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14" vs 16 " Dob

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#51 Galicapernistein

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Posted 25 April 2014 - 08:38 AM

Against what more one might see, one has to balance the hassle factor.


This was the deciding factor for me. I sold 20 and 18 inch Dobs for a 12.5 incher. With my 12.5, both feet are always on the ground, set up, take down, storage, loading into the vehicle are all easily done. The others were a huge hassle, and caused me not to want to get out and view. Not so with the 12.5.


That's a decision many people have come to, including me. The improvement in the view through a 16" scope is not proportional, for me, to the extra hassle of transporting the larger scope to a dark sky site. If I could view from my backyard it would be a different story. The jump in size from even a 12 to 14 inch scope is much more than the 2" difference in mirror size would suggest. But if transport is not an issue, I would definitely go with the 16" scope - every extra bit of aperture helps.

#52 JMW

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Posted 25 April 2014 - 01:58 PM

I used a 20 inch for five years taking to events for our club. Last summer I bought a used Webster D14 with Zambuto 14.5 inch mirror. It's easier to load and transport, I don't need a ladder and the thinner mirror keeps up with drop temperatures better through the night. I also like the wider field of few of the faster mirror with a shorter focal length compared to the f/5 20 inch Obsession.

It's best if you can help someone else setup and move around a scope of the size you are considering. Be sure you want to deal with the logistics on a regular basis. A lot of big Dobs come up for sale because people get tired of or too old to deal with the big scopes.

#53 BigC

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Posted 25 April 2014 - 05:55 PM

I'm a bit confused here (no surprise :D). For someone who's mathematically challenged, am I correct in understanding that the end result of this should be that even with the biggest possible telescope and highest possible magnifications, combined with the overall brightness of the sky (given that even in a truly dark sky there's still light from stars, the milky way, etc), that even in perfect conditions with the best possible equipment, there should be a whole class of objects that can just simply never be visually observed just due to the inherent limitations of the eye?

Brown and red dwarfs ,most of the Oort cloud ,most of the "original" asteroid belt all come to mind as unlikely in any amateur scope.Even that big 70 inch out west.You'll have to settle for a few hundred millions stars ,thousands of galaxies, and a paltry few hundred nebulas.Barely enough to keep busy for a dozen lifetimes.

#54 TCW

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Posted 25 April 2014 - 07:37 PM

Increased light gathering capacity (objective diameter) will increase brightness at a given magnification as long as the exit pupil diameter does not exceed the diameter of your pupil.

#55 Jon Isaacs

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Posted 26 April 2014 - 03:29 AM

Gut instinct is saying get the 14".... Any last thoughts before I dump my hard earned cash into the abyss?



I think in terms of thresholds rather than incremental differences in apertures... Some examples:

- The largest scope I can can easily carry mounted through a standard doorway.

- The largest scope I can easily carry in two pieces.

- The largest scope that I can allows me to sit on my most favorite Starbound chair all night long.

- The largest scope that does not require a ladder.

- The largest scope I can fit in my car without significant disassembly.

- The largest scope I can fit in my car with disassembly..

- The largest scope I can store in my garage without disassembling it...

In my mind, a 16 inch F/4.5 and a 14 inch F/4.5 are probably not a lot different in terms of the physical aspects, the comfort, the ease of setup... With the right chair, a 6 foot tall observer can observe seated though it will be a tall chair. Standing may be more comfortable and no ladder or stool will be required.

So, if that's how it looks to you, then I would go for the 16 inch because it maximizes the capability without significantly increasing the hassle factor.

Jon

#56 jpcannavo

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Posted 26 April 2014 - 03:58 AM

Increased light gathering capacity (objective diameter) will increase brightness at a given magnification as long as the exit pupil diameter does not exceed the diameter of your pupil.


Or stated in a more sobering way: the maximum achievable visual surface brightness of an extended object - i.e.when exit pupil matches dilated anatomical pupil - can not be increased with increasing aperture. But, with increasing aperture, that maximum achievable surface brightness occurs at higher magnifications, thereby rendering more detail visible.






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