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  #1  
Old 18-06-2017, 02:56 PM
Sconesbie (Scott)
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Dobsonian viewing table

Hi all. When I bought my dob it had a home made table. Unfortunately it sits outside and the mdf top has seen better days. Besides the fact I was sick of the scope moving as I spun it around from star to star therefore getting it off north or centre all the time. Just when I put it back and zero it in again, it moved again.

So, I've taken the mdf off (easier said than done as it was screwed down pretty well) and put some treated pine down instead.

To avoid the scope moving, I've drilled three feet holes so it all sits nice and still.

To help me find north when I set up, the little arrow with a saw cut into it pointing down so when I mark a line on the ground, it's easier to line up. This should hopefully save me about half an hour setting up and finding north, checking a set target and manoeuvring until it's spot on.

Just something I figured may help me. It's yet to be tested but may help others who have the same struggles.


Regards
Scott
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  #2  
Old 18-06-2017, 03:28 PM
raymo
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I'm a bit curious, why do you need to find north when setting up your dob?
raymo
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  #3  
Old 18-06-2017, 03:38 PM
Sconesbie (Scott)
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It helps me to know that if my dob is on zero and pointing north, it's much easier to locate targets I want to find. I use sky safari and it gives alt and az co-ordinates and all I do is just spin the ole dob around to whatever azimuth, up we go with altitude and bang, should be there in the eyepiece. In theory.
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  #4  
Old 19-06-2017, 10:30 AM
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mental4astro (Alexander)
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Scott,

Good thinking, every bit helps.

I'll make a couple of suggestions that will go a long way to further improving accuracy:

* Not set up on turf. Turf yields in different directions depending on where the scope is pointed, as does the soil. Plus the turf changes in consistency depending on how long and how much moisture is in the ground. Turf also expires A LOT of moisture at night, and unless your gear is made out of high grade plywood that is also appropriately varnished, this moisture and dew will have detrimental effects on your scope and gear.

* If you set up on concrete or other permanent hard surface, you can take your time to make some permanent and very accurate markings on it to indicate true north. This leaves absolutely nothing to chance when you need to remove your gear from the observing area. "Etched in stone" so to speak. Some people do this also for their astrophoto gear that they need to set up everytime in order to use it. Some people have even gone to the trouble of hiring a professional surveyor to produce these markings for them.

* You might like to place a bullseye bubble level on the rocker box of your dob. This will also greatly increase the accuracy of all your readings. You can fashion a set of levelling screws/legs on the platform. The appliance levelling feet used under stoves and the such are one such gizmo to give you an idea.

* To eliminate guess work from reading the az dial, you can put a block on the platform over the "0" north mark with a little arrow coming off it that points to the dial. It's all a matter of eliminating parallax error.

The more accurate you can make things, the better the quality of the readings. Before the advent of digital setting circles, manual setting circles were used to find stuff (if not by star hopping), for both amateur and professional astronomers. Key to the accuracy of these manual setting circles is getting everything right in terms of gear stability and accuracy of alignment. Get either one of these too off, and it's bye-bye pointing accuracy.

Alex.
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  #5  
Old 19-06-2017, 02:54 PM
Sconesbie (Scott)
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Quote:
Originally Posted by mental4astro View Post
Scott,

* You might like to place a bullseye bubble level on the rocker box of your dob. This will also greatly increase the accuracy of all your readings. You can fashion a set of levelling screws/legs on the platform. The appliance levelling feet used under stoves and the such are one such gizmo to give you an idea.

Alex.

Alex, this adjustable feet idea is a ripper. I am often running around the yard looking for sticks or stones to place under each leg to level it. Then it makes it wobbly and I can spend about 15 minutes just levelling the thing.

I don't sit the scope on the ground. Hence the table/platform. Plus it makes it a bit higher for me to use.

I have thought about using garden pavers in the back yard and would need to make it look like it belongs or would look funny (according to my wife). Anyway, it wouldn't be hard to do.

That said, I'm off to the hardware store to find some adjustable feet.

Thanks.


Regards
Scott
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  #6  
Old 19-06-2017, 04:34 PM
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Nebulous (Chris)
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Quote:
Originally Posted by Sconesbie View Post

That said, I'm off to the hardware store to find some adjustable feet.

Thanks.


Regards
Scott
If you can't find anything to suit, a simple bolt and bracket might do the job for you. 4th photo below all the waffle about the trolley I made. It takes either a 10" Dobsonian or a tripod mount.

http://www.iceinspace.com.au/forum/s...d.php?t=157987

Some hardware places may also stock the kind of sturdy adjusters that are used on top of metal stumps for adjusting the height of joists on buildings, decking, etc. Alas, my local Bunnings was not one of them... hence the bolts.

I like your arrow and scale. Nice job. I'll have to see if I can fit something similar too.

Cheers,

Chris

Last edited by Nebulous; 19-06-2017 at 05:06 PM.
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  #7  
Old 19-06-2017, 04:50 PM
Sconesbie (Scott)
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Another good idea Chris. The bolt and nut scenario could work. It'd mean that I need to have a spanner in my kit but that is no problem.

I did have some wheels I was going to put on the base of the dob to make it easier to move in and out of the house but then the thing would move on the platform I use. That said, I noticed yours has locking mechanisms on them. That would work too.


Regards
Scott
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  #8  
Old 19-06-2017, 05:04 PM
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Nebulous (Chris)
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Quote:
Originally Posted by Sconesbie View Post
It'd mean that I need to have a spanner in my kit but that is no problem.

Regards
Scott
I have found that, because they are fairly chunky nuts and bolts, that I can get pretty good stability just nipping them up finger tight. But a spanner would certainly get them tighter.

Cheers,

Chris

EDIT: I particularly enjoy star hopping and learning the layout of various areas, and I've found it very useful to have the right finder scope. The ones that come with telescopes are usually straight through types and can literally be a pain to use, because of where you have to try and crouch to see through them. I have two finder scopes with right angle erecting prisms on. These do two things for me: a) I can view through them comfortably and b) they turn the view the "correct" way up. Left is left, right is right and moving up and down all do exactly what they would if you were using binoculars or naked eyes.

One is 9x50 https://www.firstlightoptics.com/fin...nderscope.html

And the other is a lower powered 6x30 - but less magnification gives a wider view, which makes it much easier for me to follow charts without having to try and mentally reverse everything. Which gets harder at my age.... Using one of those finders usually gets me close enough to the target to then pick it up with the big telescope.
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  #9  
Old 20-06-2017, 08:50 AM
Wavytone
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Chris ... wingnuts !

The other trick for levelling is to use two large wooden wedges, one on top of the other but pointing in opposite directions. Sliding one wedge relative to the other gives a nice height adjustment, yet the top remains level. Use a Velcro strap wrapped around the ends to hold them in place.

Same technique was sometimes used for levelling floors on stumps.
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  #10  
Old 20-06-2017, 09:30 AM
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Nebulous (Chris)
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Quote:
Originally Posted by Wavytone View Post
Chris ... wingnuts !

The other trick for levelling is to use two large wooden wedges, one on top of the other but pointing in opposite directions. Sliding one wedge relative to the other gives a nice height adjustment, yet the top remains level. Use a Velcro strap wrapped around the ends to hold them in place.

Same technique was sometimes used for levelling floors on stumps.
Yes, wing nuts was my first preference but, alas, the hardware store didn't stock them in the size I needed. A good wingnut is a satisfying thing to use. Fortunately the basic nuts work just fine, and if I did ever need them tighter the toolbox is never more than a few metres away.

As you say, wedges are very useful too. I have several sets in the workshop and quite often find a use for them. Handy for such tasks as levelling doors prior to hanging, and so on. And I did consider them (mostly because they cost nothing when you can make them yourself!) But for convenience of use, especially at night, the bolts won out. I'll still keep an eye out for some wingnuts though....
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  #11  
Old 20-06-2017, 05:56 PM
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omegacrux (David)
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Yup I built it solid , except for the mdf , it's lasted well 😃

David
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  #12  
Old 20-06-2017, 06:32 PM
jimmyh1555 (James)
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Why do you want to find North? Tasssie is down under! I have an equatorial mount and I set it up as follows:
Put tripod with fully extended legs on grass lawn, I then get the axis to point SOUTH by using the same method that Capt Cookie used to find Australia - ie with a magnetic compass. I thought I would be techno and savvy and used my iphone compass. Rubbish. It pointed in different direction each time I switched it on! Use the old fashioned accurate methods - not gimmiky cheap electronics. You need to find what the magnetic variation is in your locality. In northern Tas it is near enough 14 degrees East. (Variation East, steer least. Variation West, steer best!) ie set compass to point exactly to 166 degrees magnetic. That is now pointing 180 degrees True (due South).
Once your tripod is set up Due South, mark around the feet. go and get 3 bricks and carefully cut a brick shaped bit of turf out around each foot. Lift up each foot in turn, insert brick (level with grass) and place foot down on brick. Draw a circle around each foot in paint and hey presto, you can lift tripod off and put it in shed, and next time, place feet back on marks, and you are aligned! Easy, Eh?
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  #13  
Old 20-06-2017, 06:36 PM
that_guy (Tony)
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I think Alt Az coordinate starts at 0 deg from north regardless of hemisphere. Different from EQ coordinate system
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  #14  
Old 20-06-2017, 09:05 PM
Sconesbie (Scott)
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Quote:
Originally Posted by omegacrux View Post
Yup I built it solid , except for the mdf , it's lasted well 😃

David
It's still very much something I use David.
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  #15  
Old 20-06-2017, 09:07 PM
Sconesbie (Scott)
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Originally Posted by that_guy View Post
I think Alt Az coordinate starts at 0 deg from north regardless of hemisphere. Different from EQ coordinate system
Yep. Zero degrees on north. That makes it so so simple to locate targets with co-ordinates from SkySafari.
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  #16  
Old 30-06-2017, 11:35 AM
muletopia (Chris)
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simple stable dob platform

For my wife's 8 inch dob.

End third of an old 44 gallon drum cut off and top removed inside the rim (rusted out in our case).

To set a convenient viewing height a 50mm deep circular trench dug to receive the cut off end of the 1/3 drum. Place the drum and level the top,pack the drum trench, and fill the drum to about 50mm from the top with small rocks (or whatever you have) then fill with concrete and trowel to leveled drum.

About one inch of concrete shows outside the positioned dob to take whatever markings you choose.
Cheers
Chris
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