Choosing focal length

[watzup]

I'm 15 and I've owned a 45mm refractor for a few years. I am planning(not sure yet) to build a 10" dobsonian . I want to be able to see deep sky objects like galaxies and nebulae, as well as planets(more). So what would be a good f-ratio?

BTW Are there any pitfalls of having a aperture as large as 10" ?

[Sam]

Portability would become an issue at 10" and above - not a problem if you're not going to move it much and there's lots of options on light weight designs. The f ratio will become important as the longer the f number is the taller your scope will be and the more difficult to get to the eyepiece. My 16" Lightbridge is a f 4.5 and it's still just under 6 ft in height - my kids need to stand on a chair to see through the eyepiece. The shorter f numbers are not so forgiving on cheap eyepieces so that's also a consideration. Hope that helps.

Sam

[barkis]

A 10" Dob. at f5/f6 will suit your needs. The higher the f/ratio, the higher the inherrent magnification for a given eyepiece, therefore the need to move the scope more to keep the target in view would be required. 5 or 6 is good.

The Dobsonian is a simple, but very effective instrument, and the only drawback to having a huge one, is the weight problem, although that can be overcome by using a wheelie to move it about.

A 10" f5 would be a wonderful scope for your deep sky observing.

Ron.

[Ben Ritchie]

The other thing is that collimation becomes more critical as the focal ratio decreases - so f/5 or f/6 are going to be (significantly) easier than f/4

[MikeP]

You say you are planning to build a dob, does that include grinding a mirror or will you buy a mirror set?

If you do plan to go down the glass pushing route, 10" is quite large for a first go. Also focal length will be an issue but others will have to comment on that, I have no experience.

Mike

[scotastro]

I have a Dark Star 10" f6.3 which is a good deep sky and planetary scope. An f5 scope would have a lower magnification and wider field of view than the f6 for the same eyepiece. Following up on Mike P - the longer the focal length the easier it is to figure the glass if you are considering to build your own mirror. There are ATM websites for scope designs.

Mike

[watzup]

If I'm not considering height, weight, etc, but just what I can see through the scope, then would higher would be better?

Will a f 6.~ scope produce an image with significantly less contrast than a f5 one?

[John]

If I'm not considering height, weight, etc, but just what I can see through the scope, then would higher would be better?

Will a f 6.~ scope produce an image with significantly less contrast than a f5 one?

Visually an F/6 newtonian should probably show more contrast than an F/5 for 2 reasons:

1. The secondary mirror can be smaller and therefore contrast should be a bit better, all other things being equal.

2. The scope is easier to collimate and keep in good collimation which also maximises contrast.

John