drift polar alignment procedure Accurate Polar Alignment

[MarsG76]

9 hours ago, ollypenrice said:

Getting to turn off the Dec guiding entirely is a nice idea and sounds OK in theory, but I don't know anyone who has ever managed it, and you need to consider atmospheric diffraction as I mentioned in an earlier post.

If running misaligned and guiding in one direction only, you do need to reverse the guide axis in Dec after the flip.

Olly

Very good point with atmospheric refraction... 

DEC guiding in PHD has a "resist DEC switching" option for such a reason I think. I don't remember such an option in NexGuide.

Even if DEC was still on, it can be limited to adjustments every 5, 10... 20 etc seconds I guess... so it can still keep an "eye" (or CCD) on the star in DEC but only adjust every so often when needed.

I'll tell you that I'm itching to give it a go now that I feel like the system is ready for more accurate and detailed imaging than I ever had in the past... Next 2 days I had off work and only going back on Wednesday for a late shift so perhaps I can trial the tracking and imaging in real world scenario.

[newbie alert]

On 08/09/2017 at 15:51, LightBucket said:

Surely that is only as good as how well your scope is aligned with the mount..?? With polemaster you are aligning the mount and not the scope...!  Depending on scope tube ring and dovetail alignment, or in the case of using a finderscope, how well that can be aligned with the mount with the fiddly 6 adjustment screws...

PA is aligning the mount and not the scope...look at polar scopes, theyre through the mounts axis otherwise they'd be on top like a finder..you align with the ncp  and then the stars are tracked on the RA axis

[rwg]

On 08/09/2017 at 15:51, LightBucket said:

Surely that is only as good as how well your scope is aligned with the mount..?? With polemaster you are aligning the mount and not the scope...!  Depending on scope tube ring and dovetail alignment, or in the case of using a finderscope, how well that can be aligned with the mount with the fiddly 6 adjustment screws...

Actually no, it doesn't matter how well aligned your scope/guider is with the mount axis (at least as long as you are not stupidly badly aligned - like 5 degrees or more off). SharpCap (and Polemaster for that matter) work by calculating the point about which the stars appear to rotate when you turn the mount in RA. That's the point in the sky that the RA axis is pointing to. It doesn't matter if the scope is pointing exactly along the RA axis or a degree or so off - the only point in the image that will appear not to move as the mount is turned is that center-of-rotation point.

You can test this yourself - sit on a swivel chair under a light. Look up a bit so you can see the light and then spin around. You'll see that the light that on your chair's 'RA axis' stays in the same place in your vision regardless of whether you are looking straight up or not quite straight up.

cheers,

Robin

[ollypenrice]

On 10/09/2017 at 05:43, MarsG76 said:

 

DEC guiding in PHD has a "resist DEC switching" option for such a reason I think. I don't remember such an option in NexGuide.

Even if DEC was still on, it can be limited to adjustments every 5, 10... 20 etc seconds I guess... so it can still keep an "eye" (or CCD) on the star in DEC but only adjust every so often when needed.

 

Thanks, I hadn't spotted that.

Olly

[Dixie]

Polemaster is very accurate - at least on my set up.  I use a C14 on an EQ8 and any worries about getting a good PA to image at almost 2.3 metres have been banished.  Polemaster says it can get within 30 arc secs.  Generally Phd drift assistant will report something around 1 arc minute.

[LightBucket]

3 hours ago, rwg said:

Actually no, it doesn't matter how well aligned your scope/guider is with the mount axis (at least as long as you are not stupidly badly aligned - like 5 degrees or more off). SharpCap (and Polemaster for that matter) work by calculating the point about which the stars appear to rotate when you turn the mount in RA. That's the point in the sky that the RA axis is pointing to. It doesn't matter if the scope is pointing exactly along the RA axis or a degree or so off - the only point in the image that will appear not to move as the mount is turned is that center-of-rotation point.

You can test this yourself - sit on a swivel chair under a light. Look up a bit so you can see the light and then spin around. You'll see that the light that on your chair's 'RA axis' stays in the same place in your vision regardless of whether you are looking straight up or not quite straight up.

cheers,

Robin

Thanks for that, I think my small brain is getting there...

so am I correct in thinking that if the mount is perfectly aligned with the NCP, but the scope is very slightly off that the only thing that will be out are goto’s, by the amount that the scope is off..? So the object will be off centre but it will track that object perfectly and will stay just off centre...

Also does it mean that if my finder is not perfectly aligned with my Main scope I can still use for PA with your software..?

Hope that makes sense

[tooth_dr]

48 minutes ago, LightBucket said:

Thanks for that, I think my small brain is getting there...

so am I correct in thinking that if the mount is perfectly aligned with the NCP, but the scope is very slightly off that the only thing that will be out are goto’s, by the amount that the scope is off..? So the object will be off centre but it will track that object perfectly and will stay just off centre...

Also does it mean that if my finder is not perfectly aligned with my Main scope I can still use for PA with your software..?

Hope that makes sense

I don't think that is right either. Once you centre the first star the mount 'knows' where it's at and this is irrespective of PA? My PA is not great but my gotos are on the button! 

[rwg]

53 minutes ago, LightBucket said:

Thanks for that, I think my small brain is getting there...

so am I correct in thinking that if the mount is perfectly aligned with the NCP, but the scope is very slightly off that the only thing that will be out are goto’s, by the amount that the scope is off..? So the object will be off centre but it will track that object perfectly and will stay just off centre...

Also does it mean that if my finder is not perfectly aligned with my Main scope I can still use for PA with your software..?

Hope that makes sense

Once you go through the align procedure on your handset or use the 'Sync' option on your planetarium software if you are using ASCOM it will correct for the offset from then on if your PA is accurate.

If your PA is less than accurate then your handset/planetarium may still be able to correct for the problem if you have done a two or three star align, although having accurate PA in the first place is preferable.

You don't need perfect alignment of the finder with the main scope or the mount axis for the SharpCap polar alignment to work - find out more here : http://www.sharpcap.co.uk/sharpcap/polar-alignment

One thing you do need to be careful of is that if your finder moves or flexes as you rotate he mount then you will get incorrect results - people have had problems with things like pulling cables causing this.

cheers,

Robin

[newbie alert]

7 minutes ago, tooth_dr said:

 

8 minutes ago, tooth_dr said:

I don't think that is right either. Once you centre the first star the mount 'knows' where it's at and this is irrespective of PA? My PA is not great but my gotos are on the button! 

That means your star alignment (goto) Is spot on..if the PA is out it will show early drift on the imaging subs....

[tooth_dr]

15 minutes ago, newbie alert said:

 

I know, was just replying to LightBucket, explaining that they arent linked.  Never got round to doing a really tight PA, but the mount autoguides ok, so maybe someday!