Dobs - How big is too big?

[glend]

I am considering taking the step to the "Big Dob" world, but would like some advice on usability from present owners of the big stuff.

Right now I have a 16" GSO Strut dob on a custom build lowered base (with Wheel Barrow handles) running an Astro Devices Nexus Wifi Pushto DSC system (app is Sky Safari Plus on Android tablet), which is well sorted out, works great, and I can move it around.

Looking into the future I can't resist looking at the Obsessions, particularly the 20" Classic and 22" Ultra Compact:

http://www.obsessiontelescopes.com/telescopes/20/index.php#accessories

Towards the middle of this year I will be in a position to afford one of them (thanks to Super).

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.

Is the step up to the larger aperture going to be worth the change over? Sure limiting magnitude is a little bit better, as is resolution and contrast compared to the 16" but some of that might be addressed by a mirror upgrade on the 16 (say to Zambuto) for less investment.

If I went with the Obsession I'd want the ServoCat Goto system as well, but I could put that on my 16 fairly easily.

The Zambuto mirror and Servocat for my 16" will cost less than the new Obsession, but it may not scratch that persistent Big Dob itch.

[alocky]

I use my 25" every chance I get. Setup time is only 30 mins or so including collimating and aligning the servocat. The views are spectacular and I can assure you I have never wished it was smaller.
If I was 80 and infirm I might - go and do it while you're fit enough to climb a ladder, and make sure you treat yourself to some dark skies.
Cheers,
Andrew.

[wavelandscott]

Be sure to check out SDM Telescopes before you spend your money...fine machines and by all accounts great service!

[barx1963]

Glen
I made the jump from a 12" GSO to a 20" Obsession clone. I have absolutely no regrets and I love the scope.
BUT you really need to be a dedicated deep sky observer to get value out of these scopes.
Setup time itself is not that much more than with the 12" at about 20 minutes compared to 15 with the 12, but alot of that was due to me accesorizing the 12 in ways I couldn't with the 20.
It is big and heavy and requires 2 people to safely load and unload if transporting by car. At home however, I handle it without any issues.
If looking at the ultracompact Obsessions, just be aware if you have neighbours lights, streetlights etc, the upper baffles are not super effective.
Peter at SDM gives excellent service. Even though I haven't purchased from me he has on 3 occasions allowed me to pick his brains for info and advice. If I was buying a new big dob, he would be my first port of call.

As far as the image goes, there is no comparing the 2 scopes. In the 20 I can pump the power up to 317x in the trapezium and easily see the E and F stars. Eta C looks simply amazing and faint Galaxies just pop out all over the place.

Malcolm

[jamespierce]

Another vote for Peter and SDM - I enjoy my 16" F4 built by him very much... I've seen more than a couple of Obsessions which he has rebuilt and upgraded.

As for how big is too big - I've been wrestling with this my question myself as I dream of the next big thing !!! Having looked through a range of 'big' scopes, the increase in performance becomes more and more subtle as the pain in the butt factor increases rapidly with size, weight and heigh. Also cooling is a huge issue if you live somewhere like Victoria with large day to night temp drops.

My heart would like a 30" F3.3 ... My head says 24" F4 or F3.3 with a 1.5" thick mirror is probably truthfully the sweet spot of easy of setup, movement and use.

[sn1987a]

I love my 28" but truth be told if I was to order it again I'd go the 32". A 40" f3 mirror and matching 9" secondary from Mike Lockwood is around US$44k+. I'll be very interested to see pictures of the 40" SDM are building when its finished. For inspiration goto http://www.cruxis.com/scope/scope1070.htm

:D

[jamespierce]

Factor in the same again for a structure too !

Roughly $25K for 24", $40k for 30" and $90k for 40" ... give or take a bit.

[cometcatcher]

For globular clusters there is no substitute for aperture.

