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OPT 8" f/9 Planet Pro Dobsonian

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#1 Oscar56

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Posted 19 April 2013 - 11:39 AM

While doing some "window shopping" last night I came across this dob Opt Dob

I have been searching for a review of this dob but none so far. Anyone with experience with this telescope?

A couple of intriguing aspects are:
- f/9 compared to most commercial dobs in the range of f/5,
- telrad is standard,
- weight is kept down by using sono tubes,
- spider vanes for the secondary
- low % of secondary compared to primary

What I have read so far about f/5 dobs is that you should be prepared to invest in better EPs. So does that rule not apply for a f/9 dob?

#2 Starman81

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Posted 19 April 2013 - 12:27 PM

Definitely an interesting scope, though I have never read any comments on here about it. The one review that is on OPT's website (though not for the same scope but a similar configuration) gushes about what an 8" f/9 can do.

#3 KerryR

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Posted 19 April 2013 - 12:42 PM

This topic comes up every so often. It's always the same-- intriguing concept, but no one willing to pull the trigger.

To me, it looks potentially wobbly and un-stiff. Sonotube is not THAT light, and parallel struts of that length could easily flex a bit. While f9 might tolerate the resultant de-collimation fairly well, wobbles at the ep while trying to attain precise high power focus might be problematic. Same goes for the long lever owing to the long tube-- the rocker would have to be extremely stiff to avoid flexing and oscillating while focusing.

F9 wouldn't require high end ep's, so that's nice. But, the smaller FOV of lower end ep's would require more constant guiding, which could be an issue if the tube and rocker are on the squishy side.

I know that paralell struts CAN work just fine- I use such a configuration to great effect on my 16" f4.5- but in this particular execution they're potentially on the long and small (diameter) side. Enough of a risk IMO that I was unwilling to pull the trigger myself in the past when considering this scope or a CPC11. (I went with the CPC11)

#4 Oscar56

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Posted 19 April 2013 - 01:09 PM

and parallel struts of that length could easily flex a bit.


Thanks for pointing that out. Both flex and twist might be a big issue.

#5 DJCalma

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Posted 19 April 2013 - 01:19 PM

To say the lone review of this scope on the OPT website is overly optimistic is an understatement.

The reviewer says "The sco(p)e will redefine what you thought was possible with an 8 aperture." :rolling:

#6 5u4

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Posted 19 April 2013 - 02:54 PM

I called them a couple months ago inquiring about the scope as I was interested in buying one & wanted to know the wait time, etc. Said they'd get back with me, well they never got back to me, so I bought a used 8" Portaball & couldn't be happier.

#7 azure1961p

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Posted 19 April 2013 - 11:43 PM

Post deleted by azure1961p

#8 SeattleScott

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Posted 20 April 2013 - 01:44 AM

Notice that the tube is over 6" long, which means the eyepiece will probably be about 6" tall when viewing at zenith. How tall are your eyes?

Also, I have an almost fundamental problem with a scope that is advertised as a planet killer, but doesn't go on a tracking mount.

#9 azure1961p

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Posted 20 April 2013 - 06:31 AM

Notice that the tube is over 6" long, which means the eyepiece will probably be about 6" tall when viewing at zenith. How tall are your eyes?

Also, I have an almost fundamental problem with a scope that is advertised as a planet killer, but doesn't go on a tracking mount.


Well most observers are taller than GI JOE - a slight hinderance to be sure. The simple solution fact is at the zenith one of those super light small aluminum step ladders that have but a couple steps us all that's needed. Next if you want tracking and photographic capabilities of the moon and planets get an equatorial platform. The pay off here is at a meek fraction of the price of an astrophysics 6" apo your getting similar planetary performance (some say better) and higher angular resolution. Planetary performer indeed.

There's a lot of *planetary killers* that require an 8' ladder - it doesn't mean its s design problem - it means its a behemoth with performance to match. One would imagine GI JOE would need to rappel up the ladder with Kung Fu Grip - but at the top he'd realize it was worth it. Ken would upset Barbie by not being virile enough to make the climb nor carry the tools, but he'd have a nice convertible to drive away in.

