I would ad that the return from a larger telescope is very much up to your experience and expectation and that ease of transport and setup are significant issues.
Big bright objects such as Omega Centauri and Eta Carinae look brighter but do not look substantially different in comparing views between a 30" and an 18" scope -new owners of larger scopes are often quietly disappointed by this: the differences are more subtle and clear when trying to find faint objects .
If you are not obsessed about faint fuzzies and and not prepared to house or travel to a dark sky once a month , the advantages of a larger dob may become swamped by the practicalities of trying to cool a large mirror , a large structure , and the inevitable woolly soft images that come with poor seeing and large scopes ( or is it heat from my structure or mirror ? or is it the quality of the seeing ? or is it the quality of my mirror ? )
Many large scope owners just get overwhelmed by the portability issues and never get to the bottom of these questions before their enthusiasm runs out. I know of a number of large telescopes that rarely see starlight - they are status symbols of one having reached a financial peak in ones involvement in the hobby and not much more. Theres a simple maxim - if you need help to pack or unpack your scope before and after travelling your telescope will get used much less.
I would hazard a guess that a high quality 16"-18" scope is in a sweetspot as far as all factors concerned- up until 30 years ago would have been considered a very large telescope to be in amateur hands.
If you are in the market for a large telescope do try and get a look through one and really evaluate if the images are going to be worth your while in the long run when you factor in transportation and cooling time . How are your energy levels ?
Just keepin' it real
Cannot agree more with Mark's comments...
An 18" is perfect for me. I can load everything in the ute by myself, unload it and set it up in 10 minutes max. I have looked through a 25" Obsession but my gut feeling is that it was only marginally better than my 18" under similar skies with a majority of common DSOs. Yes the larger aperture did bring out some extra detail on the occasional faint fuzzy. But I walked away totally satisfied for my 18" 4.5.
I did considered an Obsession but after factoring in the shipping options it was cheaper to buy from SDM - SDM also have supplied no end of free advice and support. I also heard of a disappointing story in regards to their (Obsession's) customer service which really made me nervous.
An 18" is perfect for me. I can load everything in the ute by myself, unload it and set it up in 10 minutes max. I have looked through a 25" Obsession but my gut feeling is that it was only marginally better than my 18" under similar skies with a majority of common DSOs. Yes the larger aperture did bring out some extra detail on the occasional faint fuzzy. But I walked away totally satisfied for my 18" 4.5.
I did considered an Obsession but after factoring in the shipping options it was cheaper to buy from SDM - SDM also have supplied no end of free advice and support. I also heard of a disappointing story in regards to their (Obsession's) customer service which really made me nervous.
I'd have to say that if the 25 didn't blow your 18 away in every respect then it had a poor mirror or the seeing was bad.
No disrespect to your 18 incher, but that's physics!
I believe that Stephenb's comments reflect what economists refer to as 'Marginal Utility'. If you were to graph the marginal improvement in scope performance, whether resolving power or limiting magnitude, along with the $ investment required to purchase said scope you will likely notice that the 'utility' ( or what you gain) falls away significantly as the cost goes up. I agree with previous comments that from a financial investment point of view there is a 'sweet spot' where performance investment uility peaks and then falls away. Investment beyond the sweet spot maybe considered as pretentious to utility derived from the performance of the scope, but for some people the exclusivity of the investment may be more important than the result it is capable of achieving.
I'd have to say that if the 25 didn't blow your 18 away in every respect then it had a poor mirror or the seeing was bad.
No disrespect to your 18 incher, but that's physics!
( If I remember correctly Stephen's scope is an SDM with Zambuto mirror )
I think psychology and utility is involved . The difference in the appearance of the big bright objects ( ie Omega Cen and Eta Carinae ) between an 18" and a 25" is marginal at best . If you are not dedicated in chasing really faint fuzzies you will not feel it is worthwhile to deal with the portability and thermal issues of a 25" let alone having to hang off a ladder a lot of the time. The 18" does lie in the sweetspot of marginal utility graph .And owners of large telescopes with mediochre optics will be aware that smaller telescopes with excellent objects can easily show fainter stars.
Looking at Barry's collection, his utility definition is probably very different to someone buying one scope (whether constrained by portability, available space, or money to spend on scopes). There are alway's 'outliers' whose view of the rational utility decision is going to be very different, and likely to only make sense to others in that same situation.
I think I have achieved what I was seeking when I started this thread, and that to gather the views of many to inform my decision through your experiences.
Clearly, I am price sensitive when it comes to scopes, and I really appreciate the knowledge shared about the sweet spot in terms of dob size. With the arrival later this year of new production dob scopes in the 18-20" range from GSO and others, this is going to potentially alter my views on the size/performance/managability/price sweet spot.
For big dob owners, did your move up in size meet your expectations and do you use the big dob as much as you did the smaller ones (assuming there were some). I am assuming that frequency of use is related to size.
I went from 12.5" to 29" F4.5 in one step back in the 1980's. I built the monster and it took several years. Used it for teaching for some time. Was it worth it? Hell yes
I could walk on the moon, count Saturns rings, watch fine detail in Jupiters atmosphere. On one night I spent 4 hours with a 4mm orthoscopic in the LMC and I didn't more the thing more than a degree. 30 years back now and I still dream about it.