[clive milne]

Glen... the point of diminishing returns is somewhere between 25" to 32".
If you want more aperture than that, get a pair of primaries and make a binocular. You could always do that with your current scope (buy another 16" from GSO) It will have a similar limiting magnitude as a 19.2" but the aesthetics of the view will be in a completely different league (better).
Go here:
http://www.binoscope.co.nz/birth.htm
I would strongly advise against a 1m f3 ... don't even think about it, it is a really really bad idea.
As for the telescope structure... build it yourself, it really isn't that hard.
If you want someone else to build it, Peter Read would be top of the list, Webster also makes a fine telescope, and Obsessions are pretty hard to criticise in any way I can think of.

c

[clive milne]

Actually... next primary I will buy will probably be from here:http://hubbleoptics.com/mirrors.html

First thing I would do is send it to Mark for testing.

[Allan_L]

Make sure you are comfortable standing on a ladder for a few hours on end first Glen!
For me, I'm not even comfortable standing on the ground.
Observing chair height for me.
Comfort at the eyepiece allows more detail to seep through!

This is just my humble opinion, and not meant to degrade anything any of the previous posters have said. But i am getting older, and balance can be a problem (even without a beer).! :P

[netwolf]

A side question what would be largest you would go in Sydney suburban skies?

[AstroJunk]

In my experience, every aperture increase makes a big difference too, but the mirror must be good. I settled on a 20" f5 which the largest I could transport at the time and the focal length made it only a few steps up.

But every time I looked though PR's 30 at Astrofest, I knew my puny 20 was beaten into the ground. I did get the pleasure of using a 24" f3.6 that was exceptional in every way and just about the same height to the eyepiece as my 20 - these new ultra fast mirrors are great with a parabola corrector. I have used a less than perfect 25" mirror that really spoilt the definition - no different to normal sized scopes really.

At the end of the day, you can sit in an observing chair looking at nothing, or stand on a ladder and gaze at the universe :rofl:

[glend]

Thank you all for those considered answers. I have been exchanging emails with David Kreige of Obession (and The Dob Book fame), re his 20" f5 Classic and the 22" Ultra Compact (which would only need a short one step stool for me). I have also considered making it myself, well not the mirror, but using David's book or getting one of Dennis Steele's kits. Dennis offered to just sell me the rings to keep the shipping cost down and I could source the rest locally. SDM is way out of my league I think but they are great examples of fine furniture as telescopes, I'd be worried about damaging the finish. I love the 25" folded dob he built, the first one I have seen and it eliminates the need for a ladder but I am sure the cost is way out there on that one. I assume it's a RC type design from the look of the front obstruction baffle.

http://sdmtelescopes.com.au/scope-showcase/sdm008f-25-f5-folded-newtonian/

Re the budget for buildings, observatories etc, my garage/shed will have to do, and there will be a space restriction there as well. I am pretty sure I can get a 20 or 22 in there easily but anything bigger is going to require throwing stuff out (not a bad thing in itself). Allan_L is right about the ladder and it's likely that anything that requires more than two steps on a short step ladder or stool is out of the question. Moving the scope to a dark site is also going to be a constraining factor. Right now I use ramps for the 16 to load it but anything larger, even if compact like the 22" UC is going to be heavier in component parts and maybe require a winch up a ramp or a dedicated trailer (and I can't tow my camper trailer at the same time). I guess I could buy a van and carry the scope in that, a fair number of people seem to have them in vans. I am getting the impression that using and living with a big Dob requires some compromises in other aspects of our lives, sort of like having a very large dog, or a horse.

BTW David Kreige mentioned that he is going to be in Australia in April for the event at Coonabarrabran (Sidings Springs), so that might be a good chance to talk to him in person.

[sn1987a]

Grab this one! :)

http://www.astromart.com/classifieds/details.asp?classified_id=843602

[glend]

That's very nice Barry, ashame it's not here in Australia. I have the feeling that even older large dobs are not going to have depreciated much if the mirror is quality and the build was good. Do people even sell them?

[bratislav]

Second that! They have lifted their game incredibly, and seem to be able to compete with big boys in quality stakes. Currently evaluating their 16" f/4.5 but weather in Melbourne has been truly atrocious. A few casual star tests show a lot of promise, beautifully smooth, clean edge and just a tad undercorrected (not visible in in-focus images). Will post bench and Roddier results as soon as the weather gets sane.
Factor in the low price and they are winner indeed.

