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Subject: Re: Image Shift Revisited
From: h h funston
Reply To: mapug@shore.net
Date: Sat Nov 25 04:26:44 1995

> When I got my 10" LX-200 f/10 used some months ago, one of the first
>things
>that I found objectionable about it was the amount of image shift that I
>encountered when focusing. After delving through months of Mapug archives
>and talking to other users, I knew that mine was definitely excessive. Using
>a 21mm
>objective I would get anywhere from 1/3 to 1/2 fov shift. I have been
>working on this problem and now have next to NO image shift at all I'm happy
>to say. I now can contribute what I think can be some worthwhile information
>to the group that some may find helpful.
> If you encounter what you suspect is excessive image shift try this test.
>Set up your scope and elevate the end with the correcter up 30 degrees. Back
>the focus out fully CCW then undo the three screws holding the focuser assy
>in place. Now, while holding the focuser knob in your hand, allow the weight
>of the primary mirror to slide down the optical tube and time how long it
>takes to reach the back of the scope. Obviously, you dont want to offer
>resistance to its
>downward slide, except to keep it in control and guide it. Try timing it 2
>or 3 times. It should slide effortlessly down the tube to the back of the
>scope in app 7 secs or less. My experimentation has been that if it takes
>longer or has to be physically pulled backward then its much to stiff. The
>result is that if the mirrors sliding action of the baffle against the optic
>tube is too great, when you attempt focus you will induce lateral motion
>into the image. It can be seen by putting a piece of masking tape across the
>front of the scope(not on the corrector plate obviously) and while looking
>down the end at the primary mirror, run the focus back and forth. You can
>see the mirror stress sideways before movement occurs. The possibility of
>running the focuser rod back and forth to re-distribute the grease could
>help, but if the grease has aged and gotten stiff, disassembly and
>replacement of the grease is probably in order. After having gone through
>this now a number of times. The lesson is this, use a lightweight,dark
>grease that wont liquify or stiffen in temperature extremes. A little grease
>goes a long way. I found that merely applying a thin coat and then wiping
>most of that away with my hand was quite sufficient. I suggest that if
>re-lubricating is done that you perform the slide test above again. The
>primary mirror baffle should slide easily down the tube and should not
>require assistance at a 30 degree incline. Nor should it obviously go
>crashing down. It occurred to me that if there were too much slop in this
>arrangement that image shift could occur here also. I suspect that too much
>slop could be removed by the application of a thicker lubricant.
> As to lubricants, the tolerances of my scope dictated a very thin
>grease and I settled on a dark Molytone grease. This grease is available in
>many places, particularly where VCR repair would be performed or electronic
>stores. I suspect that if lubrication is required that there may be some
>playing by ear required, per the tolerance of your particular scope.
> I hope this has been helpful and am willing to feedback to anyone
>requiring further information. Naturally, I came to my observations thru the
>help of previous posts to this group. I cant begin to tell you how helpful
>this group is to me. Thanks to all.
> Sam Lattuca
>
>
>lattuca@midwest.net

Sam
This is a great technical article you have provided. It indicates good systemic
diagnosis, good scientific procedure and a very workable solution to a problem
that can confront any of us LX200 owners. Thank you for your efforts and for
sharing with us the details of your work. This is an excellent example of how
a user's group should be utilized. Never mind whether it is of interest to
everyone on the address list. Never mind whether it is lengthy or short. All of
these criticisms are nothing more than attempts of a few subscribers to control
the rest of us. Many have unsubscribed because a few have attempted to
monopolize the system, often times with nothing more than their petty
rhetoric. The excellent hands-on documentation of our experience is worth
its weight in gold, especially when shared. Thanks again.
Herb Funston
MT Funston Astronomical Observatory
Hereford AZ