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10" vs 12" apertura for planetary

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#1 Ohmless

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Posted 23 September 2020 - 02:17 AM

My back has been more healthy the last couple years and I am able to carry my ~40 pound cg4(includes counterweights) without much struggle.  I can lift and lower without problem.  My back and hip issues only limit the duration of activities.  I am predominantly a seated SLaP visual observer and am curious which scope is best for me now that my health allows for some aperture.  I have a pickup so size for transport doesn't really matter.  I also noticed the truss varieties weigh roughly the same as the solid tube units.  As I am a solo observer, my bias is against a truss variety.  Also since I live on a disability pension, I have ruled out custom optics.

 

10" has f/4.9, 1250mm fl and a 2.47" (24.7% CO).  they state a lambda of 1/12 for the primary and OTA wt of 34.8 lb(mount 31.4 lb).

12" has f/5, 1520mm fl and a 2.76" (23% CO).  also with a stated lambda of 1/12 for the primary and OTA wt of 47.8 lb(mount 38.3 lb).

 

10" maximum resolution(Dawes' limit): 0.46 arcseconds

12" max resolution: 0.39 arcseconds

current 6" scope: 0.77 arcseconds

 

I have read past threads that focused on deep sky which I only uncommonly do.  This prompted me to ask specifically for experiences that fall in line with my observing preferences. 

 

Which scope do you all think would work the best for me eh?  Thanks in advance!



#2 Eddgie

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Posted 23 September 2020 - 08:27 AM

Depending on your typical seeing, the 10" will likely keep up with a 12" on many nights. If you live somewhere that has very good seeing on a regular basis, where the 12" can work close to its full resolution, then it would be a better scope.

 

But for most people on most nights, a 10" is not going to be at a disadvantage to a larger scope.


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#3 Jon Isaacs

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Posted 23 September 2020 - 09:17 AM

Ohmless:

 

Given that you're located at 44° north, the 10 inch will probably be as effective if not more so.

 

Jon


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#4 John_K

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Posted 23 September 2020 - 10:09 AM

I’d be going for a 12”. The views will be brighter, you will be able to use more magnification when the seeing is good, and for visual your best views for planetary will come with colour filters where more aperture is handy.

 

Aperture is always king in my mind.

 

John K. 


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#5 Jon Isaacs

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Posted 23 September 2020 - 11:09 AM

I’d be going for a 12”. The views will be brighter, you will be able to use more magnification when the seeing is good, and for visual your best views for planetary will come with colour filters where more aperture is handy.

 

Aperture is always king in my mind.

 

John K. 

 

Aperture in not king in my mind. It's one of several factors to consider. 

 

Michigan is in the northern US, the seeing is generally not the greatest. It cold in the winter, thermal equilibrium is important, the 10 inch will cool more quickly. 

 

The 12 inch is a foot longer and the OTA is about 15 pounds heavier.. 

 

Jon


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#6 MrRoberts

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Posted 23 September 2020 - 12:16 PM

Once upon a time....

I had an 8" and a friend a 12" (same mfg). When we saw through the 10" at an outreach event about a year after we purchased ours we both wished we had it. I for the added views without much more effort and my friend for the lesser effort without giving up any real noticeable views.

 

Just saying wink.gif


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#7 cuzimthedad

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Posted 23 September 2020 - 12:48 PM

Another vote for the 10" here.


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#8 Baatar

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Posted 23 September 2020 - 01:29 PM

Another vote for 10" which I think is a good all arounder. Size, length and weight wise there is little difference between 8" and 10", while the latter will provide better views.

Similar optical performance between 10" and 12" in average skies, but the latter will be much bigger, bulkier, heavier. Physical difference will outweigh optical performance, unless you have good and dark skies.

.


Edited by Baatar, 23 September 2020 - 01:36 PM.

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#9 Second Time Around

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Posted 23 September 2020 - 02:39 PM

10 inch for me too. The 12 inch is a lot bulkier and heavier, and in many locations the seeing will often mean that you'll see no more on planets than the 10 inch anyway. Plus as Jon said the 10 inch will cool down quicker.

Additionally, if (when?) your back starts playing up again, even temporarily, you may find the 12 inch too much to handle.


Edited by Second Time Around, 23 September 2020 - 03:05 PM.

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#10 earlyriser

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Posted 23 September 2020 - 02:45 PM

I'm in southern Ohio, and I can recall exactly one time in three years when the seeing was good enough to fully realize the resolving power of my AD10.  I'd go with the 10 inch.  


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#11 turtle86

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Posted 23 September 2020 - 02:54 PM

Another vote for the 10.  Plenty of aperture and much easier to handle than the 12.  The 12 is big enough to discourage use if you're on the fence about observing on a given night.


Edited by turtle86, 23 September 2020 - 02:54 PM.

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#12 junomike

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Posted 23 September 2020 - 04:35 PM

One thing I like about the larger 12" is the larger exit pupil at the same magnification.

 

10" @ 300X = 0.85 mm

12" @ 300X = 1.00 mm.

 

Not a huge difference but worth noting IMO.


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#13 GeneT

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Posted 23 September 2020 - 05:56 PM

You are physically better, but old age keeps moving along and I believe that for the long haul, the 10 incher will serve you fine. 


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#14 SteveG

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Posted 23 September 2020 - 10:52 PM

Its not worth the weight, size and cost difference for the subtle improvement only seen on the best nights. 10" is a good planetary scope, once cooled.


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#15 Jon Isaacs

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Posted 24 September 2020 - 05:14 AM

One thing I like about the larger 12" is the larger exit pupil at the same magnification.

 

10" @ 300X = 0.85 mm

12" @ 300X = 1.00 mm.

 

Not a huge difference but worth noting IMO.

 

If Ohmless lived in San DIego or Florida, the 12 inch would be worth considering because there would be a significant number of nights one could use 300x or more for viewing the planets.  I often do.  

 

But living in Houghton Lake, Michigan at 44 degrees north latitude, my guess is that the nights that 300x is a reasonable magnitication are few and far between. And I will say that 300x is a reasonable magnification for a 10 inch, it's still got plenty left in the tank.  

 

Jon


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#16 earlyriser

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Posted 24 September 2020 - 08:30 AM

If Ohmless lived in San DIego or Florida, the 12 inch would be worth considering because there would be a significant number of nights one could use 300x or more for viewing the planets.  I often do.  

 

But living in Houghton Lake, Michigan at 44 degrees north latitude, my guess is that the nights that 300x is a reasonable magnification are few and far between. And I will say that 300x is a reasonable magnification for a 10 inch, it's still got plenty left in the tank.  

 

Jon

The one time I've experience truly excellent seeing, it looked like I was viewing Saturn and Jupiter through a window at  210X with the 10 inch newtonian.  It was one time I really missed not having a higher power eyepiece. 


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#17 stargazer193857

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Posted 24 September 2020 - 09:09 AM

For me, bright light worsens my astigmatism. High magnification makes the image big enough and dims it enough that I don't see multiple images as much.

I've looked at the moon at 350x with a 50mm refractor. It was dim, and likely not as sharp as it could be. But my eyes (20/35) did not complain of lack of sharpness.

...

My point is, for me at least, I'd not worry about 10" vs 12" exit pupil brightness at 300x. A 6" seemed bright enough at 300x. My 4.5" dimmed and fuzzed out by then.

Edited by stargazer193857, 24 September 2020 - 02:35 PM.

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#18 Galicapernistein

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Posted 24 September 2020 - 09:45 AM

For me, bright light worsens my astigmatism. High magnification makes the image big enough and I brightens it enough that I don't see multiple images as much.

I've looked at the moon at 350x with a 50mm refractor. It was dim, and likely not as sharp as it could be. But my eyes (20/35) did not complain of lack of sharpness.

...

My point is, for me at least, I'd not worry about 10" vs 12" exit pupil brightness at 300x. A 6" seemed bright enough at 300x. My 4.5" dimmed and fuzzed out by then.

I agree. For planets, a 6” F8 would be more than enough, and probably even better than a bigger but faster scope. 


Edited by Galicapernistein, 24 September 2020 - 09:46 AM.

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#19 Ohmless

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Posted 24 September 2020 - 11:43 AM

I am not so worried about the apparent brightness of a planetary target either, but I do have floaters that reveal themselves starting around 0.7mm exit pupil.  Not a big problem until I am at 0.5mm exit pupil in my 6" f/5 scope.



#20 junomike

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Posted 24 September 2020 - 02:52 PM

If Ohmless lived in San DIego or Florida, the 12 inch would be worth considering because there would be a significant number of nights one could use 300x or more for viewing the planets.  I often do.  

 

But living in Houghton Lake, Michigan at 44 degrees north latitude, my guess is that the nights that 300x is a reasonable magnitication are few and far between. And I will say that 300x is a reasonable magnification for a 10 inch, it's still got plenty left in the tank.  

 

Jon

I only used 300X as a random figure, but the same holds true for 200X

 

10" = 1.27mm

12" =  1.53mm

 

The % difference  is the same and the larger exit pupil tends to benefit me more. 

YMMV


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#21 Jon Isaacs

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Posted 24 September 2020 - 04:22 PM

I only used 300X as a random figure, but the same holds true for 200X

 

10" = 1.27mm

12" =  1.53mm

 

The % difference  is the same and the larger exit pupil tends to benefit me more. 

YMMV

 

Don't get me wrong.  I like large exit pupils. In my 22 inch, 300x is a 1.87mm exit pupil, 200x is a 2.8mm exit pupil. The planets are bright and colorful.. And they seem supersized, not quite sure why.

 

But I am thinking overall, hassle factor, seeing, cool down, all the things that go into choosing a scope.  

 

Of course, I just hauled my 12.5 inch in from the high desert to fix some little stuff and I am looking forward to viewing Jupiter and Saturn with it tonight.. It looks like it will be clear until about 1 am when they predict the clouds will roll in off the Pacific.  

