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Downside to Dobsonians?

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#26 Kunama

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Posted 24 November 2020 - 05:04 PM

Only downside of having a big Dobsonian is getting to the eyepiece myself, seems all the owners of small scopes have developed a natural tendency to migrate to someone else's big scope undecided.gif   Why is that confused1.gif

 

 

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Edited by Kunama, 24 November 2020 - 05:05 PM.

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#27 brentknight

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Posted 24 November 2020 - 05:10 PM

I do not consider needing a coma corrector a "disadvantage"

 

I consider it a part of the cost of a big Newtonian (as it helps scopes as slow as F/6 al the way to as fast as you dare.)

It's a disadvantage when the CC costs a significant percentage of the original telescope.  But then it's still relatively cheap compared to the thousands of dollars required to correct a refractor...

 

The joys of scope ownership...


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#28 junomike

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Posted 24 November 2020 - 05:29 PM

Downfall of a Dob is the size.  When you're using it (viewing), you wish it was 50% larger, but when you're setting up or taking down you wish it was 50% smaller.


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#29 Kunama

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Posted 24 November 2020 - 05:49 PM

Downfall of a Dob is the size.  When you're using it (viewing), you wish it was 50% larger, but when you're setting up or taking down you wish it was 50% smaller.

or you can do what I just did..... I bought a vehicle that the Dob can be wheeled into without any dismantling..... a pair of telescopic aluminium ramps is all that is needed... flowerred.gif


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#30 brentknight

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Posted 24 November 2020 - 05:52 PM

Another downside that I just though of...always wanting more of them.



#31 SteveG

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Posted 24 November 2020 - 06:10 PM

They don’t look as cool?


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#32 SteveG

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Posted 24 November 2020 - 06:11 PM

Only downside of having a big Dobsonian is getting to the eyepiece myself, seems all the owners of small scopes have developed a natural tendency to migrate to someone else's big scope undecided.gif   Why is that confused1.gif

Now THAT, is a dining room!



#33 MitchAlsup

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Posted 24 November 2020 - 06:15 PM

It's a disadvantage when the CC costs a significant percentage of the original telescope.  

When this is the case, the primary mirror is too small !

 

Once you get 2-3-4 big DOBs the CC vanishes into the noise--in terms of cost.


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#34 Jon Isaacs

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Posted 24 November 2020 - 06:43 PM

Eddie, That's why I was going with a 14" f3.5, not one of the mass produced pieces.  There was a gentleman in CA that purchased a DobStuff brand DOB and it looked VERY manageable for that size. Not sure what the exact weight of the whole scope or components was though.  Very fast cool down with a sandwich type mirror as well.  Visual only.

 

Now this looks almost grab n go.  I like the styling also.  Did you make it yourself?

 

vdog, That's why I was going with a 14" f3.5, not one of the mass produced 'boat anchors'.  There was a gentleman in CA that purchased a DobStuff brand DOB and it looked VERY manageable for that size and it had a sandwich mirror for extremely fast cool down with the advantage of lighter weight.

 

Things get very tricky at F/3.5.

 

The collimation tolerance for an F/3.5 primary is 0.21mm.  That's 0.008 inches. A piece of typing paper is about 0.004 inches thick. That's very tight. You need a structure that is rigid enough that the collimation does shift from the horizon to the zenith, a mirror cell that does not shift.. That's probably not a Dobstuff.

 

And then you need the collimation skills and tools to precisely collimate to such tolerances.  This is doable but it's not something I'd recommend for a first Dob.

 

And then there's the mirror. F/3.5, very difficult. Hubble Optics might claim something, what really is... 

 

You don't see commercial Dobs at F/3.5 and you don't see many ATM Dobs or premium Dobs at 14 inch F/3.5.  A 14 inch F/4.5 has a very comfortable 63 inch focal length, the eyepiece will be about 60 inches at the zenith, and it doesnt have the optical difficulties of an F/3.5.

 

In terms of portability, the focal length of a truss Dob has zero effect, the difference between an F/3.5 and an F/4.5 is the length of the truss poles.. 

 

Personally, I've owned my 12.5 inch F/4.06 for 20 years. I'd only want a scope faster if it kept me from climbing a ladder. I'm happy with my 16 inch at F/4.4..

 

Dobs are great.. fast Dobs, and F/3.5 is very fast, have issues of their own. Best to have some experience with Dobs and fast newts before tangling up with one.

 

Jon


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#35 Pinbout

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Posted 24 November 2020 - 06:52 PM

 

So, what is the downside to DOB ownership?

you may find its easier to setup and you'll spend more time away from the wife... and she'll start complaining you don't give her enough attention anymore and she'll make you spend more $$$ to show her you still love her as much as you did when you first met and you'll go broke.

 

that's the down side.

 

a 14" would be just as easy and almost as small

 

https://www.youtube....h?v=G3xu8CNBe0k

 

 

for goto, I shoot my 301 green laser thru a 35mm eyepiece and whatever I point at, its in the eyepiece.

 

keep things simple.


Edited by Pinbout, 24 November 2020 - 06:55 PM.


#36 Sheol

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Posted 24 November 2020 - 06:53 PM

           Only issues I have ever had with my 8 inch Dob is Dobson's hole. And that can be corrected/hacked/worked around by the diligent observer.

 

   Clear Skies,

      Matt.



#37 MitchAlsup

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Posted 24 November 2020 - 07:10 PM

you may find its easier to setup and you'll spend more time away from the wife...

If you have chosen wisely, this is not a problem..........


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#38 cuzimthedad

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Posted 24 November 2020 - 07:17 PM

If you have chosen wisely, this is not a problem..........

It's been known to happen that my wife will hand me an eyepiece and say "Get out of the house" making it my choice of which scope to take with me.


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#39 Echolight

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Posted 24 November 2020 - 07:29 PM

I am going to disagree--to an extent.

 

There are advantages to speed in that size:: take my 13" F/3

 

attachicon.gif13newbase02.JPG

 

Notice that the primary is at hip height, above the dust and moisture of the ground.

Notice the assembly is so short one can hold the top and the altitude bearing at the same time, making precise altitude adjustments more precise.

 

The only downside is one needs very short FL EPs in order to operate at high power.

I love that setup. About the coolest looking telescope I've ever seen.


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#40 Tyson M

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Posted 24 November 2020 - 07:35 PM

David,

 

A premium Dobsonian has very few downsides.  High quality mirrors, smooth yet accurate motions, significant size and weight reduction.

 

Your absolutely right about the "boat anchor" Orion and SkyWatchers.  I've been there and the best thing I did was to get the base rebuilt by DobSTUFF.  But I don't think you need to go as fast as F/4 if your looking for a manageable size.  The F/4.6 in my old XX14g never gets high enough that I can't sit in my adjustable chair...

 

attachicon.gifXX14g on DobSTUFF.jpg

 

If you can go without adding motors, you'll save yourself a lot of unnecessary hassles as these scopes are much easier to move manually than the mass produced ones.  Get a Nexus or something similar if you need the Push-2 (Star-hopping works just fine for me).

Nice scope.  Love the dobstuff base.  How tall are you?


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#41 sw196060

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Posted 24 November 2020 - 07:48 PM

I remind my wife I could be spending my evenings a ta a bar with friends instead of star gazing at home.


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#42 lphilpot  Happy Birthday!

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Posted 24 November 2020 - 07:53 PM

I had a 14.5" f/4.5 TeleKit, which was hardly small (structurally) for its aperture. Even sitting on top of 5.5" of EQ platform, the eyepiece was dead level eye-height for me at zenith. Perfect. I'm over 6' tall, but there's wiggle room. ~15 inches is a nice sweet spot in scope size.


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#43 Neptune

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Posted 24 November 2020 - 08:03 PM

Thanks Guys for the info. Some of it I had not thought about.  I was definitely planning on a SIPS for coma correction.  I will continue to investigate this venture.  I am now wondering if I really need GOTO.  I started out without GOTO many years ago.  Some call it a crutch others call it a modern convenience.  I was trying to get a slightly larger scope (always wanting larger) with better portability, hence the f3.5 design. I can see how the GOTO would ad complications. Maybe a DOB drive platform is really all I need.  Here is a pic of a completed scope 14" f3.5.  Not looking to bash the scope but constructive comments would be ok if it's ok with the moderator.

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  • Dobstuff 14in f3-5.jpg

Edited by Neptune, 24 November 2020 - 08:13 PM.

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#44 Jon Isaacs

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Posted 24 November 2020 - 08:10 PM

was trying to get a slightly larger scope (always wanting larger) with better portability, hence the f3.5 design.

 

F/3.5 only makes the poles shorter, it doesn't make it any more portable.

 

This is my 12.5 inch F/4.06. It has a 52 inch focal length. A 14 inch F/3.5 has a 49 inch focal length. And no, I'm not 7 feet tall, just 6 ft 1/2".

 

6060550-Jstar in back of escort.jpg

 

And if you want it to perform at high magnifications, you need to get a premium mirror from Mike Lockwood or someone similar and then build a structure to handle the fast optics.

 

Jon


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#45 Kunama

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Posted 24 November 2020 - 08:33 PM

I think the key to a successful sub-F4 scope is to have a very, very stable structure.  As mentioned before, the collimation tolerances are very small so one needs to be sure that the design prevents the secondary shifting as a response to altitude changes and that the primary mirror similarly stays in place once collimated.

