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Old 09-01-2020, 05:09 PM
Hijynx (Benn)
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A simple question

I have what I think is a simple question.

Yes aperature is king etc etc. and yes bigger is better when it comes to visual astronomy but one must consider cost, weight, transport, storage and eyepiece height at zenith. So .......

Is 16 a respectable aperature for viewing DSO with a dob? Is there that much difference at the eyepiece when compared to 18-20?
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Old 09-01-2020, 08:11 PM
gaseous (Patrick)
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I've had a 16" dob, and now have a 20" dob. The 20" has 1.57x more light gathering capacity, and it is definitely noticeable over a 16" (although I never owned them simultaneously).


Having said that, a 16" is still a beast of a scope for DSOs - if you had one, it wouldn't disappoint. They do take up a lot of room though!
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Old 10-01-2020, 03:37 AM
Ukastronomer (Jeremy)
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Quote:
Originally Posted by Hijynx View Post
I have what I think is a simple question.

Yes aperture is king etc etc. and yes bigger is better when it comes to visual astronomy but one must consider cost, weight, transport, storage and eyepiece height at zenith. So .......

Is 16 a respectable aperture for viewing DSO with a dob? Is there that much difference at the eyepiece when compared to 18-20?
Aperture is NOT always king (non sexist also Queen)

It is pointless having a massive scope that never gets used when a smaller one will be, I would rather spend more on a good usable scope than less on a massive one I will never use
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Old 10-01-2020, 11:09 AM
Tropo-Bob (Bob)
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Previously, I had access to a 16" scope in a roll off observatory near Koah. It was magnificant. The views were better than my 12.5", which I also considered to be great. However, the 16" was a beast; I would hate to try and move it.
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  #5  
Old 10-01-2020, 02:41 PM
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ngcles
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Hi Benn,

The simple answer is to square the radius of the apertures. 64, 81 and 100. Assuming they are all used at similar magnification, in the same conditions have similar quality optics and similar contrast elements.

A 16" 'scope will produce 64% of the light at the focal plane compared to the 20".

An 18" will produce 81% of the light at the focal plane compared to the 20".

The practical effect in my experience is a little different. For many years I regularly observed with two friends and the three 'scopes were: 16", 18" and 20". The consensus among the three of us was that going from 16" to 18" didn't make an incredible difference. Neither did going from 18" to 20".

But, going from the 16" to the 20" was. That said all three are good aperture choices and provide lots of bang for your buck.

The best piece of advice I ever heard about choosing a telescope is this: The best telescope for you is the one you will use the most often.

When you do feel like a bit of astronomising, there's little point in owning a telescope that says: "Oh it's just too much bother".

The one you want, when you feel like astronomising is the 'scope that says "Pick me up, take me outside, the reward exceeds the effort expended."

You pick the one you believe you will use the most.

Best,

L.
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  #6  
Old 10-01-2020, 05:43 PM
gary
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Quote:
Originally Posted by Hijynx View Post
I have what I think is a simple question.

Yes aperature is king etc etc. and yes bigger is better when it comes to visual astronomy but one must consider cost, weight, transport, storage and eyepiece height at zenith. So .......

Is 16” a respectable aperature for viewing DSO with a dob? Is there that much difference at the eyepiece when compared to 18-20”?
Hi Benn,

As a further point in your deliberations, you need to ask yourself,
which specific embodiment of a 16" and which specific embodiment
of an 18" or 20"?

The reason I raise it is that they are not all born equal.

Others have already addressed the importance of aperture.

The quality of the figure of the mirror is obviously very, very important
as well.

Being up in Cairns you have readily accessible dark skies so you
are luckier than most. So you have the opportunity to push the optics
more than most.

But what I wanted to raise was the mounts themselves. Now and
then I've had to transport various new commercial mounts back
to the office to evaluate them as part of what we do.

I have to admit that I've even seen some 12" scope designs that
have been so bulky and awkward to transport in a car compared
to some other scopes with much larger apertures that looking
back in the rear view mirror I've asked myself, "what were the designers
thinking"

Related to this, another point I wanted to raise is the "feel" of the motion
of the scope as you slew it or nudge it or even re-focus it.

Those premium Dobs don't look the way they do just to look good.
The centre of gravity, the moments of inertia, the matching of the
amount of stiction and friction in their bearings have been calculated
and purposely engineered. It makes them feel smooth when you nudge
them.

This may seem a minor point but when you are looking at some faint
object at the limits of what you can discern, the last thing you want is
for the scope to hesitate when you nudge it, or overshoot the target
or have some undampened oscillation that you have to wait for to stop
before you start looking again. You are not seeing just with your eye,
but with your brain.

Until you experience it for the first time, it is hard to explain. But most
experience it for the first time when they are given the opportunity to
use someone else's premium Dob typically at a club new moon weekend
or at a star party.

It's a bit like when someone gives you the keys to drive their premium
sports car. At first you wondered why they paid so much for the damn
thing in that all cars are designed to get you from A to B but once
you start to really drive it and feel its poise and certainty, a cheaper
commodity car starts to feel like a shopping trolley by comparison.

You even overhear it in the dark when someone might say whilst at the
eyepiece of someone else's scope, "God, I love this focuser".

Having said that, many of the commodity scopes provide fabulous
bang for your buck.

Good luck in your deliberations. It's a good problem to have.