[Satchmo]

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 :)

[mbaddah]

Mark I totally agree with all this. Having now owned my SDM (14.5")for well over a year, I wished I had forked out the dough for a 16-18" f4 scope. Setup time would be identical between these sizes, unlike say a 12" where the rockerbox can be carried around. I tried this with my 14.5" once and gave up instantly :eyepop:

[glend]

Well said Mark and I appreciate you voicing those 'issues'. I do have reservations about the marginal increase in (utility) capability against the price point. In terms of portability it is mandatory (in my case) that I can handle loading, transport, and setup and take down by myself; and that I do not need purpose built buldings etc to store the beast. It's also preferable that I don't have to buy another vehicle or switch to a van if I can help it, which means a scope that can be broken down to fit in my 4WD and campertrailer in component form. I also want to avoid falling off ladders, which has recently happened to a friend in Canada.

For those reasons I have decided to draw the line at anything over 22" in a fast f Ultra Compact form factor. The Obsession 22" UC size is the boundary for me.
I am very interested in the Hubble 20" Ultra Light as it ticks all the boxes. It's shown here:

http://hubbleoptics.com/UL20.html

Have to wonder if it's a coincidence that Hubble is using the same website colour scheme, font, and layout as Obsession; and have moved in on David Kreige's Ultra Compact product range. Ain't Competition Grand.

Hubble's 18" introductory pricing is placing it just above GSO's 16" Stut Dob which sells really well here.
http://hubbleoptics.com/UL18.html




Cheers

[Satchmo]

At those prices and fast focal ratios and using plate glass - with no optical guarantee or optical certification that can be challenged , I would say you would have to take the claim of 0.95 Strehl with a grain of salt . The structure certainly looks light.

[glend]

Yeah, the more I think about it.....and that secondary hanging out front like that will be a problem for stray light and required shrouding around my area..and as you say performance with a grain of salt. I'll go back to Dennis Steele and see if he can do a 20" kit.

[AG Hybrid]

Hubble Optics mirrors have been tested and had their results posted on Cloudy Nights. The few that have been done are test around the 0.90-0.92 Strehl mark. But, as any mirror maker will tell you Strehl isnt everything. To be honest with ourselves, any mirror above 0.90 strehl will probably be a really good mirror that will be good enough for any of us who aren't chasing the numbers.

I will either be ordering the 18" f4 or the 20" f3.7 Hubble Optics ultralight at the end of 2014. Its exciting that there are other options than Obsession ultralights on the market. That being said. Skywatcher is scheduled to release a 18" ultralight dobsonian at the end of 2014. Interesting times ahead.

[Satchmo]

You would need to qualify that: were they mirrors in the larger ultra fast sizes like F3.7 ? And who did the tests - was it with a foucault test ? Or Null interferometer . I would go further and say that an true 0.8 Strehl mirror free of significant figure distortion problems in a large fast size would be a great performer . Unfortunately I've found a number of epic fails in 20" + sub F4 size amongst a number of USA makers - I would be very surprised if a Chinese company were doing a lot better for half the price !

The fact is that large sub F4 mirrors are much harder to make well - polishing and testing techniques that work easily for smaller slower mirrors can start to fail when applied to larger mirrors - no matter how great your enthusiasm might be .

[alocky]

I guess Barry and I must be examples of status-seeking 'non-serious' observers. Although both of us can set up in about the same time as smaller dobs ( it's the same number of components to assemble), do so singlehanded, and although I concede having a dedicated trailer for the scope is a bit more of a commitment, it makes heading out observing as simple as hitching it up and driving off. Plus you can store a lot of camping crap in the trailer with the scope - I'm self contained, big battery and inverter as well.
Still - perhaps you should ask someone who lives nearby who observes with a 20+ if you can tag along for a night? Most of us status seekers are only too happy to show them off!
Cheers,
Andrew.

[sn1987a]

I love showing off my Phallic symbol! :D

[AG Hybrid]

I think the mirror tested was 16" f4.5? I believe it was fully bench tested. I can't remember all the details but the paper work that came with the mirror had a very high strehl. Like in the high nineties. The owner didn't believe it and he was right. Apparently the paperwork that came with the mirror only tested one axis of the mirror - whatever that means. I don't know the full details of how its done - I imagine you do. But, the results were inflated. It still turned out to be a great mirror. The owner didn't want or thought it needed to be re-figured.

Who knows what the quality of their sub-4 mirrors are like? For the price I would settle for diffraction limited. I mean, if the one I get isn't to my liking. I can always send it to you for a re-figure, right? Right? :lol:

[glend]

Well today GSO has thrown a cat amongst the pigeons by indicating they will be introducing a 20" dob later this year. As per my other thread about the announcement, I had emailed the head office this morning asking about future product development in the dob range and had a response this afternoon from Susan Hsu at GSO confirming they were going to 20".

Here is the full content of her email:

Hi Glen,


we plan to make the 20” dobs later this year!



regards,



susan hsu




Then I rang Luke at Andrews Communications to see what he had heard, and after a discussion out of my earshot he said there had been some discussion about the 20". I told him to keep me in mind for #1.

So it's good to see the production big boys getting into the 20" bracket, and this follows on from GSO's recent introduction of the larger 16" truss RCs.

[clive milne]

I disagree Mark.
The highly distorted value of the Yuan gives the Chinese a significant advantage in labour costs. To put it another way, a Chinese optical company can get a full months work out of their technicians for the same price as just two days graft from an American. There is just no way that a US company can compete on those terms even at half the price. You would know yourself that you can't even buy the raw materials for making optics here for the same price as you can get the completed item including coating and shipping.

The Chinese might still be a bit behind in the development curve but they are catching up pretty fast. There is nothing (racially) that makes them any less capable of learning how to perform any given mechanical task, so while the imbalance in their currency persists I can only see the merit function leaning further their way.

[Satchmo]

Making high quality large fast mirrors is about expertise and knowledge in testing and figuring that is at a much higher level , and no amount of cheap labor can make up for the lack of that . It will happen eventually but it is not there yet .

[clive milne]

Mark, if they can successfully and routinely pump out 16" RC optical sets then they are already a lot better than opticians at the Osterhauski/Mike lockwood level (and at half the price)

[AG Hybrid]

I agree that Hubble Optics is not your run of the mill mass production mirror maker. They have got some really specialized products. Not to mention a production variant sandwich mirror for general sale.

After some reflection I might skip on the big scope and pick up a 12" mirror from these guys and retrofit my current scope some time this year. A rapidly cooling 12" mirror sounds good to me.

[glend]

Isn't it GSO (Taiwan), and not Hubble, that is the company that has started producing mass market 16" RC in this region?

[skysurfer]

Too big for me is is that I need a ladder or another step-up to reach the eyepiece. So with my height (1.96m) a maximum focal length of 1.80m is the limit. I have a GSO 40cm (f/4.5, so f=180cm) which is just not too big. Despite its mass of 64kg, I can easily transport it with a modified wheelcart.
Of course the so-called 'suitcase Dobs' or 'travel dobs' with a very lightwelight construction a 40cm f/4.5 can be even lower because the rotation axis and the counterweight (which is the main mirror) are very low.

With shorter f/ ratios a 50cm dob can also meet these requirements but then a coma corrector is mandatory.
Commercial Dobs over 40cm are rather rare particularly those below f/4.

The largest Dobson ever built was long before John Dobson was even born: Lord Rosse's (https://en.wikipedia.org/wiki/Lord_Rosse) with 1.8m main mirror but obviously it was not called a Dobson, but it is the same design.

[Satchmo]

Clive - Have you seen any tests a 16" GSO RC OPtical tube ? Is there even a complete successful 16" GSO RC in existance ? As I understand it sales of 12" RC's are only a tiny fraction of even the 10" , so I would expect the 16" RC's are more about building brand perception that $. AS I understand it the GSO RC's are automatically polished - not touched by the human hand. I'm not aware that the USA opticians offering any RC sets at least routinely.

[Satchmo]

The optical tests I have seen on HO mirrors by Wolfgang Rohr show a tendency to astigmatism and some turned edge . Good value if you get a good one. The 14" I star tested had turned edge. I have never seen any HO mirrors bigger than 14" in this country .



I wouldn't try and refigure a sandwhich mirror - they distort too much while cooling as they are not homogeneous.

[AG Hybrid]

HO do a plate and pyrex variants. Would either make any difference to its strength or suitability for refiguring?

[glend]

I agree that there are probably no GSO RC 16"s delivered as yet, based on info on the Andrews Communications website. I sighted that development as an example of their (GSOs) movement into the larger scope mass market, which is supported by their advice to me that they will be introducing a 20" Dob later this year, and their apparent competitors 18"dob comiing as well.

From what I have read I expect that most GSO optics are produced on machines, and that does provide for a certain quality level in mass produced products. They don't produce individual mirror test results but 'claim' a certain level of qualty for their products based on the processing system design and sample testing. I would never expect a mass market producer to be able to duplicate the manhours put into a hand made mirror by an experienced optical specialist dedciated to just the high end of the market. But GSO are bringing astronomy to the masses and at a quality level which is very good compared to past industry offerings and they should be commended for that. Quality wil certainly continue to improve as optical robotics are further developed. As a base for further development and upgrades a GSO scope ticks alot of boxes for many people.

Mark, et al, maybe there needs to be a separate topic thread on mirror quality issues and the cost benefit (or not) of custom hand made verse robotic.

[Satchmo]

Glen - All commercial mirrors in the world are mostly machine made - the `boutique ' low production mirrors ( ie Galaxy , Lockwood, Kennedy etc ) may get some localized retouching by the human hand with the mirror rotating face up on a machine and a human being rubbing a polisher on top ( maybe 10% ) of the total production time . This last touch up can produce a very smooth accurate surface, better than one made quickly totally by machine and an operato . Some can do this by machine only . Zambuto mirrors are 100% machine polished with no 'hand' work but a skilled machine operator .

What I was referring to by computer machine polishing is a system where the test results are automatically fed into a computer which designs the next run of machine strokes until completion - no human interpretation involved in the loop. This is the system claimed by GSO for the RC optics.

This info should obviate the need for another thread .

[David Niven]

Totally agreed and they have landed a rover on the moon too.
So it is not outside their capability.
Just bear in mind, there are good manufacturers and not so good ones.

[stephenb]

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.

[AstroJunk]

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!

[glend]

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.

[sn1987a]

I just take both and let the seeing decide :P

[Satchmo]

( 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.

[AstroJunk]

Nice one Barry!

[glend]

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.

[el_draco]

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 :thumbsup:

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 :rofl:

Would love to have that scope back; probably trade in one or more of the kids for it.... hmmm :D

In short DO IT!!!

[Curt]

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.

[Camelopardalis]

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 :D

[sn1987a]

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 never claimed I was rational :P.

[N1]

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.:shrug: 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.

Correct me if Im wrong though.

[AstroJunk]

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.

[Satchmo]

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.

[sn1987a]

You could probably land something like this with all the fruit in Oz for around 10k.

http://www.astromart.com/classifieds/details.asp?classified_id=845693

[MrB]

*drool*
If only I could afford it! :sadeyes:

[ausastronomer]

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.

Cheers,
John B

[glend]

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:

Obsession 20 : Limiting Magnitude 16.5, Resolution 0.27; GSO 16 Limiting Magnitude 15.5 Resolution 0.29.

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.

[sn1987a]

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.:thumbsup:

[m11]

Out of curiosity with the 16" Suchting mirror, did you end up putting the mirror in a different structure? :)

[sn1987a]

It's in a slightly modified Dobstuff at the moment. It's good enough for the bush until the day a naked 16" SDM or equivalent pops up out of the blue and beckons me over. Dobstuff are closing down soon, if not already, which makes me sad as Dennis there provides a really great value product and service for the budget end of the premium Dob market.:sadeyes:

I may eventually sell the 16" f4.5 Dobstuff without the mirror, focuser, ArgoNavis and secondary - just the Dobstuff body, to free up some space. It would suit someone already with a 16" f4.5 mirror.

cheers

[m11]

Ohh nice. :thumbsup:

It seems alot of astro stores and places are closing down, so sad.

Cool. I guess you could always ask Pete to build a structure. ��

I can see from previous posts you have some nice scopes. :D

[Bobbyoutback]

Hi Mel , your in a great position to talk about big dobs .

