What is the biggest sensible aperture for the UK?

[Dark Matter]

....as the old saying goes.. 'buy your last telescope (or mount) first'

The general rule of thumb is, if you live under skies of Mv3.0 or less, no larger than 8"  If you have skies between Mv4.0 and Mv5.5 or so, 10" - 12" aperture. But if you have skies to Mv6.5 or better! 12" and above. 

Ever looked through a 32" under Mv2.5 skies? I did. Nice scope, but the images were absolutely washed out to buggery. Looking at Omega Centauri  it was a bright blob, no stars really, just all this haze of light.

This young bloke was all show and no punch. He had no idea what its potential was until he was 'persuaded' to take it out into the bush. Last we heard he hadn't returned and that was a year ago!

Probably got caught up in aperture fever and didn't want to return home, thinking it may spread and become pandemic.

Once this fever reaches you, it won't let go nor go away. It absorbs into the brain and you become transfixed. You look at your telescope and say.. "this scope here, will be my finderscope on my next scope...".

Aperture Fever is contagious . Once it bites you, your brain is re-arranged and you become transfixed on faint objects. AF causes severe optic nerve damage too, as it re-arranges itself to adjust to prolong viewing of extremely bright objects between 8th and 12th mag. Aperture fever also causes sleep deprivation, which can put you into a zombie state. This leads to voices in your head saying... go bigger, go bigger

!

[nicks90]

one thing to note with really big scopes - and something not often take in to consideration - is minimum effective magnification.

if you have a big F4 dob (18"+), your maximum exit pupil is going to pretty much restrict you to a 24mm eye piece. Therefore you are starting to approach the problem of your starting magnification being near x100. Plus being an F4 you need some proper good eyepieces. Alternatively getting an F5 dob means its going to be very very long and a very long focal length. so although you may be able to use a 32mm, the longer focal length still means your starting mag will be knocking on x100 again. Not great for big widefield objects.

my 12" dob with maximum comfortable exit pupil (32mm ep) gives a mag of x46. And remember when you double the magnification you are effectively halving the brightness. So its all well and good saying a bigger dob will give a brighter image, but its only brighter at a simillar magnification! Plus having a starting mag of x100 is going to restrict your enjoyment of the more open objects you can view. Say goodbye to M81 and M82 in the same field of view, you'll only ever see a small section of M31, M45 will be missing several sisters etc etc etc

However if you are prepared to accept a more limited fov and regularly work at x100 and above - then yes it will blow the socks off a 12" dob at simillar magnification.

But working at x100 and above with such a big aperture means you are more at mercy from atmospheric conditions - which others have already alluded to.

edited to add: i;d have a 22" dob in a heartbeat...... if i could afford one + lived somewhere dark.

[estwing]

I start at 9x mag with the finder...then 97x with a 21mm ethos,does kinda smack you in the face! but also most nights only two ep's are used...

[osbourne one-nil]

I'm starting to think that telescopes are actually just a pain in the....

I just want a lightweight, large aperture, affordable telescope with a low f-ratio that provides wide field views and high-powered magnification using mid-range eyepieces, which isn't prone to dew...and is blue.

[AlexB67]

Perhaps my first observing winter with the weather is clouding my judgement, and a big scope would be nice and at some point in the future, as long as I can manage the weight and transport,  I would never go without a 10 - 12  inch as the sort of largest size I consider grab and go. Even in the last week or two my 5 inch has been a saviour, quick cooling, a great little cloud dodger for a short session that the 10 inch would not have managed. 

I can well imagine if I owned a 16 inch it would not nearly get used, with the current winter that may well have been months at this stage.   So .. a bigger scope is nice, but not a substitute for my 10 inch. I would consider jumping to that 12 inch  at some stage perhaps just to get that little extra and still have the benefits of the same grab and go nature .. but that would be the upper limit for my grab and go I use often enough I feel. 

[nicks90]

i think your 10" flextube is the perfect grab n go - as i find my 12" flextube a bit of a heavy old hector to cart about and it fits in my audi estate quite 'snugly'...

if it wasnt for the fact i got my 12" second hand, i wouldnt have bought one that big, I would have gone for the 10". Dont get me wrong, I aint complaining really - its a stunning scope thats genuinely a 'scope for life'.

[AlexB67]

i think your 10" flextube is the perfect grab n go - as i find my 12" flextube a bit of a heavy old hector to cart about and it fits in my audi estate quite 'snugly'...

if it wasnt for the fact i got my 12" second hand, i wouldnt have bought one that big, I would have gone for the 10". Dont get me wrong, I aint complaining really - its a stunning scope thats genuinely a 'scope for life'.

