#1  
Old 07-06-2015, 07:47 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
Best Polar Alignment Practice

I did a 250 point T-Point model last night (F3.8 rocks, 3 second exposures at 3x3 binning was taking very little time to do each point).

I had jut finished 2 hours of quite good round stars on the Honders, PMX,Portable pier (very solid), MMOAG, STi. All very tried and proven stuff.

I did the relatively minor mount adjustments the polar alignment report said to do.

Then I started to do some more imaging and got eggy stars - what????!!!

I then switched to Pempro Polar Alignment wizard. It had me adjusting the mount big time - what!!????

I was then getting much better stars but not as good as when I started first thing - great!

So I am now wondering just how good is a T-Point model? I have had good success with it in the past.

I guess I'll do several 35 point models tonight and adjust the mount each time until the report says it good enough and see how that goes.

What do you use?

Greg

Last edited by gregbradley; 07-06-2015 at 08:06 AM.
Reply With Quote
  #2  
Old 07-06-2015, 08:00 AM
g__day's Avatar
g__day (Matthew)
Tech Guru

g__day is offline
 
Join Date: Dec 2005
Location: Sydney
Posts: 2,807
I have just pondered the very same. Once you are down under an arc minute mis-alignment what software do you best trust and are you trying to align on the SCP or the refracted SCP?

I am soon to try and roughly determine the precision of PEMPro (my general favourite) or Tpoint over Maxpoint over PHD (with guiding switched off). Next I will give APT's DARV a shot.

I don't think I have ever seen a definitive review over which method gives optimal results for a permanent observatory once your gear is all tightly coupled and tuned and your polar alignment is already rather close to spot on.

Be very, very interested in others findings and views?
Reply With Quote
  #3  
Old 07-06-2015, 08:15 AM
glend (Glen)
Registered User

glend is offline
 
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,033
This drives me crazy. I have my mount setup on a good pier in my observatory and it is dead set pointed at the SCP, and yet every single time I do an alignment on the NEQ6 using Synscan I get errors, not much (under 1') but I don't like it. I have given up on precision in polar alignment, won't waste my time with drift alignment, and concentrate on guiding now, as the alignment gets me close enough to adust and frame the target object.

If there is anyone running a NEQ6 on a pier, that has precise polar alignment, I'd like to hear how they did (or do) it.
Reply With Quote
  #4  
Old 07-06-2015, 08:46 AM
codemonkey's Avatar
codemonkey (Lee)
Lee "Wormsy" Borsboom

codemonkey is offline
 
Join Date: Jul 2013
Location: Kilcoy, QLD
Posts: 2,058
I know nothing about the PMX, but I believe that enabling the encoders on the EQ8 causes issues with goto accuracy, which may screw with plate solving based PA. Could this be related?

I've got an NEQ6 on a pier footing (1100x1100x600mm of reinforced concrete), but still sitting on the tripod. I've spent ages on PA. Fairly happy with RA, but DEC is still a bit off. I want DEC off a touch so that I can guide in only one direction on DEC, but it's a bit further out than I'd like right now and needs more work.

PHDLab experimental feature reports a DEC drift of 2.17"/min, giving a total of 8.76' out, which is pretty far out. You wouldn't believe it, but that's actually still good enough for my widefield broadband imaging (1.4"/px) and will only give 0.22px of field rotation in a 10min exposure, and I only take exposures in the area of 4mins. Still, I'll be working on that at some point in the near future.

Interesting to note that when I originally aligned this, the DEC graph (PHD2) was dead flat. Having said that, seeing was pretty terrible that night and I wonder if that screwed with the drift trend estimation.

This last time I used Astrotortilla for a rough alignment, then followed up with PHD2 drift. AT did not work well for my EQ8, presumably due to the encoders, but seems to work better with the NEQ6.

Obvious though it may be, if you do drift align, make sure you're pointing at the right place in the sky when you drift for each axis ;-)
Reply With Quote
  #5  
Old 07-06-2015, 08:55 AM
Paul Haese's Avatar
Paul Haese
Registered User

Paul Haese is offline
 
Join Date: Jan 2009
Location: Adelaide
Posts: 9,937
Greg, I now only do half of the adjustment that is called for in Tpoint. This means i effectively sneak up to the refracted pole. Both my mounts report nil adjustment required. Dec guiding is non existent on either mount. The FSQ has 6 second guide exposures with less than 0.1" maximum movement off centre with RA. The 12" requires tighter guiding exposures, but still a maximum movement in good seeing of around 0.3"-0.4" in RA.