My first scope was 60mm ' yours was a 400mm :thumbsup:

Have Fun
Bobby

[m11]

Hey Bobby,

Haha, still a newb.:rofl:

You have had lots of fun with your scopes, which is what counts and having the passion going which you have in spades buddy. :thumbsup:

Mel

[Bobbyoutback]

Thanks Mel but I only have 7 scopes , biggest a 12" :sadeyes:
You have well over double that including a 24" dream scope .

Your no newbe with your great knowledge of optics , it's wonderful to have you as a close friend , we must have spent hundreds of hour talking about scopes & its been super fun !

Cheers & Beers
Bobby

[m11]

That is a decent amount of scopes and equipment, especially with the darker skies you get. :thumbsup:

Hahah ty buddy, I hope later on I can grind my own mirrors and test them.

Haha yah, likewise. I really enjoy the discussions with you always and its so relaxing and fun to chat to you. Time really flies when we chat.:D

Mel

[alan meehan]

Hi Glen how are youbeen awile since we last spoke good to see you are looking for another Dob Barrys one looks nice and portable A must i think when contemplating using one of these , are you not using the 16" or just want to go larger which means heavier .we have just brought a 16" skywatcher dob what a great scope complete tracking on a dob of this size i had to build a special trailer to transport it and takes 2 to lift and set up the base heaviest part being collapsable helps and holds collamation well the views are brilliant.We have been granted a space at the new digital cube by LMCC under free contract its bright as you can see but the lake side is dark no lights and the DOB is in its element both for public and club viewing ultamatly the choice is up to you just remember getting there and setting up is 80% of the battle take care my friend and if you want come up to lake for some viewing
ALAN

[glend]

Gee Alan, that Dob reference was back in 2014 (see below). I sold that 16" GSO when I built my observatory in 2015. Age does weary one, and I just could not handle it anymore, particularly for trips and switched back to imaging with smaller scopes. Macular degeneration was ruining my visual ability anyway. I did get a chance to look through Allan W's 35" Black Widow out at Coolah in April 2018, at its unveiling. Impressive Dob for sure. These days I do EAA video out of my backyard observatory, with my ED150, and that meets my needs just fine. It is still pretty dark here, as I am on the Bush at the back fence, and have a great tall hedge of conifers blocking stray light. Travel for astronomy is probably not for me anymore, especially in the present infectious environment, and having underlying health issues; but best of luck with your outreach efforts. Cheers Glen

[alan meehan]

thanks Glen didnt even look at the date

[glend]

Well seeing how I started this, way back when, why not kick it off again. Hopefully there are new big Dob fans out there, considering taking the plunge. As I said just below, I did get to witness the sky through Alan D's 35" Black Widow, in April 2018. And yes it was very impressive but this was a scope with it's own roll out shed, so portability was not a feature. Perhaps discussion could kick off with limits of travelling size, probably directly proportional to owners age and fitness level. My GSO 16" did travel a fair bit, always broken down into components, with the mirrors in the back of my Jeep Cherokee. Ramps were required to get the bottom primary section in and out of the car safely. So any travelling Dob will need a suitable vehicle as well.
So who has the current largest travelling Dob?

[gaseous]

I believe Mel, Alan and Chris Cox all have (or will have) +24" SDM scopes to lug about, which as mentioned require their own traveling circus. I believe a few of us on IIS have the 20" Skywatcher Stargate scopes. My wife got tired of me having mine in pieces stashed around the house, so I bought a 6' x 4' trailer to keep it in (disassembled), along with folding tables, chairs, and other knick-knacks required when traveling to dark sites. It can be done with one person, but you need to have a reasonable degree of upper body strength and a sound back, both of which I can imagine will start to desert me as I approach my golden years.

[m11]

I think alot of the owners have trailers to move the large scopes which I like to do as well. I have access to a van but even getting the scope to the van loading it is more a two person job for safety.
A more permanent setup would be ideal at a dark country property which also has other factors like cost and safety.
I have all my scopes fully assembled at home so I can roll them straight out to use.

I reckon go as big as you are willing to handle and have the scope generally fully assembled for ease of use.

I am seriously looking at a 30"/32" scope but need time to save for one.