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OPT 8" f/9 Planet Pro Dobsonian

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#76 careysub

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Posted 23 April 2013 - 12:25 PM

...
The "average" American stands about 6' tall - 72". Figure your eyeballs are mounted about 4" below the top of your head. So, the "average" person has a maximum standing eyeball height of 68".

On the other side, one can quickly figure a normal and comfortable seated position of body height minus the distance from knees to hips. (Think about it, or just just look at how you sit now - your thighs are horizontal and contribute nothing to height when seated). For most people this is around 40".

Thus we have a normal comfort range of 40" to 68". Any shorter and you need to "scrunch" while you sit. And on the other end, you need a 4" tall step. For a 68" radius arc corresponding elevation angles are roughly 34 degrees and 71 degrees. Attached is pdf which illustrates this. Of course YMMV by body size and how efficiently the scope is designed and built.

Maybe in Texas. Only 15% of American men are as tall as 72", half are below 69", and very few women are 72" (Census round-off error makes this number 0.0%), the half point for them is 64".

So, subtract 3" to 8" to bring the comfort figures in line with the real average values. It reduces the max height for standing 6.5 degrees and 14.5 degrees respectively (to 64.5 and 56.4 degrees).

But if you live north of 49 degrees (the U.S. Canada border in the west) the ecliptic never gets above 64.5 degrees anyway.

#77 azure1961p

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Posted 23 April 2013 - 01:20 PM

[quote name="Jon Isaacs"] [quote]Jon,

I understand your point, maxing out the aperture for a given focal length, but I found my own choice by considering what I wanted to see, what the resolution needed to be and how big an aperture and every bit as rock solid: how big a scope do i want to manage? [/quote]

Pete:

Mostly I am just trying to provide a different way to look at things, if you look at say F/3.6 versus F/8 without substantially increasing the aperture, it makes little sense as a planetary scope.

Jon [/quote

I couldn't disagree with you more here but I appreciate your comments.

Pete

#78 Jon Isaacs

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Posted 23 April 2013 - 01:43 PM

Give two guys a \$1000 to buy a complete 8" dobsonian scope. One gets an 8" f/4 scope and the other an 8" f/8. Who do you think is going to get the better high power views? The guy with 8" f/4 sure isn't going to end up with a Zambuto mirror in his scope! I'm also sure a 10" f/5 with a "perfect" mirror is going to beat out (probably not by very much) that 8" f/8 in high power planetary views ... but, at what ... 2X-3X the cost?

Think outside the box, how about giving two guys \$1000 to build the best possible planetary Dobsonian? No aperture restrictions.

It might push the cost a bit but I would put my money on a 12 inch F/5 GSO Dob with a refigured mirror, (if necessary.)

Jon

#79 Sarkikos

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Posted 23 April 2013 - 01:45 PM

Sounds good to me.

Mike

#80 Jon Isaacs

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Posted 23 April 2013 - 02:03 PM

[quote name="azure1961p"]Pete:

Mostly I am just trying to provide a different way to look at things, if you look at say F/3.6 versus F/8 without substantially increasing the aperture, it makes little sense as a planetary scope.

Jon [/quote]

I couldn't disagree with you more here but I appreciate your comments.

Pete
[/quote]

Does that mean you think an 8 inch F/3.6 does make sense as a planetary scope?

Once you move beyond comparing telescopes by aperture alone, suddenly there are many more choices and solutions.

Engineers think in terms of constraints. I have \$XXXX, is the question: "I want to build the best planetary scope I can" or is it "I want to build the best yy inch planetary scope I can?" Aperture is an artificial and unnecessary constraint.

I think it's kind of humorous that somehow this F/3.6 snuck in here. Reasonable focal ratios for an 8 inch Dob begin probably at F/5 but F/6 is superior ergonomically. If you have ever used a dob that's 34 inches long, it's not much fun.

