Dobs - How big is too big?

[glend (Glen)]

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/t...hp#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 (Andrew lockwood)]

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 (Scott)]

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

[barx1963 (Malcolm)]

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 (James)]

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 (Barry)]

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

[jamespierce (James)]

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 (Kevin)]

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 (Allan)]

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).!

[netwolf]

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

[AstroJunk (Jonathan)]

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

[glend (Glen)]

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-sh...ded-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 (Barry)]

Grab this one!

http://www.astromart.com/classifieds...fied_id=843602

[glend (Glen)]

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 (Bratislav)]

Originally Posted by 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.

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 (Mo)]

Originally Posted by 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

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

[glend (Glen)]

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