Best Regards

Gary Kopff
Mount Kuring-Gai NSW 2080
Australia
Phone +61 2 9457 9049

Last edited by gary; 10-01-2020 at 05:57 PM.
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  #7  
Old 10-01-2020, 08:17 PM
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skysurfer
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I have a 16" but live in Bortle 6 Dutch suburban skies, so use it less often than my 80mm tabletop telescope which is very versatile.
And I have a 110mm ED telescope mainly for AP.
I use that scope for travel as well, and despite the much smaller aperture, it performs well (even visually) under dark skies when I make an Australia or South Africa trip.
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Old 10-01-2020, 09:28 PM
Ukastronomer (Jeremy)
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Quote:
Originally Posted by skysurfer View Post
I have a 16" but live in Bortle 6 Dutch suburban skies, so use it less often than my 80mm tabletop telescope which is very versatile.
And I have a 110mm ED telescope mainly for AP.
I use that scope for travel as well, and despite the much smaller aperture, it performs well (even visually) under dark skies when I make an Australia or South Africa trip.
I have a 120 triplet but again I am up and using the 72mm before I could even set up the AltAz mount for the 120.

I agree that small is useful
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Old 12-01-2020, 11:37 AM
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ngcles
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Hi Benn, Gary & All,

Quote:
Originally Posted by gary View Post

... The reason I raise it is that they are not all born equal.

Others have already addressed the importance of aperture.

The quality of the figure of the mirror is obviously very, very important
as well ...


... But what I wanted to raise was the mounts themselves. Now and
then I've had to transport various new commercial mounts back
to the office to evaluate them as part of what we do.

I have to admit that I've even seen some 12" scope designs that
have been so bulky and awkward to transport in a car compared
to some other scopes with much larger apertures that looking
back in the rear view mirror I've asked myself, "what were the designers
thinking"

Related to this, another point I wanted to raise is the "feel" of the motion
of the scope as you slew it or nudge it or even re-focus it.

Those premium Dobs don't look the way they do just to look good.
The centre of gravity, the moments of inertia, the matching of the
amount of stiction and friction in their bearings have been calculated
and purposely engineered. It makes them feel smooth when you nudge
them.

This may seem a minor point but when you are looking at some faint
object at the limits of what you can discern, the last thing you want is
for the scope to hesitate when you nudge it, or overshoot the target
or have some undampened oscillation that you have to wait for to stop
before you start looking again. You are not seeing just with your eye,
but with your brain.

Until you experience it for the first time, it is hard to explain. But most
experience it for the first time when they are given the opportunity to
use someone else's premium Dob typically at a club new moon weekend
or at a star party ...


You even overhear it in the dark when someone might say whilst at the
eyepiece of someone else's scope, "God, I love this focuser".

Amen & Amen to that!

Best,

L.
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  #10  
Old 12-01-2020, 12:38 PM
Rainmaker (Matt)
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Quote:
Originally Posted by Hijynx View Post
I have what I think is a simple question.

Yes aperture is king etc etc. and yes bigger is better when it comes to visual astronomy but one must consider cost, weight, transport, storage and eyepiece height at zenith. So .......

Is 16 a respectable aperature for viewing DSO with a dob? Is there that much difference at the eyepiece when compared to 18-20?
16" is a great aperture for DSOs. The next logical step up would be 20"
Once you go past 16" then 2" steps are not that noticeable.
I would rather a excellent 16" than an ok 18 or 20.

There can be a huge variation on the actual bulk of scopes of a similar aperture. A well designed 18 or 20 can be more compact than many commercial 16s.

I would vary the saying to "Good quality aperture is King"
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Old 19-01-2020, 10:02 PM
jjjjohn9 (John)
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To your list of considerations I would also add ease of alignment for those of us struggling in backyards without fixed, perfectly aligned mounts.
I am now moving from a 120mm/ f8 achromatic refractor because of the time it takes to set up not quite properly on any night. I'm thinking a 150mm SCT with built in Goto drive/batteries etc that I can plonk on a solid stand should be an ideal solution to my issues.
Does that make sense?
John
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Old 19-01-2020, 11:03 PM
Bobbyoutback
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Quote:
Originally Posted by skysurfer View Post
I have a 16" but live in Bortle 6 Dutch suburban skies, so use it less often than my 80mm tabletop telescope which is very versatile.
And I have a 110mm ED telescope mainly for AP.
I use that scope for travel as well, and despite the much smaller aperture, it performs well (even visually) under dark skies when I make an Australia or South Africa trip.
Well said skysurfer ,
No point in having big aperture scopes were they cant perform !

My biggest scope is only a 12 " , it comes out when conditions are right .
And that's when the enjoyment starts

Bobby.
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  #13  
Old 20-01-2020, 10:37 PM
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OneCosmos (Chris)
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There is another factor to consider too -and it is a big one.

Even if you had all the money in the world and commissioned a top shelf 50 mirror and combined it with superlative precision build quality scope design and housed it ready to use in a huge comfortable observatory under perfect dark skies with excellent seeing, you still may, just may, prefer the views through the humble 16!

The reason is field of view. A 50 scope even at F3.3 which many fast mirrors are these days, would have a focal length of 4125mm (and a 50 f3.3 would hard to make and unforgiving of poor seeing and unforgiving on the wallet too). At this focal length even the mighty 21mm Ethos would produce 196X. That is starting to be decent for planets but on the whole definitely on the high side for many nebulae. Personally I find about 120-140x about right and often lower. Almost every other eyepiece no matter what the apparent fov would leave you wishing you could just have a wider view. It is satisfying to see the whole of ETA Carina not just the central part.

With very long focal lengths you have to be content with only ever seeing one bit of large nebulae - brightly for sure, but not with very wide fields.
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