Central obstruction size, Round II

OK, folks, bear with me. I keep going around in circles on this. Just when I was getting serious about minimizing my CO, a friend brought up an argument very similar to one I had made a month or so ago:

At 16" of aperture, the Airy disk is on the order of 0.5 arc-seconds, and the first diffraction ring is on the order of 1.0 arc-seconds. Up to 40% CO, most of the damage is done by throwing light from the Airy disk into the first ring (in fact, a 50% obstructed scope throws nearly an identical amount of light into the Airy disk and first diffraction ring as a 0% scope does into the Airy disk).

So, unless your seeing is better than 1 arc-second (and mine rarely is), you won't often be able to tell the difference between a large obstruction and a smaller one.

One couterpoint is that when observing planets I usually spend 20 min or more at the eyepiece, during which I'll often get several seconds of much better than average seeing. These several seconds are likely to be better than 1 arc-second FWHM, and so if the same is true of deep sky targets I still might be missing out on the few instants that the bar, or the arms, or the tidal tail pops into view.

The first point would argue for a 20" 40% obstructed scope, while the counterpoint might favor a 16" 30%. Anyone swayed by this, or does everyone still line up on the same side of the fence?

Thanks for your thoughts,
-- Jeff.

Interesting take on it.
I have always elected for more field illumination over minimizing the secondary at all myself, and never thought to much about it beyond that.

Even though they say you can't detect light fall off visually with a smaller than 100% illuminated field, I think you would be surprised by just how much overall brighter the view in the eye piece is with a larger obstruction / secondary mirror. This goes against convetnional logic some, as you would think more obstruction would mean a dimmer overall view from reduced light, but that is not how it works in reality.

Good thoughts on it and a great post though for sure. Certainly more food for thought on it.

First-off, for a 16" scope the Airy disk diameter
is about 0.25 arc-seconds.

The reason to have a 16" scope on planets is indeed
those rare times when the seeing is much better than
1 arc-second. If you want to tune your aperture to
the average seeing, then get a 4" scope.

If your main observing is deep-sky, then aperture
is the way to go. CO won't matter very much.

If you plan to do most of your viewing with a
31 Nagler (or wide-field imaging) then aiming for
uniform illumination is probably a sensible goal.

I think as someone pointed out on the CO-size, Round I,
the bigger variable in all this is the quality of the
optics. The difference between 30% CO and 40% CO
will be small compared to the difference between
1/2 wave PTV and 1/10 wave PTV optics. Maybe the
real question should be who will make the better
optics and back it up with some believable documentation.
Who will stand behind the optics, when they're not
up to specs.

"The first point would argue for a 20" 40% obstructed scope, while the counterpoint might favor a 16" 30%. Anyone swayed by this, or does everyone still line up on the same side of the fence? "

With due respect to all large Dob owners, I am willing to bet that a 20" is a bit too big for planets, that is if you expect to get its full firepower, the seeing will never allow you to even come close, even if you live in southern Florida, if you live in the snow belt then a 6" apo will certainly outperform a 20" instrument all the time, never mind the laws of physics.
Another thing to consider with a 20" 40% co., since the images are so bright you need to start considering the effects of not only the first ring but also the second and possibly the third, yes they are faint but they will be there and they will affect the contrast.
For planets a high quality 10", IMHO, is the optimum aperture, with 1/4 arcsecond resolution, seeing permitting, you can expect to capture threshold detail, once you go over that aperture the seeing starts to limit its potential.

Vahe

Most of the best planetary pictures have been with C14s which suggests that the tradeoff between larger aperture and obstruction leans to aperture even if obstructed. When the seeing is good, the planets look great in the SCTs, and when seeing isn't so great, there are deep sky objects to observe.

Jeff Young said:
....

One couterpoint is that when observing planets I usually spend 20 min or more at the eyepiece, during which I'll often get several seconds of much better than average seeing. These several seconds are likely to be better than 1 arc-second FWHM, and so if the same is true of deep sky targets I still might be missing out on the few instants that the bar, or the arms, or the tidal tail pops into view.

I was having this conversation with Joe Haberman and he had a fascinating partial refutation of my counterpoint. Evidently, the dark adapted eye has a slower transfer rate to the brain, and so I'm less likely to get the advantage of very brief moments of improved seeing when viewing DSOs.

-- Jeff.

I took another run at this from the CTF angle, rather than from the Airy disk angle. All the CTF charts (Suiter, Rutten and van Venrooij, etc.) show the central obstruction curves with a significant dip between 20% and 50% of the maximum spatial frequency of the instrument.

So what exactly does that mean for a 16"-ish f/10-ish instrument on deepsky targets?

I got out the calculator and did a little math. (It wasn't too painful.) At f/10, a 40% CO shows:

* a contrast loss around 60% at 50 lp/mm (1 arc-second with a 16" aperture)
* around 10% at 10 lp/mm (5 arc-seconds)
* negligable contrast loss at 5 lp/mm (10 arc-seconds)

So I think we can say that the CO plays little role in features between 5 arc-seconds and 10 arc-seconds, and almost no role in features larger than that.

So, how big are features? Here's a sampling:

* M51's features are large. Even the bridge is probably 40" wide or wider.
* M95's bar is about 30" wide, and its inner, nearly circular ring is about 15" wide.
* NGC3631 (a bright but small face-on spiral) has arms also about 15" wide.
* The visible granulation in the Sunflower galaxy is on the order of 10" clumps.
* The Eskimo nebula is about 40" in diameter, with some visible features in the 4" to 5" range.

No issues, with the possible exception of the Eskimo. Here we're starting to see some degradation from a 40% CO, but it may not yet be visible to the human eye.

Planets? Yes, big CO bad. DSOs? Not so much, at least at 16"+ of aperture looking at Messier/Herschel-type objects.

Comments?
-- Jeff.