So my recommendation is to try what I did. Otherwise I found that you can overshoot the pole.
Reply With Quote
  #6  
Old 07-06-2015, 11:08 AM
LewisM's Avatar
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
Quote:
Originally Posted by glend View Post
This drives me crazy. I have my mount setup on a good pier in my observatory and it is dead set pointed at the SCP, and yet every single time I do an alignment on the NEQ6 using Synscan I get errors, not much (under 1') but I don't like it. I have given up on precision in polar alignment, won't waste my time with drift alignment, and concentrate on guiding now, as the alignment gets me close enough to adust and frame the target object.

If there is anyone running a NEQ6 on a pier, that has precise polar alignment, I'd like to hear how they did (or do) it.
Glen, are you using the Synscan handcontroller and what software version?

It's not the mount, it's the software. It's still buggy.

I have a Synscan GOTO kit on my Vixen GPD2. It's SCP aligned to within 8 seconds. Just for giggles recently, using my SECOND (backup) handset with V3.37 firmware installed (I usually run 3.27B), I decided to do a polar re-alignment. After doing their routine, it reported a 173° error... yep, 173°... I used Alpha Crux as the realignment star by memory... not a star I could be mistaken with. In short, the software for polar realignment is bunk.

I just do a 5-iteration 2 star alignment (home and away), and that gets me down into the LOW seconds in error. That's from starting off with merely a compass and inclinometer.
Reply With Quote
  #7  
Old 07-06-2015, 12:24 PM
glend (Glen)
Registered User

glend is offline
 
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,033
Thanks Lewis, yes I am using the hand controller for alignment then I switch over to Sky Safari Pro via wifi. The controller software is version 3.36 I think. I gave up using the polar alignment correction routine because I know that was buggy, but I thought the normal 2-star alignment was ok.
I am re-orienting the mount on the pier today with my compass and altitude gauge and I will give it another try tonight.
Reply With Quote
  #8  
Old 07-06-2015, 09:52 PM
Joshua Bunn's Avatar
Joshua Bunn (Joshua)
Registered User

Joshua Bunn is offline
 
Join Date: Oct 2011
Location: Albany, Western Australia
Posts: 1,459
Greg, have you tried the accurate polar alignment procedure? You get visual clarification of your mounts movements this way, not relying on the tic marks of the mounts alt/az adjusters. I would try 1 model that has been super modeled, and about 50-60 samples all over the sky. Doing it this way should get you there in 1 go, it's not an iterative procedure. This is what I did some time ago and haven't looked back.

Josh
Reply With Quote
  #9  
Old 08-06-2015, 08:50 AM
Garbz (Chris)
Registered User

Garbz is offline
 
Join Date: May 2012
Location: Brisbane
Posts: 644
Quote:
Originally Posted by glend View Post
won't waste my time with drift alignment
Correct me if I'm wrong but if you're on a pier isn't a decent drift alignment the exact opposite of a waste of time? Spend an entire night getting it perfect with drift and then just enjoy the fruits of your labour for evermore?
Reply With Quote
  #10  
Old 08-06-2015, 09:18 AM
g__day's Avatar
g__day (Matthew)
Tech Guru

g__day is offline
 
Join Date: Dec 2005
Location: Sydney
Posts: 2,807
I am also pondering about tiny mechanical flexures influence on polar alignment.

For instance if I drift align by a method facing West instead of East and get different results I presume that is due to flexure.

Also I run Refractors side by side with an SCT. I generally drift align using software using the SCT - which may have some tiny mirror shift happening that the refractors don't. I ponder would I be better off drift aligning using the refractors?

Other than these considerations I presume once your drift alignment is under an arc minute its refraction that is predominantly causing the errors in your final stages of drift alignment - particularly when it comes to elevation?
Reply With Quote
  #11  
Old 08-06-2015, 11:57 AM
PRejto's Avatar
PRejto (Peter)
Registered User

PRejto is offline
 
Join Date: Jan 2011
Location: Rylstone, NSW, Australia
Posts: 1,397
Greg,

I agree with Josh. The best way now is the new method that uses fiducial stars. It takes all the error out of moving alt and azi. And, if I understand it correctly, unlike the old way, once you move the mount based on those stars you are done as the model updates itself to the new position.