Jon

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Posted 23 April 2013 - 02:22 PM

The only application I can think of where I would want an 8" f3.6 is for wide-field imaging.

#82 Mark Costello

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Posted 23 April 2013 - 04:28 PM

The last Newtonian I owned was an 8" F3 Dob. It was built primarily as a Newtonian but had a hole in the primary mirror for conversion to a Cassegrain. The company making it put it on a Dob mount. When I ordered it (over 30 years ago), I got a friend at work to build a table top tripod for it. I had the legs extended all the way for observing standing up - like I thought real astronomers should do. That was a BIG mistake, and one of the reasons why I drifted out of the hobby. But the scope itself worked really nice at low and "midling" powers, giving me my first real views of M42, M31, and the double cluster (a real favorite for me)....

#83 azure1961p

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Posted 23 April 2013 - 08:08 PM

Jon, come on now - and poking me with that stick will you(!) an 8" f3.6 is a disaster of planetary scope . It has its place in imaging to be sure but - oh come on its got to have a secondary thats atrocious. Theres no overwhelming angular res to the rescue here its simply a bad planetary scope!!!! Wonderful imager like Jarad mentions but .

The 12" f/5 - now thats a good planetary scope. Id prefer a step and a half longer but clearly its fine. The trouble with this however is the comparitively heavy mirror and OTA. You cant have a 35 lb. Ota in 12" . In and of itself its superior no doubt. Id prefer it and collimation issues be da____d. But its never in and of itself and despite the long 72" tube it is a really super easy carry and i was happy to be rid of the 10 for that reason - so a 12 never entered my mind.

If weight was never an issue and I still had a country home with garage itd be a 12" f/6 or 7 - probably a 7 to keep,my secondary neurotically small. Id ponder a 16" f/6.

Pete

#84 Sarkikos

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Posted 23 April 2013 - 09:03 PM

Jon,

I think it's kind of humorous that somehow this F/3.6 snuck in here. Reasonable focal ratios for an 8 inch Dob begin probably at F/5 but F/6 is superior ergonomically. If you have ever used a dob that's 34 inches long, it's not much fun.

I don't know. This evening I used a 6" Dob that's about 27 inches long and I had a lot of fun Moon gazing!

Mike

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Posted 23 April 2013 - 09:05 PM

You cant have a 35 lb. Ota in 12".

Now you're talking about ultralight designs. Whole new subject. But yes, you can actually get there. I have a 14.5" f4.4 scope that weighs 27 lbs (total). All carbon fiber with a quartz mirror 0.8" thick.

You have to deal with a different set of issues (stability vs. weight, etc.), but it can be done.

#86 Sarkikos

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Posted 23 April 2013 - 09:10 PM

Mark,

I had the legs extended all the way for observing standing up - like I thought real astronomers should do. That was a BIG mistake, and one of the reasons why I drifted out of the hobby. But the scope itself worked really nice at low and "midling" powers, giving me my first real views of M42, M31, and the double cluster (a real favorite for me)....

We know better now, don't we? Nothing helps an observing session like a good SIT! I usually don't even stand when I take out binoculars!

Mike

#87 moynihan

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Posted 25 April 2013 - 01:52 PM

Uncle Rod Mollise once said, "When it comes to planetary viewing, seeing is not the most important thing, it's the only thing."

In a thread like this, the first thing I do is look over and see what people live. We are captives of our own experiences so it's no surprise that someone who lives up near the great lakes or in the North East might have quite a different view than someone who lives along the coast in Florida or along the coast here in southern California.

When I respond to someone's post, I try to imagine something about their situation and hope to communicate something about my situation..

Jon

#88 moynihan

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Posted 25 April 2013 - 02:12 PM

There were references in this thread to human height averages.
They do vary by country.
FYI
In the U.S.:
Male: 5 foot 10 inches
Female: 5 foot 4.5 inches

#89 azure1961p

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Posted 25 April 2013 - 06:17 PM

You cant have a 35 lb. Ota in 12".

