Is there an easy way of aligning your scope to the South Celestial Pole, on my LXD 75 mount there is a polar scope with the Octans etched in the upper left corner of the scope , i am finding it hard to see them or find them for that matter, any help would be greatly appreciated cheers Dennis
Last edited by dennisjames1; 28-12-2006 at 07:57 AM.
I have used mounts with Polar Alignment Scopes (PAS) with the Octans "trapezium" shaped reticule, one with a field of view of 6 degrees and another with a FOV of 3 degrees and agree - in can be tough! In my back garden I have marked 3 spots where I place my tripod so I can find the Octans stars (Sigma Octans) very easily now.
Here’s how I would start off at an astro camp or new location:
Use a magnetic compass to locate magnetic South.
Now we know that magnetic South and True South as defined by the South Celestial Pole (SCP) are not the same.
Magnetic South is where the compass needle points.
True South and the SCP is the point around which the skies appear to revolve (the projection of the Earth’s axis of rotation onto the imaginary celestial sphere).
For Brisbane, the SCP is now some 10 or 11 degrees E of magnetic South. So, when we look south and have our compass needle pointing to magnetic South, the True South (SCP) is some 10 degrees to the East (or our left as we look south).
My tripod has a peg on the tripod head which sits over a leg and I set this leg and peg to point True South. I then fit my German Equatorial Mount onto the tripod and using a template that fits on the declination shaft, I adjust the altitude screw on the mount until I am at 27.5 degrees for Brisbane.
The template is a triangular piece of 12mm plywood with the long side of the triangle making an angle of 27.5 degrees to the next longest side. I place the long side against the Dec shaft then place a bubble level on the short side of the triangle and adjust the altitude screw on the mount until the bubble is level. I then know that the Dec axis is at 27.5 degrees even if the tripod head is not level.
Using these techniques, I can usually get with 2 or 3 degrees of the SCP and then locate Sigma Octans in the PAS. Hope I haven’t bamboozled you! If so, ask away and I’ll try again.
Do you have a magnetic compass? If so, you will need to find out your magnetic declination (or deviation as some call it). I will hazard a guess and suggest that for Toowoomba you will need to subtract your magnetic declination from 180 degrees to get the magnetic bearing for true south. (You should check this! ).
Stand back at least 3 metres from your scope (so it doesn't affect your compass) and shoot a bearing through your scope with the compass, make adjustments until the polar axis is aligned with the compass bearing. Tis should get you within +/-2 degrees of true south... close enough to start drift aligning (or to use your polar scope) anyway.
If you don't have a compass, I suggest you get one. It will help with setup until you get to know the sky well enough to find the SCP without it. If you want to learn how to find the SCP without a compass have a look here:
With me, I don't bother trying to locate the octans. They are pretty faint and generally obstructed by my neighbours trees.
Find out your magnetic offset, for me in Sydney it's 11 degrees. Then get a compass out and setup your mount so it's xx degrees east of magnetic south. that should put you close enough to get started, well it works for me at least.
Thank you fellows this is a great site, so i would need a reasonable compass to get an accurate reading i would imagine, a liquid filled one ?? and where to buy it from? and the latitude for Toowoomba is 27'34" i think and on my Meade LXD75 mount the markings are small and rough so i guess when the Octans are aligned i would be set at the correct latitude, is the right?
snip.
...on my Meade LXD75 mount the markings are small and rough so i guess when the Octans are aligned i would be set at the correct latitude, is the right?
Hi Dennis
Yes – if the pattern of stars in Octans matches the pattern in your Polar Alignment Scope, then you should be polar aligned. That is, if the mount has been accurately manufactured and assembled so that the PAS is truly in line with the polar axis.
Some IIS members have reported that the small altitude scale on their mounts (not necessarily Meade mounts) have been off by as much as 6 degrees.
On my Vixen GPDX mount, when I am polar aligned with a level mount, the altitude scale does read between 27 and 28 degrees so in this case, Vixen have produced a fine mount.
Cheers
Dennis
PS – It seems strange addressing one Dennis (you), and then signing off as another Dennis (me).
Yes on my mount (an EQ6) when it was set to 34 on the mount, an inclinometer actually showed it as reading 41! How they could bugger up something so simple is beyond me.
I have seen worse! A few years ago, at Qld Astrofest, I was trying to help someone use the Polar Alignment Scope in a very early model Chinese clone of the Vixen GP mount.
I had a look through their PAS and the Octans “trapezium” reticule of stars (which contains Sigma Octans at one of the vertices) was just plain wrong. It’s as if someone just randomly chose 4 points for the reticule when they were making it, without reference to the actual stars in that region, so the reticule just could not be lined up with the 4 real stars.
I think newer instruments have the correct trapezium, where the 4 stars in Octans fit nicely into the 4 vertices of the etched reticule.
