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PHD2 Drift alignment not stabile

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#1 Bill G.

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Posted 28 September 2020 - 08:15 PM

I've always polar aligned using the scope's routine. I have my scope legs set in divots on the driveway and alignment is very close. The problem is I want to do some longer focal length imaging and decided to go from close to "as good as I can". This time of year my view of the sky is very limited so an accurate alignment is difficult.....at least until the leaves are down. I've never used PHD for PA but watched the process a few times on Youtube and thought...ok I've got it, let's give it a shot.

 

The process seemed to start ok but after making an adjustment I just let it drift for awhile. The trend line slowly swings back and forth + and - a considerable amount (+/- 45degrees). Waited a few minutes but it never really seems to stabilize. Tried a few adjustments and kept going back and forth but never got a good feel for where I should be. I thought maybe the scope backlash was off and played with that a bit but just seemed to get worse, so set to back to my original settings. At one point I went to do some planetary imaging and though I was not guiding, the image was almost completely stationary so I know PA is close.

 

BTW... Using a CGX. Drift align using a 900mm guidescope and DSI I.

Seeing conditions were "OK". Not great.

 

The questions....

I have not done PEC training..... Is that needed to do this? It would make sense that the periodic error could cause this...but I can't imagine it being enough to cause the trend line to swing so much....when I couldn't see any movement unguided during imaging (done with a C8 (2000mm)) and a flea3 camera with a very small sensor.

 

Could it be seeing? I know most people do their guiding on much shorter focal length guide scopes but this is what I have and have used....for the last 15yrs.

 

Any suggestions appreciated. 

Thanks,

Bill G.

+

 



#2 Kevin Thurman

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Posted 28 September 2020 - 08:39 PM

I tried PHD drift align and although on paper it ought to be the most accurate, it was an incredibly slow process for me. I had the same issue as you and it would often take an hour to get within 2 or 3'. The problem with PHD's routine is that it takes a few minutes to get an accurate reading for each iteration, and then moving the scope from the horizon to the equator seems to shift things just enough to throw it off. I bought sharpcap and now I can have myself under 1' in about 10 minutes, which is essential for my lack of dec tracking. I can't leave my scope in place day to day so I don't want to spend an hour of precious darkness not collecting photons every single time I want to image.


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#3 Midnight Dan

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Posted 28 September 2020 - 08:41 PM

Sounds like periodic error to me.

 

-Dan



#4 Bill G.

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Posted 28 September 2020 - 09:22 PM

I tried PHD drift align and although on paper it ought to be the most accurate, it was an incredibly slow process for me. I had the same issue as you and it would often take an hour to get within 2 or 3'. The problem with PHD's routine is that it takes a few minutes to get an accurate reading for each iteration, and then moving the scope from the horizon to the equator seems to shift things just enough to throw it off. I bought sharpcap and now I can have myself under 1' in about 10 minutes, which is essential for my lack of dec tracking. I can't leave my scope in place day to day so I don't want to spend an hour of precious darkness not collecting photons every single time I want to image.

I do have Sharpcap Pro. But, guess I failed to mention, I can't see Polaris (or anything near it) this time of year to use the Sharpcap alignment. Even when I can see it there are branches in the way but at least well enough to get close. Once the leaves are down I can see enough stars east and west to get a good PA. It's now with limited sky and no north view that I'm looking for another method. In fact I was only trying to align the one direction because of this....pretty sure altitude is very close.

Just out of curiosity, what is the FL of your guide scope?

 

Sounds like periodic error to me.

 

-Dan

I didn't think about this the other night when trying to do the drift align, only while writing the message earlier. I guess the next thing is to do the PEC training and see if that helps. I'm wondering if the 900mm guide scope may be aggravating the calibration by making the PEC more noticeable. I do have it entered in PHD2.

Thanks for the responses,

Bill G.


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#5 Kevin Thurman

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Posted 28 September 2020 - 09:32 PM

I do have Sharpcap Pro. But, guess I failed to mention, I can't see Polaris (or anything near it) this time of year to use the Sharpcap alignment. Even when I can see it there are branches in the way but at least well enough to get close. Once the leaves are down I can see enough stars east and west to get a good PA. It's now with limited sky and no north view that I'm looking for another method. In fact I was only trying to align the one direction because of this....pretty sure altitude is very close.

