Continuing Polar Alignment Issues

[SStanford]

Hi All,

I had the first chance with clear skies yesterday since Christmas Eve! I thought I'd take the opportunity to get myself properly polar aligned (on Christmas Eve I set up Ascom/EQMod/Stellarium/PC controlled mount, which took up the whole evening).

As mentioned in previous threads, I don't have a view of Polaris from my covered Balcony and only views from the East to the South. My plan was two fold:

• Use the Synscan Handset to do a three star alignment, check the Polar alignment error and adjust alt/az accordingly until error reduced as close to zero as possible.
• Use the DARV alignment process via APT here

I quickly found out that of the available stars on the three star alignment method via the SynScan handset, I could only see two in the night sky. I'm hoping on a clearer night I can return to this method.

The DARV alignment method asks that you use stars at zero dec (celestial equator) in both the South and East regions.  I couldn't see any stars that were near 0 dec in the south and unfortunately it seems 0 dec in the east (which is very near the horizon) is obscured by buildings! 

My question is; how close to 0 dec do you need to be in the South and East for DARV to be effective? Is it more important that you're as close to South/East as possible or 0 dec?

S.

[Owmuchonomy]

If you have a Synscan handset then it is all in there. Do a 2 star alignment then use the polar alignment routine option built into the Synscan menu. No need for Polaris or a polarscope but use a relatively high power EP. It will probably take 2 or 3 iterations to get it close enough for imaging.

[SStanford]

Thanks @Owmuchonomy, 

Have you found 2-Star alignment is sufficient for this process? I thought you needed at least 3 for adequate alignment error...

If 2-star works then that's perfect.  I using the Crosshairs shown on liveview via APT and zooming in to increase accuracy.  Can you see any issues using this method rather than an EP?

Edited January 7, 2021 by SStanford
Typo

[KP82]

Liveview with a crosshair overlay is definitely going to be more accurate than a standard ep for star alignment.

Not familiar with the DARV in APT, but I used to do drift align in PHD2 for every imaging session in my previous house. When adjusting the azimuth, PHD2 would select a star close to 0 DEC and near meridian. There was a tree obscuring the views near the celetial equator, so I always selected a star around 7 DEC. When adjusting the altitude, PHD2 would select a star 60 degress to the East or West and still close to 0 DEC. I always chose a star at about 3 - 4 DEC as I had very limited views of 0 DEC to the west because of the garden fence. The resulted PA was good enough for imaging.

Edited January 7, 2021 by KP82

[SStanford]

2 hours ago, KP82 said:

Liveview with a crosshair overlay is definitely going to be more accurate than a standard ep for star alignment.

Not familiar with the DARV in APT, but I used to do drift align in PHD2 for every imaging session in my previous house. When adjusting the azimuth, PHD2 would select a star close to 0 DEC and near meridian. There was a tree obscuring the views near the celetial equator, so I always selected a star around 7 DEC. When adjusting the altitude, PHD2 would select a star 60 degress to the East or West and still close to 0 DEC. I always chose a star at about 3 - 4 DEC as I had very limited views of 0 DEC to the west because of the garden fence. The resulted PA was good enough for imaging.

Excellent, thanks @KP82.

Unfortunately for me, drift alignment via PHD2 is out of the equation until I get my guidescope and guidecam. Only have my DSLR and this isn't supported by PHD2 :(.

[Owmuchonomy]

4 hours ago, SStanford said:

Thanks @Owmuchonomy, 

Have you found 2-Star alignment is sufficient for this process? I thought you needed at least 3 for adequate alignment error...

If 2-star works then that's perfect.  I using the Crosshairs shown on liveview via APT and zooming in to increase accuracy.  Can you see any issues using this method rather than an EP?

For imaging one should use a 2 star alignment using 2 stars on the same side of the meridian as your imaging target. A 3 star alignment adjusts for cone error in your set up so it’s more of a compromise. So yes, a 2 star alignment is more appropriate. I use mk 1 eyeball so can’t comment on using APT software.