Now you're talking about ultralight designs. Whole new subject. But yes, you can actually get there. I have a 14.5" f4.4 scope that weighs 27 lbs (total). All carbon fiber with a quartz mirror 0.8" thick.

You have to deal with a different set of issues (stability vs. weight, etc.), but it can be done.

I was actually going to design (steal Kriegs work) an ultra compact as a carbon fiber wonder. I've got a lot of experience with fiberglass, carbon fiber and Kevlar. Larger aperture is this thing I move through as an idea from time to time then abandon it. Its cyclical.

I really really do like those scopes though Jarad. Right now Im in the abandonment part of the cycle. Summer will come with great seeing them it'll reinvigorate it, it'll hang around till the pit of winter where by this point bad seeing is as certain as gravity then Ill dump the idea again. That super large exit pupils begins to show traces of astigmatism also is a non motivating factor.

You make good points Jarad. I'd like to think one day all scopes will e these techy looking composite compacts. Its a flattering design look for a science oriented pursuit,

Pete

#90 echoes1961

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Posted 26 April 2013 - 09:58 AM

You cant have a 35 lb. Ota in 12".

Now you're talking about ultralight designs. Whole new subject. But yes, you can actually get there. I have a 14.5" f4.4 scope that weighs 27 lbs (total). All carbon fiber with a quartz mirror 0.8" thick.

You have to deal with a different set of issues (stability vs. weight, etc.), but it can be done.

#91 davidpitre

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Posted 28 April 2013 - 11:19 AM

Did you make that?

Fly Me to the Moon

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Posted 28 April 2013 - 11:49 AM

No, Craig Combes made it. It's similar to the 16" one he wrote an CN article on.

This is it packed for airline travel. The silver case is my eyepiece case. The truss poles are in the poster tube.

#93 Rachal

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Posted 24 May 2013 - 09:57 AM

Uncle Rod Mollise once said, "When it comes to planetary viewing, seeing is not the most important thing, it's the only thing."

In a thread like this, the first thing I do is look over and see what people live. We are captives of our own experiences so it's no surprise that someone who lives up near the great lakes or in the North East might have quite a different view than someone who lives along the coast in Florida or along the coast here in southern California.

When I respond to someone's post, I try to imagine something about their situation and hope to communicate something about my situation..

Jon

Jon, Based on my experience Rod is right on this one. The best view I ever had of Saturn was at TSP many years ago through a 12" scope owned by Chris Schur(IIRC) and of Jupiter through a 16" Astrosystems(?) Dob at TSP as well. This was through brief periods of exceptional seeing. Back in '95 I was able to see 8 craters in Plato and the rille in the Alpine Valley through my 8"F7 scope. The seeing was exceptional(at least by my standards) on that occasion.

#94 azure1961p

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Posted 28 May 2013 - 02:20 PM

Here's a pic of my 8" f/9 on homemade dob mount.

Pete

#95 UND_astrophysics

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Posted 28 May 2013 - 11:40 PM

I just read a review on an OPT 8" F/9 Dobson.. all I can say is WOW.

#96 derangedhermit

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Posted 29 May 2013 - 10:47 PM

...
The "average" American stands about 6' tall - 72". Figure your eyeballs are mounted about 4" below the top of your head. So, the "average" person has a maximum standing eyeball height of 68".

On the other side, one can quickly figure a normal and comfortable seated position of body height minus the distance from knees to hips. (Think about it, or just just look at how you sit now - your thighs are horizontal and contribute nothing to height when seated). For most people this is around 40".

Thus we have a normal comfort range of 40" to 68".[...]

Maybe in Texas. Only 15% of American men are as tall as 72", half are below 69", and very few women are 72" (Census round-off error makes this number 0.0%), the half point for them is 64".

So, subtract 3" to 8" to bring the comfort figures in line with the real average values. It reduces the max height for standing 6.5 degrees and 14.5 degrees respectively (to 64.5 and 56.4 degrees).

