Azimuth circle - flaws in my plan?

[BeerMe]

Since I found out I was getting a Wixey with a second-hand Dob I'm buying, I've looked in to the idea of trying to incorporate a PushTo setup using the Wixey and a homemade azimuth circle.  After eventually understanding how the gauge works (thanks everyone from that thread - massive help!) I started considering how to make the azimuth circle.

I searched through many threads and blogs, all of them with different ideas on how best to go about it, from simple paper stuck to the base to complex setups that requires machining parts, etc.  I want to do something that will last, as well as something that is easily moved around the base so that you can use any bright object to align it.  Most of the designs that enable this were the more difficult ones and usually involves placing the circle between the base and azimuth boards in a way that allow it to be freely moved by hand, but that stays put when the azimuth bearing is being turned.  This was my idea to solve the problem, without it getting too technical (although I'm going to wait for the scope itself to confirm dimensions,  etc before I go ahead)..

The azimuth circle will be made by a friend to my spec, using tough, laminated card.  The base of the scope is approximately 52cm in diameter, the azimuth circle will be 60cm in diameter and the card will be like a flange, the distance from the inner to the outer edge of the card being 6-7cm like an extra large polo mint.  Approx 2cm in from the inner edge of the card, I will stick a thin metal strip around the circumference of it.

At the bottom of the baseboard, I will run 50 small self-adhesive magnets around the circumference at equal spacing just a few millimetres in from the outer edge.  The mount will then be sat on top of the azimuth circle, and the azimuth circle can then be lifted a few inches and stuck to the magnets under the baseboard, via the metal strip I'd put round the azimuth circle earlier.  The azimuth circle should protrude about 1.5 inches around the circumference of the baseboard. 

Finally, I'll calculate the line that runs parallel with the OTA and the azimuth bearing, which will be at a point roughly underneath the carry handle for the mount, at the front end of the OTA.  I'll stick a 9v battery pack to the wood with white tape, and run a wire with a small red LED  down this centre line and over the edge of the azimuth bearing, pointing down on to the azimuth circle to illuminate it.  Once I have Polaris  (or any bright object that I know the current azimuth of) in the scope, I should then be able to turn the azimuth circle on its magnets until the desired azimuth bearing is illuminated under the LED.  Once I'm done for the night, I can remove the azimuth circle to store it safely.

Thoughts?  Flaws?

 

[Cosmic Geoff]

Having tried this sort of thing myself, I think you will struggle to achieve the degree of precision required to do anything seriously useful. Except with a very low power/wide field, the degree of precision required to drop an object in the eyepiece field is measured in minutes of arc.  GoTo mounts and digital setting circles can do this, but manual methods will struggle.  To put it another way, if the scheme you propose worked well, one imagines that many Dob owners would be using it. In practice it seems they either star-hop, or resort to expensive GoTo mounts, or (mainly in the US) to equally expensive digital setting circle/push-to systems. 

But don't let me discourage you if you want to give it a try.

[Charic]

Its just a thought, as they have the sizes available! I have a vinyl graphic office nearby, they produce vehicle graphics, and it was my intention some time back to have them design me a compass rose that I could apply over the top surface of my Dobsonian, then replace the support frame. Not only would the graphics look good, and be accurate, they would be water proof, also protecting the top  surface of the base unit from any contamination.

[Astrobits]

As I understand your description, the card disc will be underneath the baseboard. That's where the feet are placed and you might have to move those inward so they don't  foul the disc. That might make the whole telescope less stable. Also, how do you plan to make sure that on rotating the disc you keep it concentric with the rotation axis of the telescope?

Nigel

[BeerMe]

21 minutes ago, Astrobits said:

As I understand your description, the card disc will be underneath the baseboard. That's where the feet are placed and you might have to move those inward so they don't  foul the disc. That might make the whole telescope less stable. Also, how do you plan to make sure that on rotating the disc you keep it concentric with the rotation axis of the telescope?

Nigel

I had considered the problem with the feet, but I haven't seen it yet so I don't know how close the feet are placed to the edge of the baseboard.  Even 1cm of space around the circumference should be enough to get the strip magnets applied.

The laminate will only cover the card and no more round the whole internal circumference hopefully, so if I do need to move the feet it should only be by millimetres.  The plan, also, is to put castors on the bottom at some point - depending on how the feet are, it may be sooner rather than later :-)

[BeerMe]

Sorry @Astrobits about your 2nd point..

I will only need to align the disc once at the beginning  (or again if I life the base and move the scope).  The disc will stay fixed on the bottom board, and the LED will denote exactly what azimuth the scope is pointed at any one time.   So basically, as I turn the azimuth bearing, 0 on the disc will remain pointed towards Polaris. 

[happy-kat]

Reads quite complicated. Setting circles have been added to dobs, I did to my heritage. Plenty threads on this. I chose to use relative coordinates approach rather then go the way of actually always aligning the dob to polaris. The most important thing is the base must be level so either make a leveling base for the dob or add leveling feet to the base.

