Aperture Fever

I have a 12.5" Discovery split tube PDHQ. I "think" I want to go bigger. Does anyone out there have any experience with what size would be next? I would think that in a 15" you would only see a marginal difference. Definitely going truss (if I go) and I want a F5.

The 12.5" to 15" is not a big jump or change. I had both at the same time.
Probably an 18" would be the logical next size for a big jump that would be noticeable.

I would recommend doubling your mirror size, if you want to go bigger. That gives you almost a full magnitude of light gathering ability. In my experience, that's where I see a notable difference. If you are doing a side by side comparison, I bet you could see a smaller difference.

I think what may be more important are the ergonomic issues. A 15" scope with a 75" focal length is fairly good sized. An 18" seems only a little bigger, but it is 15" longer and taller.

Others have suggested 18 inches or so. I agree. I am in the same situation as you, two 12.5 inchers but I am happy at this time.

But i would definitely suggest rethinking your committment to F5. An F4 scope can provide those same sharp views with a good mirror but will be only about 72 inches long so it will not require standing on a ladder.

Jon

George,
I had only had a 12.5" truss Dob for a few months when I started looking at 18" Obsessions, which I had already been wanting anyway. After soliciting the advice of the "forum panel", I ended up getting a 14.5" Starmaster, which I have been very happy with. Of course, the 12.5" hasn't been getting much action, which is a shame.
If you'd like to tap in to some of the excellent advice I got, it's still there:
https://www.astromart.com/forums/viewpost.asp?forum_post_id=311725&poll_id=&news_id=&page=8
Have fun, whatever you end up with.
Linton

This is a long thread isn't?
My tuppence worth is that there is a mindset amongst us,thanks to Kriege and Berry,that big aperture means wheelbarrow handles and wheels pretty much.
I'd suggest thinking light.There are a few builders out there that have gone down this path and not regretted it.
I managed to make a 2" thick mirrored 20"f4 that is put up in less time than it takes me to put up a Sky90.Six screws.Two or three minutes maybe.It weighs less than a hundred pounds total.I'm near 60 now and the impetus to build it was unadulterated aperture fever.The question was if I didn't build it now,..would I ever?
The idea of a superlight 30incher has started to distract me now I confess.
There doesn't appear to be a cure for those afflicted by "AF"!

Bruce

I had a 16" dob that just "grew" too big for me to haul around. I was considering downsizing to a Nexstar C11. So one night I made an aperture mask for my 16" to create an 11" out of it and compared lots of DSO's from my backyard. I was surprised at how similar the views were. The biggest difference seemed to be on the globs. There were noticably more stars resolved in the 16". But it was harder to see much difference in galaxies or nebulae. So I went ahead and bought the C11. I made the right move. I enjoy the new scope immensely. So for me, it was the right way to go.