Celestial pole.

[MarkH]

This topic is a bit worrying. Duncan recently pointed me in the direction of the astro engineering polar mate, which looks like a great piece of kit for saving time. Now I know the celestial pole wobbles about quite a bit. I have a cambridge star atlas which obviously lists the CP. On investigation on the web there are hugely varing claims of the position of the CP, reticles etc.... So what reliable source is there? Who has referenced from who? Any informed answers?

[Mac]

Hi Mark,

The celestrial pole wobbles about by a huge margin, the good news is that its on a period of about 25000 years,
So its not going to change muchover the next 70 years. Just google precession of the equinoxes.

A bit of info.
Normally the positions of stars are given by their RA and Dec, this is normally with referance to an Epoch.
This is their alignment on the celestial sphere, with regard to the firstpoint of Aries. this is their RA
So if you see for example Beltelgeuse it has the following information.

Right ascension 05h 55m 10.3053s
Declination +07° 24′ 25.426″
J2000.

This means that the position of the star is correct at this date 2000,
and it will cross the celestial meridian when YOUR LOCAL time is 05h 55m 10.3052s
To exactly calculate the correct position for today you need to add a calculation to get you to 2014.
You can find out how to do this in any almanack or get it done by software,
So what has this to do with the celestial pole you ask.

Well the celestial pole is not going to move from its present location, the only problem is that there is not a star directly above the pole.


But there is one very close.
be it alpha ursa minor.
or as you probably know it polaris.
This star has the following info.

Right ascension 02h 31m 49.09s
Declination +89° 15′ 50.8″

So you can see that this star, as do all stars, rotates around the celestial pole,
but because it is very close to it you can align your telescope with it.

Normally when you use a polarscope, it has the following image within it.


This tells you where the North celestial pole is with regard to polaris.

If you point your scope at polaris and align it so that polaris is sitting in the small circle, then you going to have major problems.

To use this you need to know the following thing.

Your exact siderial time, now this is dependant on where you are in the world
When you know this you can normally set your telescope up so that it is
correctly set to your local siderial time.

This means that when the polar scope is correctly set to your local siderial time then
polaris circle should be in exactly the correct position

if your local siderial time happens to be 02h 31m 49.09s
then polaris would be on the celestial meridian.
and the polaris circle would be at the top of the large circle.

Any other time and the smaller circle will be further round the larger circle.

Once you have set your scope to this its a case of adjusting the scope so that polaris is sitting in the circle.

then your telescope is polar aligned. (Roughly)

Why roughly, well you can see how big the circle is and how small the star is,
Where you put the star in that circle is roughly correct, but should be good enough.

If you want to go down a more thorough route, then either get an automated piece of software (some are good, some are bad)
or go down the route of drift alignment, very good but can take a while.
or the iterative process of polar alignment which is what i normally use.

Mac.
(Still lurking in the background,
Head down fingers crossed.)

[MarkH]

Thanks Mac, that reticle diagram is what I understood to be the point, about one fith of the way back to cassiopia. Puts ones mind at rest thank you.

[Carole]

As you have announced you have bought yourself an NEQ6 this opens up a number of possibilities for you to get accurate polar alignment.

Basically you need to find out where polaris is around that little ring around the actual CP at a given time.
I use a piece of software called Polarfinder and this will show you exactly where polaris will be at your location at a given time.
If you google it it seems to come up with a different version of polarfinder but the one I am talking about looks like this:


I got the link for this on Astronomy shed Forum

This will get you reasonable polar alignment manually.

Additionally for more accurate polar alignment there is a software called Alignmaster (very cheap) which will get you very accurate polar alignment and once you have done it a few times it is very quick to do.  This however needs to work through ASCOM so you would have to be set up as if you are going to use EQMOD with an EQdir cable to use it, but EQMOD is also worth using as well, ask any of the guys who use it.  I resisted it for a long time, but recently started using it and found it excellent, it also means you can network your laptop and sit indoors and control the mount and you can pick any star for alignment or GOTO, pick a star very close to your target and sync and then your target will be spot on.  

Carole

[Carole]

Whoops, just realised it is Marek who announced he has an NEQ6, but some of my above post will be helpful to you, no experience of imaging with a fork mount I'm afraid.

Carole

[MarkH]

Mac, you might be able to clear something up here or maybe I'm missing a trick. The diagram of the reticle veiw shows the position of the CP relative to polaris, but am I correct in thinking relative to cassiopia and ursa major as well? In that reticle veiw the CP is shown on the cassiopia side of polaris. In my cambridge atlas the CP is shown on the othe side of polaris, have cambridge published incorrectly?

[The Thing]

The SkyWatcher reticle shown has been used to polar align by tens of thousands over many many years worldwide. If it wasn't right we'd know by now!

[mickw]

Loads of pictures and instructions -

http://www.themcdonalds.net/richard/index.php?title=Polar_Alignment_of_your_Equatorial_Mount

[MarkH]

Ok I'll clarify, the star atlas is hugely different by comparison to the reticle in proportion, to the spacing of cassiopia and ursa major. So which is correct ?

[Mike]

Does your reticule look like the one Duncan put up above?

[Marek]

Thank You Very much Carole for these helpfull clues ,I 've already got a polarfinder on my mobile,the alignmaster will come later i suppose when i start my guided photography,now I'm getting familiar with the mount itself,watching tutorials,readind instructions and so on.

[Mac]

Mac, you might be able to clear something up here or maybe I'm missing a trick. The diagram of the reticle veiw shows the position of the CP relative to polaris, but am I correct in thinking relative to cassiopia and ursa major as well? In that reticle veiw the CP is shown on the cassiopia side of polaris. In my cambridge atlas the CP is shown on the othe side of polaris, have cambridge published incorrectly?

Nope.

Its a Jedi mind trick to get you thinking. Just kidding.

When you look through the polarscope the image, i.e. polaris and the CP is in the correct orientation for when you align the star.
i.e it is reversed
You will never see Cassiopear or ursa major through the polarscope as the field of view needed would be huge,
but the image of the two constelation is how you would see them when you look up normally.
i.e. not inverted through a scope.

Mac.

[Mike]

They are purely there for a guide. I never use them myself and use the Kochab's Clock method or use an app on my phone. Both are easier.

[MarkH]

Ahhhhhhhhhhhhh thank you all, now I get it. 

[Mac]

I never use them myself and use the Kochab's Clock method

Never used that method.

All i did was to align my polar scope so that polaris was truly vertical and at the bottom. (its reversed dont forget)
I Imaged it against a spirit level hung on a wall and adjusted the internal recitle
so that the NCP and polaris was inline i.e vertical.
I then adjusted the external white lines so that they were also inline with the pointer

This means that now when i turn on my EQ6 (after its finally rebuilt).
All i do is to align the two white lines with the pointer.


I know now that polaris is on the meridian, All i do is set the RA dial on the scope to 02h 31M (polaris's RA)

as this is the siderial time when it crosses the meridian.

I get the local siderial time for me and then just turn the scope to that time.
Bingo polar scope is in the correct position.

I align the telescope and then spend the next few months praying for clear skies,
as the weather gods will normally allow you enough time to align and set everything up, before they send in the clouds.

Mac.