Pete

#10 Jon Isaacs

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Posted 20 April 2013 - 08:59 AM

Notice that the tube is over 6" long, which means the eyepiece will probably be about 6" tall when viewing at zenith. How tall are your eyes?

Also, I have an almost fundamental problem with a scope that is advertised as a planet killer, but doesn't go on a tracking mount.


Well most observers are taller than GI JOE - a slight hinderance to be sure. The simple solution fact is at the zenith one of those super light small aluminum step ladders that have but a couple steps us all that's needed. Next if you want tracking and photographic capabilities of the moon and planets get an equatorial platform. The pay off here is at a meek fraction of the price of an astrophysics 6" apo your getting similar planetary performance (some say better) and higher angular resolution. Planetary performer indeed.

There's a lot of *planetary killers* that require an 8' ladder - it doesn't mean its s design problem - it means its a behemoth with performance to match. One would imagine GI JOE would need to rappel up the ladder with Kung Fu Grip - but at the top he'd realize it was worth it. Ken would upset Barbie by not being virile enough to make the climb nor carry the tools, but he'd have a nice convertible to drive away in.

Pete


I don't quite know what GI Joe has to do with this. :question:

The height problem is a real issue. If they had made it F/8, the performance would have been essentially the same but a stool or ladder would not be needed.

Here's how it looks to me:

I have a 16 inch F/4.42 with an eyepiece height of 69 inches. I am 6 feet tall and if I wear thick shoes I need to stretch a little to view at the zenith. This scope is going to be somewhat taller at the eyepiece, it's got a 76 inch OTA and with the base and all, the eyepiece height is probably going to be something closer to 74 inches.

That doesn't sound like a lot but the rub is that the eyepiece height doesn't change much as you move away from the zenith, (it's the sine of the elevation) it's going to require a ladder or stool for even a relatively tall person down to 60 degrees or less.

"Behemoth Planet Killers" that require 8 foot ladders are not 8 inch scopes, I have two ladder scopes that are pretty decent on the planets but they get their performance from having reasonably big mirrors. I am fine with ladders and stools if they are necessary. But I don't see any real advantage to an 8 inch F/9 over an 8inch F/8 or even shorter, I am not messing with ladders and stools for an 8 inch scope, maybe for a 16 inch, definitely for a 25 inch..

YMMV

Jon Isaacs

#11 CHASLX200

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Posted 20 April 2013 - 09:40 AM

I grew up on ladder scopes like a 10" f/10. I would think a plain oLD 8"F/8 would do just as good as a F/9.

Chas

#12 SeattleScott

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Posted 20 April 2013 - 11:17 AM

Funny post Pete. Just to clarify, my best ever view of Jupiter was at the top of a ladder looking through a 25" Dob. However, I wouldn't get a 25" Dob primarily for planetary use.

Good point, the eq platform could solve the tracking issue, at an additional cost (and additional height). Not having a Dob, I sometimes forget about those.

#13 *skyguy*

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Posted 20 April 2013 - 11:26 AM

To say the lone review of this scope on the OPT website is overly optimistic is an understatement.

The reviewer says "The sco(p)e will redefine what you thought was possible with an 8 aperture." :rolling:


I own a 6' f/8 RV-6 Criterion Dynascope and a homemade 6" f/10 refector that has a "premium" Edmund primary mirror, 25mm. secondary mirror, curved vane spider and low profile focuser. RV-6 owners universally proclaim this scope has great optics ... and it does. However, the 6" f/10 is far superior in all categories. In direct ... side-by-side comparisons (at approximately the same magnification)... it shows a significantly darker sky background with better overall contrast and sharper stars with better detail in both planetary and deep-sky objects. The difference in the views in literally "shocking!"

If you haven't looked through an optimized, long focal length reflector ... then all I can say is: "Don't knock it if you haven't tried it!" ;)

Here's a shot of both my 6" reflectors to show the size difference between them.