But if you live north of 49 degrees (the U.S. Canada border in the west) the ecliptic never gets above 64.5 degrees anyway.

Well off on the low end too. Standard seat height of about 18", average distance to top of head 35" - 4" down to eyes gives 49" instead of 40". Yes, one can sit with one's rear 9" off the ground, but it is not comfortable.

So a more accurate comfort range for (US average) people's eye level is around 45-50" (seated) to 60-65" (standing), depending mainly on the ***** of the person.

* "The distinction between **** and gender is a concept that distinguishes ...

Lee

#97 CollinofAlabama

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Posted 29 May 2013 - 11:30 PM

Personally, I'm for freedom. If someone wants an F/9 8", all power to him. Personally, I believe the standard 8" F/6 is the best sized 8" dob, but to each his own.

... So I might end up comparing the 6" Mak to the 6" SCT, and then the 6" SCT to the 6" Newt. Comparing the Mak to the Newt would be a case of apples and oranges. However, SCT's are supposed to be Jacks of All Trades, so comparing it with each of the other two would be appropriate.

Mike, why not compare a 6" Mak to a 6" newt? 6"=6"=6" who cares for the design? They're all reflecting telescopes with a CO. They're fair game. Actually, I'd think a 120mm-110mm-102mm refractor(s) would be reasonable competition, too, since those are, to some extent, the refractor equivalents to 6" reflectors ergonomically. But I'll be interested in hearing about your shootout(s) any way you put them together.

Good luck with them.

#98 UND_astrophysics

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Posted 30 May 2013 - 12:05 AM

I would love an F/9 Like this, In fact it will be the next one I buy, but I am not sure what freedom has to do with it, I think it is more about availability, and I would love to see some planetary images with this cool scope, especially Jupiter!

#99 derangedhermit

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Posted 30 May 2013 - 01:02 AM

Personally, I'm for freedom. If someone wants an F/9 8", all power to him. Personally, I believe the standard 8" F/6 is the best sized 8" dob, but to each his own.

+1 on 8" f/6, and I think for all-around use at an overall price and performance it's better than either 48-50" f.l. alternative; 6" f/8 or 10" f4.8 or f/5. Ergonomically, for dobs, I find a longer focal length better; and then I prefer a 10" f/6 over the 8" f/7.5 (or the 10" f/5) - as long as the tube fits across the back seat or in the boot of whatever vehicle I'm driving. 12" f/6 seems kinda long to me, f/5 fits better, but I'd prefer the 12" f/6 over the 8" f/9, for the nights when the atmosphere allowed the larger aperture to provide more resolution (at the same 72" focal length, mind). It would cost more, though, there's that.

What has been raised but not really discussed in this thread that could change my mind in a hurry is the availability of the equivalent in quality of TV (and if you like, ES, Pentax, and other primo eyepieces) - eyepiece families that reduce cost and weight, but keep good eye relief and wide apparent fields (or higher mag with the same TFOV, if you prefer to look at it that way), and perform *excellently* at f/7 or f/8 - they can turn black inside at f/6. Once you have to spend more on a few eyepieces than on the telescope to get great views, it is time to rethink things. Delos or Ethos or XL or ES82 or whatever, at half the price *and weight*, with the limitation that they only work *great* at f/7+...sign me up. Maybe it doesn't add up optically, or I would think someone would have already done it for the SCT market alone...

Lee

#100 Jeff Morgan

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Posted 30 May 2013 - 09:03 AM

Maybe it doesn't add up optically, or I would think someone would have already done it for the SCT market alone...

Lee

Good point. There are lots of f/6 and slower scopes. Virtually all APO refractors and SCT's for starters.

Even more interesting would be to see a manufacturer offer a line (or variants on a line) optimized to a specific optical design (that is to say, negative or positive field curvature). As it is now, marketing stamps out the fact there is such a thing and users buy the name and may (or may not) bet getting the best match to the scope they have.

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