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Equipment Discussions >> Reflectors

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alexvh
sage


Reged: 07/29/07

Loc: South Africa
14" vs 16 " Dob
      #5857162 - 05/13/13 10:31 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


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Happy Birthday ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5857315 - 05/13/13 11:58 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!

Edited by ckwastro (05/13/13 12:10 PM)


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MikeBOKC
Post Laureate
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Reged: 05/10/10

Loc: Oklahoma City, OK
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5857331 - 05/13/13 12:05 PM

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.

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DJCalma
member


Reged: 01/17/13

Loc: Northern California
Re: 14" vs 16 " Dob new [Re: MikeBOKC]
      #5858382 - 05/13/13 07:05 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.

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Happy Birthday ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: 14" vs 16 " Dob new [Re: DJCalma]
      #5858391 - 05/13/13 07:12 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.


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: ckwastro]
      #5859072 - 05/14/13 01:58 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?


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george golitzin
Carpal Tunnel
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Reged: 02/24/06

Re: 14" vs 16 " Dob new [Re: alexvh]
      #5860524 - 05/14/13 05:26 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.


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NewMoonTelescope
Vendor


Reged: 08/31/12

Loc: Upstate NY
Re: 14" vs 16 " Dob new [Re: george golitzin]
      #5860852 - 05/14/13 07:54 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


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Bill Barlow
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Reged: 12/03/07

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Re: 14" vs 16 " Dob new [Re: alexvh]
      #5860890 - 05/14/13 08:28 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


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: Bill Barlow]
      #5861465 - 05/15/13 04:00 AM

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

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James Pierce
member


Reged: 08/11/09

Re: 14" vs 16 " Dob new [Re: alexvh]
      #5861524 - 05/15/13 06:11 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.

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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: James Pierce]
      #5861542 - 05/15/13 06:52 AM

Quote:

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


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James Pierce
member


Reged: 08/11/09

Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5861572 - 05/15/13 07:30 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.

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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: 14" vs 16 " Dob new [Re: James Pierce]
      #5861674 - 05/15/13 08:54 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.

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Happy Birthday ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: 14" vs 16 " Dob new [Re: Darren Drake]
      #5861831 - 05/15/13 10:35 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!

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".


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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: Darren Drake]
      #5861907 - 05/15/13 11:16 AM

Quote:

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


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5861970 - 05/15/13 11:45 AM

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

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Jarad
Postmaster
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Reged: 04/28/03

Loc: Atlanta, GA
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5862009 - 05/15/13 12:03 PM

Quote:

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

Edited by Jarad (05/15/13 01:46 PM)


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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: Jarad]
      #5862156 - 05/15/13 01:06 PM

Quote:


Does that help?

Jarad






Jarad.

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

Jon


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Happy Birthday ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5862298 - 05/15/13 01:59 PM

Quote:

Quote:


Does that help?

Jarad






Jarad.

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

Jon





+1


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: Jarad]
      #5862401 - 05/15/13 03:04 PM

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

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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: 14" vs 16 " Dob new [Re: ckwastro]
      #5862416 - 05/15/13 03:10 PM

Quote:

That was a simple, concise explanation...





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


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Jarad
Postmaster
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Reged: 04/28/03

Loc: Atlanta, GA
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5862438 - 05/15/13 03:20 PM

Quote:

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


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DJCalma
member


Reged: 01/17/13

Loc: Northern California
Re: 14" vs 16 " Dob new [Re: Jarad]
      #5862816 - 05/15/13 06:20 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!


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jgraham
Postmaster
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: 14" vs 16 " Dob new [Re: DJCalma]
      #5862899 - 05/15/13 06:42 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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NewMoonTelescope
Vendor


Reged: 08/31/12

Loc: Upstate NY
Re: 14" vs 16 " Dob new [Re: Jarad]
      #5862991 - 05/15/13 07:41 PM


Great explanation!


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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: DJCalma]
      #5863615 - 05/16/13 12:47 AM

Quote:


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




To your eye, there is no difference in the maximum possible surface brightness of an extended object between a 14 inch and a 200 inch. The difference is in the magnification.

