How important is physical positioning in polar alignment?

[Davey-T]

2 hours ago, Ash2020 said:

Thank you Dave. One of the problems I've had so far is that the mount will slew to an alignment star but I can't always tell, if there are a few, whether I'm centring the right star. Also, if its not already in view, I don't know which way to manually move to find it. I guess practice makes perfect. I've just got a Telrad red dot finder which I think will make everything much easier.

 

Definitely need an RDF as the first alignment star can be out of the field of view, I start with a 26mm eyepiece and centre roughly then change to a 12mm illuminated reticle to centre.

Dave

[iapa]

On 30/01/2017 at 19:36, Ash2020 said:

Hi again. I'm still trying to get my head around polar alignment. It seems to me there are 4 variables:

1. The North leg of the tripod.
2. The bubble level
3. The Dec clutch position
4. The R.A. clutch position

What is the order of importance of these? I know they're all important but, for instance, what would give more accuracy, a perfect North on the leg but not quite level or absolutely level but a degree or two out on the leg position? Likewise, if the weight bar isn't precisely 90 degrees to the horizontal, is it a problem if it is counteracted on the other axis?

Sorry if I'm being pedantic, it's just about my understanding.

Cheers

Andrew

 

2, 3 and 4 do not need to be really considered of polar alignment. It is useful, not necessary to have the mount levelled. A three star alignment should sort it out if not level. Your circles will assume you are level tho' of you were to use them to aim at your target later..

The polarscope will have it's own adjustments which are not used to do the polar alignment, but to align the polar scope with the RA axis. Doing this first is a good idea as it will help get a more accurate polar alignment.

 

I usually draw myself a line running east-west on the ground, and place the two rear legs on that, obviously with the 3rd leg pointing north. Makes for easier setup later. However as long as the front leg is more or less north - more is better:)

You will probably need to release the RA clutch to rotate the mount so the weight bar is parallel to the ground - lock the clutch before you let go . This will open up the central 'view port' that you are looking thought to see Polaris. 

You adjust altitude and azimuth bolts to make your polar alignment. This may help. EQ5 Set up & Polar alignment - YouTube

I cheat, I got the PoleMaster and adapters for my mounts, as well as the Starsense

PS I have a certificate 'Teaching Granny to suck Eggs 101'

 

[MARS1960]

On 2/2/2017 at 14:39, Ash2020 said:

Thank you Dave. One of the problems I've had so far is that the mount will slew to an alignment star but I can't always tell, if there are a few, whether I'm centring the right star. Also, if its not already in view, I don't know which way to manually move to find it. I guess practice makes perfect. I've just got a Telrad red dot finder which I think will make everything much easier.

 

I bought a cheap laser pointer off ebay and hold it up to the finder eyepiece, this i found was a cheap way to make sure i centered the correct star on the initial star aligment.

[Stub Mandrel]

I point the north leg at Polaris and roughly level the mount. This is enough to make sure Polaris is near or inside the circle in the polarscope view, any more time aligning or levelling is time wasted that can be better spent on getting Polaris in exactly the right spot.

 

[barkis]

Certainly generated a lot of comments this topic.
I always attached great importance to the polar axis of the mount being 
exactly perpendicular to  the horizontal plane, and point parallel to the local meridian to the NCP.
Sure, the  the auto tracking will take care of the guide star, but long subs will still contain 
Dodgy stars to field rotation if those criteria are not met. 
I'm happy to be contradicted, if  it can be explained what mechanism in the guiding prevents such 
an event. Stars in the vicinity of the guide star will survive, but I fear for the edge of field ones.
Now I admit I am basing my utterance on a day gone by, when guiding was not flavoured with todays modern
electronic marvels, but even so, I can't get my head around a misaligned polar axis being compensated for 
by this presumed wizardry .

I'm not accepting corrections by processing though, that's illegal .

 

 

[Stub Mandrel]

18 hours ago, barkis said:

Certainly generated a lot of comments this topic.
I always attached great importance to the polar axis of the mount being 
exactly perpendicular to  the horizontal plane, and point parallel to the local meridian to the NCP.
Sure, the  the auto tracking will take care of the guide star, but long subs will still contain 
Dodgy stars to field rotation if those criteria are not met. 
I'm happy to be contradicted, if  it can be explained what mechanism in the guiding prevents such 
an event. Stars in the vicinity of the guide star will survive, but I fear for the edge of field ones.
Now I admit I am basing my utterance on a day gone by, when guiding was not flavoured with todays modern
electronic marvels, but even so, I can't get my head around a misaligned polar axis being compensated for 
by this presumed wizardry .

I'm not accepting corrections by processing though, that's illegal .

 

 

Your point is that imperfect polar alignment will cause field rotation. Yes it will in the same way that a misaligned DEC axis will.

How much will it affect images?

Well an Alt-Az mount in the midlands has its polar alignment 52 degrees out and is generally good for ~1-minute subs, so how big is the effect with a degree or two of misalignment?

A graph I found suggests that it will be about 1 degree per hour if your PA is one degree out.

 

 

[barkis]

36 minutes ago, Stub Mandrel said:

Your point is that imperfect polar alignment will cause field rotation. Yes it will in the same way that a misaligned DEC axis will.

How much will it affect images?

Well an Alt-Az mount in the midlands has its polar alignment 52 degrees out and is generally good for ~1-minute subs, so how big is the effect with a degree or two of misalignment?

A graph I found suggests that it will be about 1 degree per hour if your PA is one degree out.

 

 

Yeah, but imagers doing serious deep Sky Deep sky exposures, for the most part  are exceeding 60 seconds.

[Stub Mandrel]

8 minutes ago, barkis said:

Yeah, but imagers doing serious deep Sky Deep sky exposures, for the most part  are exceeding 60 seconds.

I was trying to estimate the size of the effect, not making a judgement on whether it matters or not!

[barkis]

5 minutes ago, Stub Mandrel said:

I was trying to estimate the size of the effect, not making a judgement on whether it matters or not!

Fair enough me lad .