8 OR 10" DOB??

Which will be better for planets and globs a 8" f/6 or a 10" f/5 dobsonian? or is thier not much of a difference?
thx
Brian

8" f/6 gets my vote for these reasons: The 10" will have 25% better resolution and 56% brighter, but it'll be significantly bulkier and heavier without being THAT much better on planets.

Moreover, off the center of field coma starts to be a factor which is less severe at f/6 than it is at f/5. This is an issue with planetary-dob viewers (like me!) since, with manual tracking, your target is off the center of field the vast majority of the time.

Even though the 8" f/6 system would allow a smaller secondary for improved contrast, most people seem to agree that aperture always wins. If the secondary is 20% of the aperture or less, the loss of contrast will not be great enough to be significant (I'm told).

When it comes to planets, I often wonder if aperture that is *not* vastly different is the only variable in the "cosmic equation" :-(. Well Brian, as someone who has the affinity to fine optics, perhaps, Rick Ringmaster can shred some light on this ;-).
http://www.starmastertelescopes.com/html/thin-mirrors.htm

Ron B[ee]

Brian,

I'd be interested in finding out what brand of dob you were planning to purchase? I own an Orion XT10 (10" f/5) and I see very little detail on the planets. I'm beginning to think my mirror isn't of very high quality. And before anyone asks yes it is colluminated (sp?). I would sugguest getting the 8" but I would look for an f/7 or so.

On the other hand I bought a TV-102 (prolly way out of your price range) and I LOVE IT!! The detail level is much higher than the dob and I find it gives much more pleasing views.

The main point of this post is look for a high quality mirror no matter what size you decide to get. I think that is the most important factor of all. Hope I could help you some.

Rick Riley

I have actually owned both scopes Brian and at one stage had them both together for a month while I was selling the 8". The 8" is slightly easier to handle but you can definately see more with the 10" than the 8", on both DSO's and the planets. Cool down time with the 10" is significantly longer than with the 8", allow at least 30 mins for good planetary views in the 10", this is not such an issue with the 8". Also something to consider is that if your local seeing is unsteady the 8" is less affected by poor seeing than the 10". So consider the above exceptions carefully before you decide. I will conclude by saying that once in position, set up, cooled down and with good "seeing" the 10" wins comfortably everytime.

Clear Skies
John Bambury

I have the 8" Discovery DOB (not the premium one) and I love it, though sometimes I find myself looking at their 13.1" and going hmmmm....