PHD2 Drift Alignment / Synscan to the rescue

[Startrek]

Being retired with a bunch of grandkids, my wife and I are on grandparent duty 2 days week. One of my 2 year old grand daughters accidentally tripped over the tripod leg of my EQ6-R mount earlier in the week , so a polar alignment check was required.
Used the PHD2 Drift align tool last night before the big ol moon got too high , but trend lines particularly in Azimuth were fluctuating so much I couldn’t get a handle on the direction of Azimuth adjustments. It would come down to a few arc minutes then back up to 25 arc mins or so. Maybe seeing was the culprit ? and yes I was at Dec =0 for Azimuth and in the East near the c equator for Altitude with a good saved calibration.
Altitude was similar but not as bad as Azimuth. Stuffed around for 40 minutes or so and decided stuff this , I’ll use to good ol Synscan Polar Alignment routine. Swapped cameras and used my Canon 600D on BYEOS live view with alignment screen crosshair
Slewed up to Acrux and Hadar as my 2 star alignment stars ( Acrux being my polar alignment Star )
After only 2 iterations got my error down to 1.24 arc min , good enough
Plonked my 2600MC back in slewed to Hadar , pushed a 75 sec unguided exposure and perfect Star shape in the zoom box of APT
Good ol clunky, dated , hand paddle , Synscan polar alignment routine came to the rescue and got me up and running in less than 20 mins
I’m not going to discount the PH2 Drift method as a backup as it has worked ok in the past for me , but it certainly didn’t work last night
For a lot of folk who own Skywatcher EQ mounts with Synscan , the Synscan polar alignment routine in the handcontroller is either unknown to them or their preference is Sharpcap , NINA of Polemaster etc… which is great
It can certainly get you out of trouble with polar alignment and what’s even better you don’t need a view of the SCP , just 2 bright stars

Cheers
Martin

[AdamJL]

Good stuff, Martin. Sounds like a good backup if everything else fails.

Near 20mins is a fairly long time, though, no? Polemaster will do it in about 4 mins (but like you alluded to, requires the SCP).

NINA and ASI Air will do it in about the same time, maybe a minute longer. Neither need a view of the SCP either. Can't say how long Sharpcap takes as I've never used it, but circa 5 mins is about as much effort as is needed in polar alignment with all the new equipment/options about. If I can't align with NINA in 5 mins, I'll switch to the PM. All up 10mins for two options.

[Startrek]

Hi Adam,
Thanks for replying
20 mins included swapping cameras , re cooling the 2600MC and taking a test image of a Star
The 2 iterations with Synscan took about 6 to 8 mins as my 2 alignment stars were fairly close.
I assume the NINA 3 star alignment requires slewing around between all three star plus centering etc , that would take more than 5 minutes, no ??

Also folk who achieve these incredible polar alignment figures of 0.20 to 0.50 arc sec on the “lower to mid cost” mounts ( like my EQ6-R pro) and scopes with mid length focal lengths of around 1000mm seems to good to be true. The AZ and Alt bolts on these EQ6-R mounts are quite robust with heavy gauged wide pitch threads , not exactly Swiss watch precision. I get my alignment down around an arc minute and even with just slight grip pressure on the adjusting bolts black knob can’t get much lower in error. ( I lubricate all my bolts )
Maybe my resultant PA error is in fact lower say around 0.50 to 1.00 as the Synscan doesn’t have the most sophisticated algorithms plus the optics using a DSLR vs Dedicated Astrocam would be play a major part too.
The proof in the pudding is taking a 60 to 80 sec unguided exposure on a test Star with a scopes focal length of a 1000mm to 1280mm which is what I image at. At around 1 arc min error using Synscan I can usually take an unguided 75 sec test shot on a Star ( viewed in APT using zoom box ) and Star is still round with diffraction spikes still sharp. The Star compared to a guided 75sec is just slightly more bloated.
Obviously with every PA procedure, resultant error is totally in the hands of atmospheric conditions at the time.

Your thoughts

Cheers
Martin

[AdamJL]

Hi Martin

6-8minutes definitely is a lot better and makes it a viable option (in my opinion).
NINA's slews are in RA only and they take (all up) about 30 seconds at most. Actually, maybe the process isn't clear - with NINA and the ASI Air, there's no alignment done on any star. It's a slew in RA only to three separate points, and those three points are plate-solved. The plate-solve images are then used to determine polar alignment.

I can't say I'm sure whether those sub arc second alignments are true or false. To be honest, I always intentionally make sure I'm not perfectly polar aligned. I can't even remember why but I recall reading something about it on CN that perfect polar alignment can cause some guiding issues later. Maybe that's a misunderstanding but it's something I've stuck to, and since I can easily do >10min subs, I'm guessing there might be something in it :lol:

That said... I still dream of a 10micron mount that allows one to build a sky model instead! Hope those lotto numbers come in one day.

[oska]

I don't see the difficulty. The mechanism for PA on the EQ6R is exactly the same as machinists/engineers use to adjust precision machines, since forever. If you work with principles of the mechanism and mitigate the limitations of manufacture ie: shim the AZ rotation boss with shirt box lid type plastic to remove slop, it's remarkably precise and easy to use (with experience of course). For ALT I 3d printed a large knob to replace the t-handle for extra gronk and finesse, all the fine adjustment is done with the larger knob.

I routinely get way under 1 arc min PA on the EQ6R at least according to NINA. I guess NINA could be lying to me?

I always run the routine several times as a measurement is not accurate if it's not repeatable. I have confirmed PA with PHD2 several times and get <10 secs agreement.

Now whether one needs this level of accuracy for PA is another matter :rolleyes:

[Startrek]

Unless your imaging unguided , then 1 to 3 arc minutes error is fine for long exposure subs using PHD2 with focal lengths up to 1280mm which is what I use
If you chat to Bruce Waddington ( support developer) from PHD2 , sub arc minute polar alignments are of no additional benefit or value to your guiding.If fact PHD2 advises you can successfully guide up to 10 arc minutes of error but I probably wouldn’t endorse that. I’m sure field rotation and Dec drift would affect your Star field , Star shape quality at 10 arc min or slightly higher

Cheers
Martin

[oska]

No doubt Bruce's correct as to what's needed. I am seriously intrigued how one can even measure 0.20 arcsec PA error let alone hold it.