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Ultimate planetary scope?

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#1 watcher

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Posted 13 January 2013 - 12:01 AM

I think in small to medium apertures, the ultimate planetary scope would be a long focus achro, a longish APO, or a Mak Cass. Once you get over about 10 inches though, things change.For planetary and small DSO's, I'm thinking something like the Parallax (Royce Optics)Cassegrains would be the best in 14 to 16 inches. Assuming that either could be made to an F/20 spec, is there any advantage to a Classical Cass over a Dall-Kirkham for a strictly narrow field instrument?

#2 Eddgie

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Posted 13 January 2013 - 11:21 AM

First, it is difficult to exploit apertures over about 10" to 12" for planetary work unless your seeing is routinely pretty good. I do a lot of planetary observing in my C14, but it is rare for me to work at close to the full potential of the scope. When I can, I see clear detail on Ganymede, but that is a couple of times a year.

But to your question.

The problem with the scopes you mention is that if they are made to f/20, they get very large and very difficult to mount.

The 14.5" Parallex DK is 56 inchs long and weighs 85 Lbs.

The secondary obstruction is a pretty large 25%.

The mirror is made by Royce (one of the best).

The scope will have about 275mm of clear aperture.

It will cost about $13,000 and you will spend another $7K on a mount for it.

For about $4000, you could buy an Orion Go-To dob and have someone refigure the mirror and put in an optimized size (18%) secondary mirror.


If you were serious about planets, you could do as well with a 12" Go-To dob

This would give you motorized tracking, and would negate the need for a 6 foot high saddle for your 85 LB cass.

And if you plot the MTF curves for the two scopes, you will quiclky see that the contrast performace of the 12" scope with 18% obstruction will be equal to the 25% obstructed 14" Cass.

Contrast transfer is everything for a planetary scope, and the 12" optimized Newt would be the winner here.

It costs less, is lighter, provides a better viewing position, and provides better contrast transfer.

A small obstruction Newtonian gives performance that is difficult to distinguish from an equal sized APO. The APO owners might disagree, but having owned both a 6" 18% obstructed reflector and a 6" APO, I would call the difference in performnce to be within a cat's wisker.

To me, ultimate planetary scope is 12" Go-To Newt with 18% obstruction. You still come up against seeing though, but you will get a better planetary scope out of a custom 12" Newt.

If you were really serious about this, you would do an MTF plot and see quickly that the Newt is on par at a fraction of the price.

#3 Eddgie

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Posted 13 January 2013 - 11:44 AM

Here is a rough MTF plot showing the 25% obstructed 14.5" scope vs the 12" 18% obstructed scope.

The left plot shows a perfect aperture (red, the 25% obsturction contrast loss vs perfect, and a hand drawn approximation of the 18% 12" obstruction.

Notice that the MTF curves suggest similar contrast performance at the low and mid frequency detail.

The bigger scope pulls ahead for the finest level of detail, but since this is hard to exploit visually due to the very small exit pupil, for planatery observing, the low and mid frequencies are considered more important.

Also, detail represented on the right side is the very smallest detail that is the very first to suffer unless seeing is virtually perfect.

In 40 years of planeatry observing, I have never had a night of absolutely perfect seeing. Not in a larger aperture. Not one. Not Ever.

The plot on the right shows the actual MTF unadjused for max spatial frequency. The "1" on the horizontal axis would be .75 on the plot on the left indicating that the 12" scope only has .75 of fs/max of the 14.5" scope.

An 18" obstruction looses so little contrast that it is almost impossible to see.

I will concede that on paper, the 14.5" scope has an advantage, but from a practical standoint, you will spend 4 times the money and never see it at the eyepeice.

And of course if you wanted to build a custom 14" Newt with an 18% obstruction, you could have an even better planetary telescope on paper, but once again, you would never see the benefit even when standing on the ladder. LOL.

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#4 titanio

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Posted 13 January 2013 - 12:07 PM

Hi watcher
Do not forget the maksutov Newton for plantery.
I am building a super planetary telescope with Royce mirrors, as for the mount, I have changed te arms of my CPC mount, it sems to work pretty Well.

http://img716.images...ytelescopio.jpg

Toni

#5 kfrederick

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Posted 13 January 2013 - 12:12 PM

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#6 Mirzam

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Posted 13 January 2013 - 03:31 PM

I have a Parallax 12.5" f/15 cassegrain with Royce optics and it is an excellent planetary and lunar scope when conditions allow. It also takes terrific photos.

The best way to observe planets is seated using a quality binoviewer and with tracking of course.

If you can obtain a custom alt-az newtonian that is designed for binoviewing and tracking, I think that this is a very good solution. Newts on equatorial mounts are very inconvenient in larger apertures.

