Degree circles on Dobsonian how to

[Varangian]

The best way to align your Dobsonian using degree circles and a wixey inclinometer? Should I locate a bright star in my finder and then shift the degrees on my rocker base to reflect that star's known co-ords? Then go from there?

Any hints on this appreciated.

[Shark Bait]

I prefer to set my Dob up so that it is level and pointing at True North. That becomes the 0/360 degree mark for the Az setting circle. Then I test it against some obvious targets and compare the setting circles (Az & Alt) to the figures given on my iPod's Sky Safari. Even though the Dob is a GoTo model, I can use it as a push to and the setting circles make life easy.

I use a sighting compass, so that makes it easy to get with 0.5 degrees of True North. It will be a little harder without this feature on a compass but it is still possible to achieve reasonable accuracy.

Your method sounds fine as well.

[Allan_L]

I am with Stu.

I have built a platform (with holes for the dob base legs to sit in firmly) with leveling legs (plastic set of four-under $20 at Bunnings).
Find true north (I use Android app GPS Status) and align platform.
Level the platform.
Attach Dob base and scope.
Line up a known object.
Then any difference from known co-ords and setting circle becomes the adjustment factor for the rest of the night.

works for me

[mental4astro]

If you're going with setting circles on a dob, there's a couple of ways. One is by setting your ground board to True North. This becomes your permanent "Zero". This also requires your altitude bearing to be trued to either dead verticle or dead horizontal. You'll also need to have some means of leveling your scope. This link shows how to employ manual setting circles (http://www.homebuiltastronomy.com/ManualDobSettingCircles.htm). It als shows a very clever, simple and inexpensive way of leveling your dob.

Another way is with an electronic gizmo that has accelerometers in it to then spit out the alt/az co-ordinates. It also requires the scope to be aligned to true north.

Aligning on a star can be done to achieve your initial azimuth bearing too - I realised this after reading Allan's post!

I believe that there may be one or two smart phone apps that allow the phone to act as an alt/az digital setting circle. Others may be able to eleborate on this. The electronic gizmo I mentioned, Astrofix (http://www.manortronics-force-gauges.com/Astro.html), works with the same type of accelerometers found in smart phones.

Mental.

[Varangian]

Thanks guys. Is there a table showing converted alt/az co-ords? I don't use iPhones or much else with a screen other than the computer and a dgps for work really :rolleyes:

I assume not becasue we are always moving hence different co-ords?

[Allan_L]

No Table.
As 1. It is continually changing and
2. It needs to be specific for your viewing location.

Stellarium gives live Alt/Az
as does SkEye (Android App.)

But what might suit you best is a visit to a web site called Tonights sky (http://www.tonightssky.com/) where you input your viewing cords, when you will be viewing, and what you want to see.
The site then produces a viewing guide for you (select format with Alt/Az) and it gives you the alt/az for each object over the period you plan to observe, at various intervals.

[mental4astro]

Excellent suggestions of Stellarium and SkEye, Allan. I wasn't sure if Stellarium did, and you answered that for me!

John, in case you didn't know, Stellarium is also a free download, and a fantastic tool!

[Varangian]

Wow didn't know Stellarium did that either. I have the program and will check it out. Thanks.

[Varangian]

Alan, I've had a good look at this and it is just what I need. thanks kindly. Thanks to others who responded. :thumbsup:

[mark3d]

You can find a template to print the azimuth circles here: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/937949/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1. (It costs about $3 to print at Officeworks on A0 size. The Wixey for altitude costs under $40 from Amazon.)

[Varangian]

Okay, so my degree circles run 0 -360 (360 is back to 0) perfectly around the rocker base of my dob, azimuth looks good. My wixey also arrived which I bubble level set to 0, so altitude is good. I set the dob to my solar noon markings taken today with the dial over my degree circle set to 0. I levelled the dob base pretty accurately and then attempted to find M42 as a test using the below azimuth/altitude co-ords from tonights sky at 10.30pm;

22:30 pm
alt: 32° 48´
azi: 230° 36´
Local horizon set to 45?

Using Melbourne co-ords @

Melbourne, Australia 37 47 S 144 58 E

Unfortunately the Dob was nowhere near it, actually I was pointed in the opposite direction looking near crux! Also, the altitude was way too low in the sky to be anywhere near accurate. What am I doing wrong? Are these co-ords right? Is there an offset I am ignorant of? :shrug:

[Cumbrian]

s

Thanks for that, Mark. I made my own laminated degrees circle yesterday, based on your design, and it's now fixed to the base of my 10" GSO dob. I still need to do something for the declination, which is proving to be a bit of a challenge..

With regards to setting north for your scope. Try the U.S navy site
http://aa.usno.navy.mil/data/docs/mrst.php

This site gives the transit times for all major celestial objects.
Plug in your co-ordinates then find a star that's approaching due north, then at the right time set your scope onto it. From there take a bearing on a land based structure, a tv mast or something (nothing too close though) and then you're set for future set ups. You just need to set up on the same spot each night and take your reference, if not repeat the process wherever you are.

[Allan_L]

John,"Tonights sky" does not have north south etc, and is decimal, so
the Co-ords for Melb would more likely be Latitude -37.8 {note minus sign]
Longitude 145.0 {positive} (each to the nearest tenth of a degree).

That sounds like it would explain it.

Allan

[Varangian]

Thanks Alan it certainly puts me right back into the ball park.

Many thanks again.