Setting circles on a dob?

[popeye85]

I recently heard someone say that they had modified a dob and put setting circles on the base.

Has anyone else come across this? Seems a good idea tbh

[fiestazetecmk2]

I too would be interested in this.

[steppenwolf]

Yes, I have done this - I added a setting circle to the azimuth axis base with a suitable pointer and a 'Wixey' to the altitude axis.

Azimuth Card Added

 

[fiestazetecmk2]

Thanks. Was it for a 200p skyliner. Could you upload how you done it.or what would it cost to have one made to fit my 200p skyliner. Regards 

Paul.

[happy-kat]

If you use a fixed pointer you would need to effectively polar align the base when observing which is fine if always set up in the same place. If set up location varies could use a movable pointer and do relative adjustments and a setting circle on top of the top board, degrees go anticlockwise in this case.

There are a few how I did mine threads with pictures.

[steppenwolf]

26 minutes ago, fiestazetecmk2 said:

Thanks. Was it for a 200p skyliner. Could you upload how you done it.or what would it cost to have one made to fit my 200p skyliner.

This was on a homebrew Dobsonian mount not a commercial one but the requirements are:-

1. A printed card on the rotating part of the base that sits securely but can be rotated to set it

2. A pointer on the fixed part of the base   OR .......

The alternative method that I used because I had a homebrew square base, not a circular one .......

1. A printed card mounted on the top of the bearing bolt suspended above the rotating part of the base that sits securely but can be rotated to set it

2. A pointer on the rotating part of the base

Here is a link to the article that I wrote for the Sky at Night magazine to explain the project.

[fiestazetecmk2]

Many thanks.will study it and decide what to do.??

[Astro-Geek]

Thanks for that article in Sky at Night Steve.

I used  your idea to make an azimuth setting circle for my GSO 10" Dob. (I use a cheapo electronic digital level for Alt)

I couldn't find a still live download file for the scale so I made one up from other files found on the Internet.

I scaled it to exactly fit the 12" LP (what an excellent idea of yours that was ! )

It works really well, turning  with the scope attached to the central pivot locking knob, yet free to be initially  "set" at North without having to rotate the base itself.

Here's my scale file -

[steppenwolf]

5 hours ago, Astro-Geek said:

Thanks for that article in Sky at Night Steve.

I used  your idea to make an azimuth setting circle for my GSO 10" Dob. (I use a cheapo electronic digital level for Alt)

I couldn't find a still live download file for the scale so I made one up from other files found on the Internet.

I scaled it to exactly fit the 12" LP (what an excellent idea of yours that was ! )

It works really well, turning  with the scope attached to the central pivot locking knob, yet free to be initially  "set" at North without having to rotate the base itself.

It's great to know that at least one person was able to make use of the article and I really like your 'negative' dial - very easy to read! Who'd have thought that old vinyl would have such a second use? ?

[JOC]

Even if you move the telescope I imagine it would still be possible to paint a dot on the base separate to the scales suggested above so that you could align setting the telescope down with compass North and then have the setting circle adjusted in its position relative to that to give celestial North the correct number of degrees out of alignment with compass North.  This should help putting the telescope base down in the right position with just a standard compass setting at a certain location, i.e. if you always set it up in the garden, but move it between uses.  This would work wouldn't it?  Thinking here of how I could adapt this setting circle thing to my own location/situation.

[happy-kat]

If you do it like I did mine you can plonk and go anywhere. mine here

[popeye85]

Can someone please explain how setting circles work? And how useful they would be on a dob. I have always just star hopped to find targets 

[ollypenrice]

46 minutes ago, popeye85 said:

Can someone please explain how setting circles work? And how useful they would be on a dob. I have always just star hopped to find targets 

There are two positioning systems used in astronomy. First there's RA and Dec, in which an object's co-ordinates never vary because the scales are 'printed' on the celestial sphere, so to speak. RA and Dec are the setting circles of an equatorial mount. The other co-ordinate system is Alt-Azimuth in which the scales are locked to the observer's location. To use Alt Az co-ordinates you'll need a planetarium software into which you've keyed the time, the date and the location of your setup. Your object is always moving relative to Alt and Az, the more so the further from Polaris you stray, but the planetarium will, at any given time, provide you with the Alt Az co-orodinates for the object. They are of temporary value but they will tell you the compass bearing and elevation of the object so that you can find it, after which you'll need to track it by hand.

