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Equipment Discussions >> Reflectors

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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5866865 - 05/17/13 11:01 AM

Quote:

Quote:


I recall that this was once famously referred to as "geometric optics knowing about the second law of thermodynamics", as it illustrates the beautiful interconnectedness of our physical laws. Very cool ( ) stuff!
Joe




Joe:

A very nice discussion.. I have never tried your dimly lit photo demonstration, quite intriguing. Stepping back rather than forward would simulate a decrease in aperture and at some distance, the object itself would disappear.

Jon




Jon

Nice flip side to this! Also, and as is surely obvious, the "dimming" I refer to actually involves rather low indoor light levels. I once did this by manipulating amount of outdoor street lighting coming in through a window at night.

Joe


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 14" vs 16 " Dob new [Re: jpcannavo]
      #5866919 - 05/17/13 11:31 AM

Quote:

Jon

Nice flip side to this! Also, and as is surely obvious, the "dimming" I refer to actually involves rather low indoor light levels. I once did this by manipulating amount of outdoor street lighting coming in through a window at night.

Joe




One interesting aspect of viewing low surface brightness objects is just how little detail the eye sees. The Veil is quite wonderfully detailed in a 16 inch Newtonian at low magnifications and yet a camera on a much smaller scope will show as much or more.

Another worthy demonstration is using neutral density filters to view DSOs. Probably a variable polarizing filter would be best... The moon viewed through a solar filter shows how poorly the eye detects details at low light levels. It's actually quite bright as far as DSOs are concerned, about 15.5 MPSAS, much brighter than the ring... but it's kind of a detail-less blob to the eye even though the detail is there to be seen if the eye could see it.

Jon


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Greatshot
sage


Reged: 04/15/10

Loc: Norton, MA
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5867901 - 05/17/13 06:33 PM

I'm a bit confused here (no surprise ). 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?

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5867927 - 05/17/13 06:52 PM

Not a 'big' difference. More is more and less is less. If you can easily handle the extra hassle of a 16, then I would recommend it. I owned a variety of telescope sizes from 20 inches to 8, and finally decided that a 12.5 inch, F5 is just right.

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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 14" vs 16 " Dob new [Re: Greatshot]
      #5867936 - 05/17/13 06:56 PM

Quote:

I'm a bit confused here (no surprise ). 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?




I think that is a fair evaluation. An object that is significantly dimmer than the darkest skies, you will be able to detect it.

Jon


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auriga
Pooh-Bah


Reged: 03/02/06

Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5868153 - 05/17/13 09:15 PM

Hi, Jon,
In my experience a 16" is noticeably brighter than a 14" on galaxies and globular clusters , but not tremendously brighter.

I think a 16" is a very nice size scope. I have a 16" f/4 with an eyepiece that is accessible in all orientations while I am seated.

Many years ago, 6 inches was normal, 8 inches was large, and people would drive miles to look through a 10", which was considered a galaxy scope.

These days, 18 inches is a medium size scope, 20 is on the large side, and 24 is definitely large, In people's perceptions.

In terms of ergonomics, to me a 16 inch is the biggest I can handle, with help. It is compact, so to me it is large in what it can do, but medium in physical dimensions.

I have been sorry I missed the chance to buy a very compact 8' from Tom Clark.

Regards,
Bill


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Greatshot
sage


Reged: 04/15/10

Loc: Norton, MA
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5868536 - 05/18/13 12:20 AM

Huh. That's interesting. Somewhat disappointing to think that there's literally no way to ever accomplish my childhood pipe dream of "seeing all the way back to the beginning*", but interesting.

*As an adult I was already let down by the learning about the early universe that's simply not possible anyways (can't see stuff with no stars to light it up!), but I figured "ancient galaxies" was still in reach in theory, should I win the powerball Ah well. Neat anyways. Helps make sense of why light pollution inflicts so much difficulty.


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: Greatshot]
      #6485105 - 04/24/14 08:42 AM

Hi guys... So here's the update: I went to a star party and no one had any dons in this range- there was a 12" and the next jump was a 22". I am in the final stages of deciding between a 16" Sumerian optics alkaid, and a 14" alkaid.
I was looking online to see if there were any sketches of galaxies showing the increase in detail between the two mirror sizes but no luck...
Gut instinct is saying get the 14".... Any last thoughts before I dump my hard earned cash into the abyss?


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jpcannavo
scholastic sledgehammer
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Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: 14" vs 16 " Dob new [Re: alexvh]
      #6485179 - 04/24/14 09:23 AM

Quote:

Hi guys... So here's the update: I went to a star party and no one had any dons in this range- there was a 12" and the next jump was a 22". I am in the final stages of deciding between a 16" Sumerian optics alkaid, and a 14" alkaid.
I was looking online to see if there were any sketches of galaxies showing the increase in detail between the two mirror sizes but no luck...
Gut instinct is saying get the 14".... Any last thoughts before I dump my hard earned cash into the abyss?




Another way of looking at this decision (cost aside) is to realistically determine the limit of aperture for yourself in terms of physical usability/portability/ergonomics etc. and then go for the largest aperture that does not exceed it. For me a 16" F5 represents one such limit - and so that is my big scope for now. If 14 is at that limit for you then chose that, if 16 is then choose that. Bottom line, you don't want to have a scope were the inertia of set up trumps the motivation to observe.

As for assessing small differences, you want to avoid the infinite regress of small differences. i.e. 14 is close to a 16, Ill go 14. But wait! 12 is close to a 14, Ill go 12. But wait, 10 is close to 12...eventually you are down to a 60 mm scope. Now of course, this is meant to be a somewhat humorous argument but for me there something of a point here: Rather than trying to quantify such aperture differences, sometimes a better approach is to honestly and realistically assess where your physical limits are with a scope, and chose the largest scope - granting the quest for aperture as defining goal - that does not exceed this limit and then going for it.

Joe


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #6486503 - 04/24/14 08:42 PM

Quote:

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.


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Galicapernistein
super member


Reged: 09/24/07

Loc: Detroit Michigan
Re: 14" vs 16 " Dob new [Re: GeneT]
      #6487408 - 04/25/14 09:38 AM

Quote:

Quote:

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.


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JMW
Pooh-Bah


Reged: 02/11/07

Loc: Nevada
Re: 14" vs 16 " Dob new [Re: Galicapernistein]
      #6488090 - 04/25/14 02:58 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.


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BigC
Carpal Tunnel
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Reged: 09/29/10

Loc: SE Indiana
Re: 14" vs 16 " Dob new [Re: Greatshot]
      #6488560 - 04/25/14 06:55 PM

Quote:

I'm a bit confused here (no surprise ). 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.

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TCW
Pooh-Bah


Reged: 11/05/13

Loc: The North 40
Re: 14" vs 16 " Dob new [Re: alexvh]
      #6488736 - 04/25/14 08:37 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.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 14" vs 16 " Dob new [Re: alexvh]
      #6489197 - 04/26/14 04:29 AM

Quote:

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


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jpcannavo
scholastic sledgehammer
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Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: 14" vs 16 " Dob [Re: TCW]
      #6489208 - 04/26/14 04:58 AM

Quote:

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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