Dob setting circles "confused"

[haznomad (Harry)]

Hi All

I bought a GSO12" dob some time ago and have enjoyed it very much thus far , but my ability to star hop to targets is reaching its limit . I have been looking at what others have done with setting circle and Wixey and at the expense of looking like an idiot i have a couple of questions for those in the know 1) should the base circle increase in number from 0/360 to the left or right in the southern hemisphere 2) If the 0/360 point is pointed to true north and the scope also pointing north , the notch out of the scope base being on the observing side of the scope , how do you navigate to a target with this 90 degree offset , you would see 90 or 270 degrees ??? . Please tell me im missing the obvious guys , just cant seem to get my head round it

Thanks in advance Haz

[Allan_L (Allan)]

I got my setting circles from this thread:
http://www.iceinspace.com.au/forum/s...etting+circles

Just copy an appropriate file (there are plenty in the thread to choose from) and have it printed by a copy shop (these are fairly inexpensive) probably want it on A1 paper.
Get it laminated (about $20)
attach it to the base.
Set zero/360 to North
and off you go.

from memory it counts clockwise so that when 90* is at the cutout, the scope is pointing due East.

here is a photo of mine on a SW 10" (now sold)

The notch cut out is NOT on the observing side of the scope, it is in the front of the scope.
You gotta get up and look while you are re-aligning it to each object !
Small price to pay !

For altitude I used a digital incline meter form Bunnings (less than $30)
Not sure these are still available but others have found similar devices for similar prices (maybe even mentioned in that same thread).

Good Luck with it!
I found it extremely accurate and easy to locate items presuming:
A) you level the scope.
B) you get your zero degree alignment accurate
C) you have good software to give you accurate Alt Az co-ordinates (which are constantly changing of course)

You can get alt az from www.tonightssky.com
or on the fly on several smart phone apps.

[astro_nutt]

Hi Harry.
I'll try to explain as best I could. (I made my setting circle so that it moves around on a track attached to the scopes base.) Looking from above the setting circle should increase in numerical value to the left of 0. The scope is pointing North and you are observing through the focuser at 90 degrees from the front of the scope. If the setting circle was aligned at Magnetic North, the numerical value of the setting circle would be 270, and at the focuser side, being 90 degrees less should be 180. However, for the setting circle to be aligned to True North, you have to allow for the geomagnetic offset. In Melbourne the offset is 11.5 degrees West of North. So, when I set up the setting circle and after I align the front of the scope to Magnetic North and the numerical value of the setting circle indicates 270 degrees, I then move the setting circle to allow for the offset so the front of the scope indicates 281.5 degrees and is set for True North. Looking down standing in front of the focuser, it should indicate 191.5 degrees. (for altitude I use an inclinometer available from Bunnings)
I hope this helps. Cheers!

[glend (Glen)]

Gee those explanations confused me and I used circles on my last scope. If you have your circle aligned to true north - being zero, then turning it Clockwise reaches 90 degrees after a quarter turn and your now pointing East. South is at 180 degrees, and West is at 270 as you continue around clockwise. My magnetic declination was 12.3 degrees here near Newcastle. It helps yo have a good compass that has a rotating bezel so that you can set it to point to True North when the needle is pointing at magnetic. Maybe this is easier if your shown how to do it.

My degree and altitude co-ordinates always work with the various sky software Alt-Azimuth co-ordinates. So if 0 is True North the Azimuth numbers go up as you rotate the scope clockwise - its that simple.

[noeyedeer (Matt)]

hi Harry,

I don't have setting circles on my dob, but if I were I would go with Allen's way, except ... I'd have the notch cut out on the observing side with 0° showing once the scope is placed to true north (basically 90° would be under the handle) .. that would be if I rotated to scope to the right (heading east) the numbers would go up. it still would effectively tell you what degree your scope is facing without having to walk to the front each time.

matt

I'm just going to attempt a rudimentary setting circle with some masking tape and my circumference broken up into 360 and see how that works before I start hacking into my base.

[glend (Glen)]

?he reason that the notch is usually cutout under the tubr and at t:he front of the scope is so that you know the azimuth degree that it is pointing. If you put the notch some where else you will need to do a calculation every time you point the scope to a new heading gained from a software package like Skysafari, etc. Please read the various articles and posts here and elsewhere on making setting circles.

[noeyedeer (Matt)]

Originally Posted by glend

?he reason thatvthe notch is usually cutout under the tubr and atvthe front of the scope is so that you know the azimuth degree thwtvit is pointing. If you put the notch domewhere else you will need to do a calculwtion every time you point the scope to w new heading gained from a software package like Skysafari, etc.

I know but if you offset it by 90, it's still reading 0 at north with no calculating whatsoever. just means no walking back and forth to the eyepiece when realigning az.

instead of 0 being north 0 is on the west side by sticking down the setting circle that way ... but it's still showing what way the telescope is heading.

basically it's like this, the outside would be my setting circle and notch cutout at 0.
ps feel free to leave cartons of beer at that location

[Pinwheel (Doug)]

If your using a compass to set this up, shouldn't you be using Magnetic North & NOT true north. You could be out as much as 10 deg because of this.

[glend (Glen)]

It should be set with 0 degrees at True North to make the co-ordinates correct, the compass can be used to determine True North by applying the declination when you set it up. For example, here on the NSW coast the declination is 12.3 degrees, so True North is 12.3 degrees to the west (or left) of magnetic north. Many compasses have a rotating rose that allows you to set a True North pointer.

[astro_nutt]

Hi Harry.
My apology's to you and everyone. I should have explained that instead of looking at the front of the scope to point the scope to the azimuth figure when locating an object, I have an indicator on the focuser side so all I have to do is look down from the focuser to turn the scope to the azimuth figure to locate an object. Sorry guys!

[haznomad (Harry)]

Hi again all

Many thanks for all of your speedy replies , it is now as clear as mud ... only joking guys I understand it now guy , looks like there a a few ways of setting the system up , with or without calculation , cut out at front or side , cool , the part i was struggling with was if everything is pointing north how do you get to the objects az reading "calculation" , penny dropped with a huge clang , many thank again folks .

Haz

[noeyedeer (Matt)]

Originally Posted by haznomad

Hi again all

Many thanks for all of your speedy replies , it is now as clear as mud ... only joking guys I understand it now guy , looks like there a a few ways of setting the system up , with or without calculation , cut out at front or side , cool , the part i was struggling with was if everything is pointing north how do you get to the objects az reading "calculation" , penny dropped with a huge clang , many thank again folks .

Haz

with software or ra/Dec conversion into alt/az as mentioned by Allen.

for eg the sun is 21h19m 15°38'. at the moment that translates into az284° alt55°. in an hour's time it would be at 274° 43° for my location.

rotate the scope so your needle on your base lines up with 284, and use an inclimeter to set the altitude ...

matt