A 14 inch achieves an exit pupil of 7mm at 50.8x. The 200 inch would achieve an exit pupil of 7mm at 725x. You could operate the 200 inch at 50.8x, it would produce a 100mm exit pupil but since your eye would only see 7mm of the that 100mm exit pupil, it would be no brighter than the 14 inch.

Photography is a different story.

Jon


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sopticals
professor emeritus


Reged: 03/28/10

Loc: New Zealand
Re: 14" vs 16 " Dob new [Re: jgraham]
      #5863622 - 05/16/13 12:54 AM

Quote:

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.




Amen


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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: 14" vs 16 " Dob new [Re: sopticals]
      #5863971 - 05/16/13 09:16 AM

It is quite simple to demonstrate the difference in performance. The op can make a 7 inch aperture stop and put it over his 8 inch sct. Study some object for a while then take off the stop and go back up to 8 inches. The difference would be about the same as a 14 to a 16.....

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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: 14" vs 16 " Dob new [Re: Darren Drake]
      #5864421 - 05/16/13 12:38 PM

Yes, but i'd imagine that'd have some limits... the already largish CO of a typical SCT would swell rather quickly as the aperture was 'stopped-down', to where CO degradation of images would also quickly become an issue. At 7" aperture, your 35% CO would now become ~40%.


Might just work for such a comparo, or illustration, but might also be about as far as one could push that...


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Darren Drake
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Re: 14" vs 16 " Dob new [Re: Mike B]
      #5864587 - 05/16/13 01:35 PM

It could also be done with his 10 inch where the co would be less of a factor...

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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: 14" vs 16 " Dob new [Re: Darren Drake]
      #5864610 - 05/16/13 01:46 PM

Yep- missed that he had one o'dem! Perfect, as it's likely more like 25% CO to start out with... you could almost milk that illustration down to ~6-inches!


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Jon Isaacs
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Re: 14" vs 16 " Dob new [Re: Darren Drake]
      #5865298 - 05/16/13 05:38 PM

Quote:

It could also be done with his 10 inch where the co would be less of a factor...




It's easier just to put your hand or in this case, your fingers in front of that 8 inch SCT. You will see the image dim somewhat but since the contrast is not affected, the question becomes, how does it affect your perception. Since the eye is highly non-linear, the actual effect is quite small.

Jon


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Darren Drake
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Reged: 10/09/02

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Re: 14" vs 16 " Dob new [Re: Jon Isaacs]
      #5865390 - 05/16/13 06:04 PM

Quote:

Quote:

It could also be done with his 10 inch where the co would be less of a factor...




It's easier just to put your hand or in this case, your fingers in front of that 8 inch SCT. You will see the image dim somewhat but since the contrast is not affected, the question becomes, how does it affect your perception. Since the eye is highly non-linear, the actual effect is quite small.

Jon



Unless your hand becomes a heat chimney and ruins the seeing lol.


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Jon Isaacs
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Loc: San Diego and Boulevard, CA
Re: 14" vs 16 " Dob new [Re: Darren Drake]
      #5865574 - 05/16/13 06:50 PM

Quote:

Quote:

Quote:

It could also be done with his 10 inch where the co would be less of a factor...




It's easier just to put your hand or in this case, your fingers in front of that 8 inch SCT. You will see the image dim somewhat but since the contrast is not affected, the question becomes, how does it affect your perception. Since the eye is highly non-linear, the actual effect is quite small.

Jon



Unless your hand becomes a heat chimney and ruins the seeing lol.






Not a good test for planetary viewing but it's reasonable for the faint, fuzzies. Of course, for planetary viewing, the seeing has to be essentially perfect for there to be a difference between a 14 inch and a 16 inch.

Jon


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N. Ham
super member
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Reged: 02/14/11

Loc: GA
Re: 14" vs 16 " Dob new [Re: alexvh]
      #5865826 - 05/16/13 08:45 PM

Jarad, so does your explanation of magnification vs. surface brightness explain why Globular clusters seem to be improved so much by a larger scope than galaxies do, although galaxies thru an 18" scope vs. a 6" scope offer so much more when magnified more?

