quick question

[neilf]

with reflector telescope does the length of the scope have any significance?

[Merlin66]

Longer reflector telescope tubes usually means that it has a longer focal length. With normal mirrors this would also infer longer focal ratio's.

Keeping things simple: The focal length gives the image size and the focal ratio gives field of view. So an f8 would have a larger image size and a smaller field of view than the same diameter mirror working at f5. For a 150mm mirror the f8 tube would be approx 1200mm long whereas the f5 would be 750mm long.

Hope this helps,

Ken

[neilf]

yup nice one thanks m8

[brianb]

Following on .... a longer focal length is more tolerant of cheap eyepieces, is easier to collimate and more likely to retain collimation, and actually has a wider field of sharp (diffraction limited) performance. f/5 scopes have got popular for reasons that have more to do with cosmetics than performance ... if you really do need a compact tube, and are prepared to pay as much for each eyepiece as for the whole scope and/or fork out several hundred pounds for a coma corrector, maybe a f/5 Newtonian makes sense. But, in small to medium sizes, I'd far rather have an f/8 scope ... the secondary mirror doesn't need to be as big, either, which improves the contrast on planetary targets to a small extent. Even in large sizes, I'd want f/6, at least, for visual use with a Newtonian / Dobsonian.

Deep sky imaging is a different kettle of fish, something like f/4 with a coma corrector is probably the better way to go.

[neilf]

Deep sky imaging is a different kettle of fish, something like f/4 with a coma corrector is probably the better way to go.

why would a smaller scope make a difference for the imaging?

[dark knight]

Not trying to put a spanner in the works, but using a barlow will also affect the F/ratio. My understanding is say using a 2x Barlow in an F/5 scope would make it F/10...Perhaps someone can confirm this is correct.

[neilf]

ok totally confused now exactily how do the f's work and what are they?

[SteveP]

Neil

I'm not surprised you're confused! It can seem complicated (it is!) but not helped when you have some misleading information.

There are two important physical measurements of any telescope - the 'aperture', ie the diameter of the lens or primary mirror and the 'focal length' ie the length the light travels within the telescope. On some designs eg refractors, this is not too different from the overall physical length. On other designs, eg Newtonians, SCT's where the light is folded back on itself by mirrors, this can be very different.

The aperture determines how much light is collected and the focal length determines how the light is magnified by the scop. The bigger the aperture, the more light is collected and the longer the focal length, the more the object is magnified. Does this make sense?

The third term you will see is the Focal Ratio of a scope. This is merely the ratio of the focal length to the aperture eg a scope with a focal length of 500mm and an aperture of 100mm, has a focal ratio of 5 - normally shown as f/5. It's a useful shorthand for describing the 'speed' of a scope. Scopes with f/6 or below are usually referred to as 'fast' scopes. In most observing situations, the speed of the scope is however not that important although fast scopes sometimes need higher quality eyepieces because of the high refractive nature of the fast scope's design. A fast scope comes into its own when imaging. Why? Because it allows the image to be collected with shorter exposure times. Thus reducing the overall time in collecting the 'raw' images. the light isn't travelling any slower or quicker in fast scopes. It just describes the relationship between its ability to collect light (aperture) and the extent to which it is magnified (focal length)

The focal ratio has nothing to do with the field of View - that is governed by the scopes focal length combined with the parameters fof the eyepiece - specifically what is know as the Field Stop which governs the diameter of the image which is seen.

I hope that this hasn't added to your confusion.

Steve

[dark knight]

Sorry Neil, didn't mean to confuse you, have a read of this

F-number - Wikipedia, the free encyclopedia

[dark knight]

SteveP, what is the misleading information??

[SteveP]

Neil

That's a good article which DK has pointed you to.

If you want to read some more, have alook at the Televue site for the articles written by Al Nagler, one of the best known scope and eyepiece designers

Tele Vue Optics Article Page

Steve

[SteveP]

DK

The misleading part I referred to is the earlier comment by Ken on the importance of the focal ratio - '... and the focal ratio gives field of view'. FoV is a function of the scopes focal length and the EP's Field Stop and not related to the focal ratio

Sorry, i should have been more specific

Steve

[dark knight]

It's ok mate, I just wanted to understand where the topic went wrong...The F ratio determines image scale and not FOV is my understanding, is that right?

Carl

[SteveP]

Carl

There's a lot to understand in this hobby of hours and as you infer, having a good understanding is enormously helpful.

Do you mean 'image scale' in the context of a CCD? That is, how much of the image is represented by each pixel? If this is what you mean, then again, image scale is a function of the focal length of the scope, ie by how much the object is being magnified, and not the f ratio. It's usually shown as X arcsecs per pixel

Steve

[brianb]

My understanding is say using a 2x Barlow in an F/5 scope would make it F/10..

Yes, and gets you round the eyepiece issue, but doesn't work around the collimation issues - a f/5 Newt with a Barlow in it is just as hard to collimate as one without (maybe harder because of the extra image scale) and will have 1/8 of the diffraction limited field of a native f/10 Newt.

[neilf]

hehe think i got it now ill have a look at thiose sites later tonight as well

[dark knight]

Thanks Steve, it all makes more sense to me now also. Your not kidding when you say there is a lot to learn, I'm just starting down the slippery imaging route...

Thanks Brian, I also understood a slower scope is better for imaging planets, is this correct?

Neil sorry for hijacking the thread, only like you I want a better understanding lol..

[SteveP]

That's good news! Glad to be of help

There is so much for everybody to learn and if the understanding isn't therealready, then the best way to understand more is to ask questions. There's usually somebody here who knows the answer:eek:

Steve

[Merlin66]

SteveP,

You are indeed 100% correct. I was trying to over simplify.

[brianb]

I also understood a slower scope is better for imaging planets, is this correct?

Indeed - you are going to have to extend to f/20 - f/30 to get a reasonable image scale anyway so a "slow" f/8 or f/10 objective isn't a handicap at all.

[dark knight]

So my f/5 skywatcher Newt will need a 5x Barlow, to get to f/25, and the focal length of my scope would become 5m.