[astro744]

Originally Posted by bluesilver

Thanks for the replies and information.
Sorry if this is another basic question, still trying to get my head around all the right things.

So for planetary viewing, i am better off going for a longer focal length rather than larger aperture?

I thought the larger aperture would give me more detail due to the more light it lets in.

So instead of looking at say the 10 inch dobsonian i might be better of going for something like the Cassegrain Telescopes,
Something like the Black Diamond 180/2700 Mak-Cassegrain, where it has a focal length of 2700 mm ?
Or this might be a tad over the top, but the Celestron C14

There is nothing magical about a Mak. The 180mm has a smaller aperture than your 10” Dob but does have longer focal length. The longer focal length just means you can use longer focal length eyepieces to get a similar power that on your 10” requires shorter focal length eyepieces. The maximum capable magnification still goes to the 10” as it will give a higher power at a similar exit pupil.

The Mak has a 30% obstruction by diameter (used to compare contrast) whereas your Dob is closer to 25%. The Mak is a sealed system which will help with tube currents but it takes longer to reach ambient. A C14 has a larger aperture, large obstruction and long focal length but can deliver fine images if of good mirror and properly cooled. The 180 Mak too can do the same but with less bright images and less resolution.

You are correct in saying higher aperture is capable of more detail. Yes there is more light so higher powers are possible for the same exit pupil but also there is more resolution. (Of course good seeing and collimating required). I got the impression though that you believe adding an aperture mask to a larger mirror Dob retains the resolution of the larger mirror but reduces the brightness. No, adding a mask produces a smaller aperture telescope but one that is not obstructed by the secondary and spider vanes. However in moments of good seeing the larger aperture will show more detail even on an obstructed system. (A reasonable mirror assumed but doesn’t have to be first class).

Maks too can be difficult or impossible to collimate by the user and usually have to be sent to the factory. If anyone tells you they hardly ever need collimating ask them what happens when they do. If you do get a fine sample and seeing is good you will get some fine images but your current 10” should be able to do the same and a larger aperture Dob will show more given good seeing, good collimation and have a reasonable but not necessarily first class mirror, (first class excel in best seeing).

Whatever you choose, enjoy!