Just out of curiosity, what is the FL of your guide scope?

 

I didn't think about this the other night when trying to do the drift align, only while writing the message earlier. I guess the next thing is to do the PEC training and see if that helps. I'm wondering if the 900mm guide scope may be aggravating the calibration by making the PEC more noticeable. I do have it entered in PHD2.

Thanks for the responses,

Bill G.

When I was using the PHD drift align I was usoing the ZWO mini guide kit basically FL of 120mm only. Results might be better with a bigger guide scope but I'm not sure.


Edited by Kevin Thurman, 28 September 2020 - 09:33 PM.


#6 klaussius

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Posted 28 September 2020 - 09:48 PM

I tried PHD drift align and although on paper it ought to be the most accurate, it was an incredibly slow process for me. I had the same issue as you and it would often take an hour to get within 2 or 3'. The problem with PHD's routine is that it takes a few minutes to get an accurate reading for each iteration, and then moving the scope from the horizon to the equator seems to shift things just enough to throw it off. I bought sharpcap and now I can have myself under 1' in about 10 minutes, which is essential for my lack of dec tracking. I can't leave my scope in place day to day so I don't want to spend an hour of precious darkness not collecting photons every single time I want to image.


That sounds like your mount isn't perfectly level. That happens when you're not level since in that case adjustments in the axes aren't fully independent (adjusting alt will mess with az and viceversa), so you have to go back and forth a few times.
 
You can speed things up considerably by making sure your mount is perfectly level.
 

The process seemed to start ok but after making an adjustment I just let it drift for awhile. The trend line slowly swings back and forth + and - a considerable amount (+/- 45degrees). Waited a few minutes but it never really seems to stabilize. Tried a few adjustments and kept going back and forth but never got a good feel for where I should be. I thought maybe the scope backlash was off and played with that a bit but just seemed to get worse, so set to back to my original settings. At one point I went to do some planetary imaging and though I was not guiding, the image was almost completely stationary so I know PA is close.


It could be a mechanical issue with your mount. I'm trying to solve similar issues with mine. It didn't use to happen, it started happening recently. I disassembled the worm gear and found the axes were overtightened and were binding. Not completely, just enough to be stiffer at times parts of the period, and smoother in other parts of the period.

I fixed it but I haven't had a chance to put the fixes to the test. But the important thing to note here is that I'm seeing what you're seeing, but it didn't happen before. So your case could also be some kind of misadjustment that can be fixed.



#7 Bill G.

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Posted 28 September 2020 - 09:50 PM

When I was using the PHD drift align I was usoing the ZWO mini guide kit basically FL of 120mm only. Results might be better with a bigger guide scope but I'm not sure.

Kevin,

   Thanks. From what I understand, 120mm is fine for guiding and a bigger scope is not necessarily better. That just happens to be what I have from years back when the Orion guidescopes were longer FL in general as I recall.....I'm just wondering if the longer FL may be making the drift alignment more sensitive to.....well, just about everything....

 

Bill G.



#8 Kevin Thurman

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Posted 28 September 2020 - 09:51 PM

That sounds like your mount isn't perfectly level. That happens when you're not level since in that case adjustments in the axes aren't fully independent (adjusting alt will mess with az and viceversa), so you have to go back and forth a few times.
 
You can speed things up considerably by making sure your mount is perfectly level.

I do level the tripod. I can only be so accurate as a human. 



#9 Bill G.

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Posted 28 September 2020 - 10:07 PM

I fixed it but I haven't had a chance to put the fixes to the test. But the important thing to note here is that I'm seeing what you're seeing, but it didn't happen before. So your case could also be some kind of misadjustment that can be fixed.

I've only had the mount for about 1 yr. First thing I did after unboxing was take it apart and check the gear mesh and check for proper adjustment. All was good and smooth ....not to tight, not too loose... when put back together. 