Edited January 7, 2021 by Owmuchonomy

[happy-kat]

Have you seen this post

https://stargazerslounge.com/topic/362486-how-to-get-quick-and-easy-polar-alignment-and-add-a-push-to-finder-screen-to-an-eq2-mounted-scope-all-for-£20/

[SStanford]

Hi all,

I had some clear skies last night so I tried the two-star alignment process via Synscan with my EQ3 pro, adjusting the alt-az according to the pa error. I didn't have much success!

I was running the alignment on Rigel then Pollux and I'd always get very strange results on pa error. The figures would put the alt and az in +/- double digits alternating with each iteration on the two-star alignment process.

I'm thinking this may be due to my slewing of the mount via the hadset for correction: when slewing in an opposite direction it takes a while for the mount to respond (as the gears engage) but I assume the handset registers the action despite lack of movement.

Would there be anyway to offset this? I'm thinking that if I only use two directions on two stars in a single direction (I.e. From East to South or vice versa), therefore no need to backtrack the direction on the headset...

Also, in what direction am I moving azimuth when it says +/- in pa error? With altitude I'm assuming it's straight forward as it's numbered.

Any guidance would be hugely appreciated!

Edited January 10, 2021 by SStanford
Additional alt az question, rewrite for clarity.

[SStanford]

On 07/01/2021 at 16:19, happy-kat said:

Have you seen this post

https://stargazerslounge.com/topic/362486-how-to-get-quick-and-easy-polar-alignment-and-add-a-push-to-finder-screen-to-an-eq2-mounted-scope-all-for-£20/

@happy-kat I have seen this, about a month ago I followed a similar instruction as outlined by @Terrierist here.

A quicker way to get rough polar alignment for my purposes was to simply activate the reticule on StarWalk 2 APP and hold this against the polar scope until Polaris sits in the cross hairs.

[MercianDabbler]

I don't know how your mount works but if it's using 'open loop' steppers then any backlash in the geartrain could be an issue. If it is using rotary encoders to directly measure the axis position then backlash should not be an issue.

The usual way to deal with backlash (when using a machine tool, anyway) is to make sure that you always approach the desired point from the same direction. If you 'miss' and go past it then you need to back off a good way and then approach the desired point again from the 'correct' direction.

Like I said, I've no idea how your mount works. If it deals with mechanical backlash by design then your problem is elsewhere.

Good luck!

[SStanford]

Thanks @MercianDabbler; it seems clear that my issue is due to backlash (a term I've just learned this afternoon!) From further reading online I'm seeing that others have had issues with Backlash in the EQ3pro.

I was approaching the stars in different directions: from parked position to Rigel (in SE, so from North to SE) back to Pollux (in E, so from SE to E). This I assume will be accounting for a fair bit of the PA error inaccuracy.

For my next attempt I'll be sure to go in the same direction, however, I'm not too sure which two directions I should use exclusively on the hand control. Is this just a trail and error to see which gears are engaged in whichever direction you try?

Elsewhere an interesting point was raised with regards to the +/- PA error in Synscan; is the handset reporting the error, or the required correction?

[MercianDabbler]

1 hour ago, SStanford said:

For my next attempt I'll be sure to go in the same direction, however, I'm not too sure which two directions I should use exclusively on the hand control. Is this just a trail and error to see which gears are engaged in whichever direction you try?

Not sure, sorry. An extra complication with telescopes over machine tools is that gravity may also throw them to the opposite side of the backlash, so I think that being aware of the issue and doing some experimentation is the order of the day (err... night). Another factor is that your mount will always be moving in one direction when tracking the sidereal motion so keeping the backlash on the same side as the 'natural' RA motion would seem like the best bet for that axis at least.

[SStanford]

45 minutes ago, MercianDabbler said:

... Another factor is that your mount will always be moving in one direction when tracking the sidereal motion so keeping the backlash on the same side as the 'natural' RA motion would seem like the best bet for that axis at least.

Good point; I'll be sure to stay in this direction moving from the two alignment points.

[SStanford]

Sorry to keep bumping this thread!

