dobsonian weight portability v's appeture ?

[rory]

hello all. im settling in well now after six months or so with my 90mm refractor and bino's.

but planning quite away ahead ,im contemplating a dob as my next scope .

however im after some info ..best weight -portability against appeture ?

at the moment im interested in an 8" ,thinking it would be a good comprimise between the two.

most observing would be in my own garden with occasional trips out.

im assuming an 8" would give some pretty good views.

ok so enough waffling ,any thoughts welcome ,thanks

clear skies...

[brantuk]

An 8" dob is very manageable - and easily portable - smashing scope and excellent jump from the 90mm. 12" dobs are the point where they start to get bulky and heavy so you'll have no problem with 8". Make sure you see one in the flesh before buying it so you're neither disappointed nor over stretched - you may find a 10" is ok for you - more aperture the better ...

[GazK]

I've got a 10" and its an ideal balance between portability and power. I can have it in the car and ready to drive in 10 mins or set up in the garden in 5.

[johninderby]

There is a big difference in portability between a 10"and 12" dob. The 10" still falls into the easily portable category but the 12" is a lot bulkier and heavier. There's not a lot of difference in portability however between the 8" and 10" dobs, so if you could manage the 8" the 10" wouldn't be a problem.

So I'd say if you can afford it go for the 10".

John

[bish]

I've got a 10" and its an ideal balance between portability and power. I can have it in the car and ready to drive in 10 mins or set up in the garden in 5.

I agree with this. The 12" (solid tube) would have been too big to fit across my back car seat and a bit awkward to take through the kitchen. Different people have different ideas on portable though.

John also makes a good point that if you can manage a 8" you should manage a 10", but the 12" is quite a difference.

[Superewza]

Yup, portable means very different things to different people. For some it seems to mean 'easy to carry from the house into the back garden and vice versa'. Wheras i would class 'portable' as 'easy to carry to a field about ~5 minutes walk away from my house' which is free of streetlights (used to be brilliant until a warehouse on the other side of a railway decided to install security lights aimed directly onto it ).

And with that in mind, i would point out that there is a big difference in portability between 6" and 8". 6" is still a very good size for beginners. Probably the best scope for 'grab and go' would be the Heritage 130P, and while the optics on that are very good for the price, there's a difference between that and the solid tubes for not much more.

[umadog]

That's all good advice. Unless you need to walk 5 minutes, either an 8" or a 10" Dob will do just fine. Views aren't too different between the two so base your choice as much on other factors (budget, focal ratio, weight/bulk) as on the aperture. Either scope will provide a vast difference on what you're using now. M13 will very obviously turn into a ball of stars, for example.

The brightness of DSOs isn't uniformly distributed and a lot of stuff becomes visible and interesting over the 6" to 10" range. Going larger than that is nice, but it doesn't quite provide the hit you get from going 3" to 8". I have a nice story which frames, I feel, how good an 8" really is. I went out with a friend whose been observing for a year with his 8" Orion. I brought along the 18" (which at the moment is a 16", actually, but let's not go into that ) and we headed out to our ~mag 5.5 site. After a couple of hours of observing with the two scopes he didn't come away with aperture fever. Yes, there was more detail in the 18", but he came away feeling more certain of his scope and with less inclination to go bigger. So an 8" is an excellent long-term scope that will show you bags of stuff.

[rory]

thats some great advice peeps. mmm, now i need to think about focal ratios then?

cheers all...

[MatthewC]

One of the reasons i chose the 8'' over the 10'' is that the slower focal ratio of the 8'' will be less demanding on cheaper ep's and some say that at f6 stars look nicer. I can't confirm this but i'm sure someone will correct me if not true.

[umadog]

Yes, Mathew, that's true. f/6 is significantly less demanding on eyepieces than f4.7 (which is what most 10" scopes are). You will see more coma and more astigmatism at the faster focal ratio. Many people will see a benefit of a Paracorr at f/4.7 but few will bother at f/6.

In addition, the collimation tolerances are much tighter at f/5 and below. Frankly, I wouldn't use those fast scopes without good collimation gear (like the Catseye kit). At f/6, however, it's less of a big deal. For example, at f/6 the primary mirror adjustment tolerance is just over 1 mm. At f/4.7 it's half a mm. A 2x Barlow will turn the 10" into an f/9.4 scope, which will clear up the astigmatism to a large degree but won't change the fact that you need to be well-collimated. Coma is less obvious at higher powers, too.