The other night, after you did the big super model I presume you recalibrated back into it after you made the adjustments. If you didn't then the model is just messed up but easily fixed by recalibration. What you didn't say is whether you got eggy stars unguided or guided. If you didn't recalibrate then eggy stars are guaranteed unguided. But, if guiding maybe Protrack was/is fighting against guiding corrections as Protract would be constantly telling the mount to move incorrectly (since the model isn't pointing where it thinks it is). One would think that guiding would win but it would need to work a lot harder.

The arguments between T-point and drift alignment (PEMpro) are endless and I think in the end pointless (ha ha). Once you are close enough by either method a good pointing model + Protrack will give really good results. Even Patrick Wallace has agreed that a drift alignment can be good enough provided there is a good model to back it up. As best I understand drift alignment is just a compromise giving no error for just that region of the sky whereas T-Point tries to give recommendations for most of the sky one would image in + giving different recommendations to minimise field rotation, etc. taking flexure in the imaging system into the equation as well. Since all PA is a matter of compromise being close enough is good enough.

Peter
Reply With Quote
  #12  
Old 08-06-2015, 12:33 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
A question for those who have far more experience than I. I will hopefully be getting my hands on an EQ6 Pro shortly, have an ED100 and will have a ST2000XM arrive in the next few days.

I do not currently have anywhere to put a perm setup, will be imaging from both NE subs of Melb and at my dark site in Heathcote. Right now, I am trying to decide whether I should be buying PEMPro or T-Point.

Considering that it isn't a perm set up, which would be better? Most of the time I am only planning on imaging one or two subjects in a night. Unless I am doing visual in which case, I don't need to be more accurate than my compass and electronic level

Just wondering which is the best way to go at the moment? Drift alignment or all sky model?
Reply With Quote
  #13  
Old 08-06-2015, 12:42 PM
glend (Glen)
Registered User

glend is offline
 
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,033
Quote:
Originally Posted by Garbz View Post
Correct me if I'm wrong but if you're on a pier isn't a decent drift alignment the exact opposite of a waste of time? Spend an entire night getting it perfect with drift and then just enjoy the fruits of your labour for evermore?
Actually no, once a month (around the New Moon) the mount comes off the pier and goes to the dark site and onto the tripod. So I would need to do it once a month, still better than the guys who have to do it everytime they setup a tripod. My view of PA is good enough is enough, as I guide all imaging anyway.
Reply With Quote
  #14  
Old 08-06-2015, 01:56 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,716
Its somewhat reassuring in some ways to hear those with permanently mounted high end gear still have to deal with these issues.

For those of us with cheaper mounts on temp to semi-permanent piers, there is always the solace that a little bit of DEC drift is probably good to avoid backlash. Pole Align Max and drift aligning seemed the most reliable and closest in agreement for me over the years. PHD2 polar alignment module is a great piece of work for those operating outside the SB or Maxim paradigms.


Would be very interested if anyone has references to calculators for drift, and possibly maximum guiding excursion. Presumably if you fed in focal length, FOV, guiding exposure and sub exposure it would be possible to calculate:

1. Upper limit for sub exposure for a given PA error (above which field rotation expected)
2. Upper limit for guiding exposure for maximum expected PEC/guiding excursion (again, likely to only be an issue for cheaper mounts with greater worm/PE concerns).

Last edited by RobF; 08-06-2015 at 03:10 PM.
Reply With Quote
  #15  
Old 08-06-2015, 02:01 PM
Joshua Bunn's Avatar
Joshua Bunn (Joshua)
Registered User

Joshua Bunn is offline
 
Join Date: Oct 2011
Location: Albany, Western Australia
Posts: 1,459
Quote:
Originally Posted by RobF View Post
Would be very interested if anyone has references to calculators for drift, and possibly maximum guiding excursion. Presumably if you fed in focal length, FOV, guiding exposure and sub exposure it would be possible to calculate:

