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16 inch Telescope


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54 minutes ago, Chris Lock said:

Amen to that Shane, this is one of the bigger off topic tangents I've seen guys, lets agree to disagree and help the OP :)

 

I did try Chris . But Tiny obviously knows best. So Tiny can go and have a look in Scope, set/discussion about Aperture and telescope   as he knows the true meaning of aperture when it comes to a telescope (reflector in this instance)

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26 minutes ago, Timebandit said:

 

I did try Chris . But Tiny obviously knows best. So Tiny can go and have a look in telescope/discussion about aperture and Telescope as he knows the true meaning of aperture when it comes to a telescope (reflector in this instance)

I'm not saying that there isn't a difference... of course there is as the eye can only see so much and extra photons are extra photons. If using low power eyepieces in a big scope, you are going to see dim objects that you wouldn't see in the smaller scope as to get those to a decent brightness in a smaller scope, you would have to use such a low power eyepiece that in magnification terms, they would be too small to see. But that is our eyes not being as efficient as they could be and not because the scope can't see it. With the maths that I posted above, it is assumed that the reader would infer the implications of this in the real world.

You can either choose to believe the maths or not... but it is correct.

Edit: One minor correction: There will be a time when a smaller scope can't see what a bigger scope can. In the case of a 10" scope and the 130 PDS, the 130PDS will see nothing when the 10" scope collects less than the number of photons per second that ties with number of the ratio between the two scopes... so in the example I posted, when the 10" scope collects less than 3.698 photons per second.

Edit 2: Just seen the multitude of replies that took place whilst I was posting... Never mind.

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This thread has spun off in some interesting directions. To recap, the OP has had a few scopes and is thinking of jumping from a 4" to a 16". Various opinions on that, but it seems pretty much decided.

About aperture in general, we need to distinguish between imaging and visual. For visual work a scope's f-ratio is largely irrelevant. The only significance is tube length (portability) and potential aberration (coma). Image brightness depends only on aperture. With CCD it can be a different story, but I don't think the OP's intention was to use a 16" dob for that.

As to eyepieces, if you're looking at tiny faint fuzzies with averted vision then a bit of coma is not going to bother you. And if you're not looking at faint fuzzies with averted vision then why bother getting a 16"?

BTW I used "brightness" loosely: we must of course distinguish illuminance (apparent magnitude) from luminance (surface brightness). But aperture is what really counts, as every dedicated DSO observer knows, along with a sky dark enough for serious observation.

Now on with the debate, I guess...

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4 hours ago, Tiny Small said:

Not to resurrect the dead, but this is the important distinction. Whether people believe me or not, it is a fact that f5 is f5, no matter what scope or camera its on. Whichever scope you have, f5 will collect light at the same rate per unit area! A 16" scope has a lot more area so physically catches more photons but only at the same rate as any other scope of the same aperture. 

 

 

I think you are getting confused about what people are disagreeing with.

The disagreement (as far as I'm concerned) was with the actual term "aperture" It does apply to the prime collector when talking about a telescope. Photographers use the term to describe f stops or 'apertures' set by an adjustable set of blades inside the lens. As I said before a telescope does not have these blades so the focal stop is the edge of the primary collector.

I know an f/5 system exposes the same regardless of the primaries diameter, that isn't what I was disagreeing about. You said earlier in the thread that the aperture of a telescope is not the diameter of its primary and this is incorrect.  

An aperture can be measured in any way you want. Photographers choose to refer to the aperture as the focal ratio created by their aperture ring or f stop inside their lenses. Astronomers choose to refer to the aperture as a measurement of the diameter of the primary collector which is their f stop. That is why I said using the togs term on an astro forum is a bit confusing. You disagreed stating it is simple because the focal ratio is the aperture of a scope and has nothing to do with the diameter of the prime collector. This is incorrect. The aperture of a telescope IS the diameter of the prime collector as the edge of the prime collector is the focal stop. You have been trying to apply the photographic term aperture to a telescope. A telescope is not a camera lens despite the fact it can be adapted to be used as one. Therefore a telescopes aperture will always be its primaries diameter expressed as a measurement not its focal ratio or f stop value. 
 

I'm not disagreeing about surface brightness of extended objects or the photographic speed of a system. I hope that helps clear this up :) 

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4 hours ago, Paul73 said:

The point that I was making, was that you could spend £50 to get quality views. And, I'll stick with the assumption that anyone willing to spend £1,800 on a nice new 16" Dob, wouldn't be averse spending an additional £100 for a couple of eyepieces, even if the purchase may have to wait a month or two due to the scope purchase clearing out the Astro coffers. I've been there.

