Another dumb newbie question - sorry!

[Harmartolos]

Following my previous post on the welcome bit, I am probably going to get a Skywatcher or similar, thanks for the great advice everyone.

Unfortunately, and despite checking through 'Astronomy for Dummies', I still don't understand why some scopes come with the same aperture etc., but different focal lengths and f ratios. I assume that there is a good reason as they have gone to the trouble of making both.

Which is best, or is it one of those answers that depends on what I want to do with it; carry it about a lot (the shorter focal lengths seem to be smaller scopes). Also will it be important later on if I try to take photos, or imaging or whatever its called - I seem to recall f ratios being something to do with cameras.

Please, please help.

[ashenlight]

Hi Harmartolos welcome to SGL and great news you are going to purchase a scope

The focal length of a telescope is the distance traveled by light from the optic to the point where the light is focused into an image for you to look at. Whilst a longer focal length will produce a bigger image (think of the way you use an overhead projector - as you pull the projector away from the screen, the image gets bigger) the light has further to travel the bigger the focal length is, so the light will be more 'spread out', resulting in a fainter image. So whilst you might think a longer focal length telescope is a 'better' telescope, as it will produce a bigger image (because the light has further to travel down the scope tube and so spreads out into a bigger area) the image will be fainter. So in theory you could have two 5" scopes with different focal ratios. One will produce a brighter, smaller image, whilst the other will produce a fainter but larger image. I guess it is down to personal preference which one you would go for!

The focal ratio is the ratio of the focal length to the diameter of the primary mirror. So, as an example, my Skywatcher 130pm telescope has an aperture (aperture means the diameter of the primary mirror) of 130mm and a focal length of 650mm. You calculate the focal ratio by dividing the focal length of the scope by the diameter of the primary mirror - so 650 / 130 = 5. This gives it a focal ratio of 5. You can write this as 'f/5.'

Can't remember where I heard this but f/5 or f/6 are supposed to be good beginner telescopes. My scope has given me brilliant images of a wide range of objects - Moon, planet, faint fuzzies and so on, so I'd highly recommend it. For a decent sized scope it has great portability and even me (a girl, lol!) can carry it round easily - I just take the telescope tube out of the rings so it is detached from the tripod and carry the tripod and scope seperately (easy to seperate and put back together don't worry).

I can't help you on the imaging front I'm sorry - hopefully you'll get some answers on that query

[brianb]

Can't remember where I heard this but f/5 or f/6 are supposed to be good beginner telescopes.

I prefer longer .... f/8 to f/10 is a good "general purpose" scope, shorter than that can have false colour (refractor) or be hard to collimate (Newtonian). Also short focus scopes (small f number) do not work as well with cheap eyepieces as longer focus ones.

I can't help you on the imaging front

F number only affects exposure times for extended objects (nebulae), for stars what matters is the aperture (sheer light gathering power) as the image remains a simple point. A f/5 scope will record the same density in a nebula image as twice the exposure in a f/8 scope but the image scale will be smaller (unless the aperture is increased).

[ashenlight]

Thanks brian, this should help Harmartolos out alot

[Capricorn]

A bigger focal length will give a potentially higher magnification, but a dimmer image when used with the same eyepiece. The collected light is spread out over the bigger image so it is dimmer.

Not too much of a problem for planets but not so good for DSO's. No use if the primary image is too big and therefore too dim to be seen.

A long focal length will mean a smaller field of view. Believe me this can make a difference. I have a small f/5 refractor and a f/14 mak. I use the f/5 refractor most simply because it is more friendly. Basically I can find things through it when I look.

At f5-6 the light is brought to a focus that is reasonable close to the primary and so it has to be reflected at bigger angles so they can demonstrate more abberations (coma, sph and chromatic).

f/8 and above are easier to produce and (bigger f number) they show less abberations. But a bigger scope to accomodate the longer focal length.

f number does come from the camera world. As I don't image I will leave it to others. In general a smaller f number will give a brighter primary image but the image will be smaller assuming that the objective diameter remains the same.

So it comes down to a compromise between abberations, tube length, field of view, cost, usage. For the same objective diameter: A small f number is good to transport but will have more abberations. A small f number well corrected costs more. A bigger f number costs less. A bigger f number has a smaller FoV. A bigger f number is a physically bigger piece of equipment. A bigger f number should have less abberations. By and large each has advantages and disadvantages.

If you only want the advantages then you need a good supply of money, go look at the cost of an 80mm doublet compared to an 80mm triplet.

[AztecastroMcJ]

i was advised that f/6-8 was best for deep sky

while f/8-10 best for both

or f/10-12 best for moon n planets

James

[Harmartolos]

Thanks everyone, that really helps a lot. As I am still unsure exactly what I will want to be doing, I think I will probably go for the slightly longer 900mm focal length Skywatcher as it gives a focal ration of f/6.9 and should therefore be more of an all-rounder. The short tube 650mm focal length Skywatcher gives a focal ratio of just f/5 which if I have understood your comments correctly, means I will be tied mainly into deep space stuff!

[Astro_Baby]

Not exactly - the F5 version will perform well as an all rounder BUT it will be more fussy about its eyepieces. My sister has the short tube 650 and its a fine scope but I think if I were buying, knowing what I know now, I'd have the long tube 900m version.

[johninderby]

Hi

There's been a lot of good advice in the earlier posts, but if it helps at all one thing to keep in mind is that the shorter the focal length (f5 for example) the better your eyepieces need to be. With the longer focal lengths such as f8 or more you can get away with cheaper eyepieces as they're much less demanding. Also with short focal length refractors colour and other aberrations will be more obvious unless you go for an APO type.

A short focal length APO refractor with good eyepieces is a good way to go if you've got a big enough budget (more compact and portable and easier to mount), but you could also get a long focal length achromatic refractor with cheaper eyepieces for a fraction of the cost, even if it's not so convenient to carry about, and it would still give decent performance.

John

[albedo0.39]

Hi there Harmatalos

Not a dumb question at all and if you are used to photography, then the F ratio on a camera has a sliightly different meaning compared with a telescope.

It's all explained quite well here

As mentioned, the lower the F number, the more demands are placed upon the subsequent optics and set up (EP's and collimation). The former costs money and the latter time and experience !

[Harmartolos]

Thanks for the link Steve, the aperture calculator on that site is particularly useful - might have to rethink now!