Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

CPWI alignment final slew commands

  • Please log in to reply
3 replies to this topic

#1 sudeepban

sudeepban

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 01 Sep 2019

Posted 07 October 2019 - 09:14 PM

When aligning my 8SE, I've always had very good results following the advice of using the up and right buttons as the final slew commands when aligning stars with the hand controller. 

 

With my recent foray into EAA, I decided to try using CPWI for alignment, and I noticed the manual at https://celestron-si...anual_ENG_F.pdf has the same advice I've always heard:

 

Anytime you center an object for alignment, make sure you use the Up and Right buttons for the final slew commands when approaching the center of the field of view. This is to minimize gear backlash in the mount to achieve a more accurate alignment on objects.

However, I also noticed that in CPWI we don't seem to have the same inversion of up / down buttons at slew rates 6 and lower like on the hand controller, where pressing up makes the scope point down and vice versa.  So then is that advice actually correct when aligning using CPWI??  My understanding was that the whole point of the "final slew commands" advice was to match what the scope does when slewing to new targets.  If that's the case, it would seem like one should be using the down and right buttons as the final slew commands for alignment in CPWI.

 

Is the manual wrong?  Or am I missing something?


  • lkannard likes this

#2 lkannard

lkannard

    Explorer 1

  • -----
  • Posts: 58
  • Joined: 12 Jul 2016

Posted 08 October 2019 - 09:53 PM

I’m interested in reading any responses. I just started using CPWI, indoors, just to get familiar with the software and alignment steps. I haven’t used it outside yet, but I did see in the manual the same thing.

#3 sudeepban

sudeepban

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 01 Sep 2019

Posted 09 October 2019 - 06:29 AM

I contacted Celestron technical support with the question and got this response:

 

Thank you for contacting Celestron technical support. The manual is correct. Up and right are still the desired final button press directions to remove backlash even without the inversion on speed 5 and lower.

 

With EAA (Electronically-Assisted Astronomy - short exposure stacking with a camera to see more detail than you can by eyepiece), you always do the final alignment with the camera rather than the eyepiece, so what direction Up and Right move the stars depends on the camera rotation anyway.

 

Not sure this really makes sense to me, as I was under the impression that the whole reason for the specific final movements was to match the direction the front of the scope moves when doing alignment and when slewing to a new target, which is independent of the direction the stars move or camera rotation.


Edited by sudeepban, 09 October 2019 - 06:30 AM.


#4 Michael_Swanson

Michael_Swanson

    Vanguard

  • *****
  • Posts: 2331
  • Joined: 23 Apr 2003

Posted 09 October 2019 - 07:31 PM

Hi Sudeep,

 

I haven't used CPWI so I don't know for certain how it behaves at lower slew rates.  I also do not know the directions of a GoTo operation.  When centering the alignment stars, it is important that the front of the scope move in the same direction as it does during a GoTo operation in order to remove the gear backlash from the encoder readings used in the alignment calculations.  That part is certain.

 

The resulting difference isn't really noticeable on mounts with low backlash (all of the German EQ mounts, the CPC and the Evolution) but it is noticeable on mounts like the SE and SLT with their much larger backlash.  Most of the users of CPWI are owners of the GEM/CPC/Evolution so this probably hasn't surfaced until now and Celestron Tech Support may have some key detail wrong.

 

Careful observation of the GoTo behavior and physical motion when centering the alignment stars should allow you to clear this up.  

 

Best regards,
Mike Swanson
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
https://www.nexstarsite.com


  • sudeepban likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics