Problems/benefits fo a 'fast' scope?

[Aenima]

I was quite unaware of 'speed' when I bought my SW 130p Explorer and still dont fully understand the differences in 'speed'. I believe my scope is a fast one (f5? FL650mm?) but would like to know what it really means to own a fast scope.

Any info would be appreciated,

Aenima.

ps. I know there is much already written on the subject so i will specify that in general i want to just observe and view both dso and planets, vut at some point i want to image particular things but will only go as far as buying a digital eyepiece (No expensive stuff, just one of the ones for about 60 quid (anyone know if they're any good?) with the cd supplied.

Thanks.

[Mark Scott]

The very basics are....Higher F number (example F10) the larger, but slightly dimmer the object apears in the EP.

Lower F number (example F5) the smaller, but brighter the image is in said eyepiece.

Regards,

Mark

PS, "speed" does not really come into it, thats a photographic term.

[1981suede]

To add to that, fast scopes are generally more suited to viewing deep space objects and photography, whilst slower scopes are better at planetary observing.

[rowan46]

the f number is derived by focal length divided by apparture as has been said higher f numbers magnify more with the same eyepiece but consequently a narrower field of view. essentially in visual work high f number scopes are better for planetary work as they take magnification better visually a low f number gives a wider view which may be more aesthetically pleasing on dso's but it is in imaging that a low f number comes into its own.

Low f number scopes need better eyepieces to really get the best out of them

[CGolder]

These posts....

F RATIO effects exit pupils, Photography and aberations

Focal length determines the magnification

Apature determines the amount of light gathering

A 150mm f5 and a 150mm f10 at 150mag will show the same brightness. The f10 will be better for budget eyepieces as it is more forgiving on aberations.

Apart from that i wouldnt worry about it much for visual

Sent from my A101IT using Tapatalk

[Aenima]

Thanks, still new to this so any knowledge is v welcome.

Anyone know whether those digi eyepeices for about £60 are any good? Dont want to get into spending ridiculous money on CCD or DSLR kinda things.

thank you

[Moonshane]

I know nothing about imaging but can confirm the following:

you can only compare the brightness of a visual image when looking at the same object with the same aperture, at the same magnification and at the same optical quality of components.

assuming they are all the same (but the focal ratios different) then the visual image will be exactly the same in them all.

it's slightly harder to find focus with a fast system (f5 and lower numbers) than a slow system as the depth of focus is less.

I find that slow systems offer more visual contrast than fast systems but this may be in my mind; the same detail is there if you take time to look.

aperture controls brightness and also resolution levels - more = more on both counts.

all my comments are based on newtonians.

[Moonshane]

oh, I am not sure I answered your question!

fast scopes (for the same aperture):

have a shorter more movable tube

provide a wider field of view with any eyepiece

wobble less when moved about

have to use shorter focal length eyepieces to gain magnification

are a bit more fussy when using less than good to top quality eyepieces

sometimes seems to lose a bit of contrast due to larger secondary

need very accurate collimation

slow systems, reverse the entries (apart from collimation).

[swamp thing]

Practicality is also a factor.

Short f/ratio scopes are often more portable than their long f/ratio cousins unless talking about folded optical designs like Cats.

A 10" f/4.5 newt is about 4' long but an f/10 is about eight and a half feet long. :icon_eek:This is somewhat tricky to mount.

Folded optical systems like SCT and Maks (Cats) get over this as some of the focal lengths of the large versions of these is measured in metres. Yet they are just a few feet long.

Regards Steve