Setting Circles on a Dobsonian..confused.com

[Simon Dunsmore]

Hi Folks,

Could someone help to cool my overheating brain?

Thinking about adding setting circles to a 10" dob when I finally get one. I saw the idea in the attached picture but for the life of me can't get my head around how that would work. If the scale is attached to the rotating part of the base, how would you know what degree you are moving the scope to?

Unless, the scope was aligned to North (but wouldn't this have to be true north? and then the scale was fixed so that 0 degrees was at the front and centre of the base.

I guess the other alternative is to sit the whole base on another separate base with a rotating circle, so that you could align on a known star and then turn the dial to the correct degree. This would then give you a fixed point from which to navigate??

The first option seems much easier but, as I say, I can't figure it out. Also whilst we are on the subject where would one find a printable scale to fit a Skywatcher 10" dob base?

Thanks folks

 

[happy-kat]

On my heritage I have a scale on the top board, but if on top board the numbers go in reverse, and use a magnetic strip around the bottom board underneath which I attach a marker to and use the pointer to mark on the scale when I have pointed the telescope to a known star, then I can use that position to move to the next target. Needs a level siting.

[vlaiv]

First azimuth:

Scale is on top circle but hand is fixed to bottom one. Scale rotates and hand is fixed:

You setup your scope so that it is pointing north and hand is pointing to 0 degrees on scale.

Same is for altitude.

You will need some sort of application on your phone / tablet or something similar to convert RA/DEC coordinates of the object to Alt/Az coordinates for your location at a given time (calculation is not trivial - so either pre print some tables of times for each object or use electronic aid).

[Simon Dunsmore]

18 minutes ago, vlaiv said:

First azimuth:

Scale is on top circle but hand is fixed to bottom one. Scale rotates and hand is fixed:

You setup your scope so that it is pointing north and hand is pointing to 0 degrees on scale.

Same is for altitude.

You will need some sort of application on your phone / tablet or something similar to convert RA/DEC coordinates of the object to Alt/Az coordinates for your location at a given time (calculation is not trivial - so either pre print some tables of times for each object or use electronic aid).

Hi, 

Thanks for your reply. 

So with this method, I guess you have to keep tweaking the whole mount to align it? 

This may be a silly question, but will simply getting polaris in the finderscope be accurate enough for alignment? 

Thanks 

[happy-kat]

I added relative setting circle approach to mine. The second provided above is absolute setting circle approach.

[vlaiv]

4 hours ago, Simon Dunsmore said:

Hi, 

Thanks for your reply. 

So with this method, I guess you have to keep tweaking the whole mount to align it? 

This may be a silly question, but will simply getting polaris in the finderscope be accurate enough for alignment? 

Thanks 

Precision of setting circles was always a questionable thing.

Suppose that you can accurately read off half a degree of that scale. This is probably the most precision you can get as diameter of the base is probably around 50cm. This means that circumference is about 157cm and single degree is about 4mm.

With 32mm plossl you'll get x37.5 magnification or about 1.33 degrees TFOV.

In principle - you should be able to put a target somewhere in FOV of your finder eyepiece.

Similarly, regular 7x50 finder has something like 5 degrees TFOV and if you put Polaris in the center - you'll be in principle within one degree from NCP. That should still provide enough precision to land a target in FOV of finder eyepiece.

BTW - you can make a slight tweak to above design that will enable you to roughly maneuver mount in position and then adjust settings circle arm to "zero it in". Just attach it in such way that it can move/rotate slightly to provide you with couple degrees of adjustment (remember ~4mm is one degree so you really need only a few centimeters of motion).

[Pixies]

You can print off the scale here:

https://www.blocklayer.com/protractor-print.aspx

[Simon Dunsmore]

3 hours ago, vlaiv said:

Precision of setting circles was always a questionable thing.

Suppose that you can accurately read off half a degree of that scale. This is probably the most precision you can get as diameter of the base is probably around 50cm. This means that circumference is about 157cm and single degree is about 4mm.

With 32mm plossl you'll get x37.5 magnification or about 1.33 degrees TFOV.

In principle - you should be able to put a target somewhere in FOV of your finder eyepiece.

Similarly, regular 7x50 finder has something like 5 degrees TFOV and if you put Polaris in the center - you'll be in principle within one degree from NCP. That should still provide enough precision to land a target in FOV of finder eyepiece.

BTW - you can make a slight tweak to above design that will enable you to roughly maneuver mount in position and then adjust settings circle arm to "zero it in". Just attach it in such way that it can move/rotate slightly to provide you with couple degrees of adjustment (remember ~4mm is one degree so you really need only a few centimeters of motion).

Thanks for your help. Just a thought, would it work, rather than aligning the base, fixing magnetic tape around the lower base, aligning the scope on a chosen star with known azimuth degree. Then fixing a metal pointer to the correct number?

Rather than moving the base, move the pointer? 

[happy-kat]

that's what mine is setup like

[Simon Dunsmore]

5 minutes ago, happy-kat said:

that's what mine is setup like

OK, that's good to know, thanks. Does the tape work OK? What did you use as a pointer? 

The other thought I had was to stand the dob base on a metal disc and then use a magnet with a pointer attached. 

Sorry for all the questions. 

 

[happy-kat]

I used magnetic tape under the bottom board and the pointer was a jam jar lid with a soldered on arm. There are some awesome setting circle diy projects on here with how they did it and photos have a search and check them out.

[vlaiv]

I like the one where scale is glued on the bottom part and top part has part of it cutout and hand is made from simple piece of wire. Let me see if I can find a picture of it.

Something like this:

This one does not appear to have moving hand - but there is plenty of room to accommodate one. That should help with initial setup - base needs to be pointed in rough direction of north and then hand can be adjusted on a star or similar.

[happy-kat]

If you don't want to chop your mount up this is one with a relative moveable pointer, scroll down their blog.

https://stargazerslounge.com/blogs/entry/1760-sw-200-dobson-eyepieces-my-mods/