A lot of people decide that the additional expenditure to buy the wide angle exotics just isn't worth it. We each have our own budgets & priorities. I choose not to buy 100° for anything below 20mm and stick to 50° for the really hi mags. I'd like to buy the whole Ethos range, but I choose feeding the family over my hobby.

Paul

There are other reasons not to go straight for very wide angle EPs.  The ES 20/100 weighs (OK - has a mass of) 968g, the 25/100 is 1177g, and the 30/100 a whopping 2353g!  I briefly had an EP at 580g which I found somewhat bulky!  Imagine one at 2-and-a-third kilograms!  (Granted, it is a 3" fitting.)

Doug.

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Hi,

Putting the discussions about aperture to one side, I went from a 6inch home made dob to a 16inch meade lightbridge and found the difference to be staggering , in many ways. I had been observing for years though and new that visual observing of DSOs was what fascinated me.

With regard to the size of the scope they are very large and can only be moved about in two pieces (base and tube) which means storage can be an issue and setting up can be more of a chore. For me it takes about 15mins to get set up and collimated with maybe another 5 mins if I connect dew control etc. Contrary to popular belief the bases do fit through doorways if you tilt them side on (I often describe this as being similar to how you would get an armchar through a door). Since buying I have made a smaller base which is lighter and Ive fitted some DIY digital encoders.

With regard to the views they are impressive and begin to show detail taht cant be seen in smaller scopes. At this aperture objects become more obvious and extensive and the DSOs become much more rewarding to observe. I remeber showing some members of our club the supernova in M82 a few years back with a full moon in the sky and I did some outreach once and showed some members of the public the veil neb from a village green ..... none of these would be that easy with a small scope.

Eyepiece wise, you can get away with cheaper eyepieces but with premium glass you do get a better and sharper view but some of the ES82 eyepieces would be a good choice.

At present prices are high though with a 16inch lightbridge being just short of £2400 which is more than a 16inch premium mirror from Nichol optical , so if you are handy and feel up to it you could make a 16inch scope with top class optics for similar money. Second hand is also an option though which would mean considerable savings

If you have experience and know how to star hop, and are sure that a big dob is for you then you wont be disappointed. They arnt a grab and go option but can be hughly rewarding  ...... but take one out to a dark site with a clear sky and you soon realise that for visual , in those conditions, aperture is king.

 

Cheers

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2 minutes ago, astronymonkey said:

Hi,

Putting the discussions about aperture to one side, I went from a 6inch home made dob to a 16inch meade lightbridge and found the difference to be staggering , in many ways. I had been observing for years though and new that visual observing of DSOs was what fascinated me.

With regard to the size of the scope they are very large and can only be moved about in two pieces (base and tube) which means storage can be an issue and setting up can be more of a chore. For me it takes about 15mins to get set up and collimated with maybe another 5 mins if I connect dew control etc. Contrary to popular belief the bases do fit through doorways if you tilt them side on (I often describe this as being similar to how you would get an armchar through a door). Since buying I have made a smaller base which is lighter and Ive fitted some DIY digital encoders.

With regard to the views they are impressive and begin to show detail taht cant be seen in smaller scopes. At this aperture objects become more obvious and extensive and the DSOs become much more rewarding to observe. I remeber showing some members of our club the supernova in M82 a few years back with a full moon in the sky and I did some outreach once and showed some members of the public the veil neb from a village green ..... none of these would be that easy with a small scope.

Eyepiece wise, you can get away with cheaper eyepieces but with premium glass you do get a better and sharper view but some of the ES82 eyepieces would be a good choice.

At present prices are high though with a 16inch lightbridge being just short of £2400 which is more than a 16inch premium mirror from Nichol optical , so if you are handy and feel up to it you could make a 16inch scope with top class optics for similar money. Second hand is also an option though which would mean considerable savings

If you have experience and know how to star hop, and are sure that a big dob is for you then you wont be disappointed. They arnt a grab and go option but can be hughly rewarding  ...... but take one out to a dark site with a clear sky and you soon realise that for visual , in those conditions, aperture is king.

 

Cheers

 

Hello. So going from a 6" to a 16" was certainly worth it then . Obviously the 10" aperture gain difference did make a substantial gain on you viewing experience

My experience of aperture is therefore mirrored(☺) by this. And going up in aperture certainly did have a considerable improvement on visual targets on a like for like basis.  

I am not sure if the OP is still interested in going large? . But for those who have experienced or own a large aperture scope then there is an improvement on views over smaller aperture on a like for like basis . Aperture Rules ?

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15 minutes ago, astronymonkey said:

Hi, yes the difference is relative to the change in mirror area. For me a 6inch to 16inch is going from 28sq inches to 201sq inches , so over a 7 times increase which is quite a jump so easily seen.

cheers

 

Hi yes . When you start going up in aperture size and therefore the mirror area starts to increase and therefore the light gathering ability. If I remember correctly then a 14" aperture scope has 40% more light gathering ability that a 12" . And a 16" aperture scope has just over 34 % more light gathering ability than the 14" aperture one. Therefore you can clearly see(?) why going up in aperture gives you an advantage on a like for like basis and that going up in aperture does give you a visual image difference at the eyepiece on a like for like basis 

The Dob mob do have their Monsters for a reason and not just for show. Because their cannons do have superior performance because of large aperture rule. And when you have professional observatory with mirrors ranging from I understand 5meter to over 8 meter then they obviously wish to see visually DSO as far and as well than is possible and the amount of money they are prepared to spend on such super scopes obviously get results. 

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The difference larger aperture makes on deep sky objects is really noticable although I'd caveat that by saying that a 4" aperture increase is much more noticable  than a 2" one on such targets.

The difference on planets and double stars is less dramatic in my experience. My 5" refractors produce views of planets that compare remarkably well with those than my 12" dob puts up, despite the rather large aperture difference between these scopes. The dob is a good one as well with high quality optics in it and is well collimated.

When the seeing conditions are really good the larger aperture pulls more noticably ahead on these solar system targets as well though although it is never massively ahead of the refractors - the difference is usually that more subtle details can be seen more easily with the larger aperture, conditions allowing.

As I went through scopes during my time in the hobby I reckon the step from 4" to 8" was probably about the most marked that I have made. At  8" deep sky objects start to get a bit interesting, especially under a dark sky and an 8" scope can produce fine lunar and planetary views as well.

Having said that, if the original poster has the budget for a 16" scope and can store and use it then by all means they should go for it. At least this thread will have provided some excellent and balanced views on the potential pros and cons of such scopes from those who own and use them :icon_biggrin:

 

 

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The magnitude gain in going from aperture A to aperture B is 5log(B/A); for example if you go from an 8" to a 12" you gain 5log(12/8) = 0.88 mag. If you could see stars down to mag 12 with the 8" (at a given site under given conditions) then you would expect to see down to 12.88 with the 12" (all other things being equal).

To get a gain of N magnitudes you need to increase aperture by a factor 10^(0.2N). For example, if you want a scope that will reach one magnitude fainter you need to multiply your aperture by 1.585, i.e. increase by 58.5%.

All of this says nothing about whether you'll get better views of Moon and planets (which depends on resolution, contrast, atmosphere....) or galaxies (which depends on how dark your sky is to begin with, i.e. naked-eye limiting magnitude). It just says that if all other things are equal then larger aperture shows fainter stars, whether you're at a dark site or a light polluted one.

The surface brightness limit of a telescope is whatever value it takes at lowest useable power (when exit pupil diameter equals eye pupil diameter). If the telescope has perfect transmittance (no light loss at all) it is then equal to the actual surface brightness of the target. The larger aperture scope has a higher minimum magnification than the smaller, so shows the target at a larger size at maximum possible surface brightness, so you get a better view.

As you increase magnification you lower the surface brightness limit and raise the magnitude limit. You also change the target size (if it's an extended object) and the background surface brightness. The target's contrast against the background is unchanged, but if you make it large enough you can see it, and if you make it too large then it disappears, being too faint in relation to the background. The details are a bit complicated but can be calculated.

The short version is "aperture and sky darkness rule". A 16" beats a 4" if they're both in the same place. A 4" at a dark site beats a 16" at a light polluted one.

 

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.... and continuing to multiple your aperture by 58% gets very costly and hard to transport very quickly. 20" seems like a fairly (sane) sweet spot for amateurs, though 30" are not unheard of.

 

cheers

peter

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Indeed, which is why I'd suggest a sensible upgrade path to be progressive increases of (about) 50%, e.g. 8" - 12" - 18" or 6" - 10" - 16" - 24". Going up by 2" steps obviously brings diminishing returns: 4" to 6" gives an extra 0.88 mag, 8" to 10" gives 0.48 mag, 12" to 14" gives 0.33 mag, 16" to 18" gives only an extra 0.26 mag. The judgement call is whether the extra light is worth the extra weight. If scopes were all of the same configuration and density (which they aren't) then a 50% aperture increase would mean roughly tripling the weight (1.5^3=3.375). In practice it's generally more (elephants need proportionally thicker legs than mice and a 20" mirror needs to be proportionally thicker than a 2"). A 12" at a dark site can show anything in the NGC. Once I run out of NGCs I might think of upgrading my 12" to an 18". Though I guess by then I'd be too old to move it...

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1 hour ago, acey said:

Indeed, which is why I'd suggest a sensible upgrade path to be progressive increases of (about) 50%, e.g. 8" - 12" - 18" or 6" - 10" - 16" - 24". Going up by 2" steps obviously brings diminishing returns: 4" to 6" gives an extra 0.88 mag, 8" to 10" gives 0.48 mag, 12" to 14" gives 0.33 mag, 16" to 18" gives only an extra 0.26 mag. The judgement call is whether the extra light is worth the extra weight. If scopes were all of the same configuration and density (which they aren't) then a 50% aperture increase would mean roughly tripling the weight (1.5^3=3.375). In practice it's generally more (elephants need proportionally thicker legs than mice and a 20" mirror needs to be proportionally thicker than a 2"). A 12" at a dark site can show anything in the NGC. Once I run out of NGCs I might think of upgrading my 12" to an 18". Though I guess by then I'd be too old to move it...

Depends on the reason for the upgrade IMO Acey. I went from 16" to 20" but not because I really wanted more from the scope, after all a 16" provides incredible viewing opportunities and enough targets for a lifetime. It was simply because I was feeling frustrated with owning and using a mass produced commercially made scope. I wanted to custom build my own. It seemed a bit pointless rebuilding the 16" so opted for a slightly larger size for my project.:) 

I see nothing wrong with upgrading from 10" to say 12" if one is going from a mass produced product to a beautifully crafted hand made one. Or perhaps if one has a 10" and then subsequently realises that they can in fact handle a 12". 
 

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I guess it works both ways. You could go from a 10" mass produced with bog standard optics to a 12" with premium optics ? Although again I guess aperture is aperture and if the 12" isn't going to show a significant improvement in light grasp and how deep you can go then I can understand acey's logic.

Personally I passed up on a 1/10 wave 14" OO at a bargain price which many considered lunacy. No doubting I would have loved to own such an instrument and would have drooled at the views. The reality is though I have enough astro gear cluttering the halls on those seemingly endless cloudy nights with out taking up more space with a giant dob. I lack the motivation half the time to go out with my larger scopes as no sooner I set up clouds come over, so more often than not I use my ST120 on AZ4 which is out and showing me the night sky in the time it takes to walk in to the garden. Such a large dob would have created no end of transportation issues even for a mid sized car meaning any dark sky visit would have been a solo one due to flop down seat configurations. I admire those who have the motivation and enthusiasm to visit dark skies as I agree the views are amazing even in smaller scopes but I have more than once had perfectly clear skies where I am and drove 40 miles only to see wall to wall cloud. Needless to say it is a little disheartening and the MET office is no use. Going off the feedback in this thread so far I made the right choice as a drive 40 miles to escape light pollution to get the best from the aperture would have be rare.

I know the OP is set on a 16" and seems to have an idea of the dimensions of such a scope but I really don't think they will enjoy going from a 6" slow scope to 16" fast scope but that's just MHO. I'm not sure how many of the dob mob have done similar jumps in aperture and done so without having some reasonably good eyepieces to start with?

 

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Personally my recommendation would always be try a 12" scope first. A 16" is a huge increase in bulk from a 12" - see below. My 16" and 12" (both f4).

I think the 12" f4 is a wonderfully usable and manageable scope for all.

The 16" is rarely used (last time was March 2015) and I will be creating a truss / minimalist version using these optics to encourage me to use it more. The downside will be slower set up times. I use the 12" and my 120ED as my main scopes almost all the time and from my LP garden it's probably adequate.

DSCF0255.JPG

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1 hour ago, Moonshane said:

Personally my recommendation would always be try a 12" scope first. A 16" is a huge increase in bulk from a 12" - see below. My 16" and 12" (both f4).

I think the 12" f4 is a wonderfully usable and manageable scope for all.

The 16" is rarely used (last time was March 2015) and I will be creating a truss / minimalist version using these optics to encourage me to use it more. The downside will be slower set up times. I use the 12" and my 120ED as my main scopes almost all the time and from my LP garden it's probably adequate.

DSCF0255.JPG

Are the bolts on the 12" just drilled and tapped straight into the tube rings? My 12" is the same one as that and I've been thinking about making a dob mount for it. I usually use the 10" as I drive to a local dark site if I'm observing... but having the 12" handy at home, without having to set up an EQ mount has been tempting me for a while.

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@Moonshane

Really nice Dobson bases, they look great! some day I am going to build one for my tube.

On 2016-10-14 at 10:20, Tiny Small said:

Are the bolts on the 12" just drilled and tapped straight into the tube rings? My 12" is the same one as that and I've been thinking about making a dob mount for it. I usually use the 10" as I drive to a local dark site if I'm observing... but having the 12" handy at home, without having to set up an EQ mount has been tempting me for a while.

I almost had the same thoughts heheh.

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