55 replies to this topic

[chrisbourque]

Hi everyone

 

In brief, there is a 16” Sky-Watcher Go-To Dobsonian (Sky-Watcher Flextube 400P SynScan 16”) + extras available in my local area for a very good price. However I currently own an older model Meade LX50 10” SCT (non-Go-To, but motorized) on a fork mount + wedge. I am VERY interested to understand the differences I might see between the 10” and the 16”, but I am struggling finding good resources online that clearly show what the visual differences might be.

 

Is the upgrade from 10” to 16” going to produce dramatically superior views, or only “kinda” better views compared to my 10”?

 

(I am toying with the idea of asking the seller if he might show me the scope under clear, dark skies, but the weather lately has been back and forth, and I feel as though if I go to see the scope a second time, I am almost committing to buying it. So I’d rather get some ducks in a row first, if I can)

 

Here is the “TLDR” version (and to give you a sense of the research I have done to this point):

 

Just a disclaimer:
I don’t care about size, weight, bulkiness, light pollution etc etc. I have considered these aspects plenty already, and I am fine with it. Where I live gets very dark, and I am perfectly fit enough to haul this scope around my yard (I have already seen it in person, and am comfortable with this). So no need to dive into the “practical differences” of owning a huge scope. Realistically, my 10” is a beast to setup anyway. So I am no further behind in this regard.

 

I am just curious if anyone has any direct experience, or resources available that might help me determine the visual difference between a 10” aperture and this potential 16”? Specifically on DSOs.

 

With this scope being local and available for a VERY good price ($3000 CAD, and its in like-new condition + electronic dew heater, Celestron battery bank, laser collimator, some eyepieces, shroud etc), I am extremely interested. Especially considering this may be my only chance to own such a large telescope, as I may not be able to justify the costs of purchasing new in the future.

 

However, $3K is still a lot of money, for a hobby I have only started to get into. So I am still wondering what this scope might offer me above my current 10”.

 

I really like the idea of a Go-TO scope, as I struggle with star hopping in the ultra-magnified 2500mm FL 10” SCT. So “Go-To” is actually a huge draw for me. That being said, I am also considering de-forking my Meade and putting it on a CGEM that can handle it as payload (this would be much cheaper).

 

I’ve been reading and searching for resources online to help me. There are only so many “Telescope Simulators” out there, and most only simulate FOV, not object detail and brightness. However, this site attempts to do everything:

 

https://www.stelvisi...cope-simulator/

 

I’ve gone through a bunch of simulations at the above link (10” vs 16”, same Mag level) to get a sense of visual differences. And honestly, this makes it look like there is hardly ANY difference between the 10” and the 16”.

 

This can’t be true though, right? Logically speaking, the 16” is SO much larger than a 10”, the visual must be orders of magnitude more impressive?

 

So, I started researching some of the maths to calculate some of the fundamentals. But I struggle with many of the terminologies.

 

E.g. aperture area (classic areas of a circle, pi * r squared):

10” = 78.54”(sq)
16”= 201.06” (sq)

Which equates to: 201.06/78.54= 2.56 times larger light collecting area! This sounds great! More than doubling your light gathering capabilities without actually doubling the aperture diameter.

 

So 2.56x the light gathering ability must make the 16” eons better than the 10”, right?
…But still, the Telescope simulator doesn’t look all that different “in real life”. So there must be more to this story.

 

So then I started looking into “Magnitudes” and “Dawes/Rayleighs” limits etc etc.
This is also where I start to get a little lost in terminology.

 

However, from what I can find, these factors really don’t appear to be all that different (ah ha! Perhaps this is why the Telescope simulator shows very modest differences?):

 

Meade 10” LX50 (from what I can dig up):
Dawes Limit (called “resolving power” on the spec sheet I found, and measured in arc seconds)= 0.45
Limiting Visual Magnitude (approx.)= 14.5
Rayleigh limit= ???

 

Sky-Watcher 16” (Synscan 400P) Dob:
Dawes Limit= 0.29
Limiting Mag.= 15.5
Rayleigh Limit= 0.34

 

Again, I am unfamiliar with these terms, and only understand them as well as Wikipedia could explain them to me (i.e. I only kinda know what they mean).