My base works by get telescope on a known star in region of interest, move moveable pointer to az position of found star then can use that as a position from which to adjust az to object then use wixey to adjust al to find desired object. Does rely on my widest fov eyepiece. I chose this approach as I can't see polaris from my front garden due to a street light.

I'll add a link later there are a couple great threads on adding setting circles to a dob using an aligned base.

[sockgoblin]

hello there. I took the simple approach as described by happy-kat.

A couple of points to note. The setting circle on mine sits on the top board and so the direction of the ascending 0 to 360 degrees is anticlockwise as looked at from above.(If you have it on the bottom board the degrees run clockwise) I stuck a circle of ferrous tape to the underside of the bottom board , and then you only need one magnet on the pointer as it attaches to the ferrous tape and then points to the setting circle on the top board.

No need to align the scope with polaris , just aim the scope at a bright known star, note the azimuth coordinates from Stellarium ( or similar), move the pointer around the board until it reflects the position of the known star. Find out the az coordinate of your target then spin the scope until the now stationary pointer lies over the az of the target. Altitude using a Wixey. ( note that Wixeys remember being level and so you dont need to reset them each time out)

The scope doesnt have to be perfectly level as the wixey measures relative to its own zero (levellness?) , but best to try to levellish as Ive noticed in swinging large angles from the original bright star to the target sometimes the location of the taget is off. So in practice I tend to reset a new bright star in each 90 degrees of sky.

[sockgoblin]

5 hours ago, Cosmic Geoff said:

Having tried this sort of thing myself, I think you will struggle to achieve the degree of precision required to do anything seriously useful. Except with a very low power/wide field, the degree of precision required to drop an object in the eyepiece field is measured in minutes of arc.  GoTo mounts and digital setting circles can do this, but manual methods will struggle.  To put it another way, if the scheme you propose worked well, one imagines that many Dob owners would be using it. In practice it seems they either star-hop, or resort to expensive GoTo mounts, or (mainly in the US) to equally expensive digital setting circle/push-to systems. 

But don't let me discourage you if you want to give it a try.

Depends on the location of the target if its an easy hop from a bright star then star hopping it is . The setting circle approach is very useful when combined with star hopping . If your sky is not very dark and in an area of less obvious constellation signposts then the setting circle approach can put the target within the FOV of my panaview 32mm say 1 degreeish. Clusters and othe brighter dsos are usually then obvious , fainter targets require a final bit of star hopping. Certainly worth a try for a small outlay

[BeerMe]

2 hours ago, sockgoblin said:

hello there. I took the simple approach as described by happy-kat.

A couple of points to note. The setting circle on mine sits on the top board and so the direction of the ascending 0 to 360 degrees is anticlockwise as looked at from above.(If you have it on the bottom board the degrees run clockwise) I stuck a circle of ferrous tape to the underside of the bottom board , and then you only need one magnet on the pointer as it attaches to the ferrous tape and then points to the setting circle on the top board.

No need to align the scope with polaris , just aim the scope at a bright known star, note the azimuth coordinates from Stellarium ( or similar), move the pointer around the board until it reflects the position of the known star. Find out the az coordinate of your target then spin the scope until the now stationary pointer lies over the az of the target. Altitude using a Wixey. ( note that Wixeys remember being level and so you dont need to reset them each time out)

The scope doesnt have to be perfectly level as the wixey measures relative to its own zero (levellness?) , but best to try to levellish as Ive noticed in swinging large angles from the original bright star to the target sometimes the location of the taget is off. So in practice I tend to reset a new bright star in each 90 degrees of sky.

Very nice, that really looks the part!  

I noticed that the feet on your scope are set back a little from the outer edge so you were able to get a ring of magnets on it, did you have to physically move the feet to achieve this or were they set in that position when you received the scope? 

I already have most of the materials on order to do it the way I had planned, although I like the vinyl and the markings are very clear.  There might just be a change of plans in the offing...;-)  is it easy to read in the darkness? 

[happy-kat]

I've got a button battery switched holder used with an led that I painted red that will be used to illuminate my marker as it can be fitted to it so the light moves with the marker. I chose magnet tape strip from halfords that goes all around my base then have a my marker that can go anywhere on it (metal jam jar lid with pointer) and points to the setting circle on the top board.

[sockgoblin]

57 minutes ago, BeerMe said:

Very nice, that really looks the part!  

I noticed that the feet on your scope are set back a little from the outer edge so you were able to get a ring of magnets on it, did you have to physically move the feet to achieve this or were they set in that position when you received the scope? 

I already have most of the materials on order to do it the way I had planned, although I like the vinyl and the markings are very clear.  There might just be a change of plans in the offing...;-)  is it easy to read in the darkness? 

the  feet are as standard on this model. the  ring is metal tape not magnetic tape. I use red torch to see the pointer

[BeerMe]

1 hour ago, sockgoblin said:

the  feet are as standard on this model. the  ring is metal tape not magnetic tape. I use red torch to see the pointer

Yes of course, the magnet is attached to the pointer.   Silly me :-)

A ring of magnets and metal on the pointer would work too I spose, be a shame to waste those 50 magnets I've ordered lol.