How can a telescope be fast?

[spurius]

Also what does things like F/5 mean and so on. Also im a bit thick so can you please not use long words when diminutive ones are adequate.

Also what aperture telescope do you need to see M101 the hurricane galaxy or what ever its called. Its near the large saucepan constellation by the handle apparently but I cant see it in my 8incher. Light pollution is a bit rubbish hear and the faintest thing I can see in my telescope are the galaxies either side of andromeda, I can just about make them out on skies that are particularly dark for my location. Same question but for martian features when mars is nearest to earth.

[spurius]

ah [removed word], how do you edit posts?

[Moonshane]

you have to have a certain period of membership/number of posts to allow edits.

light pollution will kill galaxies although if you can see M110 then it's not as bad as at my site. a darker sky will show you M101 with your current scope and the darker the sky the more you'll see but these face on galaxies will always be quite dim. In light polluted areas, it's often best to wait for the objects to reach the zenith (right overhead) as the effects of ligh pollution is least here.

I could easily see polar ice caps and surface markings on Mars last opposition with my 6" newt so I'd expect you to see good details if your scope is collimated and cooled. Light pollution does not affect planetary, doubles or lunar observing although heat rising from chimneys/rooftops does.

For visual observing the detail visible through a scope is only affected by aperture not speed. A 6" f11 scope will show the same detail as a 6" f5 scope to the eye. Faster scopes are harder on eyepieces and you need to buy better quality to get the same result as with a slower scope - e.g. sharp stars to the edge of field. I find that slower scopes do have a little more contrast than slower ones but the detail is the same with patience.

The speed of the scope is more important for imaging as a faster scope collects photons more quickly than a slow scope and is also easy to guide due to shorter focal length. I know almost nothing about imaging but this is the generally accepted truth.

[JamesF]

The "f" in "f/5" refers to the focal length, and "f/5" (the focal ratio) is the size of the aperture relative to the focal length, so given a 1200mm focal length f/5 scope, for example, the aperture would be 240mm.

All telescopes with the same focal length create the same size image at their point of focus, so if you increase the aperture and leave the focal length the same (reducing the number in the focal ratio) then the telescope can collect more photons to create the image in a given period of time so it is described as "faster" than the original. If on the other hand you make the aperture smaller then you increase the number in the focal ratio, the telescope can collect fewer photons per unit time compared with the original and you get a "slower" telescope.

People often make a fairly arbitrary decision about what is a "fast" or "slow" telescope when talking about them. I'd suggest that f/6 to f/7 might be considered the mid-point, though that's just my opinion.

James

[Spec-Chum]

As an analogy, think of a running water tap and the water as photons (light) and your eyepiece (or camera) as a small cup with a diameter of ‘x’.

The faster the scope the more pressure so the more water comes out per second filling your cup up quicker (important for cameras) but this also creates a wider stream of water (important for eyepieces).

Problems arise when the pressure gets so much that the wideness of the water stream becomes bigger than the diameter of your cup. Imagine trying to fill a bottle with a very thin neck; the slower you have the tap the easier it is to get all the water in the bottle, but put the tap on full whack and quite a lot of it misses and just splashes everywhere. This is astigmatism; you’re getting the core of the stream of water in your cup but the edges are going down the plug hole and from this analogy you can see that the slower the scope the smaller x has to be to catch all the water.

Cheaper cups have a smaller x then expensive cups.

Televue cups are in fact Mugs

[swamp thing]

Hi

Observing faint galaxies is all about the sky you observe from rather than aperture of scope. LP kills galaxy observing stone cold dead. There's no easy way to put it ,LP= no joy, when looking for galaxies. You must have dark skies to have any real chance of observing them with any success.

M101 is arguably THE toughest of the messier galaxies as it has the lowest surface brightness. This makes it the toughest with any LP. I struggle to see it with my 10" from a LP site. From a dark location this is a relatively easy target and shows beautiful spiral structure in larger scopes.

[Catweazel]

What they said!...or, roll it down a hill and see how long it takes to get to the bottom