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14" vs 16 " Dob

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#1 alexvh

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Posted 13 May 2013 - 09:31 AM

Does any one own / or has anyone looked through both a 14" and 16 " and compared the views? does the extra 2" make a big difference? I once looked through a 17" and was amazed that my 12" held up so well against it. but i wonder if it was because it hadnt cooled down enough, or perhaps the collimation was out.

Thank you for taking the time to help...
A

#2 ckwastro

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Posted 13 May 2013 - 10:58 AM

I used to own a 14.5" and a good friend of mine had a 16" and there wasn't much difference at all in general practice. Could we see the difference if we deliberately went looking for it? Yes, but it wasn't much.

Between a 14.5 and 16 the 16 has about 21% more light gathering and 10% extra resolution.

To give you an idea we both trained our scopes on M42 one evening before it was completely dark and waited to see when the first stars that are buried within the nebula first became visible in each scope. He saw the first one about 10 seconds sooner than my scope. We've observed together for years & I can say our visual acuity is very close. Even observing faint galaxy clusters we never really could see any details in the 16 that weren't readily visible in the 14.5. The only other times we found a difference was when we were actually doing star counts in clusters. There the 16 would reveal a few faint stars at the edge of seeing for its aperture that the 14.5 wasn't seeing, but again, very little difference.

If you are buying a new scope and looking for a significant increase in the information available at the eyepiece over your 12", you should consider the 18" to 20" range. The 20" will go a full magnitude deeper than the 12, and at that level is significant. However if you are looking to just upgrade from a 12 to a 14, personally, I wouldn't bother. I'd even have to think a while on going from a 12 to a 16. You would see some difference over the 12, but it still would not be a "wow factor".

Good luck!

#3 MikeBOKC

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Posted 13 May 2013 - 11:05 AM

I have set up my 14 next to some slightly larger scopes (16 and a 17.5) and don't really see a significant difference. I agree -- 16 is great, but not really worth the slight increase from 14.

#4 DJCalma

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Posted 13 May 2013 - 06:05 PM

I often compare side by side with my 14.5" and 16". I do see a difference, but it's not what you'd think. Not all things are equal in my case. My 14.5" has far superior optics, a small secondary, and enhanced coatings. At very low powers I do not see a difference at all. At higher powers the 14.5" is noticeably better. All things being equal, there may be other, more important considerations such as size, portability, and cost. The difference at the eyepiece will be negligible at best.

#5 ckwastro

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Posted 13 May 2013 - 06:12 PM

Very true, quality optics make all the difference.

I should clarify to the OP that the 14.5" & 16" to which I referred in my previous post were both made by Starmaster with Zambuto mirrors. The scopes were about 18 months apart in manufacture and virtually identical in every respect, except for the aperture of course. :grin:

#6 alexvh

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Posted 14 May 2013 - 12:58 AM

Thank you gentlemen i really do appreciate the help. I must say i am surprised!!! I really did think 16" would show more detail.
My main interest is galaxies !
Are the objects brighter though?

#7 george golitzin

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Posted 14 May 2013 - 04:26 PM

I owned a 16-inch f/5 for several years, and moved up to an 18-inch f/4.2 a little over a year ago: that's a smaller percentage increase (26.6%) than you are asking about (30.6%). I find that even this small increase amounts to a notable difference between the two; this is particularly apparent when dealing with threshold objects (very faint galaxies in a galaxy cluster, for example), but also is apparent in the detail seen in brighter DSOs. The arms of M51 pop out a little more; the face in the Eskimo nebula is more readily apparent, etc. The view is noticeably brighter. So for me, the rule when deciding on your big scope is, "what is the largest aperture that I can handle, and that won't get in the way of my observing?" Part of that equation may be weight, part might be eyepiece height--I don't think the hassle factor of a 16 is a lot more than a 14, and the eyepiece height can be dealt with by going to a faster mirror: my 18, for example, is actually quite a bit shorter than my old 16.

If you're a casual tourist of the sky, get the smaller scope. If you want to wring out every bit of detail you can, then get the biggest scope that you can afford and that you can handle comfortably.

-geo.

#8 NewMoonTelescope

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Posted 14 May 2013 - 06:54 PM

Often, several from our club (the SAS) observe under dark skies in upstate NY. We've compared the views in all sorts of sizes, but I would say that the views through a 14" were a little brighter than a 12.5", but a 16" showed significantly brighter views than the 14". Obstructions of all three were 18-21%, but I am not sure about the coatings on the 14", as both the 12.5" and 16" were enhanced. All three sizes were shown up by the 20" and 24" though!