Attached Thumbnails

  • 5810791-6_f10_SCOPE.jpg


#14 DJCalma

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Posted 20 April 2013 - 11:37 AM

Shouldn't the quality of the optics be a far greater factor in resulting image quality than focal length at a given aperture? I've seen Zambuto powered scopes operating at F/3.6 that would smash the idea that long focus = sharper image, better contrast.

The quality of the mirror is primary ( pun intended). :waytogo:

#15 Scanning4Comets

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Posted 20 April 2013 - 11:48 AM

Shouldn't the quality of the optics be a far greater factor in resulting image quality than focal length at a given aperture? I've seen Zambuto powered scopes operating at F/3.6 that would smash the idea that long focus = sharper image, better contrast.

The quality of the mirror is primary ( pun intended).


+1

A good reflector on planets can have a short focal length. The real criteria is a smooth polish and a good figure. The scope does not have to be a long focal length to get decent planetary images. I get excellent images in my 10" F/4.7 which is a short FL scope by any means.

People go on about CO, etc....and I have seen excellent planetary images in an SCT which has a large CO.

I love my 10" scope because I can fit it along the back seat of my car perfectly and it isn't too long....I can also use it comfortably when standing when I have it set up on a small stand. No ladders here.

Only way I'd go with a ladder would be a 16" scope or larger. Even then, if I had the bucks to get a 16"...I'd go with a short FL so I wouldn't have to do the ladder thing.

I own a 6' f/8 RV-6 Criterion Dynascope and a homemade 6" f/10 refector that has a "premium" Edmund primary mirror, 25mm. secondary mirror, curved vane spider and low profile focuser. RV-6 owners universally proclaim this scope has great optics ... and it does. However, the 6" f/10 is far superior in all categories. In direct ... side-by-side comparisons (at approximately the same magnification)... it shows a significantly darker sky background with better overall contrast and sharper stars with better detail in both planetary and deep-sky objects. The difference in the views in literally "shocking!"


Darker sky background depends on exit pupil size / magnification.

Cheers,

#16 *skyguy*

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Posted 20 April 2013 - 12:09 PM

Shouldn't the quality of the optics be a far greater factor in resulting image quality than focal length at a given aperture? I've seen Zambuto powered scopes operating at F/3.6 that would smash the idea that long focus = sharper image, better contrast.

The quality of the mirror is primary ( pun intended). :waytogo:


Absolutely ... the quality of the optics is a huge factor in getting a quality image. However, it's far easier and far, far cheaper to get a high quality image in a longer focus mirror. It's very difficult, time consuming and expensive to figure a perfect parabola in a "fast" mirror.

In my world, the health of my wallet is primary to me! :)

BTW ... If you notice, the 8" f/4.5 Zambuto mirror costs the same as the complete OPT 8" f/9 dobsonian!

#17 *skyguy*

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Posted 20 April 2013 - 12:15 PM

I own a 6' f/8 RV-6 Criterion Dynascope and a homemade 6" f/10 refector that has a "premium" Edmund primary mirror, 25mm. secondary mirror, curved vane spider and low profile focuser. RV-6 owners universally proclaim this scope has great optics ... and it does. However, the 6" f/10 is far superior in all categories. In direct ... side-by-side comparisons (at approximately the same magnification)... it shows a significantly darker sky background with better overall contrast and sharper stars with better detail in both planetary and deep-sky objects. The difference in the views in literally "shocking!"


Darker sky background depends on exit pupil size / magnification.

Cheers,


If you notice in my post ... the comparisons done on both these scopes were at almost the same magnification to avoid this issue.

#18 azure1961p

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Posted 20 April 2013 - 12:44 PM

Notice that the tube is over 6" long, which means the eyepiece will probably be about 6" tall when viewing at zenith. How tall are your eyes? 4

Also, I have an almost fundamental problem with a scope that is advertised as a planet killer, but doesn't go on a tracking mount.


