you want by dividing the f/l of the telescope with the f/l of the eyepiece

focal length of telescope = Magnification
focal length of eyepiece

So the 800mm telescope when used with a 20mm eyepiece will give you a 40x image. And a 5mm eyepiece used with the same scope will give you 160x magnification.

Theoretically, you shouldn't use a magnification of greater than 2.4x the aperture of the scope in mm. So the maximum you can go with a 80mm scope is about 190x.

*phew* that was a handful, and i hate maths, yet i showed and demonstrated 2 formulae... gee...

[weixing]

Hi,
You should add the formula of finding True Field of View(TFoV):
True Field of View (TFoV) = Apparent Field of View(AFov) / Magnification

From this formula, you will realise that:

Generally, the "faster" a scope, the wider the field of view (fov).

is not true. The True Field of View(TFov) is depend on the Apparent Field of View(AFov) of Eyepiece and the Magnification.

By substituting the Magnification formula, we get the below:
True Field of View (TFoV) = (Apparent Field of View(AFov) * Eyepiece Focal Length) / Telescope Focal Length

To make thing clear, may be we should put in another way:
1) The Shorter the Focal length, the Wider the True Field of View(TFoV) With the Same Eyepiece

2) The Bigger the Aperture, the Brighter the Image With the Same Magnification

Have a nice day....

[Quasar]

Ah okay, i stand corrected...
I never managed to get that part straight... too technical
=P

[IceAngel]

is it true that long fractor will give lesser false colour than a short tube??

[Quasar]

Hmmm, now we're getting a bit technical and controversial.
False colour is also known as Chromatic Aberration

Theory goes like this: Refractor works by using at least 2 lenses at the objective... 1 to diverge the light, the other to converge the light

But since it's nearly impossible to make 2 lenses that match each other exactly optically, chances are the light would not converge together at the same point.

1 way to get past the problem of false colour is to make the focal length longer so that the colour becomes less obvious.

2nd way is to exotic materials to make the lenses so that they dun introduce false colour, that's where flourite and extremely low dispersion(ED) lenses come in. They reduce the amount of colour to near negligible in a refractor. So that you can have a shorter f/l refractor with nearly no colour

Of course, another way adopted by some manufacturers would be to spend lots of time making a pair of objectives that match each other so perfectly that there won't be a need for expensive additives in the lenses, but then it adds up on costs.

Kinda why a chinese refractor can cost $400 OTA only whereas a Televue can cost $2000 OTA only even if it's not using flourite lenses.

Anyways, you usually wouldn't see chromatic aberrations in a telescope unless you're looking at bright objects like the planets, and the moon. Other objects like stars have too few photons for the aberration to be visible.

Of course, if you're really bothered by colour, dun use a refractor, and stick to Maks, SCTs, Newts, Dobs' no lenses that diverge light, chromatic aberrations!

[kcy]

The point is that Iceangel what are you using ya scope for? DSO's(deep sky objects)?

Most dso's can be seen in large FOV, therefore a shorter focal length, most of them are also quite bright, therefore a 4"-5" will do.

Planetary or ya like to see things BIGGER? A longer focal length which catadiopic scope will provide(SCT,MCT, not so sure about Newts and Dobs), and a bigger app. Its not nice having dim objects at high mag.

AP-thats another story~

From what I know, first determine what ya intend for the scope before setting up the budget.Dont want to buy a scope and regret right? 8)

my 2 cents worth~

[ChaosKnight]

Problem is most of us have a limited budget. We want a good scope, and we want to see planets, DSOs, the sun, the moon, the pretty babe/hunk in the block opposite...etc.

For most beginners, i think the choice of scope will be limited by budget. Anyways, for most practical purposes, the false colour can be tolerated. The inexperienced astronomer may not even notice it. Heck, i can't see any false colour in my C102, even although everyone says there is.

Now, with a range of eyepieces (40mm~2mm), you can pretty much have any magnification you want. For my scope, the fl is 1000mm. With a 40mm eyepiece, the magnification is 25X. Good enough for the DSOs. At 5mm, the magnification is 200X. Good for lunar and planetary. All these for a medium focal ratio scope (f/9.8). And nowadays the eyepieces are so much cheaper. I remember the old days, when a 0.965" 25mm SMA could cost $150. On my pocket money i could only survive on Kellners, because the SMA blew a big hole in my finances. Now for almost half the cost, we can get those new wide-angle eyepieces that put the old expensive Plossls and SMAs to shame.

What about maintainence? My first scope was a newtonian. Aligning the mirror was a bitch. Sweated for 1/2 hr, and the mirrors seemed aligned. But later, when tested on a star, the defocused image was oval. Like an egg. And i hate eggs.

There is no such thing as an ideal scope. If there is, there wouldn't be so much discussion in the telescope forums. All telescopes are a compromise in some aspects. Show me a perfect telescope and i'll show you it only exist in your dreams. You can read all the FAQs now, and get your "ideal" scope, but chances are, 2 yrs down the road, you think of getting another, because the present scope don't seem so ideal anymore. Experienced astronomers out there, just look at the number of telescopes you own and you know what i say has some truth to it. From your very first toy scope with plastic lens (you thought it was the coolest thing back then, didn't you?) to the big expensive apo in your room (or in your bed. No kidding. i'm sure you thought of this at least once).

Bottom line is, you can get caught up with all the jargon. And you lose sight of your initial reason for getting a scope: the fun factor. Instead of enjoying the sight of the moon, you try to pick out false colour. Very little interests on what you can see with your telescope. Rather, you are more concerned with the equippment performance. Then you have changed from an astronomer to a telescoper.