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Calling all Large Dobsonian Owners

Moonshane

By Moonshane
in DIY Astronomer

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Moonshane

Moonshane

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Hi all

I am in the early planning stages of putting together an 18" dob in 2011. Got a long way to go and am reading Kriege and Berry at the minute. I also hope to get to see and see through a few large dobs at the SGL starparty in the spring and compare with my 12".

What I am hoping for from this thread is comments from people who own or have used for a period of time large dobs - 14" and bigger perhaps - and especially but not exclusively those who have built their own (not including glass which I'll buy).

If you can please share your experiences and maybe provide such details as :

  • What you have?
  • Would you do it again (make or buy a large dob)?
  • Did it meet your expectations in terms of performance?
  • Does it fit in your car?
  • Any regrets, problems etc?
  • Do you have light pollution where you live and does this affect things badly?
  • Any advice for a first time builder (I have a joiner/welder father in law so don't worry about me being a mad fool to take on such a project!).

Thank you and best wishes

Shane :(

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sixela

sixela

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What you have?

Self built 16" scope with Oldham primary salvaged from a David Lukehurst Dob (and Antares Optics secondary).

Would you do it again (make or buy a large dob)?

In a heartbeat

Did it meet your expectations in terms of performance?

Yes.

Does it fit in your car?

Yes.

Any regrets, problems etc?

No.

Do you have light pollution where you live and does this affect things badly?

Yes. Observing at home limits itself to small planetaries, the moon and planets.

Any advice for a first time builder (I have a joiner/welder father in law so don't worry about me being a mad fool to take on such a project!).

Read K&B's book. Don't take it as gospel, though, and see some lighter and more modern designs from European ATMers.

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Doc

Doc

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Is it being used with an EQ Platform if so why not build the ground board to be a part of the Eq platform.

I saw one once on CN and it looked really good, I see if I can refind it.

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Moonshane

Moonshane

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I wouldnt get or make any scope thats so large I need a step or a ladder to view through it.... but thats just me :eek:

thanks Shaunster

I completely agree with you. assuming f4.7 and 18" I should get away with being on my feet mainly but will need a small step for zenith views. that's the plan anyhow.

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Moonshane

Moonshane

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Self built 16" scope with Oldham primary salvaged from a David Lukehurst Dob (and Antares Optics secondary).

In a heartbeat

Yes.

Yes.

No.

Yes. Observing at home limits itself to small planetaries, the moon and planets.

Read K&B's book. Don't take it as gospel, though, and see some lighter and more modern designs from European ATMers.

cheers Alexis - much appreciated.

K&B is a great book but I agree there are some superb designs from Europe too. I will possibly end up with a chimera of the two.

re your LP comments, is it the LP or the size of the scope that's the main reason for your comments? eg would a 12" scope in your situation show more objects?

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Moonshane

Moonshane

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Is it being used with an EQ Platform if so why not build the ground board to be a part of the Eq platform.

I saw one once on CN and it looked really good, I see if I can refind it.

hi Mick

cheers for that - I think I saw that article and may well do as you suggest. seems to make complete sense.

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Peter Drew

Peter Drew

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I've got 17" and 30" self built Dobs with a 42.5" in long term progress. The existing pair were built over 25 years ago and are "clssical" no frills constructions calling for very little in the way of expensive materials or woodworking skills. The 30" is to be rebuilt to more modern standards for integrating video work which places less demands on drive accuracy. The 17" has given great lunar, planetary and DSO views and certainly lived up to expectations. The 30", built in 1985 to view Halley's Comet, has been disappointing for lunar and planetary performance due mostly to the aperture being very susceptical to seeing conditions. DSO's of course were a different matter, bringing into view objects that are beyond smaller apertures, however, for objects that are well seen in smaller telescopes, the crisper star images and more stable focus seemed to me to give a more pleasing view. Downsides include small field of view, forget looking at M31, M42 or large star clusters, the great height to the eyepiece calling for unweildy "ladder" constructions and if not used in an observatory, problems with a breeze let alone wind. If you like a challenge or would like to see DSO's normally out of reach then the project is worthwhile. Overall though, something half the size would get more use.

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ollypenrice

ollypenrice

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Our 20 inch was ATM built (not by me) from the book you mention. It is great. Beacon Hill Optics, f4.1. Wow factor is huge. I brought in down here in an Astra estate but it is permanently mounted. To travel regularly? Well, I guess that is why it was for sale...

Pic here, astronomy page. Sorry, it won't link directly for some reason. :: Sun * Star * France ::

Olly

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Moonshane

Moonshane

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I've got 17" and 30" self built Dobs with a 42.5" in long term progress. The existing pair were built over 25 years ago and are "clssical" no frills constructions calling for very little in the way of expensive materials or woodworking skills. The 30" is to be rebuilt to more modern standards for integrating video work which places less demands on drive accuracy. The 17" has given great lunar, planetary and DSO views and certainly lived up to expectations. The 30", built in 1985 to view Halley's Comet, has been disappointing for lunar and planetary performance due mostly to the aperture being very susceptical to seeing conditions. DSO's of course were a different matter, bringing into view objects that are beyond smaller apertures, however, for objects that are well seen in smaller telescopes, the crisper star images and more stable focus seemed to me to give a more pleasing view. Downsides include small field of view, forget looking at M31, M42 or large star clusters, the great height to the eyepiece calling for unweildy "ladder" constructions and if not used in an observatory, problems with a breeze let alone wind. If you like a challenge or would like to see DSO's normally out of reach then the project is worthwhile. Overall though, something half the size would get more use.

cheers Peter - this is great information. I have worked on the basis of the max I can realistically get through my doors and in the car myself - hence 18". I'd sooner avoid ladders too and I reckon a small step will be all that's needed for me (at 6'3") to use at Zenith. I am interested in the seeing/aperture comments and am keen for this reason to retain either my current 6" or my planned smaller 8.5" f8 dob (I bought the mirrors for this at a snip) for planetary /lunar use.

Our 20 inch was ATM built (not by me) from the book you mention. It is great. Beacon Hill Optics, f4.1. Wow factor is huge. I brought in down here in an Astra estate but it is permanently mounted. To travel regularly? Well, I guess that is why it was for sale...

Pic here, astronomy page. Sorry, it won't link directly for some reason. :: Sun * Star * France ::

Olly

thanks Olly.

One question - how tall is the person in the pic (is this you?) and is the scope on the ground? OK that's two questions. It looks taller than I imagined a 20" f4.1 would be.

cheers

Shane

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sixela

sixela

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would a 12" scope in your situation show more objects?

It would show even less.

All the data from Blackwell and analysed by others (Clark, Nils Olof Carlin, Mel Bartels, John Tatarchuk) confirms it too.

A small scope won't help you, unless the object is so big that it doesn't fit on the larger scope.

But the same theory also predicts that small high surface brightness objects are going to be better with a lot of light pollution than larger objects with a low surface brightness (and thus low contrast ratio with respect to the background). And that is also true.

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ollypenrice

ollypenrice

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cheers Peter - this is great information. I have worked on the basis of the max I can realistically get through my doors and in the car myself - hence 18". I'd sooner avoid ladders too and I reckon a small step will be all that's needed for me (at 6'3") to use at Zenith. I am interested in the seeing/aperture comments and am keen for this reason to retain either my current 6" or my planned smaller 8.5" f8 dob (I bought the mirrors for this at a snip) for planetary /lunar use.

thanks Olly.

One question - how tall is the person in the pic (is this you?) and is the scope on the ground? OK that's two questions. It looks taller than I imagined a 20" f4.1 would be.

cheers

Shane

It is I! I am a strapping gnome of five foot five. The Dob is standing on a lowish concrete block but new guests often just laugh when they see it. An F5 is seriously tall. We had one visit last year.

Olly

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Moonshane

Moonshane

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nice to see you in the real world Olly!

This makes me happier as at 10 inches taller than you my eye would not be too far from the EP at zenith with your scope so it's much as I hoped it would be when I translate this to my own planned specs.

yeah, a 20" f5 would be a bit tall for me too, 100" focal length as opposed to 84" for my plans.

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Moonshane

Moonshane

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probably more than I'll get too (weekly).

the good thing about this size is that it's manageable (can fit through my doors) and in theory if I build it right should take less time than the scope takes to cool, to set up properly.

still only in the mind stage but I am keen and determined to see it through.

An EQ platform for my current dobs is first though.

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swamp thing

swamp thing

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Hi Shane

To answer your original questions:

~I have a 16" lightbridge

~i would not buy another big dob. I would build my own and will when finances allow.

~It filled my expectations (knew it would need tweeking) but the views are awesome.

~ I have a van so no probs there. Plus I am currently converting an army truck to transport and house it

~I only regret I bought one rather than built my own.

~ I live in moderately LP area so i always transport my scope. up and down stairs, through doors, then 45 mins in the van, you name it, it don't bother me. i would carry it all the way to the observing sight if i had to for the views. Personally i think it defeats the object to have a big dob then use it under street lights but thats just me.

~My only advice would be, it aint hard to build your own. I built my 10" after i bought the 16 and didn't reallise it would be so easy and so rewarding.

Hope this is of some help.

Regards Steve

ps. I am new to computers so don't understand the smilie face etiquette so please don't be offended that i haven't used one.

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acey

acey

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I wouldnt get or make any scope thats so large I need a step or a ladder to view through it.... but thats just me :icon_eek:

Me too. For an 18" anything slower than f4 (and higher than an "ultra-portable" profile) would require steps when looking near the zenith. For a long-tube scope that didn't need to be transported my preference would be something like James Nasmyth's design.

post-14602-133877517633_thumb.jpg

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