Well most observers are taller than GI JOE - a slight hinderance to be sure. The simple solution fact is at the zenith one of those super light small aluminum step ladders that have but a couple steps us all that's needed. Next if you want tracking and photographic capabilities of the moon and planets get an equatorial platform. The pay off here is at a meek fraction of the price of an astrophysics 6" apo your getting similar planetary performance (some say better) and higher angular resolution. Planetary performer indeed.

There's a lot of *planetary killers* that require an 8' ladder - it doesn't mean its s design problem - it means its a behemoth with performance to match. One would imagine GI JOE would need to rappel up the ladder with Kung Fu Grip - but at the top he'd realize it was worth it. Ken would upset Barbie by not being virile enough to make the climb nor carry the tools, but he'd have a nice convertible to drive away in.

Pete


I don't quite know what GI Joe has to do with this. :question:

The height problem is a real issue. If they had made it F/8, the performance would have been essentially the same but a stool or ladder would not be needed.

Here's how it looks to me:

I have a 16 inch F/4.42 with an eyepiece height of 69 inches. I am 6 feet tall and if I wear thick shoes I need to stretch a little to view at the zenith. This scope is going to be somewhat taller at the eyepiece, it's got a 76 inch OTA and with the base and all, the eyepiece height is probably going to be something closer to 74 inches.

That doesn't sound like a lot but the rub is that the eyepiece height doesn't change much as you move away from the zenith, (it's the sine of the elevation) it's going to require a ladder or stool for even a relatively tall person down to 60 degrees or less.

"Behemoth Planet Killers" that require 8 foot ladders are not 8 inch scopes, I have two ladder scopes that are pretty decent on the planets but they get their performance from having reasonably big mirrors. I am fine with ladders and stools if they are necessary. But I don't see any real advantage to an 8 inch F/9 over an 8inch F/8 or even shorter, I am not messing with ladders and stools for an 8 inch scope, maybe for a 16 inch, definitely for a 25 inch..

YMMV

Jon Isaacs


For a six inch long ota is suitable or Mr. Joe indeed.

Pete

#19 azure1961p

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Posted 20 April 2013 - 01:01 PM

Shouldn't the quality of the optics be a far greater factor in resulting image quality than focal length at a given aperture? I've seen Zambuto powered scopes operating at F/3.6 that would smash the idea that long focus = sharper image, better contrast.

The quality of the mirror is primary ( pun intended).


+1

A good reflector on planets can have a short focal length. The real criteria is a smooth polish and a good figure. The scope does not have to be a long focal length to get decent planetary images. I get excellent images in my 10" F/4.7 which is a short FL scope by any means.

People go on about CO, etc....and I have seen excellent planetary images in an SCT which has a large CO.

I love my 10" scope because I can fit it along the back seat of my car perfectly and it isn't too long....I can also use it comfortably when standing when I have it set up on a small stand. No ladders here.

Only way I'd go with a ladder would be a 16" scope or larger. Even then, if I had the bucks to get a 16"...I'd go with a short FL so I wouldn't have to do the ladder thing.

I own a 6' f/8 RV-6 Criterion Dynascope and a homemade 6" f/10 refector that has a "premium" Edmund primary mirror, 25mm. secondary mirror, curved vane spider and low profile focuser. RV-6 owners universally proclaim this scope has great optics ... and it does. However, the 6" f/10 is far superior in all categories. In direct ... side-by-side comparisons (at approximately the same magnification)... it shows a significantly darker sky background with better overall contrast and sharper stars with better detail in both planetary and deep-sky objects. The difference in the views in literally "shocking!"


Darker sky background depends on exit pupil size / magnification.

Cheers,


Sorry - waving the quality wand doesnt suspend physics or bend it. Itmerely makes the most of a compromise. Theres nothing particuarly wonderful about overly bright diffraction patterns and the inherant light scatter to say nothing of the invisibility of faint companions in close doubles blotted out by the brighter rings. Or the effects of coma, or theneed fir a paracor or the need for even more glass inexpensive oculars. Quality is an ingredient not a solve-all or physics bypass.
.
Large aperture has less issues here than medium to small.
Ive not looked through a 10" f/5 or faster that can equal the planetary contrast performance. A 10" f/7 is another matter.

