Sanity check - bigge scope

[earth titan]

Just to check on my undestanding, before i start the saving up and budgetary justification process!

Now I'm bitten by the bug and bigger scopes are within price range, I am condiering buying a 10" / 12" dob. For use in the back garden / patio.

My current 6" newtonian is fab. Shall continue to useit as it is portable but assistance and advice would be great.

As I understand it, buying a bigger scope (mirror size) doesn't neccesarily increase magnification. All it does is increase light gatheirng properties so the the detail you can see at a given maginfication is greater.

E.G. Looking at the Ring Nebular last about I could just about make out a 'hole' in the cente using my 25mm EP with a 2x Barlow and averted vision. When I drop a 10mm EP in the whole nebular dissapears. Therefore a bigger scope, which gathers more light, would allow either a brighter image with the 20mm EP or still abe to pick it up with the 10mm Ep in. I would therfore still be able to see the nebula and therefore see the hole more clearly. The 20mm EP would then be just a brighter image.

Have I got it about right?

[yeti monster]

That's about it. Depending on the Focal length of the scope, magnification with each EP may change somewhat, but the general idea is, more aperture, more light, more light, brighter image/clearer under magnification.

So, go for the largest scope you can get.......... away with.

[John]

If the bigger aperture scope also has a longer focal length then the magnification will be higher. A 12" F/5 newtonian has a focal length of 1500mm so the 20mm eyepiece would be giving 75x magnification. Your 6" scope is 750mm in focal length so the same eyepiece is giving 37.5x magnification.

A 12" aperture scope gathers four times as much light as a 6" one does so the views are significantly better all round.

[Moonshane]

as others have said, the more aperture the better until the portability issue kicks in - I reckon that if you have to carry it out to the site then a 12" is at the maximum for portability. if you can roll it then maybe a 14" / 16"; any bigger and then I reckon you need an observatory with a permanent set-up.

more aperture can increase the effects of light pollution sometimes but I have loved the view of everything through my 12" dob and I live between Manchester and Stockport. most targets are just so much brighter and more detailed in the 12".

The exception to this is double stars and some planets (Saturn was fab through the 12" but Jupiter just better in my previous 120mm refractor). As a result of this I investigated the 6" f11 dob I now have and this is great on planets; consistently much better than the 12" on Jupiter.

If I had to get rid of one it would be the 6" but I reckon you'll still have a use for your existing scope as you say.

there's no doubt a larger scope will open up more targets for you and I'd wholeheartedly recommend you get one but try to get a peek through one first to see what it's like and also guage the size of it.

here's a pic of my two dobs together so you can guage the difference that doubling the diameter of the mirror makes. either of them fit in a VW Golf.

[Capricorn]

If the focal length doesn't alter then yes the image will be the same size but brighter.

Remember that an f/5 12" has a longer focal length then an f/5 10" and both are longer then an f/5 6".

It sounds as if you may be suffering from poor eyepieces as much as anything. That includes the barlow.

If you get an f/5 scope at 10" or 12" then budget for good eyepieces as well. The budget plossl's that are given away with the scope simply won't do it justice.

[michael.h.f.wilkinson]

Remember that the bigger the scope, the higher the minimum magnification which is useful: the minimum can be found by dividing the diameter of the objective (say 300mm) by the size of the fully dilated pupil (5mm if you are older than 35, 7mm for youngsters). For me this would mean 60x. To get a decent true field of view requires investing in wide field EPs.