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Ultimate planetary scope?

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#26 ValeryD

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Posted 17 January 2013 - 04:50 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.


I strogly (but politely) disagree!

14.5" with 25% telescope will be MUCH better than 12" with 18% c.o.

1. The contrast will be equal ONLY at middle frequencies. At higher frequencies, where details are the most numerous, the larger scope will have higher contrast.

2. larger scope will have higher resolution and will show significantly more small details.

3. Larger telescope will show brighter image at the same magnification and in such will win in contrast and colors rendition.

4. Photon noise will be lower in larger telescope - it will win in CCD welcoming.

I am wondered, that so experienced amateur astronomer can write such a nonsense, as you did, Eddie.

#27 ValeryD

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Posted 17 January 2013 - 04:58 AM

I can add, that for planetary telescope the key is not the size of c.o. Very often, in most cases, there are two main kings: seeing and wave front precision and smoothness.

One can have a scope with, say, 20% c.o. and another guy has a scope with 30% c.o. (both same diameter). First scope has moderately figured optics (even if a low resolution interferogram states that it has 0.95 Stehl) and second telescope has ultra-smooth optics with 0.99 Strehl. The latter scope will easily win.
Microripples on an optics, well seen on a phase-contrast test will smear small and low contrast details, while an optics with ultra smooth wave front will work at it's best.

#28 Gord

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Posted 17 January 2013 - 09:33 AM

So Valery,

What happens when you start changing diameters in the comparison? At what point does a decent (or good) but larger optic out perform a smaller but perfect optic? Is it +2", +4"?

And I'm not so sure it's an easy task for the smaller optic because as you say there are some inherit advantages to the larger optic (as you outline in the post above, extra brightness helps contrast, color, resolution, etc.). My 10" newtonian has essentially perfect optics, incredibly smooth. My C14 has good optics, but I can see what is likely some roughness compared to the newt.

In practical use, with as much details as I have ever seen through the 10", the 14 shows more. Color is one of the most striking differences, but all details just seem to be easier to see (and seemingly, all the time).

Of course I think the best would be to have both! :lol:

Clear skies,

#29 ValeryD

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Posted 18 January 2013 - 04:13 AM

So Valery,

What happens when you start changing diameters in the comparison? At what point does a decent (or good) but larger optic out perform a smaller but perfect optic? Is it +2", +4"?

And I'm not so sure it's an easy task for the smaller optic because as you say there are some inherit advantages to the larger optic (as you outline in the post above, extra brightness helps contrast, color, resolution, etc.). My 10" newtonian has essentially perfect optics, incredibly smooth. My C14 has good optics, but I can see what is likely some roughness compared to the newt.

In practical use, with as much details as I have ever seen through the 10", the 14 shows more. Color is one of the most striking differences, but all details just seem to be easier to see (and seemingly, all the time).

Of course I think the best would be to have both! :lol:

Clear skies,


A smaller telescope is preferable according to such a rule: for the image construction an undestroyed diffraction picture of larger size is better than destroyed (heavily) diffraction picture of a smaller size. So, there are two first main factors: seeing and optics perfection. All other advantages of larger aperture are still valid and play their roles. So, it is impossible to give you a solid exact answer. 1" can be a huge difference for 3" and 4" apertures and absolutely insignificant for 32" and 33".


Valery.

#30 Alan A.

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Posted 18 January 2013 - 03:17 PM

I would like to echo Mike's comments on large dobs being the ultimate planetary scope if the seeing is right. I've had my best planetary views through my 24" starstructure ( one evening of excellent seeing many of my club members told me they had their lifetime best views of saturn through it)

I believe Alvin Huey has said his lifetime best view of mars was through his 30", and Paul Alsing (a friend and extremely accomplished observer) had his lifetime best view of Saturn through the McDonald 82".

#31 iluxo

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Posted 19 January 2013 - 10:16 AM

The f/23 tri-schiefspiegler design, with unobstructed all-reflecting optics folded into a manageable box. Similar are the Stevick-Paul design, or the Yolo.

