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Magnitude difference between 15" and 16" Dob

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#1 EZ73

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Posted 24 November 2021 - 05:25 PM

Hello Y'all

 

I've been using and enjoying a nice GSO made 10" dob now for a little while and now I'm looking to upgrade to a premium model ( New Moon ) 15" or 16" F4.5. I've read quite a bit here that these are the " sweet spot " apertures that really take DSO observing to another level! I'm keeping my 10" because its such a good scope that's not to terrible to move around.

 

I understand the move to a 16" over the 10" is a FULL magnitude more, and will be profound at the eyepiece ( thanks to folks here who have shared these experiences ) 

 

Would having a 15" really be that much different?? I realize I might be splitting hairs here, but I would really like to hear everyones opinions on this! My rationale is the 16" is bigger, heavier and a little more expensive, knowing the very real phrase " get the scope you will be able to use the most " 

 

I'm 6'  48 years young, pretty fit, and have a 4x4 truck for moving the beast haha.  

 

I regularly drive to a Bortle 2 sky about an hour away from Salt Lake City, so I going to be lifting the scope into the bed of the truck. Galaxies, Galaxy clusters, PN will be the main targets with the new scope.

 

Would I be missing much going with the 15" or is it, well your already getting a Big one you mind as well get the 16" wink.gif

 

Thanks for reading, 

 

Bryan



#2 havasman

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Posted 24 November 2021 - 06:08 PM

The performance difference in a 15" and 16" aperture premium Dob is real but will be subtle. Yes, it's splitting hairs. If the cost difference is important, get the smaller scope. The weight/mass difference too will be subtle. If all else is relatively equal, get the 16".

 

I encourage you to strongly consider f4 for the 16". That roughly 4" difference in the height of a 16" scope places the f4 in a really comfortable range where a standard height observing chair like a Starbound works great and the entire travel range of the focuser is in flat footed territory for a 6' observer. I wish my f4.49 Starmaster was f4. Going f4 will lighten the package too as the moment arm of the weight of the UTA (structure, focuser, finder(s), dew control, etc.) will be smaller and require less balancing ballast on the mirror box. And you gain max FOV while keeping a long enough focal length to still easily get high magnifications if your sites allow them.


Edited by havasman, 24 November 2021 - 06:10 PM.

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#3 Redbetter

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Posted 24 November 2021 - 06:48 PM

15" vs. 16" is only a 0.12 magnitude difference, not much.  I would be weighing other factors to determine what was the best fit compared to this small incremental difference.  Those sort of things depend on how one observes and ergonomic preferences.    

 

Among potential "other factors" would be if a person knows he/she has the tendency to want to keep upgrading...    There is somewhat of a fork in the road to consider:

  • Trying to head off the urge for the next aperture size...perhaps an 18", and therefore getting the 16" so that the performance gap to the next instrument is smaller/harder to justify.
  • Anticipating going to a larger increment later, and therefore choosing the 15" so that the next increment at say 18" has an easier to justify magnitude improvement than going from 16" would.

I went from an 8" first scope 26 years ago, to a 20" second scope 18+ years ago, so I don't suffer from the aperture bug/spending itch the way some do.  I like to find something that works, and stick with it.  I do plan to max out aperture for galaxies one of these days, but it isn't planned as a portable scope and will be part of a lifestyle change.   May never happen...I find it easy to procrastinate and not spend money when I have something that works.  My truck will turn 20 years old in a few months, bought it partly with the 20" in mind while it was still only a long term plan.


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#4 TOMDEY

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Posted 24 November 2021 - 07:41 PM

Reminds me of those peculiar Coulter Odyssey mirror sizes >>> 13.1", 17.5", and 29". I had and enjoyes all three!    Tom



#5 ausastronomer

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Posted 24 November 2021 - 08:13 PM

The performance difference in a 15" and 16" aperture premium Dob is real but will be subtle. Yes, it's splitting hairs. If the cost difference is important, get the smaller scope. The weight/mass difference too will be subtle. If all else is relatively equal, get the 16".

 

I encourage you to strongly consider f4 for the 16". That roughly 4" difference in the height of a 16" scope places the f4 in a really comfortable range where a standard height observing chair like a Starbound works great and the entire travel range of the focuser is in flat footed territory for a 6' observer. I wish my f4.49 Starmaster was f4. Going f4 will lighten the package too as the moment arm of the weight of the UTA (structure, focuser, finder(s), dew control, etc.) will be smaller and require less balancing ballast on the mirror box. And you gain max FOV while keeping a long enough focal length to still easily get high magnifications if your sites allow them.

 

Dick has pretty much got it spot on.  The differences are detectable, but very subtle.  If there are major reasons why the 15" works better for you go that way for sure. If the reasons are minor, go with the 16".  A 15" with a clean mirror, will go deeper than a 16" with a dirty mirror. Similarly, a 15" with new coatings will go deeper than a 16" with 3 or 4 year old coatings.

 

I have 14" and 18" scopes side by side all the time at Ozsky and with 4" extra aperture, the difference is significant.  Not so going from 15" to 16".

 

Cheers



#6 EZ73

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Posted 24 November 2021 - 11:45 PM

Definitely some good things to consider here. With not much of a difference in performance, weight etc. sounds like the 16" would be the right way to go then. I didn't think to much about a 16" @ F4, I like to stand while observing, I thought F4 would be to short, so thanks for pointing that out as well as max FOV.  Also I like your line of thinking Redbetter, find what works and stick with it! I want this to be my last purchase so the move with the 16" makes more sense! This light bucket with my other scopes will keep me happily busy for the rest of my life! waytogo.gif


Edited by EZ73, 24 November 2021 - 11:50 PM.


#7 Bill Weir

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Posted 25 November 2021 - 12:17 AM

For really only a few hundred bucks more go f/4. Balance is so much easier as well as the truss structure ends up being more rigid due to the shorter poles. I too like to observe standing but no matter what once you reach a certain altitude a chair is needed almost no matter what. Something also that I enjoy about the shorter f ratio scope is the smaller footprint required for the observing radius. Where I usually observe from level areas are limited and even with a 20” scope I don’t need a lot.

 

As to whether 15 or 16” if you can swing the funds go 16. Then you will never question, “what if I had bought the 16” instead of the 15?”

 

Bill


Edited by Bill Weir, 25 November 2021 - 12:18 AM.


#8 berkleystg

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Posted 25 November 2021 - 03:53 AM

I have a 16” f/4 NMT. It is very easy to load and unload from the car. I don’t think the weight difference between the 15” and 16” would be much with a NMT. Ask Ryan the weights between the two. He will tell you.

#9 Darren Drake

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Posted 25 November 2021 - 07:20 AM

The 16 inch will gather 13.8% more light than the 15 inch. Also the additional light the 16 gathers over the 15 inch is equivalent to a 5.5 inch scope to put it all in perspective...



#10 Allan Wade

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Posted 25 November 2021 - 07:37 AM

If you are committed to f/4.5, then get the 15”. But a 16” f/4 is one of the very sweet spots in all of astronomy. It’s big aperture, with big performance and seated observing all the way.


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#11 EZ73

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Posted 25 November 2021 - 02:36 PM

Excellent insight here everyone! I really appreciate the feedback! This is starting to look like an easy decision! I briefly thought about if going with a 16" maybe I should do F4, now this is looking like the sure way to go! Actually getting the Full magnitude over my 10", not much weight difference than the 15", having better balance, rigidity @ F4 the larger FOV! Not insignificant stuff here waytogo.gif

 

Great point with, well if I had only gotten the 16" too, I don't need that in the back of my mind lol.gif

 

Darren that is incredible, a difference in light gathering of a 5.5" scope.. Holy cow, I had no idea. That IS some perspective

 

Berkleystg that makes me fill great about the weight on this scope since you have this exact model, I want to be able to use this for many, many years to come!

 

If anyone wants to chime in on there use with a coma corrector for these Big Dobs please do, I use the GSO coma corrector with the .75" blue fireball extension on my 10" and it works great.

GSO says its good to F3, will I be fine using this? Thoughts

 

Thanks again everyone, your insight has been invaluable wink.gif  

 

Bryan



#12 Don H

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Posted 25 November 2021 - 02:54 PM

A used 15" f/4.5 Obsession Classic might be a more comfortable and satisfying scope to use than many foreign 16" scopes. 15" and 16" side by side look very similar. I used to have a 16" and 14.3" at the same time and sold the 16, due to the visual difference not being offset by the extra weight and bulk. But my 16" was an f/5, so it was a bit more hassle... 



#13 vdog

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Posted 25 November 2021 - 06:41 PM

Yes, get the f/4.  I'm glad my 16" is an f/4.  I can't imagine not being able to spend most of my sessions on my Starbound chair.


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#14 TOMDEY

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Posted 25 November 2021 - 10:10 PM

Here are ~some other ways~ to think about the trade-space that reveal nuances seldom considered.

 

Inductive Reductionism > If 15 is essentially indistinguishable (so therefore as good as) 16 --- then one must also conclude that 14.06 is as good as 15 --- hence, by induction naked eye is as good as any telescope. So we astronomers should give up our telescopes entirely, and just enjoy the heavens freely, without the needless hassle and expense of falling for Galileo's and Newton's midnight marketing pitches.

 

If the Inductive Reductionism argument is logically-flawed, then one must logically conclude that any larger telescope is better than any smaller than itself. This is consistent with customers' willingness to pay more and more and more for bigger and bigger and bigger telescopes. "Serious" amateur and professional astronomers are anxious to finance telescopes to the limit of their cash stash + credit line + future tax revenues.... and keep asking for more. Evidence Q7, 16" ACF, 14" RASA, 50" NMT, Kecks, Hubble, JWST.

 

So the two parameters that matter most are the slope of the cost/benefit curve and the depth of the funders' pockets.

 

Final inexorable conclusion >>> Each and every one of us should get a second job to fund our next telescope.     Tom


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#15 Mike Wiles

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Posted 26 November 2021 - 09:42 AM

I'm 6'  48 years young, pretty fit, and have a 4x4 truck for moving the beast haha.  

 

I regularly drive to a Bortle 2 sky about an hour away from Salt Lake City, so I going to be lifting the scope into the bed of the truck. Galaxies, Galaxy clusters, PN will be the main targets with the new scope.

In this similar situation back in 2001 I was looking for a large aperture scope to take to Bortle 1/2 skies and I was trying to decide between a 15" f/4.5 Obsession Classic and a 16" f/4.5 Night Sky.  I chose the 15" Obsession for one simple reason....I am 5'6" and the 15" kept my feet on the ground and was a comfortable eyepiece height over nearly the entire sky.  I never regretted that decision.

 

If I was 6' tall and trying to select between those two scopes....I would choose the 16" f/4.5 for the same reason.  At 6' tall, a 16" provides a more comfortable eyepiece height than the 15". And it does provide a little extra aperture.  

 

Assuming you're physically strong enough to lift the slightly heavier 16" mirror box into the truck, I'd take the slight extra advantage of the aperture and the more comfortable eyepiece height and I'd never look back.  You mentioned New Moon as the builder....and Ryan's scopes tend to be lighter built than the older dobs that I was selecting from....so I'd think that the 16" would be just fine to hoist into the truck.  

 

About 18 months ago a couple of us put an 11" and 12" scope next to each other in Bortle 2 skies, insisting that there'd be no noticeable difference in the aperture.  There is.  It'd be hard to notice unless they're right next to each other.  But when you have two scopes with 1" difference in aperture....you *will* notice the difference.  The larger scope does go deeper and it's visible at the eyepiece.  

 

Buy the 16".  The only thing that stopped me from doing so back in 2001 is that I wasn't 6' tall back then.  And now, despite 20 additional years of eating my vegetables and getting plenty of rest....I'm still not.  wink.gif


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#16 Mike Spooner

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Posted 26 November 2021 - 11:02 AM

Not to sidetrack too much but I’ve mentioned before that for me I’d consider the proper f.l. with an eye towards possibly putting a DOB on a platform drive. Nothing like having the focuser an inch too high to rob the fun from an observing session. YMMV.

 

Mike Spooner


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#17 havasman

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Posted 26 November 2021 - 03:40 PM

If anyone wants to chime in on there use with a coma corrector for these Big Dobs please do, I use the GSO coma corrector with the .75" blue fireball extension on my 10" and it works great.

GSO says its good to F3, will I be fine using this? 

My 1st coma corrector was the Astro-Tech (GSO), used in the XT10i with primary widefields ES82 30 & 18mm. The 1st time it was in the focuser was an OH WOW moment and I've never observed w/o a CC in a Dob focuser since. I used it with no knowledge of the need for setting up an eyepiece to max out the correction and it worked well just dropping the ep in and going with it.

 

When I picked up a 16" Starmaster I just thought that if there was a better option then that scope would be better served by it so I picked up a Paracorr 2 and have used it for years with fine success. The gain was incremental but real and present. The field has been coma free. My ep kit requires changing settings from H to D to A so it's not that diff. All the way one way, all the way the other way or in the middle works and isn't so diff.

 

Then with the last Dob added to the scope stable I was able to configure it from scratch and selected the SIPS system. Now THAT's an upgrade! Set it once to the scope optics and forget it AND get the best corrected field available. I figure I'll disassemble and clean the SIPS optics every year or so and that's it. Simple. If you're building your lifetime scope then the SIPS should be strongly considered. Yeah, it's not cheap but it's pretty much the nuts. 

 

It sounds like you're well aware of this but the only serious error available is to set up a < f5 Newt w/o a coma corrector.

 

_________________________________

references for SIPS

 

Televue, with link to Starlight Instruments (Feathertouch) page  -  https://www.televue....id=61&Tab=_sips

Loptics.com link = Lockwood's SIPS instructions for setup/use  -  http://www.loptics.c...uments/SIPS.pdf


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