 

The more I think about this along with adding in the comments, I'm more and more thinking this may be a combination of the PEC training not being done in combination with a long focal length making it all that much more visible.

 

I'm gonna try a few things next time out (hopefully Thurs.). First, I will do the PEC training. If still an issue, I may put the guide camera on the finderscope  just to see if the oscillation is still visible.

.....if still a problem.....will go from there.

Thanks,

Bill G.



#10 Michael Covington

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Posted 28 September 2020 - 10:11 PM

I do level the tripod. I can only be so accurate as a human. 

And it would take several degrees of tilt to cause substantial difficulty with that.  So no worries there.

Responding to your original question, I don't think it's periodic error.  Periodic error causes east-west drift (only).  Drift alignment looks for north-south drift (only).

 

How much alignment error do you have?  It's common for the reported polar alignment error to be about 1 arc-minute and to fluctuate or swing back and forth because of atmospheric effects -- unsteady air between you and the star you're tracking.

 

Apart from that, look for mechanical looseness somewhere or maybe a cable pulling on something.


Edited by Michael Covington, 28 September 2020 - 10:12 PM.

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#11 KenS

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Posted 28 September 2020 - 10:30 PM

With such a long FL guide scope I'd also be looking at the possibility of flexure. For instance, are you using those accursed adjustable guide scope rings?

Maybe a pic of your setup would be useful.

Also, can you attach the guide log where you were doing the drift alignment? The angle of the graph means nothing. e.g. If it was just driting 1" up and down that would be nothing to worry about


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#12 Alex McConahay

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Posted 28 September 2020 - 11:15 PM

I can't say from here what is going on with what I have been told.  But I do have some questions and observations. 

 

>>>>>>swings back and forth + and - a considerable amount (+/- 45degrees)

Yup, at first it does.....but then it settles. It can continue to settle for five minutes or more. However, usually within two to three minutes per trial it is close enough to figure how much you need to adjust, or at least which direction to adjust. 

 

>>>>>>it never really seems to stabilize.

 

What does this mean? On the same run, does it point above the Y axis for a while, then move below, and then back above, then back below, (or vice versa)? Or does it start out on one side, then drift maybe to the other, but generally, after about two and a half minutes stay on one side, with slight variations up and down but generally in the same place? That second behavior is somewhat normal. 

 

>>>>>>moving the scope from the horizon to the equator seems to shift things just enough to throw it off.

 

No, that is not a "throwing it off." In fact when you get the celestial equator/meridian fix on your azimuth, moving the star to the (20-30 degrees above) horizon and adjusting your altitude actually also means you need to adjust the azimuth again. It is not as if something slipped or something was thrown off. The Azimuth setting was correct only for the original altitude of the mount. Now that you have changed the altitude, you have thrown the azimuth reading off. The altitude change affects the azimuth reading. That is why you have to go back to the equator after the horizon. (And if you are being really picky, back to the equator, and back to the horizon and.......until there is no drift wherever you point.) 

 

>>>>>Seeing conditions were "OK". Not great.

 

Take longer exposures. Four seconds minimum. It should not matter too much in this application, but if your seeing is not good, generally, go longer in exposures. 

 

>>>>>scope backlash was off

If your scope is perfectly balanced and there is a lot of slop in the dec gear train maybe. But just shove the scope to one side or the other of the dec movement (or drive it for a second or two in one dec direction before starting the test run). It should not be moving from there on, and therefore backlash should have nothing to do with it. YOur dec movement is disabled during a drift run, and your RA is pushing constantly one direction, so backlash has no place to play.  (And besides, your RA is not being considered. _)

 

>>>>>I have not done PEC training.

 

As Michael pointed out, that should not matter. Of course it is nice to have a mount that is tracking well. But, PHD2 separates north-south drift (what you care about) from east-west.

 

>>>>>>That sounds like your mount isn't perfectly level.

It is true that mounts do not have to be level to get a good polar alignment. But it is a lot easier to get it done that way, for the reason pointed out (that with a non level mount, an adjustment in Azimuth affects altitude , and vice versa than in a level mount ). 

 

>>>>>it was an incredibly slow process for me.