Barry Adcock of the ASV in Australia used to have a 12" and routinely took amazing photos of Mars, Jupiter and Saturn that rivalled shots from far larger professional observatories.

From a practical perspective if you need a single portable scope I'd stick to an f/15 Mak using the Gregory design which uses a very small secondary, as adopted by Questar.

#32 Rick Woods

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Posted 19 January 2013 - 01:08 PM

From a practical perspective if you need a single portable scope I'd stick to an f/15 Mak using the Gregory design which uses a very small secondary, as adopted by Questar.


Is that correct? I was under the impression that Questar secondaries are on the order of 32%, not particularly small. (I'm under this impression because that's what a Questar-owning friend told me.)

#33 watcher

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Posted 19 January 2013 - 04:16 PM

The Questar might have a bigger CO, because it is an F/13.4. It would have a smaller obstruction at F/15 though, than a Rumak, because of the secondary holder on the Rumak design. The Gregory is just a "silvered" spot on the corrector.

#34 Glen A W

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Posted 19 January 2013 - 04:17 PM

The f/23 tri-schiefspiegler design, with unobstructed all-reflecting optics folded into a manageable box. Similar are the Stevick-Paul design, or the Yolo.

Barry Adcock of the ASV in Australia used to have a 12" and routinely took amazing photos of Mars, Jupiter and Saturn that rivalled shots from far larger professional observatories.


That must be like a dream. My old Meade 8" f/6 Newt would give some pretty awesome views when the sky was calm - very pure in color. I can't imagine an unobstructed scope of such size as you are talking about. GW

#35 BillP

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Posted 19 January 2013 - 06:08 PM

I strogly (but politely) disagree!

I am wondered, that so experienced amateur astronomer can write such a nonsense, as you did, Eddie.


Just a gentle observation...the tone of these two statements, do seem to be at odds :(

#36 nathanstl

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Posted 19 January 2013 - 09:11 PM

I don't want to go off topic here too much, but am I understanding this right that the larger aperture scopes are more impacted by seeing? I thought it was equal. I'm thinking of getting a goto dob Newtonian and was thinking the 12", but now I'm thinking maybe the 8 if it will work on more nights here in the Midwest.

#37 stevew

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Posted 19 January 2013 - 09:47 PM

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

For the sky conditions in my area [ Pacific Northwest ] I'd have to agree. Half of the year even my excellent C8 won't perform at it's best.
The most detailed planetary views I have seen from this area were in our clubs 16 inch Galaxy mirror while using it at a star party around 6300 feet in elevation, but they weren't that much different than a custom made 10 inch F-6 near by.

Steve

#38 stevew

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Posted 19 January 2013 - 10:02 PM

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!
Mike Harvey


:bow: :bow:
Wonderful story Mike. I wish we all had skies with superb seeing. You are very fortunate indeed.
Most of us in this hobby will never witness images as you have described.

Steve

#39 Sarkikos

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Posted 22 January 2013 - 09:16 AM

So far, my best views of planets - most detailed and most contrasty - have been through my 10" f/4.8 Dob. Better than my 8" f/6 Dob or 150mm f/12 Mak, better than any larger scopes I've used at my dark site, better than the 8" Alvin Clark refractor in a nearby city. My 10" is just an off-the-shelf Celestron solid-tube on a Dob mount with no tracking. I've fully flocked the tube, added a flocked dew shield, and suspended the fan with a baffle below the primary. Collimating very closely, binoviewing and keeping the eyes near photopic are great helps.

Before reading this thread and Eddgie's excellent graphs and advice, I was thinking about upgrading to a 14" for both deep sky and planet/lunar. Now I'm considering a 12" instead to cover both areas. But then there are the contrary opinions from ValeryD and others. :thinking: I would miss a larger aperture for DSO at the dark site. A 14" can get seriously into galaxy clusters.

Mike






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