Polar Align - Was it a fluke?

[SniffTheGlove]

Hello all,

Over the last few weeks I decided to build a pier in my garden as my health has been getting worse and the prospect of walking down the garden, opening up the Obsy was getting to much and provide many hours of frustration so hence build a pier right outside the back door. The position actually gives me a much much better view of the sky as I was limited just to south in the Obsy, now I have south, west, north and a little of east.

Anyway, last night I had finished the pier and with all the remote electronics build into the pier, run active USB into my home PC and switched everything and it worked great under EQMod.

Anyway when in the Obsy I was never able to polar align due to trees etc, but I can now and this is where I am wondering if I got very good polar align first time?

I had put the EQ6 on the pier and just tightened up the bolts to lock into place, unscrewed the polar scope cover and looked through the polar scope and polaris was right their on the circle line. I rotated the RA to align with Ursa Major in the sky and polaris just moved right into the little circle. Could it be perfect first time.

I rotated the RA complete 360 and polaris stayed right on the big circle line and ended up back into the little circle after the 360.

So no adjustment of az or alt. Was it really just a one off fluke or am I missing something?

[mapstar]

Sounds really lucky, did you check with polar finder on the pc just to see if that's exactly where polaris was in transit?

[SniffTheGlove]

No I did not, I'll find it and then if check on next clear night.

[Daddystu]

If you rotated RA and Polaris stayed on the line then yes you got lucky. It happened to me a few weeks back and I was able to get unguided 4 min exposures with no sign of trailing. For an hour or so anyway until the air reached dewpoint temperature yet again.

[IanL]

Missing something I'm afraid:

- As it was the first time one your new pier it was a fluke that you got Polaris on the reticule line with no adjustment. Second time out on a pier it would be expected assuming you didn't change the Alt/Az adjusters or make any other changes.

- It might be luck that you can rotate 360 degrees in RA and keep Polaris on the line. For that to happen the reticule must be properly centred with the RA axis. Either you were lucky and it came from the factory like that (many don't), or you had previously adjusted it yourself to get it centred.

- Beyond that, keeping Polaris on the line through 360 degrees is meaningless. Well not meaningless actually, it is the mathematical definition of a circle! You could put any star you like on the reticule line, and assuming the reticule is centred with the mount's axis of rotation as above, it will stay on the circle when you rotate in RA.

- The only time it way it would be a perfect fluke is if Polaris ended up on the reticule line at exactly the right hour angle for the date/time and observing location.

In short, just because you get Polaris on the line, or even rotate in RA so the Polaris circle covers it, that doesn't mean you are properly polar aligned! You need to determine the correct hour angle, rotate the little Polaris circle to that hour angle and then adjust in Alt/Az to centre Polaris in the circle. You can do that using the calculator circles on the polar scope, or you can use software to tell you the correct hour angle.

[IanL]

Just to make it clear, you were probably not far off but relying on the constellation patterns in the reticule is not going to get you a particularly good PA since you are doing it by eye and could easily be a few degrees off. For example you could have put the reticle in any of these positions:

The red cross shows the position of the NCP. The green line shows the correct hour angle (which you need to calculate as it changes as the Earth rotates on its axis), and so you need to position the black reticle with the small circle at this hour angle. Now when you put Polaris in the small black circle using the Alt/Az adjustment screws, the NCP is at the centre of the big black circle. Assuming the reticule has been properly set up at the factory (or by you if it wasn't), the centre of the blg black circle is lined up with the mount's RA axis and you are polar aligned.

Conversely, you could put the large circle in any position, including the three different dotted lines I have shown, rotate the small circle and place it over polaris, but the NCP would not be in the centre of these circles.

Since the RA axis rotates about the centre of the reticule, if you put Polaris on the line, it will stay on the line when you rotate in RA regardless of whether you are polar aligned or not. If you wait a bit though, it will drift off the line (unless you are polar aligned, when by definition Polaris will follow the reticule line).

In your case it would not be as extreme but unless you had lottery winning type luck, you'd still be off by some fair margin. Probably OK for visual but not for imaging.

[Daddystu]

Correct me if im wrong but once you have Polaris on the circular line can you not just launch Polar finder(or EQAlign) and then using alt/az place polaris in the circle as shown by that? An easy check to make sure Polaris isn't in one of the 'other' circles you demonstrate above is (once aligned) to check that the NCP offset is roughly in the correct direction as shown by the constellations in the reticule.

All of course assuming that the polar scope reticule has already been perfectly aligned with the mount. Also assuming that the polar finder settings are correct (I sometimes get a discrepancy between stellarium and polar finder).

