Polar Alignment & Plate solving

[fwm891]

I really like the polar align feature with my light rig when I can position to see Polaris. However what I would like is to be able to use this function from my observatory rig which has now view of the Polaris area. Would it be possible to incorporate an on-line plate solver so areas of sky well away from the pole star are useable.
Currently I drift align after changing set-ups which is usually fairly easy but not always. A plate solve option would be ideal.
Hopeful...

[admin]

Hi,

Unfortunately the algorithm that SharpCap uses to do the polar alignment wouldn't work correctly with the reference images a long way away from the pole - firstly it would require a full accounting for spherical geometry which can be largely ignored if you restrict the area in which you work to within a few degrees of the north celestial pole. Secondly it's my opinion that trying to apply the same idea to images that are taken many degrees away from the pole will lead to significantly less accurate results, which is obviously the opposite of what we are trying to achieve.

Some of the GOTO based polar alignment tools should be able to work in the situation that you are in where you cannot see Polaris, but I have never been desperately impressed by the accuracy of those (running them repeatedly without making adjustments showed estimates of the polar error that vary significantly). I think there is also polar alignment tool in AstroTortilla that is a bit like a speeded up drift alignment – you move the mountain the RA direction and it uses plate solving to work out how much 'drift' in declination has been caused by the RA move, allowing it to calculate the polar alignment offset. The downside of that approach is that you have to adjust altitude and azimuth separately.

Cheers, Robin

[fwm891]

Thanks Robin,
How far out beyond the current 5°(degrees radius) is it possible to take your plate solve routine? Would you consider expanding the area...
Francis

[admin]

Hi,

The answer is not very far afraid. I tested and found that basically things started to break down beyond about 7° from the pole - the current database goes out to that limit because there is basically little point in going any further

Cheers, Robin

[turfpit]

Francis

What mount do you have?

I have a Celestron AVX and the handset has a facility called All Star Polar Align (ASPA). With this I can check polar alignment of the mount using any star in the sky. Indeed, this is always my last step in the alignment process.

Dave

[fwm891]

Hi Dave, I have an iOptron CEM 60 Standard (non EC) on a Tri-pier in my observatory. I'm happy enough with drift alignments which I do every month or so just to make sure everything is OK. The CEM 60 has a number of alignment routines also which are a good alignment check.

[turfpit]

Francis

From the manual it looks like Polar Iterate Align is iOptron's equivalent facility to Celestron's ASPA. I always carry out the ASPA using the camera (rather than eyepiece) as I have found that results in better alignment (no changes in the optical train and the capture program reticule is available to help accuracy).

CEM60-manual-page-23.JPG (103.34 KiB) Viewed 4071 times

CEM60-manual-page-29.JPG (114.22 KiB) Viewed 4071 times

Dave

[fwm891]

The Polar Iterate Align is a great tool but it helps if your reasonably close to start with. Also a good idea to use a bull's eye or cross hair overlay for consistency. With my smaller CEM25P I can use the polar scope to get a very close alignment and then tighten the PA with a run of PIA.

[givoly]

+1

I do not have NCP/Polaris visibility from nearly any of the places I typically observe from. When I use SharpCap, I first do polar alignment using ASIAIR's all sky polar alignment. Can this feature not be incorporated into SharpCap?

I much prefer not to use ASIAIR just for polar alignment, and then move to SharpCap to continue my observation session...

Thanks, Robin, for the fantastic software, and the amazing new planetary live stacking feature. My only request there - add drizzle, to allow improving resolution... (sorry, out of context - you can strike it)...

[admin]

Hi,

now that SharpCap has plate solving built in, I have more scope to think about alternative polar alignment tools.

As I've mentioned in posts before, I'm not going to do a GOTO based alignment tool as I feel that the accuracy can be suspect if there are any errors in the GOTO movements.

A rotation based tool is possible, where the mount is only moved in RA is possible, but you still need to collect measurements over a fairly wide range of hour angle positions. Ideally you'd collect data over 90 degrees or more of rotation, but whatever obstacle blocks the view of the pole is likely to cause issues when you move more than 90 degrees from the meridian, and you need to flip when moving past the meridian, which makes what should be a relatively simple measurement much more complex to arrange.

cheers,

Robin