I've just been informed elsewhere that the 2 or 3-star alignment method will produce "very poor results".  Quote: "If you are aiming for AP (which is the goal of this specific forum), you will have very poor results with this method you're trying to implement, even if you get a zero PA error as reported by synscan. 2 and even 3 star alignment has cone error and other things affecting it's accuracy. If it was as simple as using a 2 star alignment then we would all just be getting PA from plate solving somehow. It is not how it works."

So without the view of 0Dec with the DARV alignment in the East and with the Synscan PA error function being useless, which other options do I have?

[SStanford]

Just to continue my previous post; with regards to DARV, I have view of stars in the east starting at about +15°-20° from the horizon (and no view of the west). As this is quite a bit away from 0Dec do you think this will be an issue?

[kens]

That should be fine. Its too murky down there anyway

[SStanford]

2 minutes ago, kens said:

That should be fine. Its too murky down there anyway

Music to my ears! I'll try DARV when next have clear skies. 

[SStanford]

Quick update on this:

Clear skies tonight; setup eager to get the DARV process underway and PA accordingly. Not as easy as it looks! Its clear theres some significant backlash on the mount as in slew rate 1 there is no noticeable trailling as to make adjustments as outlined here.

In addition to this, Slewing West as instructed makes stars trail in an downward direction, rather than left as outlined in the tutorial.

With no success with DARV I gave Synscan PA error another bash too, selecting stars first from the East then to the SE/S. Still getting wildly incorrect PA error.

With no other options avilable its clear I can only buy my way out of the PA alignment hole 🤷‍♂️. I'm hoping that using drift alignment via PHD2 and a guide scope (eyeing this) will resolve this one and for all!

[kens]

Whenever you slew you should finish up by slewing a small distance north and west at the slowest slewing speed to remove backlash. Most goto mounts do this automatically on a goto  but not for a straight slew.

RA will be up/down if your camera is mounted in landscape orientation in the home position: (pointing at the pole, counterweights down). If you mount it in portrait orientation then RA will be left/right. Not that it matters for DARV nor for imaging.

What you could do is simply take a long exposure, say 5 minutes. Then measure how far the stars trail by counting pixels. If you expose for 10 minutes you will also see your periodic error. in RA. You can then work out how long you can expose unguided.

[SStanford]

8 hours ago, kens said:

Whenever you slew you should finish up by slewing a small distance north and west at the slowest slewing speed to remove backlash. Most goto mounts do this automatically on a goto  but not for a straight slew.

RA will be up/down if your camera is mounted in landscape orientation in the home position: (pointing at the pole, counterweights down). If you mount it in portrait orientation then RA will be left/right. Not that it matters for DARV nor for imaging.

What you could do is simply take a long exposure, say 5 minutes. Then measure how far the stars trail by counting pixels. If you expose for 10 minutes you will also see your periodic error. in RA. You can then work out how long you can expose unguided.

Thanks for this.

Could I then take a long exosure without slewing the mount and adjust the alt/az until the trailling star is a single point?

The problem I seem to be having is the mount isnt even resonding to insructions to slew on the lowest slew rate, which I'm hoing is a backlash issue (I've created a seerate topic on this).

With FLO in stock of the guide scope and cam bundle (here) I ordered this last night so I'm hoping using PHD2 drift alignment function this will put my PA problems to bed! Of course now I'm concerned that my backlash problem will affect guiding and go-to in general 😬.

[kens]

I doubt you'll ever get to a single point . At some point you will reduce the PA error enough that ther will belittle drift in Dec but then you will get RA trailing from periodic error. Guiding is the way to correct for that.

If the mount isn't responding my first guess would be backlash. Any time you reverse direction you will experience it. With guiding there are ways to mitigate the worst effects. The simplest it to make dec guiding corrections in one direction only - opposite to the drift caused by PA error.

[SStanford]

Thanks again @kens,

Hopefully then I'll be able to reduce the error using PHD2 and the guide cam moving only in one direction from parked? 

If I can find a way to reduce the backlash too then all the better. Completely new to the issue so hoping I can find a decent guide somewhere for backlash reduction.