How to Equatorialize a dobsonian

[EA2007]

Hey kids,

After a long and thoughtful time, yesterday evening was dedicated to finishing mounting an 8inch Celestron Starhopper on a DIY EQ-type mount. And here it is:

Luckily the thing is balanced pretty well at all angles and doesn't fall to one end when positioned either east or west. However, this did come from a few modifications as seen here:

The main issue was getting the scope to stay in place, for some reason the tension handles won't tense anymore, the brass nuts have come loose, so, the handles are just there for extra security when leaning it from side to side. The main item I found useful in giving friction was duck tape! The silver stuff you see here is on both sides of the scope and the black tape works well in keeping it positioned. Also, you may note that I have added another 'nylon-type' wheel to hold the tube in place when is tilted past 90 degrees north.

If anyone has any ideas on how I could possibly add a motor in the future, can you please let me know, also knowing the 'sidereal' rate would be good.

[Sam]

That looks great EA2007! motorising would be tricky, It'll be interesting to see how you get on - might have to copy for my 16".

Sam

[acey]

Looks great! What's your latitude? I'd always assumed that this kind of mount would be a no-no for British observers but maybe not. I'm tempted to see if I can get my scope tilted to 55 degrees without disaster...

You mention sidereal rate for a motor - for one revolution it'd be 24 hours minus 4 minutes.

Andrew

[Astronut]

Interesting. I hope it's well balanced and secure. I'd die if my dob fell over from that angle.

I was thinking of getting a driven purpose-built EQ dob table sometime in the future as they don't seem to lean the 'scope over at 50+ degrees like a DIY wedge mount. Not sure how they work yet though given the small angle that dobs mounted on these EQ platforms seem to lean.

There's a purpose-built alt-az twin motor driven system I've seen for dobs called ServoCAT from StellarCAT (USA) but it seems more suited to Obsession mounts than Skywatcher mounts and it's expensive. Coupled with an Argo Navis digital setting circle system (which I'm seriously considering as a push-to add-on) it would be dob heaven

[The Warthog]

You could equatorialize it the same way they make 'barn-door' type camera platforms follow the sky. Put a nut in place on the edge of the turntable, and drive a shaft in that nut at a speed that will cause the turntable to turn at the correct speed. People use digital stepping motors for this, and you could probably Google some ideas.

For example: http://www.astronomyboy.com/barndoor/

[timetraveller]

what about driving the base with a stepper motor with a friction wheel mounted on the spindle you could bolt the motor onto the side of the mount

[Jamie]

EA2007

I commend you on your idea about equatorialising a dob.

But!

With all due respect, to what end???

A dob is a dob for a reason, (ease of use / simplicity)

Why would you want to equatorialise a dob in the first place?

[timetraveller]

EA can I ask a silly question how is the OTA actually mounted on its base is it in large tube rings because the thought struck me that if it isnt how are you going to use the focus tube or the view finder to see what your aiming at as at some points both will be either right at the top of the OTA or underneath facing groundwards. my reason for asking is with my eq mount when i get into those positions I have to physically rotate the OTA round in its rings just a thought bud regards Pete

[The Warthog]

That's a very good point. You are right of course. Somehow the dob tracking platforms avoid this issue, but they track in a different way that I don't understand and therefore attribute to magic. I understand how this equatorializing method works, however, so it will probably have problems.

[timetraveller]

hi EA

I have been having another close look at your wedge design and to be honest mate I'm even more confused than ever as to how it allows you track objects.By looking at your photo all the addition of the wedge appears to have done is in effect rotated the base around the Altai axis by the angle of the wedge in effect you have just increased the tube angle relative to the ground. I guess what I'm trying to get at is even with the wedge you still only have one genuinely rotatable axis which is on the base the Altai axis is non rotatable unlike an Eq mount which does have two. With your system and your scope with the wedge aligned to the pole star how do you actually get your tube to point south. from both the picture and from what i saw at the star party the OTA cannot go past the vertical on that type of scope as it is stopped by the base itself . This has got me really intrigued >Also how do you counterbalance the tube to allow for the extra weight when a camera is added regards Pete

[EA2007]

Ah lots of questions.....

Firstly, thanks for the comments. In reply to Acey and Lightbucket first, when it was being constructed I needed an angle of 54 degrees, if you look at the image you will see some wedges which hold the unit up, now, when we placed th dob. on the mounting it was way way past where it should have been, i.e. it was angled too much, I know from my garden where Polaris is and this was pointing more towards the level of the moon (as of yesterday evening), so I was thinking, hmm, this is wrong, so, with the wedge being a triangle it actually works out that the wedge needs to be used the other way around, so the scope is placed on the 36 degree side (36+54+90 = 180), this way it points towards polaris. So if you decide to copy my idea then be sure to use a wedge which is 90 degrees + your latitude + the other side! (hope that makes sense) If were talking normal then you would assume that 54 degrees is from the equator, however it is indeed from the North pole, so the North Pole is 0 degrees and the equator is 90 degrees. If you don't understand...trust me!

Now onto Jamie's comment... I have had my dob. for a few good years now and well, not currently wanting say an EQ6 etc, I thought it would be possible to put the dob on a wedge. And it is possible, if you look at the Meade Scmidts then all they are is the OTA, on two forks on a wedge. My idea is the same, its the OTA on two forks on a wedge, but I have the advantage of a nice 8inch newtonian. I don't see any problem with modifying the scope, besides I wanted to see if it was possible. If you look at this scope at mount wilson http://www.astro.virginia.edu/class/oconnell/astr511/im/60-in.jpg then its pretty much the same idea (albeit alot more complex). Whilst not necessarily brilliant for my 8 inch scope, those of us who have say 16 inch dobs, may find this idea pretty unique and get good tracking on a big scope (some steps may be required to reach the top of the scope, but they will have to decide)!

Now onto Timetraveller, I know that using an EQ mount has issues regarding the OTA, you have to get into some awkward positions, mine kinda eliminates this, with the viewfinder most of the time in an upright position. The tube is held in the forks via 2 plastic disks (one on each side), these are held in place via 2 nylon rollers (now 3 on each side) and some hanldes (originally tension handles, but don't tension anymore). The reason I used 3 rollers is that now it is angled at 36 degrees then the OTA was likely to fall out of the forks, so now I can tilt it past veritcal and rotate it north and then tilt it anywhere and it stays in place. There are no issues with focusing or viewing.

In reply to Timetravllers later question, I assure you it tracks well and is held in place brilliantly, suffering from no sliding when angled oddly, the unit can easily point south, in that image it is infact pointing south, to point north, i.e. past polaris, all you do is rotate at the vertical and then tilt down. How about I make some more pics to show you all, or perhaps a video.

[timetraveller]

yep that would be good EA can you do one so that it points to all of the cardinal points then i can see exactly how it moves in relation to the wedge and im not being picky here im looking at a possible way of mounting a camera and big photo lens here for use in astrophotography, Ive seen the barn door type set up and im not happy about using that with a big telephoto attached to the camera but i might be able to modify the dob type of base with a wedge so asmany shots as you can would be good please regards pete

[GazOC]

http://www.roundtableplatforms.com/

I use one of these on my 14" Dob, it works a treat.

[EA2007]

Thats fine Pete, I don't mind having questions asked, especially if there is a major flaw in something, here are some pics and as far as I can tell it points to all the cardinal points.

South:

South East:

East:

North East:

North:

North West:

West:

South West:

Here's it pointing to say, near Vega at 10pm-ish:

Here's the disks where it is mounted to the OTA:

And here's how I have adapted the nylon rollers to take the weight of the scope at any angle, along with the handles which slot into the disks:

Here's a gif. of the rotation from east to west, say tracking Jupiter or something in that area (click to view I think):

[EA2007]

A little note, to those of us who have an eagle eye, you will notice that the wedge has since been painted and also the eypeiece is in a suitable place at all angles, pretty much!

[Sam]

You've done a fine job there EA!!!

I think it's obvious why you want to do this and it is certainly going to give you a few more options like imaging etc so good on ya :thumbright: , in fact for the same reasons I am going to do something like this for my 16" eventually.

One question though, how have you found the balance and stress on the original dob mount - I know my dob would be in trouble if it was put under any sideways stress?

Sam

[EA2007]

Cheers Sam,

Its okay I think stress wise, it doesn't creak or anything and tbh the tube isn't overly heavy and its held well in place.

I would guess that with a 10 or 12 inch tube then you may have issues, a 16 inch scope would probably not work. then again think all the 16 inch dobs are made by Meade and Skywatcher and have a different setup i.e. the OTA's are supported at the base whereas mine is supported in the middle. I haven't as yet addes a camera, I assume it would need balancing but I can always take off the finder scope to balance it out, plus I want a CCD, so that should be lighter anyway.

[EA2007]

Just thought I would put this in.... been messing around with a GIF animator. Gonna make a proper one sometime, like assembly and what have you.

I think you may need to click on it and it may take a few seconds to load and have one slow run through but after that it shows it properly:

[haitch]

Looks great! What's your latitude? I'd always assumed that this kind of mount would be a no-no for British observers but maybe not. I'm tempted to see if I can get my scope tilted to 55 degrees without disaster...

Further North the better with these things - it's not 55 degrees tilt it's 90-55=35 degrees tilt.

[haitch]

You could equatorialize it the same way they make 'barn-door' type camera platforms follow the sky. Put a nut in place on the edge of the turntable, and drive a shaft in that nut at a speed that will cause the turntable to turn at the correct speed.

Only for a very short time though I would have thought. The threaded shaft, being straight, will vary in it's distance from the centre of the axis you are turning as the nut moves along it. A given speed at different radii will equate to different rpm.

Not sure whether it would be up to long exposure imaging but I'd imagine it would be useful at keeping things in the fov for visual observing.

[The Warthog]

You are quite right, and the barn-door trackers are usually used for exposures of less than an hour, with widefield camera lenses. With a dob, the time available would be somewhat shorter because of the magnification. A better idea would be to drive the circular base with a little rubber wheel.

You could equatorialize it the same way they make 'barn-door' type camera platforms follow the sky. Put a nut in place on the edge of the turntable, and drive a shaft in that nut at a speed that will cause the turntable to turn at the correct speed.

Only for a very short time though I would have thought. The threaded shaft, being straight, will vary in it's distance from the centre of the axis you are turning as the nut moves along it. A given speed at different radii will equate to different rpm.

Not sure whether it would be up to long exposure imaging but I'd imagine it would be useful at keeping things in the fov for visual observing.

[acey]

The wedge is a great idea - and I can see now why it obviously has to be at 36 deg rather than 54. If you're at the North Pole then any dob is automatically an equatorial (with a tilt of zero, and the Pole Star directly overhead). The wedge compensates for latitude and makes you "parallel" to how your scope would sit at the North Pole.

What I wonder is how you get it accurately aligned: ie getting your dob azimuth axis exactly collinear with the polar axis.

Andrew

[The Warthog]

Not to be pedantic, but it is 54 degrees from the horizon.

[EA2007]

Yes, lol Warthog your quite right there.

I think I explained further up that to the layman, perhaps 0 would be the north pole and the equator 90, but its the other way around. Hence me originally having it sat at 54 degree's, way over where it should have been.

[acey]

How do you get it aligned? I'd like to try this.