 

Jon


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#22 Spikey131

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Posted 24 September 2020 - 06:54 PM

I agree. For planets, a 6” F8 would be more than enough, and probably even better than a bigger but faster scope. 

I can always see more detail on planets with my 12.5 f/7 dob than my C8 or TV NP101, even if the seeing is marginal.  The view is not always pretty, and the details may come and go in waves of thermal distortion, but I can see more.  Often I like the way the planets look in the smaller scopes more, but aperture wins when it comes to seeing details.

 

And on those rare nights when the conditions are just right, the views in the dob can rival photos.


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#23 stargazer193857

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Posted 24 September 2020 - 08:29 PM

I can always see more detail on planets with my 12.5 f/7 dob than my C8 or TV NP101, even if the seeing is marginal. The view is not always pretty, and the details may come and go in waves of thermal distortion, but I can see more. Often I like the way the planets look in the smaller scopes more, but aperture wins when it comes to seeing details.

And on those rare nights when the conditions are just right, the views in the dob can rival photos.

I love hearing data points like this.


I think there are 2 aperture thresholds, which vary with seeing.
1. The aperture above which views don't improve often with more aperture. For many places, that is 8-10".
2. The aperture above which increasing aperture makes the image worse than a smaller scope at the same magnification, on most nights. For many places that aperture is 13".

The first threshold is caused by imperfect air cells that can't be corrected even if frozen in place.

The second threshold is caused by the eye not being able to accommodate the focus changes needed for the larger aperture to get best focus in the rapidly changing air cells, so that even if the sky clears at some focuser position, you'd have to be able to turn the knob fast enough at the right split second to catch it.

At least that's what I scrounged off the net from posts and my imagination.

Edited by stargazer193857, 24 September 2020 - 11:22 PM.

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#24 John_K

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Posted 24 September 2020 - 09:43 PM

Aperture in not king in my mind. It's one of several factors to consider. 

 

Michigan is in the northern US, the seeing is generally not the greatest. It cold in the winter, thermal equilibrium is important, the 10 inch will cool more quickly. 

 

The 12 inch is a foot longer and the OTA is about 15 pounds heavier.. 

 

Jon

I don't think we are talking about the difference of a 10" and a 20".

 

Having owned a 10" and currently a 12.5" for visual planetary for the last 15 years, I would disagree I'm afraid. I also live in Melbourne, which is notorious for having 4 seasons in one day. But when conditions are good, then that extra bit of aperture, especially when using colour filters for visual observing of planetary features, is very very handy.

 

There are a number of books pre-CCD age by a range of authors including the classic "Observing and Photographing the Solar System" by Willmann-Bell which suggest a 12" Newtonian as best a good compromise all around for planetary.

 

There are multiple ways to actively cool Newtonians, and this should be done with all scopes irrespective of size to optimise their performance.

 

Clear skies.

 

John K.


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#25 Jon Isaacs

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Posted 25 September 2020 - 06:39 AM

I don't think we are talking about the difference of a 10" and a 20".

 

Having owned a 10" and currently a 12.5" for visual planetary for the last 15 years, I would disagree I'm afraid. I also live in Melbourne, which is notorious for having 4 seasons in one day. But when conditions are good, then that extra bit of aperture, especially when using colour filters for visual observing of planetary features, is very very handy.

 

There are a number of books pre-CCD age by a range of authors including the classic "Observing and Photographing the Solar System" by Willmann-Bell which suggest a 12" Newtonian as best a good compromise all around for planetary.

 

There are multiple ways to actively cool Newtonians, and this should be done with all scopes irrespective of size to optimise their performance.

 

Clear skies.

 

John K.

 

I am not taking about the differences between a 10 inch and a 20 inch either.  I was just pointing out that I do appreciate the advantages of a larger aperture, of a brighter exit pupil.  

 

I have two Dobs at my home in San Diego. One is a standard GSO 10 inch F/5 Dob that I have had for 17 years, the other is a 13.1 inch F/5.5 with a Royce mirror, a Feathertouch focuser and all the good stuff.  I am less than 5 miles from the ocean and the seeing is often very good, well under an arc-second.

 

In my situation, the larger scope does generally provide the better views, it is very close to being an optimal planetary Newtonian. It is a very nice scope.  On the other hand, the 10 inch still provides excellent views of the planets, cools more quickly, is more easily setup and just less hassle.  

 

In a place like Houghton Lake that's at 44 degrees north, things are very different. Melbourne is more like San Diego and you have the planets in your hemisphere.  The coldest temperature ever recorded in Melbourne is 27F.  In Houghton Lake,  the average January high is 28F, the average low is 9F.  

 

https://www.usclimat...states/usmi0406

 

In general, the northern US does not have good seeing, many prefer refractors because they have fewer thermal issues, they perform nearly their best nearly all the time.  My sense is that for Ohmless, a 10 inch would be more effective more of the time than a 12 inch.  

 

Jon


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