You could not have put it better nicks, it is why I ended up with it when all was said and done and I made my final decision  , it was all down to the best comfy fit for my car mainly with room to spare, though the 12 inch flexi would have fitted too, the base itself quite a bit bigger.  I do fear the 12 inch flexi may be a tad heavy for comfort, though manageable for around the garden. 

If I were to get a 12 inch, it will only be when I end up with a different car in few years time I think. Now that I am into  astro, buying a car will be more carefully considered next time.   I am not sure I would buy the flextube 12 inch myself, perhaps, but  since there is absolutely no rush I feel I want such an upgrade, since I lurvvve the 250 flexi.   A scope in that aperture would be a scope for live for me ( bigger scopes aside if that should ever come in the future).  With that in mind it is likely I'd be looking at an orion optics one, which would be lighter or about the same weight as my current flexi.

[Moonshane]

an OOUK 12" f4 on a OOUK dob base is a lot smaller and lighter overall than your current 10" Skywatcher.

[AlexB67]

an OOUK 12" f4 on a OOUK dob base is a lot smaller and lighter overall than your current 10" Skywatcher.

That makes it even more attractive   On the other hand it will also costs 4 times as much new mind you

[chiltonstar]

an OOUK 12" f4 on a OOUK dob base is a lot smaller and lighter overall than your current 10" Skywatcher.

How much do the tube and base weigh (getting a bit interested...)? 

How about optical figure....I assume the SW is probably 1/4 wave, while the OOUK is better according to price?

Chris

[John]

My 12" F/5.3 OO tubed dob on Shanes version of an OO dob mount weighs around the same, or just a little less than the Skywatcher 250P dob I think. The lowest quality optics that OO do now are 1/6th wave PV I believe. Some GSO's and Skywatchers may well be optically that good though, some however, may not.

[laser_jock99]

Big aperture, fast F ratio and dark skies is the combination to aim for. Anything else is surely a compromise?

[chiltonstar]

My 12" F/5.3 OO tubed dob on Shanes version of an OO dob mount weighs around the same, or just a little less than the Skywatcher 250P dob I think. The lowest quality optics that OO do now are 1/6th wave PV I believe. Some GSO's and Skywatchers may well be optically that good though, some however, may not.

It would be interesting to know how much a 1/6 wave or lower mirror increases the size of the Airy disk, compared with a better figured mirror...?

[jnb]

Most of the comments here are right but not specific to the UK, large apertures are not useful with low magnifications visuallybecause of the large exit pupil but that's equally true of the UK or the Atacama desert. Equally a huge newt is an awkward lump to move regardless of where you are on the planet.

Considering visual resolution in the UK then there probably isn't much point above a 6" scope. There are good reasons to have a larger scope but they are not UK specific. So I'd say for visual resolution 6", for image brightness perhaps 12 - 16" but for AP and spectroscopy there's no upper limit (other than practical limits)

[Mike73]

Most of the comments here are right but not specific to the UK, large apertures are not useful with low magnifications visuallybecause of the large exit pupil but that's equally true of the UK or the Atacama desert. Equally a huge newt is an awkward lump to move regardless of where you are on the planet.

Considering visual resolution in the UK then there probably isn't much point above a 6" scope. There are good reasons to have a larger scope but they are not UK specific. So I'd say for visual resolution 6", for image brightness perhaps 12 - 16" but for AP and spectroscopy there's no upper limit (other than practical limits)

Wow! Theres not much I agree with there but hey were all welcome to our own opinions.

[jnb]

Most of the comments here are right but not specific to the UK, large apertures are not useful with low magnifications visuallybecause of the large exit pupil but that's equally true of the UK or the Atacama desert. Equally a huge newt is an awkward lump to move regardless of where you are on the planet.

Considering visual resolution in the UK then there probably isn't much point above a 6" scope. There are good reasons to have a larger scope but they are not UK specific. So I'd say for visual resolution 6", for image brightness perhaps 12 - 16" but for AP and spectroscopy there's no upper limit (other than practical limits)

Wow! Theres not much I agree with there but hey were all welcome to our own opinions.

My rationale is that seeing in the UK is rarely going to be better than 1" so that's a 6" scope. A 16" scope needs a magnification of at least 67x to bring the exit pupil down to something that you can take full use of which makes the scope prohibitive to use at a range of visual magnifications. As soon as you are into AP and spectroscopy then sheer numbers of photons are what matters.

[John]

My rationale is that seeing in the UK is rarely going to be better than 1" so that's a 6" scope. A 16" scope needs a magnification of at least 67x to bring the exit pupil down to something that you can take full use of which makes the scope prohibitive to use at a range of visual magnifications. As soon as you are into AP and spectroscopy then sheer numbers of photons are what matters.

My 12" dob has shown features on the Moon less than an arc second in angular size and split binary stars with a sub-arc second separation. The seeing conditions need to be quite good to allow this but it's happened on a few occasions over the past year that I've had the scope and I've appreciated having access to that sort of performance when things do come good.

Everyone has to their own mind up about whats best for them though 

[scarp15]

Yet if your skies are dark enough, exit pupil becomes less of an issue. I currently use a low power magnification of 62X, with an exit pupil of 5.6mm - 14" scope and this yields superb contrast. I have used low power at 44X with the same scope and dark sky conditions, however the exit pupil had become a little too large, 7.8mm where I had lost some contrast. This particular eyepiece however had worked very well with my 8" scope, again at a dark location.  

[Mike73]

My rationale is that seeing in the UK is rarely going to be better than 1" so that's a 6" scope. A 16" scope needs a magnification of at least 67x to bring the exit pupil down to something that you can take full use of which makes the scope prohibitive to use at a range of visual magnifications. As soon as you are into AP and spectroscopy then sheer numbers of photons are what matters.

Ok well most people who buy large aperture scopes do so to view DSO's where seeing really isn't a problem, its all about sky transparency. I have never packed up early from DSO's because of bad seeing.

Your right in a way that you do lose TFoV in a large scope because of exit pupil but this is why most people with 14"+ scopes will have at least one other scope to give them much better wide field views.

There isn't really a 'one scope does all' in astronomy, its all about trade offs and how you manage them according on your own personal tastes.

My advice to you is if you ever get a chance to look through an EP of a big scope somewhere dark then jump at it because I think you'll change your mind.

[jnb]

Ok well most people who buy large aperture scopes do so to view DSO's where seeing really isn't a problem, its all about sky transparency. I have never packed up early from DSO's because of bad seeing.

Your right in a way that you do lose TFoV in a large scope because of exit pupil but this is why most people with 14"+ scopes will have at least one other scope to give them much better wide field views.

There isn't really a 'one scope does all' in astronomy, its all about trade offs and how you manage them according on your own personal tastes.

My advice to you is if you ever get a chance to look through an EP of a big scope somewhere dark then jump at it because I think you'll change your mind.

Largest I've used is a 26". By the time I reach that size I want to do something other than visual work which is where I said there is no upper limit. Visually I found tge seeing could never do the 26" justice

[Moonshane]

but assuming it was a 26" f4, a 26mm nagler would give 100x and a .82 degree field

[AlexB67]

My advice to you is if you ever get a chance to look through an EP of a big scope somewhere dark then jump at it because I think you'll change your mind.

I've always been in two minds about that because I have no doubt it would blow my mind, but I fear  somehow that DSOs will not look the same again. There again, we only live once and the chance should not be refused at some point when it arises. I always feel like if I am going to look through something I am doing so because I am getting close to the point where I am ready to buy, so it is in reach. 

For me to look through a 20 inch Dob right now would be like dangling a bit of bait on a stick for my cat, she jumps for all her worth, but she can never quite reach it = cruel :0)

[swamp thing]

When I set up my 10" and 20" alongside one another, the 10" doesn't lack brightness what it lacks is image scale at the larger exit pupils. When I double the magnification to match the view through the 20" I immediately lose brightness so any detail becomes much harder to pick out. The difference between the two is not only obvious it's huge. On DSO the 20" murders the 10" on any object you need to magnify (this is pretty much any DSO you look at).

Next time you are out observing. Every tine you magnify beyond maximum exit pupil it means you are looking at an object where an increased aperture will help.

[Stevp]

...If 6' 2" build like the proverbial outhouse...

That made me laugh far more than it should have! Think this weather is making me a little stir crazy!

[AlexB67]

The difference between the two is not only obvious it's huge. On DSO the 20" murders the 10" on any object you need to magnify (this is pretty much any DSO you look at).

Rub it in, not that I ever doubted the difference is substantial, but I'll stay well away from the 20 inch swamp for now.