Otherwise, the cassegrain will work well, and avoids the thermal problems associated with closed tubes.

JimC

#7 watcher

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Posted 13 January 2013 - 04:22 PM

I have a small set of my two scope solution already. My 6" F/5 R35 refractor for wide field deep sky, and my IM715 for the narrow stuff and planets. In a few years, though, I would like to get out from under the jet stream and to a darker place to live out my life. I wouldn't consider a Mak-Newt for the same reason I wouldn't want a Mak-Cass in larger apertures. They just have a rough time keeping up with temperature changes. That, and I just plain don't like Dobs and Newts. A fork mounted Cass sounds like a really nice ride. Is that mount going to be enough When the scope is complete?

#8 titanio

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Posted 13 January 2013 - 08:41 PM

Hi Watcher,
So far the fork mount will be enough for my 16” cassegrain, the telescope is almost finished and the total weigh is around 20 kg. Nevertheless if God wants I would like to buy a nice mount for it in a few years.



Toni

#9 orion61

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Posted 14 January 2013 - 06:59 PM

My experience after 42 years, the large scope 10-12" have given me good results, I must say I have had far greater success with 6-8" scopes. The reason being (in MY area)
Midwest, there arent that many nights of good seeing that supports larger scopes.
I had a 6" F10 Newtonian once that was killer for Planetary.
Secondary was only about 1/2" I mounted a DC fan on rubber bands, no currents.
Also The Meade 8" F6 was superb as was my F7 8" Cave.
I still have my old RV6 F8, and it still make people gasp!

#10 GeneT

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Posted 14 January 2013 - 09:29 PM

In my opinion, a 12 incher is a great planetary. I own a 12.5 inch, F5 Dob. It yields great planetary images. Large enough to resolve detail, yet so large where I am dealing with cool down and other issues.

#11 Glen A W

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Posted 15 January 2013 - 05:50 PM

The VMC-260 by Vixen is excellent on planets. It's the only visual use I'd recommend it for, though, due to long focal length and other factors. It regularly gives me views of Jupiter which are just awesome, and with lower-cost eyepieces, too.

That said, I too had a Meade 8" f/6 Newt with a USA made mirror which was a killer. Most times, it would beat larger scopes and the color was much more pure than the Vixen, which effectively has a filter in it as the views are very warm. But on a good night, the bigger scope will still prove its worth.

#12 mistyridge

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Posted 15 January 2013 - 06:55 PM

Same here 12.5" f/5 Dob. with a high quality mirror cools rapidly. I have a 8"edge HD that has very good optics that is very good on planets. it is small enough that cooling is not an issue. The only drawback is the large CO.

#13 azure1961p

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Posted 15 January 2013 - 11:20 PM

Ed very interesting graphs. It made sense when I understood how to overlay the two in my head, the .75 being 1, etc. . The seeing has its own cap of course. It d be interesting to compare the two if they were space telescopes essentially than the finest details and exit pupils could be exploited, sans the floaters and space suit.

Interesting how real world practicality shapes the graph, or what to extract from it.

Pete

#14 Jon Isaacs

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Posted 15 January 2013 - 11:46 PM

To me, ultimate planetary scope is 12" Go-To Newt with 18% obstruction. You still come up against seeing though, but you will get a better planetary scope out of a custom 12" Newt.



For planetary, GOTO is unnecessary, tracking is handy...

There are those who believe/find that an Equatorial mount is superior to a Dob mount for planetary viewing... Tubes seem to be the way to go... thermal control, fans...

There are those who believe/find that any most scope is a good planetary scope if the seeing is excellent.

Jon

#15 Rick Woods

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Posted 16 January 2013 - 02:18 AM

Well, my vote is for the Yerkes 40" refractor.

#16 watcher

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Posted 16 January 2013 - 02:53 AM

I think for my purposes, goto would be a big benefit. I like a 2 scope solution. My second scope will be a big fast achro, for wide fields, and the Cass would be for everything that works in a narrow field of view, not just the planets. Hunting down small galaxies with a 14 inch F/20 can be pretty tedious without goto. :crazy:

#17 highfnum

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Posted 16 January 2013 - 05:56 AM

I agree with edgggy its more sbout seeing conditions
The break point is about 10 inches

#18 pstarr

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Posted 16 January 2013 - 09:03 AM


To me, ultimate planetary scope is 12" Go-To Newt with 18% obstruction. You still come up against seeing though, but you will get a better planetary scope out of a custom 12" Newt.



For planetary, GOTO is unnecessary, tracking is handy...

There are those who believe/find that an Equatorial mount is superior to a Dob mount for planetary viewing... Tubes seem to be the way to go... thermal control, fans...