 

But from those specs, it appears these two scopes are not “vastly” different in terms of visual experience.

Dawes Limits show resolving powers that appear to be in the same general ballpark. Unless a difference of 0.16 arc seconds is huge?

And the limiting magnitude between the two is only a spread of 1.

Which means what, exactly? I can supposedly now see objects with a visual brightness magnitude of 15.5 instead of 14.5? (I know I know, this is some kind of a Log scale, so a difference of “1” is fairly major, but still. Going from 14.5 to 15.5 doesn’t seems crazy extreme for how much larger the 16” is). 

Is this Mag. Diff of 1 actually something to write home about? I just don’t have the experience to know.

 

 

So I am now lost in a sea of possibly contradicting information. And I am hoping some folks on here might be able to put this all into perspective for me.

 

On the one hand, we have a scope that is MUCH larger, a little more than double the light collecting capability, and lots of support from fellow astronomers who support that larger 16” dobs are amazing.

 

On the other hand, I see specification limits which don’t seem all that impressively different, and a Telescope simulator which supports this idea.

 

So I am lost! Haha.
Do I upgrade my 10” SCT to a CGEM, or do I purchase the 16” Dob Go-To?

 

Any words of wisdom would be greatly appreciated.

 

Also thank you anyone who has stuck it out with me to this point in the post! It is very appreciated any help you can provide.

-Chris

[Jon Isaacs]

I have a number of Dobs including a 10 inch and a 16 inch.

 

In terms of deep sky performance, there's no real comparison, the 16 inch is much more capable.  Forget the numbers, you've seen too many of those.  Forget the simulators, looking through the eyepiece is the key.  

 

In terms of planetary and double star performance, that does depend on the seeing. But when the seeing is stable, the bigger scope is the better performer.. 

 

If you are wondering if the difference between a 10 inch and a 16 inch is meaningful, I'd say it's mind blowing.  And I am not one for hyperbole.. 

 

Jon

[sg6]

You do have a significant increase in aperture, 2.56x so that will be noticable.

After that difficult to say as really they are 2 different instruments but both called a "telescope". How they go about delivering the final product and what the final product is will be different.

 

Doubt that anyone can realistically say which will tick the most boxes for you.

[TOMDEY]

Yeah, It's way better for most Deep-Sky targets. You're on the threshold of over-analyzing --- time to decide, one way or the other.    Tom

[JOEinCO]

.....So, I started researching some of the maths to calculate some of the fundamentals. But I struggle with many of the terminologies.

 

E.g. aperture area (classic areas of a circle, pi * r squared):

10” = 78.54”(sq)
16”= 201.06” (sq)

Which equates to: 201.06/78.54= 2.56 times larger light collecting area! This sounds great! More than doubling your light gathering capabilities without actually doubling the aperture diameter.....

 

You do have a significant increase in aperture, 2.56x.....

Don't forget to subtract the area of the secondary when doing the calculations.  

 

That said, and since your backyard is dark, the 16" makes good sense for you. Get a hand truck to roll it into position and you'll be golden. Easier than the 10" forked SCT beast.

[havasman]

I moved from a 10" Dob to a 16" Dob and my observing was transformed. Galaxies are my preferred observations. The 16" facilitates observation of tremendous volumes of extragalactic features not seen with the smaller scope. Ranges of detail not seen in the smaller scope are present in many objects. The impact of moving from a 10" SCT to a 16" Dob will be even more dramatic.

 

The central star in M57 is a common sight for me, observed at magnifications not productive in the 10". Examining the interior structure and detail of nebulae such as NGC1499 is fairly easily accomplished whereas the smaller scope allowed much less to be seen. Obscure objects such as Gyulbudaghian's Nebula become familiar instead of being invisible.

 

Here's an excellent presentation of the differences aperture makes to an observation - https://www.astronom...e/steve.ngc.htm  It relates one extremely accomplished observer's experience with the entire NGC catalog and often includes descriptions of an object observed via apertures from 8 to 48". See M33 (NGC598) and NGC891 for instance.

 

And yes, give the numbers a rest. If possible, go look via larger aperture. If not then either choose to make the leap based on others' experience or let it go. But you will very certainly not be let down by the capability gain the increase in aperture and in system efficiency moving to the 16" Dob will make.

[stoest]

I went from a 9.25 SCT to an 18" dob so it was a little bigger move but pretty comparable I think.  The difference is astounding.  It's hard to convey just how much difference there is.  I would agree that if there's any way you can get  some views through it and see for yourself that's the best way.  I used the simulator when I was going through the same decision and it doesn't really do the change justice. 

[starcanoe]

If you have decent or even better darker skies....well...like Jon said....a 16 inch is going to blow away a 10 inch pretty much.

 

A fairly common consensus is that a 12 incher is where a lot of the big name deep sky objects go from being detectable/some detail to spectacular/lots of detail. Some folks lean more towards the 8 to 10 inch line and some more towards 14 inch. A 16 inch could easily be a lifetime deep sky scope.

 

Assuming you've done the research and have an accurate assessment of the fair market value....if it IS a deal....IMO just buy it.

 

For decades of my life I often didn't buy something because I wasn't SURE I wanted it. Eventually I realized I was being stupid...because...well duh...if its a good deal I can just sell the darn thing...and maybe even make a profit !

 

My only caveat is you need to make sure the coating on that mirror is good. AND you need to make sure you know how to store and clean a mirror properly to keep the coating good.

[star drop]

There is a significant difference.

[starcanoe]

One thing to consider....does this 16 inch dob have a WORKING goto system or one that sorta works?

 

Now keep in mind my experience regarding goto dobs basically amount to being downwind of a few of them at one time or another.

 

But my impression of goto systems for dobs is that they are often more problematic than a goto system on a SCT or gem mount or the like.

[TOMDEY]

Here's a couple of cartoons I created a while back. They correspond to telescopes that I've enjoyed, over the years... Click on the firt one there; you can see the progression... bigger shows more.    Tom

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[spereira]

This topic is a better fit in the Reflectors Forum, so I'm going to move it there.

 

smp

[MikeHC8]

I'll be the odd one out, I feel 10 inch is better because in long term us it is much easer to move 10 inch instead of 16 inch.  At the beginning you'll use the 16 inch a lot, but after a while you'll find the 10 inch is better just for setup time and removal.  No trying to be a kill joy, just a realest.  

[treadmarks]

I’ve been reading and searching for resources online to help me. There are only so many “Telescope Simulators” out there, and most only simulate FOV, not object detail and brightness.

 

So 2.56x the light gathering ability must make the 16” eons better than the 10”, right?
…But still, the Telescope simulator doesn’t look all that different “in real life”. So there must be more to this story.

I think you've pinpointed it. Have you ever looked through a big telescope? It's time to get out of the simulators and into real life. As you've said, $3K is a lot of money to spend when you don't know what you're getting.

 

Speaking from my own experience, big telescopes of course can show more, but that doesn't mean it's worth it. In a lot of cases, "more" = "more of the same." It depends on the object. Adding more stars to an already rich star field doesn't seem worth it to me. But in a lot of cases, they'll make visible a galaxy that is invisible in a smaller scope, or bring out more detail in a nebula, like stimulating color perception. Those are the cases in which a big telescope seems worth it to me.

[Sketcher]

I'm not a fan of "large" apertures, but I would have to agree with those who say/imply there's a significant difference at the eyepiece between a 10-inch and a 16-inch.

 

To see the difference, try this with your 10-inch:  Make an off-axis, circular, aperture mask whose diameter (clear aperture) is equal to (10-inches minus your central obstruction diameter) divided by 2 and place it off-axis, over the front of your 10-inch SCT so you get an unobstructed, circular aperture.  Make another off-axis, circular aperture mask whose diameter is equal to that of the first mask divided by 1.6.  Then try observing different objects while swapping the two aperture masks back and forth.

 

The difference between your two masked views will be comparable to the difference between a 10-inch and a 16-inch telescope -- all other things being equal (which really never are.  In general, a Newtonian should have a tad bit brighter image than an equal-aperture SCT).

 

That being said, I would just use a 5 or 6-inch refractor under a dark sky and be as "happy as a clam".   Like I said, I'm not a fan of "large" apertures; but if you are, you ought to enjoy the better views offered by a 16-inch (or even larger!) telescope.