[IanL]

Yes indeed you can. You could:

- Use the software to calculate the Hour Angle and use the manual circles to set it (as shown in the SW mount manual).

- Or use the on-screen display in the software to set the hour angle by eye, which would probably be more accurate than relying on the constellation markers, but prone to error still.

- Use EQMOD to set the hour angle for you. This is probably the easiest and most accurate method. The key thing is that you have to put the Polaris indicator in a known position (as chosen by the 12, 3, 6, 9 O'Clock selector in the software). Assuming you have levelled the mount properly and your reticle is properly centred this is easy to do. You just centre Polaris on the cross, then use either the Alt adjusters only to raise or drop it to the 12/6 O'Clock position on the big circle, or the Az adjusters only to move to 3 or 9 O'Clock. You then rotate the mount in RA to align the small circle with Polaris.

Doing this allows EQMOD to rotate the mount to the correct hour angle when you press the go to HA button (since it is starting from a known position that you have determined mechanically), then you centre Polaris in the little circle using only the adjusters and you're done. The key to this working is a level mount and a centred reticule, but if you have those I find I do not need to drift align or make any further adjustments to get good (guided) imaging. If you are imaging unguided then you would probably need to drift align or use software to adjust futher.

[SniffTheGlove]

Thanks for the replies.....

1) I had previously centred the polar scope a year ago when I brought the scope and followed a few of Dion's Tutorials.

2) As I use EQMod, just read IanL that it has it's own Polar Alignment routine. So looking up Chris's YouTube videos.

3) If all holds out I hope for a better (slower) attempt to do things as I was just to ambitious last night to get it all working then ended up doing my back in more. I find it difficult to sit on a small box and look through the polar scope with my back. I think I might put a Dion mod for an webcam polar cam.

4) Thanks IanL for the diagram.

5) I wish that Synta put stepper motors onto the az and alt adjustment bolts so you could do everything via EQMod or PC etc without having to move from PC to mount and back again to check.

6) Been playing with it all day getting used to it in the new position and getting everything structurally aligned (scope/finderguider/finder and Canon 350d widefield). From new position I can pick out a fence post on the north hill and align onto that where as in the Obsy I had no physical landmarks to align onto (just stars at night). Doing this in the day is easier especially when it is warm..

[glowjet]

Hi Sean, that could be a case of serendipity, but it seems you have established that your Polar scope will revolve accurately about the central axis of your mount. If you would like to try a little practical experiment, then try this... There is a polar finder article written by Dr Clay Sherrod called "Kochab`s Clock" where he uses certain times of the year to establish quite accurately the position of Polaris and mount to Polar align. It is based on the assumption that Polaris lies in a straight line with Kochab, Beta Ursa Minoris, passing through the NCP, I will not go into details here, but you might like to look it up.

Using part of this method, first make sure your mount is totally level, then without the OTA and counterweights, just leaving the bar. set the Polar scopes little circle at the 6.00 position, the same as you would do for a Polar transit, align the counterweight bar, do this in daylight with a plum bob if you want to be precise, Lock all up so now that your scope`s RA will revolve with the Ppolar scope in unison with the bar.

First clear night sit behind your scope, and for this you may need binoculars, establish where Kochab is, it should not be difficult to now turn the bar to align through Polaris to establish the angle with Khochab, now adjust slightly to align with UMI 5, that star and UMI 4, lie fairly close by Khochab. I did say previously that this was based on an assumption concerning Kochab. If you consult any good star map you will see that Polaris and the NCP do NOT lie in a straight line with this star, but to all intents and purposes, it does with UMI 5. This angle will remain a constant as it will follow the circular route of the Pole star around the NCP and can be used to set Polaris alignment at any time of the night.

All that remains to be done is using the Alt/Az just adjust Polaris into the circle to position the mounts central axis on the NCP and complete your scope assembly.

I have mentioned this method, which I have adapted from various Polar alignment procedures, as it may help those who do not have the facility of software and computer control. As to its accuracy?, Clay Sherrod is quite happy that his Kochab alignment will bring established centering of a target for some considerable time. This is slightly more accurate using UMI 5, I have not had a chance to try it, but I shall as soon as circumstances permit

[SniffTheGlove]

Thanks John.

I have now managed to run through the EQMod Polar Align process. Very easy and will be come easier over time. I was out a bit yesterday!

[baggywrinkle]

Envy, I am still struggling with a SCP alignment. I have the polar scope but Sigma Oct is so faint from my garden, mainly due to Perth being south of where I live and causing a glow ....

[Daddystu]

It must be difficult to align stood upside down