There are those who believe/find that any most scope is a good planetary scope if the seeing is excellent.

Jon


I'm one of "those"
10" F-6 Zambuto mirror, 1/30 wave secondary, 18% obstruction, curved spider, fan, big eq. mount 2" shafts w/tracking, solid, fully flocked tube.

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#19 Jon Isaacs

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Posted 16 January 2013 - 09:32 AM

Well, my vote is for the Yerkes 40" refractor.


What it, 1/4 inch of focus between the colors?

Jon

#20 Sunspot

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Posted 16 January 2013 - 09:33 AM

For me I guess the question is if one is doing visual viewing of the planets or imaging. The best visual view of the planets I ever had was with a 16" F/12 Newtonian but no way I'd do imaging with that beast. My Mewlon 250 has given me (inch for inch) the best images of the planets I've ever gotten. BTW, the 24" Clark at Lowell Observatory wasn't bad either except I never seemed to have good seeing when I used it.

Paul

#21 watcher

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Posted 16 January 2013 - 10:45 AM

Well, my vote is for the Yerkes 40" refractor.


What it, 1/4 inch of focus between the colors?

Jon


Yea, If I ever bought a refractor that big, it would have to be an ED! :roflmao:

#22 Gord

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Posted 16 January 2013 - 10:59 AM

Well, my vote is for the Yerkes 40" refractor.


What it, 1/4 inch of focus between the colors?


I think it's worse than that even. 1.5" or more I think!

Clear skies,

#23 Rick Woods

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Posted 16 January 2013 - 01:01 PM

Ahhh, you guys are just jealous because you don't get those colors in your reflectors.

#24 bierbelly

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Posted 16 January 2013 - 01:32 PM

Well, my vote is for the Yerkes 40" refractor.


What it, 1/4 inch of focus between the colors?


I think it's worse than that even. 1.5" or more I think!

Clear skies,


On the plus side, they didn't need filters!

#25 Mike Harvey

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Posted 17 January 2013 - 03:06 AM

My experience after 42 years, the large scope 10-12" have given me good results, I must say I have had far greater success with 6-8" scopes. The reason being (in MY area)
Midwest, there arent that many nights of good seeing that supports larger scopes.


I think your comments trump most other arguments vis a' vis scope-type and aperture. ANY scope is going to be limited by local seeing-conditions.

My planetary observing was more "sight-seeing' for the first half of my 50 years of astronomy (God, that sounds OLD...but, hey, I got my first scope at 11...OK?). Truly large, top-quality scopes were simply not available. Grasping planetary details such as those described by the great astronomers of the 19th and 20th centuries was something I never expected to experience. Even when I managed to move up to a GIANT (at the time) 12-1/2" I was disappointed because I had the misfortune to be living under the jetstream at the time!

Then came greater financial success and a move to Florida and EVERYTHING CHANGED! Our seeing was/is usually so good that you can often use every inch of any scope likely to be in amateur hands!

Vic Menard, one of my frequent observing friends, did a study of our local skies a few year's back and came to the conclusion that the largest aperture
that could be used to full effectiveness on the most nights was 20".
I'm convinced he's right. But I'll take a few extra inches and give up a few nights! :)
I had the great good fortune to own a 24" f/4.3 by Carl Zambuto that he claimed was "probably the best large mirror I ever made". I actually tracked this scope down and bought it from the original owner even though I ALREADY HAD A 24" ZAMBUTO OF MY OWN!
Observations of detail in the Martian polar cap during the opposition of 2003 produced details that no other amateur I could find (even online) could confirm. I made certain that a number of my observing companions were seeing the same things I was, just so I wasn't a victim of 'averted imagination'. A month later, our observations were confirmed - BY IMAGES FROM THE HUBBLE SPACE TELESCOPE! It just happened to be snapping pix at almost exactly the same time we were observing!

I became so spoiled by this scope and our Florida seeing that, if I couldn't use at least 800X with perfect clarity and steadiness - I didn't even bother to observe!

I still regret ever letting this scope go...but aperture fever was still raging.
It was sold to finance a 28" f/3.6 Starstructure, which I've now had for almost 8 years and have never once grown itchy for any more aperture.
It's planetary performance doesn't match that 'perfect 24" Zabuto but it's still so good that I find myself gasping at some of the views.

I still enjoy a good "look" at the major planets with smaller scopes...but it's only for grins. Once you've gone "big scope/superb skies" you just can't go back. If you've never had the good fortune to observe with an optically excellent 24"-28" scope in these conditions you just don't know (and probably can't even imagine) what you're missing.

Come down and see us at Chiefland sometime...we'd love to show you.

Mike Harvey






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