4" APO or 5" Achro???

OK Planetary Peoples:

I currently own a 4" apo that I am very pleased with. My question is simple. For the upcoming Mars apparition, I am wondering if it would be worthwhile to purchase a good quality 5" achro.
I have read in numerous places that 5" seems to be the magic aperature for planetary viewing. This aperature seems to make the best use of atmospheric seeing on most nights.
With this in mind, would it be worth it to consider a 5"achro. I could never afford a 5" apo. The 4" apo was a stretch and I got a steal on it. Its more of a travel/quick look scope and it works well for that.
Or, and its a big or, should I just stick with the 4" apo. I also have an optically excellent 12.5" F6 dob that gives mind blowing veiws so please don't suggest other scopes. The question is whether a 5" achro would give me a big advantage over the 4" apo.

Thanks in advance for the excellent input I know this question will recieve.

Clear Skies - Rob E.

If it were me, and the 4-incher was an excellent scope, I wouldn't bother with a 5-inch achromat.

On average-to-mediocre nights, I'd be perfectly happy with the 4-incher. It will show *LOTS* of Martian detail, so long as conditions allow.

During those rare nights when the seeing had me giddy, drooling, and/or stunned, I'd leave the little guy in the house, haul out the big Newtonian, and spend most of that time nudging that bad boy across the sky.

In either case, I'd make every attempt to stare and study Mars as much as time would allow. I'm a steadfast believer in the following equation: care + time = more detail seen.

Best wishes, and enjoy the upcoming show.
Dan

I'm in a very similar position, Rob. Actually, if you use the 4" APO with patience and practice, you'll begin to see much more on Mars. I had a very decent view last opposition
with my 4" TV-102. But I'm searching for a larger scope (a reflector) to hedge my bets this year.

Since you've already got the 12.5" Dobs, I don't see that you need another scope. But then, it's really a personal decision.

Perhaps something in-between then like a 6" MN61/MN66 Maksutov Newtonian, reputed to have the most refractor like view? Many observers seem to consider 6" aperture as the sweet spot for planetary viewing.

Best of luck on your quest.

Ron B[ee]

If your 12.5" Dob has a 2.5" diagonal or less, why not use a 5" off-axis mask? You'll get 5" apo-like views for the cost of a piece of cardboard and a can of flat black spray paint. I do this with a C-14. Works great once the tube currents have settled down. Dom Q.

IMO, the "magic" of a 5" refractor has more to do with its increased resolution and light grasp than dealing with aperture limiting seeing conditions.

Dave Mitsky