1. Upper limit for sub exposure for a given PA error
2. Upper limit for guiding exposure for maximum expected PEC/guiding excursion (again, likely to only be an issue for cheaper mounts with greater worm/PE concerns).
Rob,
Not exactly what you were asking about , but related. The polar alignment report in the Tpoint add on to TSX has the following recommendations based on your Tpoint model. Mine is attached bellow for my polar alignment model...
Attached Thumbnails
Click for full-size image (Capture.JPG)
65.0 KB51 views
Reply With Quote
  #16  
Old 08-06-2015, 03:14 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,716
Quote:
Originally Posted by Joshua Bunn View Post
Rob,
Not exactly what you were asking about , but related. The polar alignment report in the Tpoint add on to TSX has the following recommendations based on your Tpoint model. Mine is attached bellow for my polar alignment model...
Thanks Joshua. SB bit out of my price range currently, but nice to know what's available if I win the lotto and buy a MEII

Interesting they quote different values for correcting unguided drift, but also "simulate the drift method" - lower values. Something to do with correction for refracted pole?
Presumably if you plate solved to find the pole you would actually be pointing at the refracted pole, when the true pole would be required for best guiding. Drift aligning near the median, surely you'd have little refractive effects?....
Reply With Quote
  #17  
Old 09-06-2015, 07:52 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
Quote:
Originally Posted by Joshua Bunn View Post
Greg, have you tried the accurate polar alignment procedure? You get visual clarification of your mounts movements this way, not relying on the tic marks of the mounts alt/az adjusters. I would try 1 model that has been super modeled, and about 50-60 samples all over the sky. Doing it this way should get you there in 1 go, it's not an iterative procedure. This is what I did some time ago and haven't looked back.

Josh
Quote:
Originally Posted by PRejto View Post
Greg,

I agree with Josh. The best way now is the new method that uses fiducial stars. It takes all the error out of moving alt and azi. And, if I understand it correctly, unlike the old way, once you move the mount based on those stars you are done as the model updates itself to the new position.

The other night, after you did the big super model I presume you recalibrated back into it after you made the adjustments. If you didn't then the model is just messed up but easily fixed by recalibration. What you didn't say is whether you got eggy stars unguided or guided. If you didn't recalibrate then eggy stars are guaranteed unguided. But, if guiding maybe Protrack was/is fighting against guiding corrections as Protract would be constantly telling the mount to move incorrectly (since the model isn't pointing where it thinks it is). One would think that guiding would win but it would need to work a lot harder.

The arguments between T-point and drift alignment (PEMpro) are endless and I think in the end pointless (ha ha). Once you are close enough by either method a good pointing model + Protrack will give really good results. Even Patrick Wallace has agreed that a drift alignment can be good enough provided there is a good model to back it up. As best I understand drift alignment is just a compromise giving no error for just that region of the sky whereas T-Point tries to give recommendations for most of the sky one would image in + giving different recommendations to minimise field rotation, etc. taking flexure in the imaging system into the equation as well. Since all PA is a matter of compromise being close enough is good enough.

Peter
Thanks Peter and Josh.

I did use accurate polar alignment on Friday night and yes it did give me round stars. I was a bit sceptical of it as the PA report showed only minor adjustments and the accurate one had me moving it more. So I was worried I was moving away from the actual point.

I think tonight I'll do a 200 point model and use the accurate polar alignment recommendations. Pempro has something similar where you do a drift alignment and then next step is to click on a star in an image and move the mount so the star is at the end of an arrow. It seems strange to adjust it even more once you stopped drift. I guess its another level of accuracy.

Greg.
Reply With Quote
  #18  
Old 09-06-2015, 07:53 AM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 17,877
Quote:
Originally Posted by Paul Haese View Post
Greg, I now only do half of the adjustment that is called for in Tpoint. This means i effectively sneak up to the refracted pole. Both my mounts report nil adjustment required. Dec guiding is non existent on either mount. The FSQ has 6 second guide exposures with less than 0.1" maximum movement off centre with RA. The 12" requires tighter guiding exposures, but still a maximum movement in good seeing of around 0.3"-0.4" in RA.

So my recommendation is to try what I did. Otherwise I found that you can overshoot the pole.
That's quite a smart approach. I like that. I may do that myself. Those are very low guide errors and also further proof you get superb seeing.
You wouldn't see those numbers in poorer seeing.

Greg.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 08:37 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement