Dobs

[kevy02]

OK guys, why are there so many 10" and 12" dobs for resale at the moment.I have only owned equatorial and fork mounted scopes in the past,but intended buying a large dob in the near future.Is there a problem with manuevoring the scope around the heavens,particularly if no drives? I need a bit of positive input before i commit.Thanks. Kevin.

[Liz]

Hi Kevin .... one reason is because of the great range of flex-dobs that have come out. I have my 10" GSO Dob for sale, cos I bought a bful flex dob, that folds down. Dobs still rule :thumbsup:

[AG Hybrid]

Chances are they are upgrading to bigger dobs or decided they want to move into astro photography - which is an expensive move and maybe they are trying to raise funs? Of course some might just want to move out of AA all together because they have no time for it.

[casstony]

It's not just dobs - there's a bunch of nice refractors in the classifieds too, and quite a pile of good gear generally. I don't think I've seen the classifieds more busy. I'm selling my dob just because I've decided I prefer SCT's - all types have their strengths/weaknesses and we all develop different preferences.

[erick]

A dob-buyers' market - Isn't it great!

No Kevin, there is nothing philosophically wrong with a dobsonian-mounted reflector. There are advantages and disadvantages to each form of scope/mount configuration. Everyone has their preferences. In my view the only thing more intuitive than a dob is an SCT or refractor on a fork mount, but not by much. A cheap mass-produced dob will work, but may be better with a little bit of DIY on the bearings. Plenty of threads on such matters here.

Of course, there is the infamous "dob hole"! Eeeek! But there are easy ways around that.

[leinad]

You'll likely just find that most users started out with 8",10" or a 12" Dob, and have since upgraded to newer more expensive equipment, hence are selling their old equipment they no longer use.

I would say Dob's are "the best way" to get started. Views.. amazing!, Ease of use/Simplicity... awesome! Beginner astrophotography(solar/deep sky).. suitable with accessories.

Nothing wrong with standard dobs at all. Flex Dobs? Try mounting one on a GOTO mount.

Just last year I recall there were eight to ten Meade 12" LX200's up for sale almost 2 weeks in a row!! :P

[supernova1965]

Hi Erick

I may be sorry for asking this but "What is the dob hole":scared3:

[Screwdriverone]

Its the bit of sky directly overhead where the Dob cant see when it is pointing at Zenith (I think). Not so much cant see, but just cant comfortably view without tube contortions which go against the normal nudges, try it and you will see what I mean.

I havent had the issue, but its there apparently.

Someone will correct me if I am wrong, I am sure...

Cheers

Chris

[supernova1965]

Now I know why Erick was scared of it I have had one experience of it most annoying :thanx:

[erick]

"Dob hole" - Pointing a dob to the zenith then trying to track the object by hand. You have typically lost mechanical advantage in the Azimuth direction so it is hard to move the OTA as you wish.

Solutions:-

1. Go and look at something else for an hour or so until the object of interest has moved away from the zenith.

2. Prop up one side of the base board on a brick, therefore moving the "hole" away from the zenith (where the object of current interest is).

3. Perfect a way of moving the OTA in the azimuth direction with a two-handed approach. Keep one hand on the top of the tube. Reach down with the other and grasp the base at one of the corners and rotate it in azimuth that way - grab it where-ever you can get some mechanical advantage. If you have a stalk for an Argo Navis or something connected to the base (eyepiece rack?), that can be a good point to use.

[mental4astro]

Don't forget, you can always 'Dobinise' your eq. mounted reflector too.

The three I've made, two 4.5" and one 8", work much more smoothly than commercially made mounts. Soooo nice to use.

You can also control the amount of Zenith play with a DIY mount, reducing the challenge of the 'Black Hole'. You won't totally eliminate it, but it just becomes a 'trick' with swinging the scope around.

[floyd_2]

I started with a 4" tasco reflector (which hooked me well and truly when I saw saturn through it), moved to a 10" dob, then to a 15" obsession, then a 4" TV refractor, then a 10" SCT. This all happened over quite a number of years of course.

I must admit to looking at all of the dob ads in the classifieds and thinking that I would like to get one for pure visual. There's something about a dob that's nice and easy to use. You just plonk it down and get to it! With my SCT, I wheel it out, polar align it, drift align it, hook it up to a computer, set focus on the guidescope and main scope and on it goes. It's not simple like setting a dob up and having a browse around the heavens.

It's all horses for courses though. If you want to do imaging, setting up isn't so much of a problem as a necessity, and I don't mind doing it at all. For pure visual, however, you can't beat a big dob and a chair to star hop around the heavens with!

[gary]

Hi Kevin,

Great to hear you have aperture fever.

One advantage the Dobsonian concept has is that the mirror size can be scaled up to
large apertures whilst keeping the cost of the mount itself low. Whereas there will
be many readers of this forum who will remember the not so distant past when a
scope of 12" aperture would be the envy of many others in their club, these days
Dobs with apertures of 10" and 12" are highly affordable and have become
ubiquitous. Their ease of portability and the "bang for the buck" they provide mean
they have become commodity items in the telescope world.

As a far as ergonomics are concerned, many people tend to find Dobsonian
mounts easier to push around the sky compared to Forks and GEM's.
Though the low cost, mass produced Dobs coming out of the Far East do the
job as far as pushing them around goes, when one grabs hold of a premium
commercial mount or a well designed and constructed home-built mount and
pushes them around, the difference in "feel" can be stunning. If you have
ever driven a performance motor car and compare its handling to the commodity
family sedan, you will know what I mean. A good Dobsonian mount does
not come about by accident. It comes from good engineering, combined with
the right combination of materials and careful construction.

By way of example, if you get the opportunity to go to a star party, ask a owner
of a premium large aperture Dob whether you may be permitted to give it a bit
of a push around. David Kriege of Obsession Telescopes uses the expression
"buttery smooth". The bearing surfaces have just the right combination of
friction and stiction so that when you push the scope, it does so without
hesitation and when you stop pushing the scope, it stops moving. When the
apertures of the scopes become large enough that you require a step ladder,
say in the 18" to 48" class, this becomes a highly desirable attribute.
You want to be able to nudge the scope whilst at the eyepiece almost without
being conscious of it. Another premium scope builder is SDM in Victoria and
there are many other excellent commercial premium scope builders.

As I mentioned, some users go the hard yards and build their own. "The Dobsonian
Telescope" is a title of a book by Kriege and Berry which is a must-read for any
prospective Dob builder. Dobsonian design has evolved over the years and
drawing upon the experience of those who have gone before is recommended.

As Eric mentioned, when Dobs point toward the zenith, they are getting close
to a singularity that makes them awkward to push - the so called "Dobson's Hole".
You will know from your experience of other mount types that ergonomically they
have a region of the sky that makes them awkward in some way to use. For
example, an equatorial mounted fork can be troublesome to use near the
celestial pole.

Beyond maneuvering a scope, the other consideration is whether you need to
physically transport it, namely in a car. Whereas a 10" or 12" solid tube scope
can fit across the back seat of most cars, for larger apertures, a Dob that
is designed to be easily transported enables it to be taken to dark sky locations.

Best regards

Gary

[Louwai]

4. Argo & Servo-Cat.. ;)

[erick]

I'll stick with my three and put the $ difference (the Servo-Cat) to other uses :D