Darker skies,

Ryan

#9 Bill Barlow

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Posted 14 May 2013 - 07:28 PM

I would think that going from a 12" to a 16" Dob, you would notice the extra light grasp and resolution at the eyepiece, especially for fainter objects. I own both 12" and 14" SCT's and can see a difference between the two, especially on faint/distant galaxy groups like the Hickson's and Arp's. I have seen that some galaxies are only seen about 25-50% of the time in the 12" using averted vision while the same ones are seen 50-70% of the time in the 14" using direct vision. I would think a move up to a 16" would be noticeable to you, especially if observing galaxies is what you like to do.

Bill

#10 alexvh

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Posted 15 May 2013 - 03:00 AM

Hmmm some very different opinions... I wish i had the oppourtunity to look through them myself!

#11 James Pierce

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Posted 15 May 2013 - 05:11 AM

I've always felt that 16" was where deep sky objects got interested. The number visible and the level of detail in the brighter stuff seems to really pop at 16 inches. I've never felt like 18, 20 or even 22" were a huge improvement in the same way. Better sure, but not remarkable in the same way as a 16" feel vs smaller scopes. For planets and very bright objects there will be no serious difference - you have to compare marginal objects to see it.

#12 Jon Isaacs

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Posted 15 May 2013 - 05:52 AM

I've always felt that 16" was where deep sky objects got interested. The number visible and the level of detail in the brighter stuff seems to really pop at 16 inches. I've never felt like 18, 20 or even 22" were a huge improvement in the same way. Better sure, but not remarkable in the same way as a 16" feel vs smaller scopes. For planets and very bright objects there will be no serious difference - you have to compare marginal objects to see it.


Personally I find DSO's quite interesting in an 80mm. Different DSOs to be sure, stuff that cannot be seen in a larger scope..

Anyway, these sorts of questions are tough to answer.. There is a difference between a 14 inch and a 16 inch, but most anything one sees in a 16 inch will be seen in a 14 inch. Against what more one might see, one has to balance the hassle factor. And then one also has to consider existing equipment.

My own pile of junk includes 10 inch, 12.5 inch, 16 inch and 25 inch Dobsonians, each has it's place, it's role in observing the night sky. Alex mentions he has a 12 inch though his list does not show it. I don't see the move from a 12 inch to a 14 inch as worthwhile. A 16 inch is a nice step up and is a nice size, these days it might even be considered medium sized because it can be easily managed by one person and at F/4.5 it will not require a ladder.

Jon

#13 James Pierce

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Posted 15 May 2013 - 06:30 AM

Yes that is perhaps more the point. A 16 F4 or F4.5 is really the largest 'small scope' After that you quickly need ladders, ramps, trailers, friends and stuff just for basic movements.

#14 Darren Drake

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Posted 15 May 2013 - 07:54 AM

Put another way if you compared a 14 inch to a 16 inch by taking the extra light the 16 has over a 14 and made it into a scope it would be a 7.75 inch aperture scope. That's not at all insignificant. All else being equal I say it's best to go for the biggest aperture realistically possible.

#15 ckwastro

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Posted 15 May 2013 - 09:35 AM

To the OP, as you can see, everyone is different on how they perceive the views in a 12.5 vs a 14.5 vs a 16.

For me personally, like I mentioned prior, the differences are there, but certainly not enough to warrant spending a lot of money on an upgrade from a 12.5 to a 14.5 or 16. Others might feel differently. I tend to follow upgrades by "magnitudes deeper". Using the two scopes I have now, the Mewlon 210 goes almost two magnitudes deeper than my 92mm refractor. The next step for me will be a minimum of a 12.5, which will go one mag deeper than the Mewlon. The next step from the 12.5 would be at least a 20", which is yet another mag deeper. I'm not sure I want two more scopes so more than likely my next purchase will be a 16" or 18", and I'll be done......for a while anyway! :lol:

My point is these types of magnitude jumps are very significant and will offer completely new perspective over the smaller scope(s). For me the mid-range increases in aperture (e.g. 8" to 10", 10" to 12.5" or 14, 12.5 to 14 or 16) are just not worth the cost. You might feel different about that.

The best thing you could do is spend a little more time with your 12.5 next to both 14" and 16" scopes, even 18s and 20s if available, and make the decision that way. Keep in mind that optically it might be worth it to you to go big, but don't forget the hassle factor. A 20" scope for one person can be a lot more work than a 16".