 

I addressed those aspects in my 18"F3.5 build by making a welded aluminium structure with very stiff but lightweight carbon poles and a relatively lightweight UTA using carbon/kevlar/titanium and aluminium components....  (unfortunately this has a negative effect on one's wallet)

So far it has shown that the extra effort was worthwhile as I have no noticeable collimation shift between zenith and lower altitudes.  My primary suspension is by a sling that hangs from linear bearings.

 

If you're planning a simple wooden structure I would suggest staying at F4 or F4.5 to minimise collimation stability issues.  As Jon mentioned, the length of poles is the only change regards portability and the structure can be made lighter....


Edited by Kunama, 24 November 2020 - 08:50 PM.

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#46 MitchAlsup

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Posted 24 November 2020 - 08:39 PM

I love that setup. About the coolest looking telescope I've ever seen.

One might think that a scope up that high would be unstable. I have not found this to be a problem because the plane of the leg-pairs intersect the CoG of the scope, transferring forces directly into the ground. The vibration damping time is way under a second.

 

In my tool kit, I have a trio of hardened steel "nails" I made to fit holes in the ground feet. I can pound the nails into the ground and it would take a tank to move the scope location. These nails have a double head making their removal easier wiht the back side of a normal hammer.



#47 brentknight

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Posted 24 November 2020 - 09:28 PM

Nice scope.  Love the dobstuff base.  How tall are you?

Thanks!

 

I'm 5'-9".  Replacing the Go2 mount shaved off over 70 lbs and 6" of height.  Standing up and at zenith, the eyepiece is right at my eye level.


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#48 brentknight

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Posted 24 November 2020 - 09:50 PM

Thanks Guys for the info. Some of it I had not thought about.  I was definitely planning on a SIPS for coma correction.  I will continue to investigate this venture.  I am now wondering if I really need GOTO.  I started out without GOTO many years ago.  Some call it a crutch others call it a modern convenience.  I was trying to get a slightly larger scope (always wanting larger) with better portability, hence the f3.5 design. I can see how the GOTO would ad complications. Maybe a DOB drive platform is really all I need.  Here is a pic of a completed scope 14" f3.5.  Not looking to bash the scope but constructive comments would be ok if it's ok with the moderator.

Go2 is here to stay, so if it's something you want then you should get it.  I personally see it as extra hassle - something that makes me think twice about taking the scope out.  I have the same feelings about tracking and the need for a platform.  If you get a scope that balances well and has fine motion, tracking can be done with just one hand.  I would try it without first and then decide if you feel the need.  There's something to be said about just wheeling the scope out to the driveway and using it...

 

Dennis worked with me very closely to get my mount just right.  He did all the cutting and engineering, and I slapped it together.  I have no hesitation recommending his work.

 

Actually, on advice from Jon, I stuck with the Orion OTA because of the stability inherent in the full truss design.  If you go with a complete build from DobSTUFF, I'd recommend the truss over the poles and I'd highly recommend sticking with something in the F/4.5 range. 


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#49 lphilpot  Happy Birthday!

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Posted 24 November 2020 - 10:03 PM

Go2 is here to stay, so if it's something you want then you should get it.  I personally see it as extra hassle - something that makes me think twice about taking the scope out.  I have the same feelings about tracking and the need for a platform.  If you get a scope that balances well and has fine motion, tracking can be done with just one hand.  I would try it without first and then decide if you feel the need.  There's something to be said about just wheeling the scope out to the driveway and using it...

I agree that goto / tracking adds complexity. Personally I value tracking much more than goto, but if you're talking pure alt-az, tracking pretty much implies goto in most scenarios. However, an EQ platform (and yes, there are limitations and disadvantages) is nice in that it's a separate component - You can use it, or not. If you integrate the platform into your scope, i.e., it becomes the "ground board", then the option is gone. But if you just sit your scope on top of the platform, you can have tracking when you want or not mess with it when it's not needed.

 

But I have to say, once (even roughly) aligned and running, the way an EQ platform becomes invisible -- other than resets -- is great. I've gotten very attached to having an object sit in the eyepiece while I peruse a chart, look up info in SkySafari, etc. That said, if I'm just heading out for an hour or two, I don't worry with it (I always "head out" - observing from home with my Dob is a waste of time and effort).

 

This is in no way meant to trash ServoCat or other alt-az tracking systems, since my "experience" with them is relegated to observations of a few friends with them, but... I can't tell you how many times I've seen them on their knees with a red light after dark, trying to diagnose and fix some issue in the drive system. All the while my platform purrs. :)  Then again, all my sites (and frequent star parties) are within platform latitude range, so anywhere else I'll leave the platform at home.


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#50 Pinbout

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Posted 24 November 2020 - 10:18 PM

My 12” packed for travel 

 

https://youtu.be/tlb_pfZRY8Q

 

211EC089-9CFD-4F63-8BB2-F4A4A9168F46.jpeg

 

DCDABD46-26DF-46E3-AD04-FA331A2073E1.jpeg
 

 


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