Im not placating a situation of ownership though with rationales based on brand name. I HAD a great 10" f/5. I didnt make excuses for the lackof planetary contrast, or slogans. I went to local star parties saw the difference and opted for. There are planetary specialized scopes where the knows the difference and then theres huve CO medium aperture instruments where its a game of rationilizing.

Life's to short.

Pete

#20 azure1961p

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Posted 20 April 2013 - 01:15 PM

If you mean to tell me that one diffraction step to 9 from 8 is too prohibitively taxing to justify the one grueling step on a stool then perhaps this s a topic beyond astronomy. Too in real world seeing the notion that a 16" is preferable to an optimized 8" on planets is a little too sanguine to say the least. Itll have its moments but its no given.

That id be standing in grass instead of ladder step 8" off the ground could be seen as detracting in anywy from viewing pleasure is a struggling point - particularly if its snowy, or wet.
Lol im sorry Jon Ive never climbed that woeful step and been disappointed when I reached the *summit*.

Pete

#21 Jon Isaacs

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Posted 20 April 2013 - 02:00 PM

It's very difficult, time consuming and expensive to figure a perfect parabola in a "fast" mirror....

BTW ... If you notice, the 8" f/4.5 Zambuto mirror costs the same as the complete OPT 8" f/9 dobsonian



Well...

It would be a 8 inch Zambuto mirror, certainly better than what OPT offers. Interestingly, if it were 8 inch F/9, the price would be the same.. Must not be that hard after all.

But in any event, we are not talking F/4 mirrors here, talking F/6, F/7, F/8, F/9, these days excellent mirrors in these focal ratios should not be a problem. Why pick an F/9 when an F/8 keeps you off the ladder/stool?

Jon

#22 Jon Isaacs

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Posted 20 April 2013 - 02:16 PM

If you mean to tell me that one diffraction step to 9 from 8 is too prohibitively taxing to justify the one grueling step on a stool then perhaps this s a topic beyond astronomy. Too in real world seeing the notion that a 16" is preferable to an optimized 8" on planets is a little too sanguine to say the least. Itll have its moments but its no given.



I suspect that your "real world" differs significantly from mine. My real world has good enough seeing that if a 12.5 inch F/6 won't do the number on an 8 inch anything, it's time to look at DSOs. :ubetcha:

Again, this is not really about you climbing your ladder, you have your 8 inch F/9 or me climbing mine (the 16 inch doesn't need a ladder on the 12.5 inch and the 25 inch) and I am not in the market for an 8 inch planetary scope.

It's about the wisdom of designing and marketing a scope that will require most observers to use a stool or ladder when it could have been easily avoided without an optical compromise.

Jon

#23 johnnyha

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Posted 20 April 2013 - 04:26 PM

There are so many variables, this is an interesting discussion! I would argue that an 8" F9 should be solid tube to be classified as a "planet killer", and the tube should have at least 1" clearance either side of the mirror = 10+" diameter. This would be far better and easier to handle as a split-tube than a truss imho.

#24 DJCalma

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Posted 20 April 2013 - 04:41 PM

I recall Starsplitter Telescopes (no longer in business) used to make an 8" F/8 with a Zambuto primary. They called it the "APO Eater"
A silly name, but at least I knew the optics were great. Maybe OPT should have called their scope the "Starsplitter APO Eater, EATER." :roflmao:

#25 Jon Isaacs

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Posted 20 April 2013 - 06:15 PM

There are so many variables, this is an interesting discussion! I would argue that an 8" F9 should be solid tube to be classified as a "planet killer", and the tube should have at least 1" clearance either side of the mirror = 10+" diameter. This would be far better and easier to handle as a split-tube than a truss imho.


Johnny:

You're right, it should be a split tube scope...

Jon


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