Posts Made By: Lorne Johnson

The reral advantage of the 15" is deep sky. I doubt that you will see much improvement on planetary. And, given what it takes to set up a 15 in dob on a tracking platform, I think that you'd actually see a lot less.

You are not wrong, but I 've used a 16" f/6, and it doesn't get used much. It's awfully hard to work with. The result has been that I've dropped the idea of a 16" and am vaguely pursuing the idea of a 14.5" f/5 or a 12.5" f/6.

I think that you have to make your own. This is why I have ended up being so happy with a quality 8".

1. The focuser may not be perfectly square to the tube.

2. The secondary may not be perfectly square to the focuser. (Check this visually, and if you can see anything wrong asume it's just fine for an f/8 system)

3. The laser may be out of adjustment (and with an f/8 system is more likely to cause problems than to fix them.

The first thing I would do is a star test, once you've go the concentric circles the optics are in line with each other. The go back and shim the focuser until things line up better. then check the views and do another star test (which should not have been changed by fiddling with the focuser).

There is a rule of thumb that a focal length of double your scopes f/ ratio gives "ideal" deep sky views,which in your case would be something like a 22mm panoptic, or other widefield of your choice.

That would certainly be a good starting point. But, there are a lot a variables (contrast, seeing, transparency, light pollution) around this, and observing experience is one of them. When I first started with my 8" f/6 objects were beginning to fade with a 12.5 mm eyepiece, now I might go down to 9 or even 7 mm on sat M 81 and 82. I also found that I used a 15 a lot. And, of course the venerable 22 mm panoptic gets a lot of use as well.

You might try to find some stars on charts that you can just see unaided and that will give you a sense of how good your local sky gets. If it's around mag 4 you might find that a light pollution filter would help.

I like 8" f/6 because it is big enough to show a fair bit, and small enough to get used a lot. However, if you really want to see more 14" f/5 is supposed to be another good point for usability versus light gathering power. I have had some experience with a 16" f/6 (not my own). It certainly shows a sky littered with NCG's, but it's hard enough to use that It rarely gets out. At f/4.5 or even 5 it would be better, but also more specialized.

My personal ambition alternates between a 14.5" f/5 and a 12.5" f/6.

Your top image looks a lot like what I usually see of Jupiter visually (sometimes I can see more than that) depending on the seeing conditions. I have never visually seen the quality you have in the second image.

Yu Gu has apparently taken images like your lower one with a 9.25" Celestron, I would suspect from a fairly high altitude.
at rfroyce.com there is among other things, a series of images of jupiter intended to show different mirror quality. You might find it very useful. He is very partial to 10" reflectors and to simplicity. An Orion xt 8, or a Discovery 8", at f/6 (or their 6" f/8) will simply blow away the ETX 90. You might look at the dobsonian section.

The Panoramic will work with the Sky 90, but at 12.5 I would go to a Gibralter. My 2 bits.

I think that I have, but I also think that I am lying. Perhaps that deserves some explanation. I started observing the vail and NA last summer using a lumicon UHC filter on an 8" dob. The vail was wonderful, and later I found the NA. It so big and so faint that it is very hard to spot unless you are right on the edge with the best contrast. With practice it gets easier. last fall i tried my new MN56 on it (very contrasty scope, it actually worked better, mostly because the almost 3 degree view meant that I could see much of it and that allows the nebulosity and shape to emerge. So a five inch scope will do it, and a good 4 inch refractor certainly should as well.

Last week I was trying out the B&W 30mm 80 degree eyepiece, and looked at both the veil and NA with the UHC filter (and it wasn't that dark a night, about mag 5 or perhaps a little less. I put the B&W back in the focuser while I put the filter away, and then went back to look through for yet one more sense of its edge correction in the star field there (quite good, by the way). I could have sworn there was nebulosity in the field when I wasn't looking for it. The more I tried to confirm this the more it dissappeared. But finding the NA without a filter, I never have, or even felt that I might come close.

AS long as this is a thread about hype, we might want to consider Zambuto hype. Let's assume as a start that Carl Zambuto is the best mirror maker in the world. Still, there are several others very close behind (and remember that our assumption is just that). One can measure the differences between those mirrors with instruments in a laboratory, possibly. But, you're not going to see them at the eyepiece, not when we're talking about strehl ratios consistently above 96%. Resale and bragging rights are separate matters, and are actually based on hype.