Focal Length v Focal Ratio

[MasterC]

Hi all

I currently have a 3 inch 'scope, which I love, but I have caught aperture fever, and I a new scope.

I have been eyeing up a Celestron 6 or 8 SE (or evolution if I get a cash windfall) but don't quite have the budget, just yet, and I have noticed the SkyWatcher Star Discovery 150P.  There is a fair difference in the speeds of the scopes.  

I quite looking at planets, but really want to get into deep sky objects as well.  

Would the cheaper SkyWatcher Star Discovery 150P, be reasonable for both, or should I stick with my guns and go Celestron 6/8 SE? I also like a Goto scope.

TIA

Chriss

[ronin]

Speed is a term from photography and I suspect from pre-digital also to an extent.

The speed of a scope can or will determine certain characteristics - a fast achro will show CA, a fast newtonian will be more reliant on being well collimated.

For deep sky you want aperture, they are faint and you need apeture to collect the available light and squirt it into your eye.

To an extent a slow large aperture newtonian is non-sensible in amateur terms. A 12" f/8 newtonian on a dobsonian would put the eyepiece about 90-96 inchs high, even on tip toes you are not going to get to look through that. So a big newtonian has to some extent be fast, they are unusable otherwise. A 12" f/8 newtonian would be nice but not much use to most.

If you want a 6SE or 8SE then wait and get one, unless you can afford both or wait a bit longer for the nSE and have 2 scopes.

Potential problem of a 6SE or 8SE is keeping the magnification down and so the brightness up when looking at DSO. Many start out dim and making them bigger and so dimmer does not always help. Also some are simply big and highish magnification mean narrower fileds of view.

A Star Discovery 150P matches the 6SE for aperture, should therefore see much the same with a similar resultant magnification, the 8SE should show more, it's bigger.

Any intention, even little ones hiding in the background, of going imaging ?

If so do a rethink.

[MasterC]

Thanks ronin.

There is a very small part of me thinking about imaging, but its tiny, and a long way off, so i think i can leave that consideration for the time being.  Its an excuse to get a new 'scope if needs be!

[ollypenrice]

In visual astronomy the effective focal ratio is what matters - that of the EP and scope combined. The F ratio of the scope only (or the EP only) matters little. However, the focal length of the scope matters a lot. With a long FL scope you stand (in more or less 'real world' terms) to gain a bit of image quality at high magnification while you lose, totally, the possibility of a really wide field of view. Personally I would rather have a shorter FL scope for visual and have the option of a wide FOV even if the quality at high power were compromised. But if the long FL comes as a Mak or an SCT you have to bear in mind that, if you lose on field of view, you gain on portability and compactness.

Oh for the perfect scope!

Olly

[LukeSkywatcher]

Honestly if you have to pick between the 6se and the 8se..........the 8se wins hands down. Honestly the narrow(er) FOV on the 8se does not bother me one bit compared to my other scopes. The large aperture and portability is what sold it for me when i bought.