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CEM70 Zero position question

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#1 mintakaX

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Posted 06 August 2022 - 10:28 AM

I've been getting by for several weeks with my CEM70 mount. I have been through several polar alignments and now have the mount permanently mounted and have a question regarding the zero position. 

 

Say that one has re-set the zero position, done a precise PA and then later releases either (or both) the RA and DEC "clutches" (say for balancing adjustment) and does not re-engage them in the exact same location.  Will the home position change ? If so, this will affect the PA, correct?

 



#2 Notdarkenough

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Posted 06 August 2022 - 10:35 AM

Which CEM70 model are you using? A, G, NUC, EC?


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#3 Boeglewatcher

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Posted 06 August 2022 - 11:22 AM

Hi Dan,

polar align (PA) should not depend on the position of the RA or DEC position of the mount. However, when the mount is not in zero position (counterweight shaft down and scope pointing to north) then adjusting the screws for PA will not move the cross in the same way since the camera is now rotated compared to home position.
IF you see that after a PA the cross is moving when moving RA or DEC, then the camera is not centered well and your PA is a bit off.
then, you should reset the center of the camera using the ipolar software.

Hope that helps,

cs joachim 



#4 Cfreerksen

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Posted 06 August 2022 - 11:25 AM

No, there are not absolute encoders on a CEM70. You will have to re-home it. The PA should not change by what you described.

 

Chris


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#5 mintakaX

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Posted 06 August 2022 - 02:30 PM

Hi Dan,

polar align (PA) should not depend on the position of the RA or DEC position of the mount. However, when the mount is not in zero position (counterweight shaft down and scope pointing to north) then adjusting the screws for PA will not move the cross in the same way since the camera is now rotated compared to home position.
IF you see that after a PA the cross is moving when moving RA or DEC, then the camera is not centered well and your PA is a bit off.
then, you should reset the center of the camera using the ipolar software.

Hope that helps,

cs joachim 

Thank you,I've started the PA with IPolar and then I make a Tpoint model in The SkyX. I have found the pointing with the Tpoint model to be considerable more accurate than the ipolar system. I'm not sure why except that there are many dozens of points plate solved over the whole sky. I know the Tpoint model accounts for more than just PA, but at the end of doing a model, PA adjustments are suggested. I have been doing the suggested adjustments and then redoing the model and each time the suggested PA adjustments have decreased until no more adjustment is necessary. 



#6 mintakaX

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Posted 06 August 2022 - 02:31 PM

Which CEM70 model are you using? A, G, NUC, EC?

Regular plain vanilla CEM70, no EC, no G, no NUC.



#7 mintakaX

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Posted 06 August 2022 - 02:39 PM

No, there are not absolute encoders on a CEM70. You will have to re-home it. The PA should not change by what you described.

 

Chris

Thank you. Just to be certain I understand..... I could set the zero point (CW axis vertical, DEC axis at top), disengage the clutches, substantially move the scope, re-engage the clutches at whatever position I wanted and when I told the scope to go to zero point it would arrive at the same position that it was set in? I could have (and should have) experiment with this but I don't want to disturb things now that I have things setup.  When I first obtained the brand new mount, the zero position was tens of degrees off in RA, so I've been a bit wary of exactly how it is registered.



#8 Cfreerksen

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Posted 06 August 2022 - 03:19 PM

Thank you. Just to be certain I understand..... I could set the zero point (CW axis vertical, DEC axis at top), disengage the clutches, substantially move the scope, re-engage the clutches at whatever position I wanted and when I told the scope to go to zero point it would arrive at the same position that it was set in? I could have (and should have) experiment with this but I don't want to disturb things now that I have things setup.  When I first obtained the brand new mount, the zero position was tens of degrees off in RA, so I've been a bit wary of exactly how it is registered.

No, You would have to enter "find zero position". Goto Zero assumes that find zero position has been performed and the scope clutches have not been disengaged. You can execute the goto zero any time after the find command and the scope clutches have NOT been disengaged. Even after a power off. This is how I use mine over and over, over multiple nights of imaging. Even after parking and powering down. (disclaimer, My CEM60, The CEM70 I have had for 10 months has just got back for the 2nd time from Ioptron. Haven't had a single night of successful imaging. Guiding is junk on one side of meridian) They operate essentially the same.

 

The find zero position uses index sensors and are not perfect. Mine are are less than a degree off. You may have to do a 1 star or more alignment to get good pointing. Or just platesolve.

 

Chris


Edited by Cfreerksen, 06 August 2022 - 03:45 PM.

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#9 mintakaX

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Posted 06 August 2022 - 06:00 PM

No, You would have to enter "find zero position". Goto Zero assumes that find zero position has been performed and the scope clutches have not been disengaged. You can execute the goto zero any time after the find command and the scope clutches have NOT been disengaged. Even after a power off. This is how I use mine over and over, over multiple nights of imaging. Even after parking and powering down. (disclaimer, My CEM60, The CEM70 I have had for 10 months has just got back for the 2nd time from Ioptron. Haven't had a single night of successful imaging. Guiding is junk on one side of meridian) They operate essentially the same.

 

The find zero position uses index sensors and are not perfect. Mine are are less than a degree off. You may have to do a 1 star or more alignment to get good pointing. Or just platesolve.

 

Chris

Thanks Chris-- That all makes sense now. I'm sorry that your CEM60 is misbehaving so badly. My last mount was a Paramount ME and I miss it very much. 

Something associated with my CEM70 has caused the pointing to deteriorate. After a hundred star Tpoint run a couple of weeks ago, I enjoyed a couple of nights of very accurate pointing, then all of a sudden the pointing is crap, close to 0.5 degrees off. The mount is on a very sturdy Paramount ME permanent pier bolted to well seasoned concrete. The Alt and AZ adjustment knobs are not loose, the scope has not been shifted, the scope doesn't have a mirror that moves, no CEM70 alignment points have been used. It appears that nothing physical has changed with the mount and I just can't explain what has changed. 

I'm going to do another Tpoint run and see what terms in the model change.



#10 Der_Pit

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Posted 07 August 2022 - 06:32 AM

The find zero position uses index sensors and are not perfect. Mine are are less than a degree off. You may have to do a 1 star or more alignment to get good pointing. Or just platesolve.

Yes, those are HW sensors on the main gear.  So what you should do is:

  1. Do a 'Find Home'
  2. If the telescope isn't pointing properly at the pole, open clutches and adjust.
  3. Do a star alignment.  Either via HC, or via plate solve.
  4. Do a 'Goto Home'. You should notice that it is now properly pointing at the pole.
  5. Do a 'Store current position as home'.

The mount stores the home position, also over power cycles.  So if you open the clutches and are not sure, repeat the above.  Likely you can stop after 2, as the difference between two gear meshes is large enough to spot it as error.

 

Edit: At least that is how it works on the CEM60.  I'd assume it to be the same for the 70...


Edited by Der_Pit, 07 August 2022 - 06:33 AM.

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