Steve's system is very elegant. I missed that article. 

Olly

Edited October 25, 2018 by ollypenrice

[popeye85]

Ok cheers. So how wood you use that on a dob mount and what setting up is required?

[JOC]

3 hours ago, popeye85 said:

Ok cheers. So how wood you use that on a dob mount and what setting up is required?

Well with planetarium software at any moment for any object you can find degrees right or left and degrees up and down can't you?  So I would have thought if you something to show rotation on the horizontal axis and something that can show degrees up and down-ness on the vertical then surely from moment to moment you could quickly find something in the sky?  Now my 200P comes with a vertical up and down degrees thing, so all I would need is something to show how much it rotated on the horizontal and surely you could find something in the sky.   Isn't that what a setting circle would give?

[saac]

4 hours ago, popeye85 said:

Ok cheers. So how wood you use that on a dob mount and what setting up is required?

Popeye take a look at stepenwolf's article in BBC Sky At Night (6th post on this thread)  it tells you how to use the setting circle together with a "Wixy" (digital angle finder). Here's and extract of Steve's article 

" First, point the telescope at Polaris (nominally 0°) or centre a known star in the eyepiece. Then rotate the setting circle until the pointer aligns with the star’s azimuth. The scope can then be set to any azimuth by rotating the rocker box until the pointer aligns with the required angle. The telescope’s altitude angle can be set by placing the digital inclinometer on the rocker box base and setting it to zero then placing it on the telescope and adjusting the telescope to read the correct angle."  BBC Sky At Night Article

 

Jim 

[popeye85]

2 hours ago, JOC said:

Well with planetarium software at any moment for any object you can find degrees right or left and degrees up and down can't you?  So I would have thought if you something to show rotation on the horizontal axis and something that can show degrees up and down-ness on the vertical then surely from moment to moment you could quickly find something in the sky?  Now my 200P comes with a vertical up and down degrees thing, so all I would need is something to show how much it rotated on the horizontal and surely you could find something in the sky.   Isn't that what a setting circle would give?

Thanks JOC. At the minute I am still trying to get my head around allot of terminology. When words like 'azimiuth' , 'rocker box' and 'polar aligning' are thrown around i get completely lost so was looking for someone to basically explain how this would work using layman terms and I think I have got it now. Cheers 

Edited October 27, 2018 by popeye85

[saac]

2 hours ago, popeye85 said:

Thanks JOC. At the minute I am still trying to get my head around allot of terminology. When words like 'azimiuth' , 'rocker box' and 'polar aligning' are thrown around i get completely lost so was looking for someone to basically explain how this would work using layman terms and I think I have got it now. Cheers 

Polar align  - point your telescope at polaris and adjust it so that polaris falls in the centre of the eyepiece (that will give a rough polar align in a dob)

Azimuth  - the angle ( 0 - 360 degrees) that you have rotated your telescope from North through East direction.

Rocker Box - the part of the Dob telescope that holds the tube and allows it to rotate both up and down (altitude) and around (azimuth).

There are some really helpful tutorials on youtube for general Dob use and use of setting circles. I'm trawling through some at the moment to get information on setting up setting circles using encoders and arduino on the school's Dob. You Tube really is helpful as well as the Hackster site for info, I'd be lost without it.   

Jim 

[popeye85]

Thanks! Think I am getting there! Between this and collimation my head is starting to hurt! Next up is post aligning a eq mount!-that's for another day though!

[DAVE AMENDALL]

I don't think that this is a project for the faint hearted. It requires quite a measure of skill to achieve accuracy in order to make them useful. The astronomers on SGL are to be highly praised for succeeding. here is what could be a useful link on this.......................Dave       Manual Setting Circle for Dobsonian Telescope_ 4 Steps.pdf

Manual Setting Circle for Dobsonian Telescope_ 4 Steps.pdf

[knobby]

An essential read if you're into 'Dob modding' 

 

[Geoff Lister]

These are the additions that I made to my Heritage 130P Dobsonian mount.

For the altitude scale, I used a steel tape measure to work out the curved length of a 90 degree rotation, then used MS Paint to generate a graduated scale, and then adjusted the printer scaling to get the required length on my laser printer. A bit of glue and a coat of varnish gave me a scale. This is my scale, and any SGL members are very welcome to copy and paste, to save re-inventing the wheel.

For azimuth, I bought a magnetic compass that had a diameter about the same as the inside of a toilet roll cardboard tube, cut the tube, and added a second layer - to give a snug fit on the compass; and with a bit of PVA wood glue, stiffened the cardboard and stuck it to the mount. With the telescope aligned on Polaris, I rotate the compass body to read North, (so removing the error between magnetic an celestial North), and then leave the compass untouched for the rest of the session. To point towards the azimuth angle of an object, I rotate the mount until the compass needle is pointing at (360 - [the desired angle]), so West reads 90, South is unchanged at 180 & East is 270 degrees. This saves producing a large printed scale to go round the base.

Geoff

[Chefgage]

I went down the digital setting circle route using a digital rotary encoder and wixey angle gauge for altitude.  The write up of the mods i did are here

 

[JOC]

I wouldn't mind doing something similar to my Dob to get over some of the problems when the Goto plays up - it would also help me to pick up on which calibration stars to use as I could look up their degrees up and down and left and right on Stellarium before I send the Goto off to find them.  Luckily where I am celestial North is within a degree of compass North so more often than not a shunt in the direction of compass North should do for starters.   Accordingly I have produced an old compass and have tied it to the mount.  However, I've been noticing that it is reticent to provide a consistent North point for me.  With the Goto Dob mount is there enough electronics in the base to throw off a compass?  I've been toying with the idea of adding a degree scale around the edge of the part of the base that doesn't move.  The problem is how to know how to create the correct distance between the points to get 360 divisions around the diameter - I guess I need to take a measurement and divide by 30, and then find someway of printing out a set of paper printed tapes at the right size to stick together to mount around the edge.  Then I was just going to mark a central pointer between the two upright legs to mark a centre point which can be twisted to 0 degrees when the compass says North.  Would that work?

[Geoff Lister]

56 minutes ago, JOC said:

it would also help me to pick up on which calibration stars to use as I could look up their degrees up and down and left and right on Stellarium before I send the Goto off to find them

I tend to use the Synscan's "Brightest Stars" 2-star alignment. The first star requires a manual slew to a location shown on the handset, but the handset does not show any indication of mount movement whilst this is happening. I always try to start with the mount & OTA horizontal & pointing roughly North. I set the target altitude, using the mount's scale, and then, standing behind the OTA, slew in azimuth until the OTA looks as though it is pointing at the target. This usually gets the target star in the finder, and sometimes in the FOV of my 32mm Plossl. The second star is an automatic slew.

I was fed up with the handset suggesting a second star, behind a fence, tree or house. So, I spent some time with Stellarium, and produced a table of 14 of the brightest stars visible from my patio, at various times in the year. I then used Stellarium with the time and date for dusk in the middle of each month of the year, and noted the direction (N, NW, W etc.) and the altitude, of 3, 4, or 5 of those stars to give me roughly 90 degrees azimuth and 30-ish degrees altitude difference, to give me good GoTo alignment. I repeated the process for 1 hour before dawn, in case I am up for an early session. Vega, Altair & Arcturus feature strongly, as a first star in my table, each for about 6 (but often different) months of the year.

The "Brightest Star" alignment will also work well as a Planet + 2-star, particularly at early dusk. As long as you can see the planet with the naked eye, (Jupiter earlier in the year, and now Mars) for the manual slew, the mount will perform the other 2 slews automatically and these stars should then be visible in the finder-scope or EP. So you can complete the alignment and let the mount track, whilst you go in for tea, and wait for it to get "properly" dark.

56 minutes ago, JOC said:

With the Goto Dob mount is there enough electronics in the base to throw off a compass? 

I found that the main OTA tube deflected the compass needle more than the Az & Alt motor boxes. On my 250PX, I added a little shelf on the outside of the upright, on the other side from the Alt box, and a couple of inches up from the base. I tend to North-align the mount before attaching the OTA.

Geoff

Edited October 31, 2018 by Geoff Lister
punctuation errors