#16 Jon Isaacs

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Posted 15 May 2013 - 10:16 AM

Put another way if you compared a 14 inch to a 16 inch by taking the extra light the 16 has over a 14 and made it into a scope it would be a 7.75 inch aperture scope. That's not at all insignificant. All else being equal I say it's best to go for the biggest aperture realistically possible.


The difference between a 14 inch scope and a 16 inch scope is 0.3 magnitudes. By comparison, the difference between an 8 inch scope and a 10 inch scope is about half a magnitude. That's for stars.

For extended deep sky objects, since it's not a question of brightness or resolution but simply increased size, greater magnification at the exit pupil, the difference is about 14%. Given the non-linear nature of the eye's response and the fact that increased aperture does not change the contrast of faint objects, the difference between a 14 inch and a 16 inch is small.

Of course all this is just hand waving to support one's experience at the eyepiece. In my experience, small differences like this are relatively unimportant and most anything visible in 16 inch will be visible in a 14 inch.

Jon

#17 alexvh

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Posted 15 May 2013 - 10:45 AM

Sorry for my ignorance but why does apperature not increase contrast and brightness??

#18 Jarad

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Posted 15 May 2013 - 11:03 AM

Sorry for my ignorance but why does apperature not increase contrast and brightness??



It's not ignorance, it's actually a fairly complicated issue.

For extended sources, the surface brightness never gets brighter than what you see naked eye. Let's use a 10" scope (254mm diameter) as an example. If your eye can open to 7mm, and you use the scope at a 7mm exit pupil, you get:

Magnification = 254mm/7mm = 36.3x
Total light gathering = 36.3^2 = 1318 times as much as your eye.
Area of an extended object at 36.3x = 36.3^2 = 1318 times as much area as at 1x.
Surface Brightness = 1318 times as much light / 1318 times as much area = 1 (same surface brightness as naked eye).

A bigger scope will result in higher magnification at a 7mm exit pupil, and more total light, but they cancel out for surface brightness. So a bigger scope doesn't make an extended object brighter, it just makes it bigger at the same brightness (which does make it easier to see, but not because of brightness or contrast).

As you increase the magnification (go to smaller exit pupils), the amount of light doesn't get bigger, so the surface brightness drops (more area, same total brightness). Contrast of extended objects stays the same (both the background and the object will get dimmer, but the ratio of the two will be constant).

For point sources like stars, aperture does increase brightness and contrast. That's because all of the light stays in the airy disk (the object is too small to magnify), so increasing magnification doesn't spread the light out any more until you magnify it so much that you can resolve the airy disk itself. So for stars, increasing aperture increases the brightness of the star, which also increases the contrast against the background (which is extended). For a given aperture, increasing magnification also increases contrast for stars, since it makes the background dimmer but not the star, until you get down to around a 0.5mm exit pupil and start seeing the airy disk itself.

Does that help?

Jarad

#19 Jon Isaacs

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Posted 15 May 2013 - 12:06 PM

Does that help?

Jarad



:goodjob:

Jarad.

That was a simple, concise explanation... glad you did it and not me... :)

Jon

#20 ckwastro

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Posted 15 May 2013 - 12:59 PM


Does that help?

Jarad



:goodjob:

Jarad.

That was a simple, concise explanation... glad you did it and not me... :)

Jon



+1 :waytogo: :waytogo:

#21 alexvh

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Posted 15 May 2013 - 02:04 PM

Jarad thank you for the info! I am a medical doctor by profession so this is a different science for me!

#22 Mike B

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Posted 15 May 2013 - 02:10 PM

That was a simple, concise explanation...

:ubetcha:

Now, tell us the part about the "bigger telescope" again! :jump:

#23 Jarad

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Posted 15 May 2013 - 02:20 PM

I am a medical doctor by profession so this is a different science for me!


I'm a biochemist who got sucked into biostats, so it's a nice break for me, too. :)

Jarad

#24 DJCalma

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Posted 15 May 2013 - 05:20 PM

Logically, if there is no difference between a 14" and 16", there is no difference between 14" and 200".

Just the same as taking the extra light from the difference between a 14" and 16", we end up with a 7.75" scope, we would also end up with a whopping 20" scope from the extra light between a 200" and 201" scope. That doesn't sound so significant anymore, does it?

How to resolve this issue? Get out there and do some comparing at star parties!

#25 jgraham

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Posted 15 May 2013 - 05:42 PM

For general observing those extra 2" may not mean much, but they become more important as you push the limits of what you are trying to observe. The extra light grasp also becomes important when using filters and binoviewers.






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