I'm not saying mass-produced 10" scopes aren't good. I frequently see encouraging comments on these scopes and, indeed, have a fast 10" myself. It's just that I wonder if they're the most suitable thing for a beginner on a budget.

[rory]

OKAY MATT AND UMADOG. very sound advice. i wont be getting one for at least another 4-6 months at the earliest. but its good to get an idea. overall with info provided so far ,im convinced an 8" dob is ideal . although i can be persuaded with the old app fever !! lol.

one reason i need one is the refractor on a e/q mount is difficult to use when observing high up, near on impossible at times.

[Moonshane]

I am a real fan of newtonian scopes and dobs in particular.

I have owned the following scopes and in this order:

120mm f8 achro refractor

6" EQ mounted newtonian

12" f5 dobsonian

and now own

6" f11 dobsonian (together with the 12").

16" f4 dobsonian (together with the 6" f11)

90mm f5.5 achro refractor

In general I agree totally with the above comments. I see portability as ability to carry through the house (either by hand or on a sack truck) to the garden or to get the scope in the car.

my dobs have all had a tube length of 60" and all just fit (individually of course!) in my Rover 200 hatchback with the back seats down and the tube diagonally.

for me there's nothing like good aperture and after owning a 6" I felt that I'd always wonder about the 12" if I bought then 10". as it happens I managed to save and buy a 16" and it's a massive step up in performance and size from even the 12".

the 12" was not that heavy but the 16" is very heavy - perhaps 80 pounds and I use a sack truck to move the tube. it's also very bulky.

personally I'd say buy the biggest dob you can afford and manhandle and you'll be very happy indeed.

don't forget your telrad/rigel quickfinder though - essential gear!

[rory]

cheers moonshine. im planning on a telrad for my current scope anyway, being as so far ive managed with no finder at all !!! , i think the maximum budget would be 10" but im hearing the 8 to 10 jump isnt that noticable,and with a more leanient focal ratio im still in an 8 " corner. probably change my mind 100 times before i buy one mind ,lol.

gaz's pic's ? the third picture from the left,what the hell is that ???

[Moonshane]

no worries. either scope will be excellent - just use and enjoy whichever one you get.

I believe Gaz's scope you mention is a 6" 'portabowl' design which is home brewed and made very cheaply from bits and bobs - superb for younger observers.

[rory]

oh right. looks weird ....thanks.

[dweller25]

I have tried the 8" and 10" Skywatcher Dobs and a MAJOR factor for me is that I have a bad back.

The 10" definitely aggravated my back. The 8" for me is perfect and it's F/6 so more forgiving on collimation and kinder on eyepieces.

Used to have a 6" but it didn't have enough resolution or light gathering.

[JamesM]

The best thing to do is to go along to an observation evening or even a shop if one is local to you and take a closer look for a reality check. What is heavy for one might be different to another, is it a quick carry out to the garden or are we loading up for a camping trip? I observe with a group where there are at least 3 12" dobsonians which have all been transported out to the countryside. I would say the owners aren't particularly young or even especially fit and have never heard of any great struggle in carrying the kit when setting up or packing it all away - its relative to the individual.

James

[umadog]

"personally I'd say buy the biggest dob you can afford and manhandle and you'll be very happy indeed"

I do agree with this, but it's important to stress that you want the biggest scope you'll move about easily. Bigger is not always better. If it's too big, it will put off setting it up unless you can view from your back garden and can simply wheel the scope out of a shed. Once you enter >=12" territory you really want to supplement that scope with a 6" to 10" grab & go, as Moonshane has done. Quite apart from the weight and set up time, these big scopes have narrow fields of view.

[rory]

yep,thats true, certainly much to consider. which is why im planning way ahead of purchase. unlike my first purchase where i went in blind with excitment,although im not entirely unhappy with my scope by any means , im learning all the time. but it wouldnt be my choice now.

good advice from you all ,thanks. and clear skies...

i see theres another two or three threads with simlar questions now. sorry if its been repetitive :-/

[umadog]

Don't worry about it. That's what the beginner's forum is for: ask away. As you've noticed, however, you'll probably get the answers faster just by spending a couple of hours trawling the old threads. Have a little read and ask again if you still have questions.