Yup, it does seem slow. However, although couterintuitive, you actually get quicker with it the more you do it. One would think that since you need a couple of minutes on each trial, there would not be much of a way to speed it up. But, with experience, it gets faster.  At first there is a lot of bumbling around (which way to turn the adjuster, how much, how long to let it run before you can decide it is stabilized, and all that). Once you get over that, it moves from incredibly slow to just slower than some other methods. Still, it is probably the most accurate of methods. 

 

>>>>>>being done in combination with a long focal length making it all that much more visible.

 

Were you doing it visually, I'd grant you that. But PHD2 has sub pixel resolution. Using more focal length will probably not make that much difference. 

 

 

At any rate, take a moony night and just mess around with it at your leisure. A little practice goes a long way. 

 

Alex



#13 Bill G.

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Posted 29 September 2020 - 09:49 PM

Wow,

      Lotta good comments and suggestions, some for me, some for Kevin. Let me try to comment on the ones that applied.

 

Responding to your original question, I don't think it's periodic error.  Periodic error causes east-west drift (only).  Drift alignment looks for north-south drift (only).

 

How much alignment error do you have? 

Michael,

   Good points, really hadn't thought about what the SW is doing in detail. Makes sense. Error was swinging +/- 9 arc minutes.

 

With such a long FL guide scope I'd also be looking at the possibility of flexure. For instance, are you using those accursed adjustable guide scope rings?

Ken,

     Don't think so. Have used this setup for quite awhile with long FL with no problems, generally where I had good sky visibility and could get good PA. I think I'm doing something wrong. As I said, using PHD2 for drift align is new to me (but not PA) so I have to assume it's me or at least something I'm overlooking.

 

Alex,


I can't say from here what is going on with what I have been told.  But I do have some questions and observations. 

 

>>>>>>swings back and forth + and - a considerable amount (+/- 45degrees)

I may not have given it long enough. I don't think I ever went beyond 5 min.

 

>>>>>>it never really seems to stabilize.

 

What does this mean? On the same run, does it point above the Y axis for a while, then move below, and then back above, then back below, (or vice versa)?

YES!

 

 

>>>>>Seeing conditions were "OK". Not great.

 

Was taking 5 sec exposures

 

>>>>>scope backlash was off

Again, a clearer understanding of the SW process helped me realize you are correct ...probably not backlash either.

 

>>>>>I have not done PEC training.

 

As Michael pointed out, that should not matter. Of course it is nice to have a mount that is tracking well. But, PHD2 separates north-south drift (what you care about) from east-west.

Good point, after realizing what the SW is doing from the above comments, I understand that PEC is not likely.

 

>>>>>>That sounds like your mount isn't perfectly level.

Not me. My mount is level.

 

>>>>>it was an incredibly slow process for me.

Not me.

 

>>>>>>being done in combination with a long focal length making it all that much more visible.

 

Were you doing it visually, I'd grant you that. But PHD2 has sub pixel resolution. Using more focal length will probably not make that much difference. 

hhhmmm, gotta think about this.....

 

 

At any rate, take a moony night and just mess around with it at your leisure. A little practice goes a long way. 

That's the plan!!

 

Thanks everybody for your comments. Hopefully I will get this straightened out with all the help.

Bill G.


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#14 Michael Covington

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Posted 30 September 2020 - 08:32 AM

BTW, a non-level mount would not cause swinging back and forth.  It would cause the altitude and azimuth adjustments not to do quite as much good each time, when you perform them.  But it would not cause swinging back and forth of the measurements, because regardless of what position the mount is in, it should be holding still at any given time.

I am wondering if something is settling or flexing or swinging.  Look for mechanical looseness.



#15 Bill G.

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Posted 02 October 2020 - 09:01 PM

Well, got out the other night to keep working with it. Started by rechecking everything. Only thing I found was the alt pivot bolts that hold the head to the mount were not real snug. Snugged them up fairly tight, but not so much that I couldn't make alt adjustments.

I waited longer each time per Alex's recommendation and that did seem to help to some extent.

