Jump to content


Photo

Image Shift

  • Please log in to reply
16 replies to this topic

#1 kljw

kljw

    Lift Off

  • -----
  • Posts: 23
  • Joined: 24 Jan 2013
  • Loc: Scotland

Posted 03 April 2013 - 09:32 AM

Hi
i have recently (Feb 2013) took delivery of a LX200 12". last night was my first time out, due the need to get a buggy made to move the scope about.
when trying to focus, using the focus knob on the scope, i noticed that the image appeared to move across the eyepeace, so by the time it was in focus i had to centre the object again.
when i fine tuned the focus, with the meade electronic unit, the image stayed central.
can any one offer a reason for this please?

many thanks

#2 Spacetravelerx

Spacetravelerx

    Surveyor 1

  • -----
  • Posts: 1756
  • Joined: 23 Dec 2012
  • Loc: New Mexico

Posted 03 April 2013 - 10:14 AM

Keith,

You are experiencing the very famous "Focusing Shift". This happens when focusing you are mechanically moving the mirror and the mirror tilts very slightly as it is moving up and down the tube - hence the shift in your eyepiece.

The Meade Zero Image Electronic Micro focuser of course does not shift the mirror at all, hence no image shift.

Note, the new LX600 and LX850 have Internal Crayford-Style 7:1 focusers to eliminate focus shifting. The LX850 also includes the motor focus for good measure.

#3 kljw

kljw

    Lift Off

  • -----
  • Posts: 23
  • Joined: 24 Jan 2013
  • Loc: Scotland

Posted 03 April 2013 - 10:32 AM

Andrew
thats fine, as long as i have not broken things, i can live with it.....

many thanks.

Keith

#4 jrcrilly

jrcrilly

    Refractor wienie no more

  • *****
  • Administrators
  • Posts: 33733
  • Joined: 30 Apr 2003
  • Loc: NE Ohio

Posted 03 April 2013 - 10:36 AM

as long as i have not broken things, i can live with it.....


No, nothing is broken - and you can easily minimize the effect by always finishing your focusing in the direction that is lifting the mirror. I think I remember that on that model it is counterclockwise. That way the mirror will always return to very nearly the same position and thus so will the object. No need to recenter.

#5 kljw

kljw

    Lift Off

  • -----
  • Posts: 23
  • Joined: 24 Jan 2013
  • Loc: Scotland

Posted 04 April 2013 - 01:08 AM

John C
many thanks for the info, i will give that a try.

#6 thesubwaypusher

thesubwaypusher

    Viking 1

  • *****
  • Posts: 909
  • Joined: 08 Jul 2004
  • Loc: New York City

Posted 21 April 2013 - 08:46 PM

[/quote]
counterclockwise.[/quote]

Yes.

OP, welcome to the world of large mirrors! :jump:

#7 Qwickdraw

Qwickdraw

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 03 Mar 2012
  • Loc: Ann Arbor, MI

Posted 05 August 2013 - 05:27 AM

I just bought a used 12" LX200 and it appears to have very bad focus shift. Even in a low power EP the image travels most of the way across the FOV. I have to wonder if this also affects culmination?

#8 Kraus

Kraus

    Apollo

  • -----
  • Posts: 1145
  • Joined: 10 Mar 2012
  • Loc: Georgia.

Posted 05 August 2013 - 08:23 PM


This may be why my declination circle has its error when the tube is tilted well north or south. My mirror might be rocking. Oh oh. I best go back to using the mirror lock.

#9 Rick Woods

Rick Woods

    Voyager 1

  • *****
  • Posts: 14391
  • Joined: 27 Jan 2005
  • Loc: Inner Solar System

Posted 05 August 2013 - 08:58 PM

Don't try to use the focuser when the mirror lock is engaged.

#10 redlinedb16a

redlinedb16a

    Mariner 2

  • -----
  • Posts: 227
  • Joined: 17 Oct 2010

Posted 08 August 2013 - 11:06 AM

I regressed my mirror slide and it cures the image shift for about a year

But I have no problems tearing my ota appart
Some do

#11 Qwickdraw

Qwickdraw

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 03 Mar 2012
  • Loc: Ann Arbor, MI

Posted 09 August 2013 - 11:32 AM

I regressed my mirror slide and it cures the image shift for about a year

But I have no problems tearing my ota appart
Some do


I need to try this, I just bought this Feathertouch focuser from Starlight hoping this would help. Although I love the focuser it did not help with the severe image shift. Does anybody know if the Peterson EZ Focus kit will also fit the feathertouch unit?

#12 Qwickdraw

Qwickdraw

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 03 Mar 2012
  • Loc: Ann Arbor, MI

Posted 09 August 2013 - 11:33 AM

I regressed my mirror slide and it cures the image shift for about a year

But I have no problems tearing my ota appart
Some do


Cured or lessened?

#13 redlinedb16a

redlinedb16a

    Mariner 2

  • -----
  • Posts: 227
  • Joined: 17 Oct 2010

Posted 10 August 2013 - 07:19 PM

Not cured but thick film coat corrects it for a limited time

After it moves a lot it will need to be re applied

I'm working on a solution
Installing a brass shim strip between the two with grease
The brass will lessen the machined gap
Thus removing image shift

#14 faltered

faltered

    Viking 1

  • -----
  • Posts: 617
  • Joined: 01 Apr 2005

Posted 12 August 2013 - 01:41 PM

get yourself an SCT focuser. Something like this...

http://www.telescope...ctdualspeedf...

Then once you have your telescope focused roughly, you can lock the mirror (then dont use the main focus knob when its locked) and this accessory will allow you to focus without any image shift.

#15 Qwickdraw

Qwickdraw

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 03 Mar 2012
  • Loc: Ann Arbor, MI

Posted 12 August 2013 - 02:43 PM

The only issue with using the focuser is that I have a binoviewer. Binoviewers don't like focusers.

#16 faltered

faltered

    Viking 1

  • -----
  • Posts: 617
  • Joined: 01 Apr 2005

Posted 13 August 2013 - 12:42 PM

ah.. i have not tried binoviewer with a focuser - i assume it will not allow you to reach focus that way?

#17 Qwickdraw

Qwickdraw

    Surveyor 1

  • *****
  • Posts: 1717
  • Joined: 03 Mar 2012
  • Loc: Ann Arbor, MI

Posted 13 August 2013 - 03:42 PM

ah.. i have not tried binoviewer with a focuser - i assume it will not allow you to reach focus that way?


That and the extra weight are a problem.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics