whats the correct angle

mario chiera · #1 23-12-2006, 10:10 PM

Hi All,
I want to take my scope out in abit . Its a heq5 6" mak with goto.blow me down I just noticed the tilt angle is set at 22 degrees.well yesterday I was advised the angle should be 32 degrees.
Can someone please come back and advise me what the correct angle is please.

Mario
perth

saturn c (Leo) · #2 23-12-2006, 10:31 PM

perths latitude is 32 degrees so set it on 32 then the goto will werk properly once u have it alligned. I was pretty confused with mine wen i first got it. Im still learning how to use it . have fun

Dennis · #3 23-12-2006, 11:35 PM

Hi Mario

By "tilt" I assume you are referring to the angle of the polar axis on your mount? If so, then as SaturnC writes, the tilt should be the same as your Latitude and the mount should be pointed at the South Celestial Pole (SCP).

Why? Well, first of all, let’s imagine that the Earth is at the centre of a huge celestial sphere and for convenience, let’s also imagine that all the stars appear as points of light inside this celestial sphere and we are inside, at the centre, looking out.

Just as the Earth has an axis of rotation with a N and S pole, our imaginary celestial sphere also has an imaginary North Celestial Pole and a South Celestial Pole (SCP). If you now imagine sticking a knitting needle or skewer through the N-S axis of rotation of the Earth, then in the South, the skewer would touch our imaginary celestial sphere at the SCP. This is where we want to point our mount.

We know that the Earth rotates once in 24 hours (approx) and if you stand outside (at night) and look south for long enough, you would see that the stars appear to revolve around this imaginary point of the SCP. That is, they would rise above your horizon E of S, be highest above your horizon when due S and finally set below your horizon when W of South. (Except for those stars that are within 32 degrees of the SCP - these will never set in Perth, they are called circumpolar).

So, the SCP is an imaginary point on our celestial sphere around which the stars appear to rotate once every 24 hours due to the Earth’s daily rotation. If we make a drawing and use simple geometry, we can show that the Latitude of our observing location is the same angle as the angle of the SCP above our true horizon. In Brisbane the SCP is 27.5 degrees above my true horizon. If the Latitude of Perth is 32 degrees south, then the SCP would be due south of you and be 32 degrees above your true horizon.

Setting our mount so that the right ascension axis points to the SCP allows the mount to use its motors to track the stars as they appear to rotate due to the Earth’s daily rotation.

Hope that rather long winded explanation helps!

Cheers

Dennis

mario chiera · #4 24-12-2006, 11:11 AM

thanks guys. saturn c i am new to astronomy so I appreciate your encouragement.
Dennis i just hope i can be as good as you one day.you lost me a bit but please keep up your advice with us novices.
one other thing because i had it set at 22 degrees would that be the reason why my goto was not aligning properly.
I tell a few months ago I was having a hell of a time and to be honest I gave up using the scope.
What do you guys do to align it south.I have acompass ,angle meter and other stuff.how important is it to get this right.I done a 6 week course and it got drummed into us .Apparently the scope needs to be lvel as well.
Some advice please woul not go astray.I might even try tonight.

mario Perth

Dennis · #5 24-12-2006, 12:45 PM

Hi Mario

I’ve been at it for over 12 years, so don’t worry, the knowledge will come as you get more time and experience under the stars! If your mount was set to 22 degrees but it “thinks” it was at 32 degrees then yes, that would likely stuff up your GoTo’s.

I’m not sure of the details of your scope/mount combination and how it works, so I cannot give you any specific advice. Here is a suggested step by step procedure largely based on how my mounts work, so be mindful that your mount could be quite different.

JOB #1 – Lat, Long & Time.
I am going to assume that your mount does not have a built in GPS to obtain your Latitude, Longitude and local time, so I will assume that you have to enter these details manually into your controller.

For my GoTo mounts, I have to enter the following details via the hand controller:

My Latitude and Longitude.
My Local Time.
My Time Zone.

So, for my location at Brisbane, we have:

27 30' deg South and 152 deg East.
11:32am AEST (the time as I write this post).
+10 Hours, which is the Standard time zone for Brisbane, meaning that Brisbane is 10 hours ahead of Universal Time, which is the equivalent of the old Greenwich Mean Time.

For Perth, you would have something like:

31° 58' South and 115° 49' East.
Your local time, by your wrist watch.
+8 hours meaning that Perth is 8 hours ahead of Universal Time.
Note: If Perth currently has Daylight Saving Time in operation, it means that you are +9 hours ahead of Universal Time.

Also, my hand controller has a small, built in battery which means that the mount can store and remember these details even when I disconnect the power. Some mounts do not have this and force you to enter these details every time you start up again.

Now that we have told the mount where it is and what the local time is, we can begin to point it in the right direction.

JOB #2 – Set up the mount in the correct orientation.
My mount can be set to 3 different modes as follows:

Alt-Az (like a dobsonian).
Equatorial non-aligned.
Equatorial polar aligned.

Let’s assume that you are going to use Option 2 - Equatorial non-aligned. What does this mean? Well, it means that you have roughly set your polar axis to 32 degrees and using a magnetic compass, you have set up the mount so that it is pointing approximately south.

But wait; there are at least two South’s – magnetic S by the compass and “true” S which is the one we are interested in. True S is based upon the South Celestial Pole which is the axis around which the stars appear to revolve. For Perth, it seems that magnetic S is about 2 degrees W of the SCP, so when using your magnetic compass, you will need to allow for this by pointing your mount 2 degrees E of where magnetic S is indicated on your compass.

So, we have now achieved the following:
JOB #1 – told the mount where it is and what the time is.
JOB #2 – pointed the mount at the correct azimuth (due S) and the correct altitude (+32 degrees).

JOB #3
You now need to read your manual to see how you perform an alignment based on this information. With my mount, I select 3 stars from a menu and align on them and the mount then uses all this information to compute its exact orientation relative to the heavens.

Cheers and good luck!

Dennis.

NOTES:
Although it is helpful to have the mount level, it is not really essential, as long as it points to true S and the polar axis is at 32 deg.

mario chiera · #6 24-12-2006, 01:14 PM

thanks dennis i will give it a try.i will let you know how i get on .
by the way my scope is a skywatcher pro 6" mak on a heq5 mount with goto.
My set up is llike yours .
cheers

mario