Neal


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SACK
member
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Reged: 08/11/11

Loc: TX
Re: 14" vs 16 " Dob new [Re: N. Ham]
      #5866098 - 05/16/13 11:03 PM

Now I understand this finally and why obsession uses the globular pics to show the gains from increases in aperture on their website, and thus not galaxies.
Thanks Jarad and J. Isaacs!


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: 14" vs 16 " Dob new [Re: SACK]
      #5866295 - 05/17/13 01:29 AM

Snap jonathan!

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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: SACK]
      #5866507 - 05/17/13 07:07 AM

Quote:

Now I understand this finally and why obsession uses the globular pics to show the gains from increases in aperture on their website, and thus not galaxies.
Thanks Jarad and J. Isaacs!




This is one of the most broadly interesting topics in visual AA (amateur astronomy that is!).

So, yes, the implications of brightness gain for point sources with increasing aperture are more straightforward. With increased aperture, globs resolve and the individual stars start to pop out of the darkness as they brighten. This makes for easy web page depictions as in the Obsession page.

What happens with galaxies is a bit subtler. With increasing aperture, magnification can increase, while maintaining surface brightness (i.e. at the same exit pupil). As such, the visual detectability of faint extended regions increases.

Much of the explanation for how this "increased detectability" obtains can be made in terms of visual processing at the retina. But what falls out from the relevant physiology is this: A typical faint, low contrast, potential visual detail in an extended object (such as a spiral arm in a galaxy) will only become visually detectable once its real image on the retina achieves a certain size. So as the increase in aperture allows larger images on the retina without sacrificing surface brightness, visual details that were previously invisible now come into view.

This "isobrightness" (if you will) gain in visual detail provided by aperture can be demonstrated with a very cool (I think!) experiment.

Find a reasonably large, highly detailed, photo of a galaxy, where you are more or less familiar with the telescopic view of that object . Place it in a room and stand at a distance so that the view of the picture approximates some visual image scale typically seen at the EP. Now dim the lighting in that room until visible detail roughly matches what you see at the EP. (You may need to give your eyes 10 minutes or so to somewhat dark adapt, and keep decreasing lighting as needed to match degree of visible telescopic detail) Now, decrease your distance to the picture, while maitaining the same dimmed lighting. This decrease in distance will simulate the increase in image scale exactly as would some inverse increase in linear aperture with a telescope at a given exit pupil, on the observing field observing that galaxy. So for example, if you cut the distance in half, you would simulate a doubling of telescopic aperture.

If you try this, what you will observe is that as you get closer to the picture in the dimmed room (for us older folks we assume well corrected vision!), the brightness of the object does not change, yet detectable visual detail clearly increases! (one can argue in fact that the amount of visible detail increases with the square of aperture). If you try this, you will appreciate what aperture does for deep sky observing. You can also simulate the expected gain with different aperture jumps. Of course, you can just get out there and look through a bunch of scopes! But experiments like this are fun nonetheless.

Just for the fun of it, I can't resist one more comment closely related to the "unincreasibility" of surface brightness with increasing aperture issue (roughly (AKA the optical invariant, lagrange invariant, conservation of etendue etc.).

It can be shown from a purely ray tracing (i.e. geometric optic) approach that it is impossible to design an optical system that will yield a real image at its focal plane that has a greater surface brightness than that of the extended object it is providing a real image of. This obtains no matter how much light gathering power is attempted, and no matter what the final image scale is. (In optical engineering terms, etendue cannot be decreased by simply manipulating lens/mirror design). But now, as a thought experiment, lets suppose that this were achieved; i.e. you have an object radiating electromagnetic energy with some given surface brightness, and now you obtain a real image radiating at a higher surface brightness. Well - in very rough terms - you have just violated the second law of thermodynamics, since the radiant temperature of the real image is higher, and you have now passively transferred heat from a lower temperature region to a higher one!

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


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Jon Isaacs
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Re: 14" vs 16 " Dob new [Re: jpcannavo]
      #5866773 - 05/17/13 10:20 AM

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


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jpcannavo
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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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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
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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
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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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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
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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
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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
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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
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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
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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
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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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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
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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
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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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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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Re: 14" vs 16 " Dob new [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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