Polar scope - most accurate in SH?

Amaranthus (Barry) · #1 13-02-2015, 05:36 PM

Last night I was using my AZ-EQ6 as a guiding platform for my Sony NEX-3 camera, shooting at f/5.6-55mm. This motivated me, for the first time, to try out my polar scope, which actually works really well in my dark skies of southern Tassie.

Alignment with the polar scope was pretty straightforward. I typically drift align, and had not adjusted my elevation settings since the last drift session (nor moved my location). But I was surprised to find my elevation was slightly out, at least according to the polar scope. (I did need more adjustment in azimuth, but some final tweaking in elevation was also required).

That got me thinking - how accurate is the polar scope in the southern hemisphere? Although we have the distinct disadvantage of no bright pole star, we do have a fairly distinct Octans asterism that is useful if your skies are fairly dark. (There is also 6.9 mag BQ Oct, which is just visible in my polar scope and is only 10.5' from the pole).

The interesting part here is that the reticle lines up with 4 stars in the SH, not one like in the NH, which in my reasoning should give a much more precise alignment, because there is more 'leverage constraint' in the model.

Anyone agree/disagree? I'm now wondering how accurate I might get my alignment with the polar scope. It's certainly a LOT faster than drift aligning, even when I'm using the beaut PHD2 procedure!

Amaranthus (Barry) · #2 14-02-2015, 08:18 PM

*Bump* In case anyone missed this and wants to comment

Camelopardalis (Dunk) · #3 14-02-2015, 09:43 PM

The cap on my polar scope ain't come off since I've been here (besides one accident!) and given how faint the asterism is, there's next to no chance of seeing at anywhere except a good dark site.

Given the FOV of the polar scope, it'll get you in the ball park no doubt, but once you get the focal length of a scope onboard you'll highlight how far off it could be. The last couple of times out, I've taken my C8 along for polar alignment, before switch scopes for imaging

Camelopardalis (Dunk) · #4 14-02-2015, 09:46 PM

Sorry, wasn't really a useful comment

Logieberra (Logan) · #5 14-02-2015, 10:10 PM

I would use a software program first. Something like Pempro's Polar Align Wizzard. Get the mount spot on the pole, preferably from the comfort of home. Pempro provides real time results, so you'll known when it's locked in. Then make physical adjustments to your polar scope alignment screws, as you look through it, to align the reticle on the appropriate stars. Then, and only then, would I trust a polar scope for polar alignment out in the field

Amaranthus (Barry) · #6 14-02-2015, 10:30 PM

Well, the key advantage of the 'raw' polar scope is that you don't need a PC in the field! (e.g. when I had my DSLR mounted on the GEM. When I'm imaging with my PC, I'll always finish with Drift Alignment for safety

But I'm still interested on feedback on my idea about the SCP reticle actually being more accurate than the NCP one...

Logieberra (Logan) · #7 14-02-2015, 11:04 PM

Never mind. You missed my point.

Amaranthus (Barry) · #8 14-02-2015, 11:47 PM

Sorry Logan, I do see what you mean now. Calibrate with a highly accurate method like Pempro, and then cross-check the accuracy of the quicker polar scope against that absolute reference. Good thought. I could do a similar thing with a precise Drift Align too.

Logieberra (Logan) · #9 14-02-2015, 11:50 PM

Eden (Brett) · #10 15-02-2015, 12:14 AM

G'day Barry,

I've only looked through the polar scope on my AZ-EQ6 a couple of times, never tried to actually do a polar alignment with it since it needs to be "calibrated" and I'm not a fan of fiddling around in the dark with a 2mm allen key.

It would be useful in the field if you've just got a DSLR on your mount, but for anything else I would use the Synscan polar alignment routine, followed by a barlowed drift align, preferably with a camera. I have a feeling I won't be faffing around with polar alignment as much from now on, though.

alistairsam · #11 15-02-2015, 12:34 AM

Hi,

I do use Pempro but it has let me down with inconsistent results so has phd2 and alignmaster. This is an eq6 with a belt drive with very low backlash.
so I fixed a cctv lens to my guide cam, a lodestar, used a 25mm piece of pipe to fix it to the polar scope and it has been the best thing ever.
I've been imaging for about 2 years so have drift alignmed with various methods including the darv method, but the polar scope method has by far been the most consistent and easiest method for me and I do 40min guided subs with virtually no drift. very minor over 30mins.
using a sensitive cam on the polar scope means I can easily see the asterism from light pollution central.
I did test it on a friends mount after he aligned with alignmaster and it was noticeably off even after 3 iterations of alignmaster.
just my experience.

Cheers
Alistair

Logieberra (Logan) · #12 15-02-2015, 07:31 AM

Alistair, great idea! A pic of the tube setup would be helpful.

Re your Pempro results, that does surprise me. I've heard of people using very complex PA modelling programs (e.g. like SkyX) involving hundreds of all sky points, only to return to Pempro b/c the results are so trustworthy. AP mount users swear by it, as they should - it comes bundled with the mount.