Polar Alignment and Guiding

[Starwiz]

Does anyone know how to determine the level of PA accuracy needed?

I'm taking narrow-band subs of 6 minutes using a SW200P on NEQ6 and guiding using OAG.

My last PHD2 guide log says PA is 1.0', but I don't know if this is good enough or not.  It would be good to calculate maximum permissible error based on my set-up.

Thanks

John

[MarkAR]

If the log says 1 arc minute then there's room for improvement. I use Ekos rather than PHD2 and easily get less tham 12 arc seconds. Thats under 12".

The better your PA the better your guiding will be and that's the important factor. With good PA on a NEQ6 you should be able to get under 1.0" RMS guiding.

[Starwiz]

48 minutes ago, MarkAR said:

If the log says 1 arc minute then there's room for improvement. I use Ekos rather than PHD2 and easily get less tham 12 arc seconds. Thats under 12".

The better your PA the better your guiding will be and that's the important factor. With good PA on a NEQ6 you should be able to get under 1.0" RMS guiding.

Thanks, I may try a drift alignment tonight to see what I get.

John

[alacant]

3 hours ago, Starwiz said:

PA is 1.0'

Hi.

That's more than acceptable. In fact you may want to increase the error so that DEC guides in one direction only. Guiding apps will happily cope with a large pa error. I keep my eq6 around 3' to 5'. 

Cheers

[Realtimedoctor]

Polar alignment accuracy and what is a good number is very debatable. Some people deliberately keep it around 5 arc and guide Dec in one direction as @alacant described.

Some like me, like to get it as low as possible. My usual PA error is ~2arc min, if I'm very fussy, and have time then try to get it as close as physically possible. You can't get a zero error (due to multiple factors beyond your control). 

In practice as long as your PA is less than 5arc min, you should be able to guide that out. Though, with an off axis guider you should have good balance, especially as you are guiding at 1000mm FL. 

You can also try and guide the Dec only in one direction irrespective of the PA error, depending on which direction your mount guides smoothly. 

I've an EQ5, way smaller mount that yours and also guide a 200p with a Finder guider. I can easily manage to guide at ~1 - 1.2 arc sec RMS (some would say its not good enough, for me works perfectly fine) 

Don't pay much attention to the numbers, see what your subs look like. That should be your benchmark, rather than an arbitrary number to aim for. 

 

Good luck. 

[alacant]

1 hour ago, Realtimedoctor said:

subs look like

Couldn't agree more. The numbers mean nothing if you're satisfied with your final images;)

Cheers

Edited August 14, 2020 by alacant

[Starwiz]

17 hours ago, alacant said:

Hi.

That's more than acceptable. In fact you may want to increase the error so that DEC guides in one direction only. Guiding apps will happily cope with a large pa error. I keep my eq6 around 3' to 5'. 

Cheers

 

17 hours ago, Realtimedoctor said:

Polar alignment accuracy and what is a good number is very debatable. Some people deliberately keep it around 5 arc and guide Dec in one direction as @alacant described.

Some like me, like to get it as low as possible. My usual PA error is ~2arc min, if I'm very fussy, and have time then try to get it as close as physically possible. You can't get a zero error (due to multiple factors beyond your control). 

In practice as long as your PA is less than 5arc min, you should be able to guide that out. Though, with an off axis guider you should have good balance, especially as you are guiding at 1000mm FL. 

You can also try and guide the Dec only in one direction irrespective of the PA error, depending on which direction your mount guides smoothly. 

I've an EQ5, way smaller mount that yours and also guide a 200p with a Finder guider. I can easily manage to guide at ~1 - 1.2 arc sec RMS (some would say its not good enough, for me works perfectly fine) 

Don't pay much attention to the numbers, see what your subs look like. That should be your benchmark, rather than an arbitrary number to aim for. 

 

Good luck. 

 

15 hours ago, alacant said:

Couldn't agree more. The numbers mean nothing if you're satisfied with your final images;)

Cheers

Thanks.  I tried a drift align last night and this morning the PHD2 log said it was 7.5', so a lot worse.  🤣

[alacant]

Were you on stable ground and at both the meridian -az-  and at 90° -alt- to it?

I find pHd2's traditional drift reliable although the eq6 altitude adjustment leaves a lot to be desired; I always need to finish with the south bolt clockwise and take up any slack with the north bolt, otherwise it moves.

HTH

Edited August 15, 2020 by alacant

[Starwiz]

27 minutes ago, alacant said:

Were you on stable ground and at both the meridian -az-  and at 90° -alt- to it?

I find pHd2's traditional drift reliable although the eq6 altitude adjustment leaves a lot to be desired; I always need to finish with the south bolt clockwise and take up any slack with the north bolt, otherwise it moves.

HTH

Yes, on my terrace that has stone tiles.  I also put some of my diving weights onto the mount's eyepiece holder to make it even heavier.

I've given up trying to adjust it by the bolts, so I just alter the height of the back leg of the tripod.

I'm probably just not very good at drift alignment yet.  😀

John