Started by intentionally moving it pretty well off alignment. Then restarted drift alignment (7sec exposures). It was about -28 arc minutes out. Waited at least 10 minutes and made an adjustment. Each time I made an adjustment and restarted the drift, it would get about 5-6 mins closer and seemed predictable....right up until I got around 3 arc mins. Then it got flaky again and kept drifting back and forth a couple arc minutes.

Just to try something, I made another adjustment, restarted drift, waited at least 10min. Said it was about +10min off. Hit the clear button to restart the trend line but made no adjustment...now after 10min or so, it was about 5 min off. Cleared it and waited another 10mins, and now it was 2min off. 

By this time I'd been playing with it quite awhile and was out of ideas. Just for grins, did some guiding...was terrible. With previous CPWI software PAs when a lot more sky was visible, guiding was very good. Did the CPWI polar alignment the best I could with the stars that were available. Did show it was off....but only altitude...which I assume was from tightening the bolts.. and the fact that I have to reset the mount in the divots each time I take it out. But, I did not retry the guiding...which in hindsight was dumb!!

 

So, at this point not sure what is actually going on. All seems to be tight. No cables being pulled or tangled. Will try again but may just do a couple alignments with CPWI and then see how it guides first. In another month or so, leaves will be down and will be able to see Polaris again.... but, really want to know what is going  on here.

Thanks,

Bill G.



#16 Michael Covington

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Posted 02 October 2020 - 09:52 PM

If you had said it was alternating between +2 and -1 arc minutes, or something like that, I would have said that's the inherent uncertainty because you're imaging the guide star through unsteady air.  But +10 and -10 is way too much.  A polar alignment accurate to 2' is about as good as you'll get.   10' isn't.  

But even with a 10' or 20' polar alignment error, you should be able to autoguide well.



#17 Bill G.

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Posted 02 October 2020 - 10:00 PM

Thanks Michael,

     I'm still thinking it's something I'm doing wrong. I'll go back to my previous methods that worked well.....I was just hoping I could improve on it when my view is limited.

 

"If it ain't broke, don't fix it!!" 

 

Bill G.



#18 Bill G.

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Posted 02 October 2020 - 10:01 PM

Thanks Michael,

     I'm still thinking it's something I'm doing wrong. I'll go back to my previous methods that worked well.....I was just hoping I could improve on it when my view is limited.

"If it ain't broke, don't fix it!!"

Bill G.



#19 KenS

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Posted 02 October 2020 - 11:48 PM

Are you able to post the guide log? That would make it easier to work otu what is going on



#20 michael8554

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Posted 03 October 2020 - 06:52 AM

When you start Drift Align from scratch, you only need to wait long enough, about 30 seconds, for the RA line to get back onto the horiz axis, to see which way Dec is drifting.



#21 Madratter

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Posted 03 October 2020 - 07:34 AM

Your getting help from some published authors:

 

Michael Covington

 

https://www.amazon.c...,aps,140&sr=8-1

 

Alex McConahay

 

https://www.amazon.c...01728419&sr=8-1



#22 Michael Covington

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Posted 03 October 2020 - 09:38 AM

Your getting help from some published authors:   ....

And at least one of them is still quite puzzled!



#23 Madratter

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Posted 03 October 2020 - 10:16 AM

And at least one of them is still quite puzzled!

 

Me too. :)



#24 Alex McConahay

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Posted 03 October 2020 - 11:20 AM

>>>>>>And at least one of them is still quite puzzled!

Both of them.....

 

A couple of comments, though. 

 

A couple of arc minutes is not a whole lot. Depending on your focal length, and target declination, that might take a long exposure to show up in a shot. (As always, check this out by heading over to http://celestialwond...ErrorCalc.html 

 

As Michael has already pointed out, that may be getting down to the limiting factors of seeing, and other weirdnesses. 

 

Have you checked focus of the guide star? If the energy is spread out a bit much (because of poor focus) it may be harder to get the actual centroid. 

 

Alex



#25 rgsalinger

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Posted 03 October 2020 - 11:57 AM

Looks like the Frank Barret has made some improvements on the CW site - here's a replacement URL for the calculator. 

 

http://celestialwond...xErrorCalc.html

 

Rgrds-Ross




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