 

Some who can see this:

 

 

with a 1-inch telescope can be quite content with a 5 or 6-inch that has 25 to 36 times the light-grasp and 5 or 6 times the resolution!    For others, there's always larger apertures.

[havasman]

One thing to consider....does this 16 inch dob have a WORKING goto system or one that sorta works?

 

Now keep in mind my experience regarding goto dobs basically amount to being downwind of a few of them at one time or another.

 

But my impression of goto systems for dobs is that they are often more problematic than a goto system on a SCT or gem mount or the like.

That does not at all match my experience with my own and others' go-to Dobs. I have spent numerous hours observing galaxies via 16" Starmaster and 6mm Delos (350x and 12 arc' field) moving from sight to sight w/o needing to change to a widefield "finder" eyepiece. Widefield eyepieces are then used for observing larger objects and wider fields.

 

Larry's 20" is at least as capable and repeatable and Allan's observatory mounted 32" is 100% spot on. Even the XX12g another club member uses is an excellent tracker that finds objects very easily with the stock Orion system.

 

And they're up and running more quickly and easily than the equatorial mounts in the field too. I've seen more broken, failing fork mounted SCT mounting systems than any other type of scope and I've never seen a failing Dob mount.

[starcanoe]

That does not at all match my experience with my own and others' go-to Dobs. I have spent numerous hours observing galaxies via 16" Starmaster and 6mm Delos (350x and 12 arc' field) moving from sight to sight w/o needing to change to a widefield "finder" eyepiece. Widefield eyepieces are then used for observing larger objects and wider fields.

 

Larry's 20" is at least as capable and repeatable and Allan's observatory mounted 32" is 100% spot on. Even the XX12g another club member uses is an excellent tracker that finds objects very easily with the stock Orion system.

 

And they're up and running more quickly and easily than the equatorial mounts in the field too. I've seen more broken, failing fork mounted SCT mounting systems than any other type of scope and I've never seen a failing Dob mount.

 

Fair enough. Like I said no real practical experience personally. Since the OP isn't specific on the details of the dob of interest 's ACTUAL goto system...we don't know whether it is a top notch system or a low budget system or an aftermarket system installed in a not so good manner.

 

My main point still stands though....if the goto system is big part of the attraction AS WELL AS significant part of the cost/market value then he needs to be sure it WORKS.

[happylimpet]

If youre confident weight and hassle isnt an issue, then do it! The difference with DSOs will be substantial. And if the optics are decent, for planetary also.

[PETER DREW]

Buy it before someone else does!

[Spartinix]

You've done the math and everything has been said I feel..the simulator only shows fov differences, NOT the detail in the objects nor the perceived brightness differences. They say a 16" really starts to open up the sky. Going from a 14" to 19.5" some years ago I can say it's true. Sweeping the sky should be easier, and if the goto is working properly then you'll be in a very happy place. Just make sure to have some decent eyepieces and perhaps a coma corrector to enjoy it to the max. Buy 😝.

[chrisbourque]

Wow, what awesome responses, thanks very much everyone for providing such excellent (and fast!) feedback and resources! It is all well received.

 

And all very good points to keep in mind. I think pretty much everyone agrees that a 16" should handily show a dramatic difference from a 10" (despite what the numbers and sims are saying). I kinda figured, logically, this MUST be the case. But very nice to hear from the people who really know. Although as some have pointed out, there are always a few important caveats and watch outs that must be navigated.

 

Many thanks again to you fine folks!! 

[Jon Isaacs]

I'll be the odd one out, I feel 10 inch is better because in long term us it is much easer to move 10 inch instead of 16 inch.  At the beginning you'll use the 16 inch a lot, but after a while you'll find the 10 inch is better just for setup time and removal.  No trying to be a kill joy, just a realest.  

 

This really depends on the individual. I bought my 10 inch in 2003 because it was smaller and handier than my 12.5 inch. I built my 16 inch in 2007. Since 2007, the 10 inch has stayed at home, never been to dark skies. I use it at home for the planet's and doubles.

 

My own experience has been that the greater capability of the 16 inch makes the increased effort to setup more that worth it.. 

 

Jon

[chrisbourque]

That does not at all match my experience with my own and others' go-to Dobs. I have spent numerous hours observing galaxies via 16" Starmaster and 6mm Delos (350x and 12 arc' field) moving from sight to sight w/o needing to change to a widefield "finder" eyepiece. Widefield eyepieces are then used for observing larger objects and wider fields.

 

Larry's 20" is at least as capable and repeatable and Allan's observatory mounted 32" is 100% spot on. Even the XX12g another club member uses is an excellent tracker that finds objects very easily with the stock Orion system.

 

And they're up and running more quickly and easily than the equatorial mounts in the field too. I've seen more broken, failing fork mounted SCT mounting systems than any other type of scope and I've never seen a failing Dob mount.

 

 

Fair enough. Like I said no real practical experience personally. Since the OP isn't specific on the details of the dob of interest 's ACTUAL goto system...we don't know whether it is a top notch system or a low budget system or an aftermarket system installed in a not so good manner.

 

My main point still stands though....if the goto system is big part of the attraction AS WELL AS significant part of the cost/market value then he needs to be sure it WORKS.

 

 

@havasman Its great to know you've had such good experience with dobs! This makes me feel more confident in them for sure, as I don't really know what to expect from a "typical" computerized dob.

 

@starcanoe You also make a good point though, in checking the electronics before I buy. Certainly I would insist on seeing this in action before I fork over $3K. We did test the motors at least, when I visited. They operated smoothly and cleanly in both axes. However this does not guarantee, certainly, that the computerized portion aligns and tracks well. As this is a major draw for me, I would like to see this in action.

In terms of identifying the Go-To system of interest, its actually just whatever stock motorized unit + computer control that comes with the Sky-Watcher SynScan 400P Flextube. Nothing aftermarket, as it should be ready "out of box" presumably. If there's any feedback or watch-outs on this particular system, that would be greatly appreciated!

[Jon Isaacs]

Chris:

 

One good thing about Skywatcher is their support for the owner. Kevin Legore (Skyward_eyes on CN) is an avid amateur astronomer and also part of Skywatcher USA and he makes sure that problems are taken care.

 

When I see someone with a problem with a Skywatcher, if Kevin hasn't already responded, I'll shoot him a PM and he's right there taking care of it.

 

Jon

[Starman1]

Wow, what awesome responses, thanks very much everyone for providing such excellent (and fast!) feedback and resources! It is all well received.

 

And all very good points to keep in mind. I think pretty much everyone agrees that a 16" should handily show a dramatic difference from a 10" (despite what the numbers and sims are saying). I kinda figured, logically, this MUST be the case. But very nice to hear from the people who really know. Although as some have pointed out, there are always a few important caveats and watch outs that must be navigated.

 

Many thanks again to you fine folks!! 

You gain a full magnitude in depth with a 16" over a 10".

The number of objects visible approximately triples because the extra reach covers so many galaxy groups.

The brightness of every object at the same power will be significant, and noticeable.

The size of every object at the same exit pupil will be larger, making details easier to see.

You will have a significant improvement in resolution: Dawes limit changes from 0.45" to 0.28", which will have an appreciable effect on what you can see on Moon and planets.

Maximum magnification will go from approximately 500-600x to 800-960x.

A 1 magnitude jump is always noticeable and profound, the in between steps (10" to 12.5", for example), not so much.

 

So what do you lose:

--weight and size:  The 16" is larger and heavier and more difficult to move, but if you can handle it, no problem.

--speed of mirror cool down:  You'll have to pay more attention to cooling the mirror with a fan or fans.  It will take longer to cool down to ambient temperature and yield sharp images.

--collimation tolerances will be tighter, so you will have to pay more attention to collimation.  You might want to upgrade your collimation tools.

--a coma corrector at f/5 is not critical.  By f/4.5, it is, so you will need to think about adding one right away, especially if you plan to use ultrawide eyepieces or wider.

--stiffness with a coma corrector and heavy eyepiece in the focuser.  You may end up adding some wire reinforcement to the upper parts of the scope.

 

All that said, I certainly wouldn't discourage you from doing so.  I think the sweet spot in telescope size is somewhere between 12.5" and 16".

You will NEVER run out of objects to view.

Edited by Starman1, 12 September 2019 - 12:50 PM.