Here is a drawing of the FOV of the Vixen GPDX reticule.
dennisjames1, here is a 'cheeky' method that I can not vouch for, but will save you the cost of a good compass............if it works that is
Why the uncertainty? I am currently trying this method but clouds are complicating the issue at the moment so I can't say 'been there done that'.
I have 2 main problems to overcome, firstly my south celestial pole area consists of a pine tree, and secondly a compass reading is out by about 40deg because of colour bond fences and patio, so.......
I guestimated where south pole should be. I skipped alignment and performed a goto the Moon. I then parked the mount and powered off.(this was to establish a datum point in memory) I then powered the mount back on and did a goto the Moon again. at this point the cloud rolled in, but I was at least somewhat close to the Moon, same part of the sky anyways.
What was intended to follow next was to physically move the mount to center the Moon in the EP, without having powered down the mount electronics.. Will this method work? I have no idea and wont until clear skies allow me to finish trying it. It should work in theory because I will be using the on-board atlas to find SCP by default. If you have clear skies you might like to try this method and let me know if I'd be wasting my time or not. Of course I will still need to drift align to do any imaging, but the above should get me ball park alignment in a 10 minutes or less.
I should add that the above method assumes that the correct date, sidereal time, Latitude and longitude have been saved in memory so that the performed goto is calculated on the correct parameters.
I have an EQ6 (SkyWatcher) which comes with comes with a built in Polar Alignment scope.
I've never actually used it to align but have looked through it and I can see both Octans and Northern Hemisphere stars for aligning (Polaris... the constellation Cassiopeia (Sorry about spelling)
My question is: can the PAS be rotated/adjusted to line up with the Octans asterism?
The Vixen GPDX mount also has a fixed or non-rotating PAS and this is okay, it’s designed that way. Here is how I polar align.
Find the correct field through the PAS.
Make sure you have enough adjustment in the Azimuth screws by grossly moving the mount first, if so required.
Rotate the mount around the RA axis (DEC clutch locked) until the PAS reticule is approximately the same orientation as the Octans asterism.
Use the mount Alt Az adjustment screws to then move the mount so that the reticule and asterism overlay, making the final (small) RA axis rotational adjustments as necessary.
Note that sometimes, I have to remove the 'scope off the mount to stop it crashing into the mount or pier.
During certain seasons and depending on whether it’s early evening, mid night or early morning, the reticule overlays the Octans asterism with minimal rotation of the RA axis. At other seasons/times, I have to rotate the RA axis of the mount almost 180 degrees to obtain the alignment.
Cheers
Dennis
PS – By fixed or non-rotating PAS, I mean that the PAS does not rotate independently in the mount. To rotate the PAS reticule you have to rotate the RA axis of the mount.
Last edited by Dennis; 28-12-2006 at 06:40 PM.
Reason: Clarified non-rotating statement
Hi Dennis, yeah it does sound like you are talking to yourself, that view through your vixen polar scope shows the Octans in there correct orientation as they appear in the sky through your pas. is that right? cheers Dennis
Hi Dennis, yeah it does sound like you are talking to yourself, that view through your vixen polar scope shows the Octans in there correct orientation as they appear in the sky through your pas. is that right? cheers Dennis
Hi Dennis
The Octans asterism or trapezium rotates around the SCP approx once every 24 hours and so will move or rotate during the night.
Also, if you view the asterism at the same time each night, the position of the asterism will change (rotate) in the field of view of the PAS with the season of the year as well, just like the constellations.
The EQ6 PAS does not rotate independant of the RA axis, however it does have its own setting circle, or at least mine does.
Preliminary results seem to indicate that my rude and crude method works, but I'll need to wait for a clear night sky to use stars instead of the moon.
When I tried the mount out at 3pm standard time today it went to within about one lunar diameter of target, without going through an alignment cycle. This a good result for not being able to even use a PAS at my location. Of course the reverse is known to be true. An accurately polar aligned goto scope will automatically goto a selected target without the need for alignment...been there done that with the Meade LX200.
I should really have added that there is the (24hr) daily rotation of the Octans asterism around the SCP due to the daily rotation of our Earth, as well as the (annual) “creeping” of the Octans asterism around the SCP due to the difference between the mean solar day (24h) and the sidereal day (23h 56m).
The annual “creeping” is caused by the stars rising 4 mins earlier each consecutive day, so the Octans asterism slowly creeps clockwise around the SCP approximately 1 degree every day, making a complete revolution in 1 year.
Yes on my mount (an EQ6) when it was set to 34 on the mount, an inclinometer actually showed it as reading 41! How they could bugger up something so simple is beyond me.
Hmm, that is poor workmanship, unless the mount was not level, so that the base of the mount was tilted down 7 degrees towards the south, which would be quite a big tilt.
to IIS!
).
