Digital Compass vs Analogue Compass

[floyd_2]

Hi guys,
I'm currently using an analogue compass to assist with the initial stages of polar aligning my SCT. However, I have noticed that the needle is influenced heavily by a few factors:

1. Being too close to the ground (blue metal dust under terazzo tiles)
2. Being too close to my back fence (colourbond)
3. Being too close to my telescope

Unfortunately, my telescope is close to a colourbond fence and is sitting on the blue metal dust / terazzo tiles.

I was wondering if anyone here knows if digital compasses are as easily influenced as the analogue ones.

Dean

[danielsun]

Hi Dean, Funny thing is I just bought an anologue compass because my digital one is easily affected mainly by metal but the anologue is just a cheapie and not very consistant for accuracy at all. Maybe either way I may need to spend a little more for better quality, though after having a little look on around there is only very cheap or very expensive, no in between.

Cheers Daniel.

[gary]

Hi Dean,

At their heart, electronic compasses rely on precisely the same physics,
namely a magnetic field influencing a set of sensors. So they can be disturbed by
stray magnetic fields in precisely the same way as their purely mechanical
counterparts.

For navigational purposes, magnetic compasses are relatively crude
devices and even when you compensate for magnetic declination
and in the absence of other extraneous ferromagnetic fields, you would
be doing well if you obtained a consistent reading that was any more accurate
than about half a degree.

So on their own, compasses, either mechanical or electronic are of little
use except for the crudest of polar alignment and with a little experience
many users can eyeball where the SCP will be by reference to a few
bright stars to equivalent levels of accuracy.

[netwolf]

Dean, we had problems at my mates place with polar alignment with compas's and we thing the main influence was possibly the High voltage power lines. The only reason we found out is that my mate decided to use the Sun to draw a True North South Line based on the shadow of a vertical stick at exactly solar noon for his location. There is a website you can use with your GPS cordinates to find out to the minute the solar noon for your area. At first we went with the compas for rough alignment and after drifting we realised that we were way off with the compas and the the shadow line was spot on. We should have trusted the age old method first.

Found this via Google. They use a plumbob to ensure the shadow is cast from a true perpendicular line (to the tangent of the earth at that point).
http://www.wsanford.com/~wsanford/sundials/temp/JohnCarm_Finding-True-North.pdf

[mithrandir]

Bother - netwolf hit enter first. Here's a rundown of the method. It won't work on the Tropic of Capricorn on the December Solstice, or on the Tropic of Cancer on the June Solstice.

Get a good time reference - accurately set watch, GPS, radio, NTP synced computer clock. You don't have to be dead right, but better is better.

Find the time of local solar noon - eg http://www.ga.gov.au/geodesy/astro/sunrise.jsp and select "Sun Transit Time"

Hang the plumb bob where you want the centre of your mount to be.

Wait for solar noon and mark where the bob touches the ground and as far along the string shadow as is practical.

Extend the two marks to form the north-south line - one tripod leg will sit on that. Mark an east-west line (builders square, 3-4-5 triangle, etc) where the other two legs will be.

Make your final markings however you like - paint, impact drill, etc.

[floyd_2]

Thanks Gary, you confirmed my suspicions. Hopefully it'll be sunny this coming weekend, as I'll be marking out true south on my observing pad as detailed by Fahim and Andrew. I have a funny feeling that Daniel will be doing the same thing :). Glad I asked as you've saved me the wasted cost of purchasing a second compass.

Thanks once again everyone.

Dean

[g__day]

Here's a suggestion.

Get two laser pointers.

Build a holder for one you can mount it somewhere adjustably rigidly but easily removable in your backyard that has a view of the SCP - say attached to a balcony or a fence. Mount the second on your telescope's SCP alignment viewer (if your mount has one - else mount it to the OTA aligned to the light path).

Do a really carefully drift alignment once - even if it takes you all night.

Now once your mount is well aligned if the laser is on your OTA - point it to the SCP then switch on its laser. Finally turn on your second laser pointer and adjust it so it points where your aligned mount is showing the SCP to be.

Now on any other night when you want to roughly align your mount - you simply do the procedure in reverse. Switch on your seperately aligned laser sight that is pointing to the SCP then switch on your mount's (or OTA's) laser sight and adjust the mount until the two lasers "touch at infinity".

This should give you pretty good alignment (within a few arc minutes) night after night in just a few seconds.

To improve things - can you put pegs or some sort of permanent markers in your backyard where you want your mount's tripod legs to go? - Like surveyors pegs if it's grass or even chalk or paint circles if you're on hard ground? Because once you've drift aligned well - and really nailed the lattitude setting for your (I suppose German Equatorial) mount - then provided you keep the mount absolutely level on each set up - you should only have to adjust its East / West orientation.

Hope this helps,

Matt

[Barrykgerdes]

I guess I am lucky. I have my house plans and the coords and angles of the boundaries are marked on it. From this and a bit of high school trigonometry I was able to place a north/south string line right through the centre of my polar mount. Precision less than an arc minute, accuracy about 10 arc minutes!

If you use the sun and a plumb bob don't forget to check the actual time of the meridian pass for your particular location and if you are at a place where there is no shadow at noon make two passes one at 10 am and one at 2 pm and bisect the angle!

Barry

[JimmyH155]

I used dollops of concrete under each leg - once I had got the alignment spot on. Dug a hole in the grass, filled it with concrete, placed a bit of plastic sheet over it, then, one at a time, I lowered each leg onto the concrete to make a little dimple. Then next leg, checking level etc very carefully. Worked a treat. Of course you never alter the tripod settings after this.
Put the concrete below the level of where the mower could hit it. :D:D

[mithrandir]

Local solar noon +/- 3 or even 4 hours would give you a wider angle and longer shadows, both of which help you to improve the accuracy of the bisection.

Of course the method won't work in Sydney these days. No sun to cast shadows unless it's a work day and I'm in the office.

If there is some easily identified object you can see from the viewing ground you could use Google Earth to plot your yard and the object, work out the true bearing, and use that as a starting point.

[floyd_2]

I asked my wife to take a pic of the back yard at 1.01pm today to check just how long the shadows were at solar noon. They were disappointingly short. At least, however, there were shadows at all where my observing pad is. I think I'll take the best of both worlds on Saturday and mark out 3 hours before solar noon, exactly solar noon, and 3 hours after.

No doubt I've cursed all Sydney siders for the weekend now, and I would like to apologise in advance for any cloudy skies over Sydney for this coming Saturday and Sunday!

Dean

[ChrisM]

I used Meade's Autostar ephemeris to look up the Sun's transit time (this relied on knowing my lat/long). The winter sun cast a shadow from a set square that was resting on top of a spirit level. I used the resulting shadow line so that I could fix my new home-made wedge to the pier and get it oriented approximately N-S.

The result was pleasing - no adjustment required for visual observing; objects stay in the eye piece for 20 minutes or more.

Chris

[Wavytone]

Couple of comments:

1. The plumb-bob method works well enough to get you within half a degree, provided you know your longitude and can calculate the time the sun passes over the meridian (this is usually not noon).

2. Regarding the laser pointer idea to repeatably align the mount in azimuth - it's sound - but you can get by with only one laser. You can even compensate for misalignment between the laser and scope.

Assuming

a) you have got the mount accurately aligned by one means or another, and
b) the laser and the scope are optically aligned (eg by checking optically on a star)

the procedure is:

a) Turn the scope so the dec axis is horizontal and the scope is pointing at the south celestial pole.
b) Turn on the laser.
c) With the RA locked, swing the scope downwards (using dec axis only) until the laser hits something (on a wall or fence etc) due south of the mount, where you can place a permanent marker. Even a peg in the ground will do.
d) Flip the scope over in declination so the laser is pointing north, and place another marker on the north side (on a wall or fence etc).

Now you have two marks - one north and one south.

Next time you set up the scope, assuming the laser will stay repeatably aligned with the scope all you have to do is clamp the RA axis such that the dec axis is horizontal, and turn the mount in azimuth so the laser hits the south mark.

Then check to see if the laser also strikes the north mark when the scope is flipped 180 degrees in dec. If it doesn't, it means the laser is not in the same alignment with the scope as it was when you made the initial marks. To correct for this, adjust the azimuth of the mount so that the laser mises the marks by an equal and opposite amount.

[floyd_2]

Ok Matt and Wavytone, now you've really done it. I want to get a laser :) I think it's a great idea to reduce setup time. I've read through the "I want to buy a laser" thread and there's some good information there. I'm thinking a gigawatt should be about right for me :eyepop:plus some welding goggles (although I might draw the alignment dot on the next door neighbour's house).

Now the search begins for the right laser.

Dean

[RobF]

You really don't have to get too carried away with this as long as you can identify a spot on hard ground you can always set up on. I've marked my backyard tiles for the 3 tripod legs (with nikko pen) , then put another pen mark across the base of the tripod once I had the azimuth adjustment correct for my site.

For visual AND imaging I have no trouble at all lining up tile marks and checking azimuth is ok (in case I've been offsite to a dark sky site). You'll probably need to mark your tripod legs with a black line too showing where they need to be to get the mount level.

The key to finding the correct spot is drift aligning though. There is no better way in the end. The webcam method with K3CCD tools is pretty damned good if you have a webcam. Failing that, its worth spending the time with a high power eyepiece to get it right once. Remeber you have to get the altitude of your mount set right too, and drift alignment will sort this out too.

Its nice to have a compass, electronic level, etc for when you set up at an new "unknown" site, but I never use mine at all now at home......
(one of the reasons I'm always mumbling and fumbling in the dark at a dark sky site trying to remember how to get polar aligned! :lol:)