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PHD2: How many degrees is "rotate more than a few degrees"?

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#1 dx_ron

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Posted 03 December 2021 - 06:51 PM

I know it only takes a few minutes to recalibrate, but I just did and then noticed that I need to rotate the camera 3° to line up with my previous captures. Being lazy, I'm not going to recalibrate for just that, and I guess 3° is probably OK anyway (unless guiding sucks, in which case I will try recalibrating to see if that improves it).

 

But where roughly is the line at which you really should recalibrate?



#2 ChrisWhite

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Posted 03 December 2021 - 08:17 PM

I cant answer your question... but ponder this:  We go through so very much effort to get everything working well, gear, software, etc... and then we put so much effort into processing that data... why not take advantage of a "free" 5 minute action to ensure you get better data?


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#3 dx_ron

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Posted 03 December 2021 - 08:23 PM

That is indeed a good point.

 

Also ponder this: How many of the tweaks we spend time and effort on make zero discernible difference to the final image?

 

Anyway, an hour into the session the answer appears to be that 3° rotation has no discernible effect on guiding accuracy compared to what I am accustomed to with this mount.



#4 TelescopeGreg

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Posted 03 December 2021 - 08:53 PM

Um, wait.  The guide camera has no influence on the orientation of your imaging camera.  It can be in any orientation, as long as it's generally pointed in the same direction as the imager.  PHD2's calibration is only dependent on the orientation of the guide scope and camera.  It only needs to be re-done if the guider is moved, and there's no reason to move it unless you're taking stuff apart.  Mine hasn't moved in several years.

 

Similarly, the orientation of the imaging camera has no influence on the guider's operation.  Aim and rotate it wherever your target dictates.  No need to recalibrate if you do so.

 

What are you trying to align?


Edited by TelescopeGreg, 03 December 2021 - 08:55 PM.

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#5 Mike in Rancho

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Posted 03 December 2021 - 09:05 PM

Yep.  A week or so ago I paused my calibrated PHD2, moved to a new target, rotated the imaging camera a good 45 degrees, and started up again.  All good!

 

The first image was more of a tester, and I knew I would have to reset things for taking flats for target 1, so both camera orientations were marked.

 

Now, maybe if OAG it makes a difference, depending on exactly what gets rotated and if the sticking-out-thing must also be moved so as to not end up in the image rectangle?  But as you can tell I know little about OAG. lol.gif



#6 ChrisWhite

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Posted 03 December 2021 - 09:07 PM

Um, wait.  The guide camera has no influence on the orientation of your imaging camera.  It can be in any orientation, as long as it's generally pointed in the same direction as the imager.  PHD2's calibration is only dependent on the orientation of the guide scope and camera.  It only needs to be re-done if the guider is moved, and there's no reason to move it unless you're taking stuff apart.  Mine hasn't moved in several years.

 

Similarly, the orientation of the imaging camera has no influence on the guider's operation.  Aim and rotate it wherever your target dictates.  No need to recalibrate if you do so.

 

What are you trying to align?

Guess we need the OP to clarify.  I was answering the question as if the guide camera was being rotated. 


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#7 TelescopeGreg

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Posted 03 December 2021 - 09:13 PM

Guess we need the OP to clarify.  I was answering the question as if the guide camera was being rotated. 

Right.  And as Mike points out, my reference to the independence of the two cameras only works if one isn't using an OAG where the two are mechanically tied together.


Edited by TelescopeGreg, 03 December 2021 - 09:13 PM.

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#8 dx_ron

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Posted 03 December 2021 - 09:59 PM

Ah - sorry for the information gap! I've not yet updated my gear to the OAG I've been using the past month - my bad.

 

So yes, I rotated the camera 3° which also rotated the guiding orientation.

 

I guess this is ultimately a question for the PHD2 developers. I assume (well, hope) that there was some math behind that general statement that you should recalibrate if you rotate the guide camera "more than a few degrees".


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#9 Michael Covington

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Posted 03 December 2021 - 10:20 PM

There is.  If X is the angle through which you rotate the guide camera, basic trigonometry tells you that a guiding correction that would originally have moved Y distance in RA (or dec) now moves Y cos X in RA (resp. dec) and Y sin X in dec (resp. RA).

So if X is 3 degrees, then cos X = 0.998 and sin X = 0.052.  That is probably lost in the noise.  

If X is 10 degrees, cos X is 0.984 and sin x is 0.173.  The second of these in particular is getting big enough to be a problem.

 


Edited by Michael Covington, 03 December 2021 - 10:22 PM.

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#10 Michael Covington

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Posted 04 December 2021 - 12:08 PM

I should say that more clearly...

 

If you have rotated X degrees, then a movement that would originally have been on one axis, is now cos X times that amount on the intended axis, and sin X times that amount in a direction perpendicular to that axis.

As you go up from 0 to small angles, cos X decreases slowly from 1.0, and sin X increases somewhat more rapidly from 0.  So the sine term (the component of motion perpendicular to the intended axis) is usually going to be the trouble.


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#11 dx_ron

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Posted 04 December 2021 - 12:14 PM

Thanks for the very clear explanation!


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#12 Michael Covington

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Posted 04 December 2021 - 12:17 PM

Actually the sine term is *always* going to be the trouble.  If the motion were merely a little smaller than intended (the cosine term), guiding would still work; without even realizing what was going on, you would tweak the aggressiveness, etc., to get it right.  The problem is of course that you are also introducing some motion in an unintended direction.


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