Only draw back is that they are massive and astrophotography is difficult. Some things you cant look at like Sirius.... burn in on the retina is a b itch
Would love to have that scope back; probably trade in one or more of the kids for it.... hmmm
Ive had the pleasure of observing through Barry's collection of scopes. All of them are fine tuned machines. As a relative newcomer to astronomy it is amazing to see the difference in a mirror made by a true professional compared to a bang for your buck mirror. The views through Barry's 28" Webster, plettstone and lightbridge is nothing short of amazing all A1 optics. Compared to my 16" gso mirror the difference is chalk and cheese. My jaw drops when l look through the eyepece in his 28". I will never forget the veiw of the veil nebula one night
it's takes a dedicated observer to handle such a large scope, but If the passion is there then why not go as big as you want.
I would love a large 25"+ scope but simply do not have anywhere to store it. I have just ordered a 16" telekit from astrosystems, same size as the lightbridge jut a little more refined in my eyes. Just need a premium mirror to go with it now.
See I've often thought about a bigger scope...I'm sure the sky has more to offer me than my C11 is showing me, and I have no real idea of how good or not the optics are. Anyone got the kit to test one?
And I'm convinced the day will come where a big Dob gets unloaded from the car to soak up a dark sky - say, 18-20". It's just that it's a big investment which ever way you cut it, and if I'm going to invest I'd want to be certain that it's got good optics. The clincher is that I've got to be able to lift and assemble single handedly. I can manage this comfortably with the C11, but the day will come when I won't be able to lift it into the saddle without risking injury. It won't get replaced until I find 'the right scope'. So whatever options are out there, I'll be watching closely to see what's happening in the lightweight space
My 28 tracking Jupiter in my backyard right now, red spot fest. How much utility do you want?, my back door is 4 meters away, slap on the wheel barrow handles and roll it in. The sting of the cost has long since faded, these are the days of cream and gravy.
I'd have to say that if the 25 didn't blow your 18 away in every respect then it had a poor mirror or the seeing was bad.
No disrespect to your 18 incher, but that's physics!
I may be wrong here, but aren't the exit pupil and magnification at a given exit pupil in a linear relationship with the aperture? If that is the case, then the 25" would show the same object at the same apparent brightness just under 1.4 times bigger that the 18", assuming the apropriate eyepieces are used. From experience, I know that going from 100x to 140x doesn't exactly open up new dimensions. A factor of 1.4 in power increase may be "interesting", but leaves my socks firmly on my feet, sorry. Heck, my eyepiece increments are more than that. I can see why Stephen was underwhelmed by this improvement, given that going from 8" to 12" gives a greater improvement than going from 18 to 25. Not to mention the dollars. Diminishing returns here big time, it would seem.
The exit pupil calculation is an interesting (and useful) thing. Basically, there is a minimum magnification that a telescope can achieve before the exit pupil is larger than the eye's pupil can accommodate at which point light is being lost. Its age dependent too as the ability of the pupil to open up gets progressively worse over the years So as long as the exit pupil is of a reasonable size then no precious photons are being lost.
The next is comparing like with like. Comparisons should be made using as close to the same magnification as is possible for the reasons you make.
It is true that the down sides of a larger focal length instrument are increased magnification and narrower fields of view, but the increased colour and resolution is simply unbeatable in my opinion, and anyone who has enjoyed the Veil nebula with a 30"er would be forced agree.
I may be wrong here, but aren't the exit pupil and magnification at a given exit pupil in a linear relationship with the aperture? If that is the case, then the 25" would show the same object at the same apparent brightness just under 1.4 times bigger that the 18"
Mirko - you are absolutely right of course - if your pupil is exactly filled by the telescope , the Eta Carina nebula for example has exactly the same surface apparent brightness with the naked eye as it does in a 2 meter telescope - only the magnification and resolution changes When people get this they have an `aha ' moment in their understanding of how telescopes work.
It is certainly going to be a clearly superior product to anything Skywatcher or GSO will be able to produce in that aperture class, in the short term. The Asian manufacturers will certainly get there but I still think they have some learning to do in terms of scopes in this size range.
I am not sure it is that 'clearly superior', and it is going to come down to being able to use that performance.
I did a performance number comparison between the 20" Obsession (linked below) and my 16" GSO and there is not a lot of useable difference in my mind. For example:
In the real world where we normally observe most of that performance difference cannot be used due to atmospherics and Sky Quality. The real power of the larger dobs comes into it's own at very dark sites.
The Big Dobs have a great advantage in that they usually have a great mirror system, and money gets you that, but the cost of putting a great mirror system in an existing scope is going to be much less than buying the larger dob (and will probably get used more often as well due to the smaller package). This will bring them closer together, if not equal, in performance.
I am holding off on my upgrade until I see the design, pricing and specs on the 20" GSO.
Mate been there, done that, it's a false economy. I have a beautiful 16 Suchting mirror sitting in a Lightbridge when it really deserves an SDM. I spent more than 3 times the original cost getting my Lightbridge up to speed and at the end of the day it's still a Lightbridge POS. Stretch, borrow, steal, sell yourself, do whatever you can, find the dough and when you're finally out under the stars with a well built observing machine you'll be so glad you did.
Heck, my eyepiece increments are more than that. I can see why Stephen was underwhelmed by this improvement, given that going from 8" to 12" gives a greater improvement than going from 18 to 25. Not to mention the dollars. Diminishing returns here big time, it would seem.
So as long as the exit pupil is of a reasonable size then no precious photons are being lost.
