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19546[William Optics] Re: Focuser Sag

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  • Greg
    Mar 15, 2009
    • 0 Attachment
      Thanks for everyone's comments and sorry for taking so long to do so.

      I am positive the R200SS is rock solid. Using a Barlowed laser collimator I can go on months of outings without needing to tweak collimation. That would not be possible if there was any shift of anything in the image train. The guide scope mounting should be solid as I am using Losmandy dovetail plate and guide ring tightened snugly.

      I did take apart my Mergrez 90 focuser again and found something interesting. There was a 0.017" shim glued to the arched piece that holds the roller bearings for the draw tube. This was on the outside of the arch but did not reach all the way to the edge of arch by the rollers. This left a gap that I believe allowed the arch to flex under load from the rollers.

      I built up some shims using heavy duty aluminium foil to 0.018' and put them under the unsupported ends. It was immediatley clear that there was less flexure as it was much harder to reassemble the draw tube into the focuser assembly than it has been previously. Now tightening the focuser tension set screw has a much better defined effect. Trying to wiggle the draw tube with it nearly fully extended seems to show much les flex. Even the nylon tipped focuser lock screw seems to work a little bit better.

      I will now have to get out under the stars and see if ther is any actuall improvement in differential flexure.

      Greg

      --- In William-Optics@yahoogroups.com, Rod Mckay <rodnscope@...> wrote:
      >
      >
      > Thinking about this one further, have you ruled out movement within the Vixen scope optic train?
      >
      > Mirror movement is common with SCTs, so could it be that the primary or secondary mirrors in the Newt is moving slightly during the imaging session?
      >
      > My reasoning (as pointed out by others) is that you'd be pretty lucky to have 2 WO scopes that have matching sag, and the common element in your sag problem is the Vixen Newt => I'd suggest it's a possible cause.
      >
      > Cheers
      > Rod
      >
      > --- On Tue, 3/3/09, Rod Mckay <rodnscope@...> wrote:
      >
      > > From: Rod Mckay <rodnscope@...>
      > > Subject: Re: [William Optics] Re: Focuser Sag
      > > To: William-Optics@yahoogroups.com
      > > Received: Tuesday, 3 March, 2009, 1:34 PM
      > >
      > >
      >
      > > It's definitely worth looking at how the scopes are
      > > mounted in relation to each other, as that may be a factor.
      > > I would expect a piggy-back arrangement to result in less
      > > differential flexure than a side-by-side arrangement.
      > >
      > >
      > >
      > > That said, I've got an FLT110 and piggy-backed ZS66SD,
      > > both of which exhibit flexure in the focus tube if you pull
      > > down on them. On the ZS66SD it was particularly bad until
      > > the supporting bearings and focuser roller were tightened
      > > quite aggressively, but it's all but gone now.
      > >
      > >
      > >
      > > On the FLT110 it was a similar story, but I still have a
      > > little flex left - you can see it via the graduations on the
      > > focus tube when you pull down on the end of the tube.
      > > It's only about 1/2 the width of a stripe now, but it
      > > used to be a lot worse.
      > >
      > >
      > >
      > > I'm guessing that it's the FLT that causing a 1-2
      > > pixel drift between 10 minute exposures, due to the weight
      > > of the camera hanging off it, and the change in sag as the
      > > mount slowly rotates. I can almost live with this, but it
      > > would be nice to get rid of it if I can. It was 4+ pixels
      > > per 10 min exposure, and that was definitely a limiting
      > > factor.
      > >
      > >
      > >
      > > The other potential source of sag is the focus tube to
      > > camera tube compression ring. Even though there is a ring
      > > it still operates as an almost point source of pressure. It
      > > would be better if there were 2 or more tightening screws
      > > around the connection point, as I can still (just) move the
      > > camera extension tube when I've done the single
      > > tightening screw up with a pair of pliers!
      > >
      > >
      > >
      > > If anyone has found a simple solution to tube sag or
      > > compression ring failure please don't hesitate to let us
      > > all know!
      > >
      > >
      > >
      > > Cheers,
      > >
      > > Rod
      > >
      > > NZ
      > >
      > >
      > >
      > > --- On Tue, 3/3/09, Timm Bottoni <t.bottoni@comcast.
      > > net> wrote:
      > >
      > >
      > >
      > > > From: Timm Bottoni <t.bottoni@comcast.
      > > net>
      > >
      > > > Subject: [William Optics] Re: Focuser Sag
      > >
      > > > To: William-Optics@
      > > yahoogroups. com
      > >
      > > > Received: Tuesday, 3 March, 2009, 11:43 AM
      > >
      > > >
      > >
      > > >
      > >
      > > >
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      > > >
      > >
      > > > Hi Greg,
      > >
      > > >
      > >
      > > >
      > >
      > > >
      > >
      > > > Is there a way to eliminate a variable and actually
      > > measure
      > >
      > > > what you
      > >
      > > >
      > >
      > > > think is focuser sag? It may be that it is related to
      > > the
      > >
      > > > Vixen
      > >
      > > >
      > >
      > > > product. Perhaps it is really an alignment issue that
      > > is
      > >
      > > > not visible
      > >
      > > >
      > >
      > > > but this is just a guess.
      > >
      > > >
      > >
      > > >
      > >
      > > >
      > >
      > > > It seems more likely to me that this might be the
      > > direction
      > >
      > > > to look
      > >
      > > >
      > >
      > > > since, the 66 and 90 WO scopes have completely
      > > different
      > >
      > > > designs,
      > >
      > > >
      > >
      > > > focal lengths, focuser designs, etc. If the two
      > > products
      > >
      > > > act the
      > >
      > > >
      > >
      > > > same, and the are so different, it would lead me think
      > > that
      > >
      > > > the
      > >
      > > >
      > >
      > > > problem lies somewhere else, since what is the
      > > probability
      > >
      > > > that those
      > >
      > > >
      > >
      > > > two completely different products would exhibit the
      > > exact
      > >
      > > > same problem
      > >
      > > >
      > >
      > > > with the Vixen, but not with each other. Maybe
      > > through
      > >
      > > > process of
      > >
      > > >
      > >
      > > > elimination and careful measuring you can figure out
      > > the
      > >
      > > > root cause.
      > >
      > > >
      > >
      > > >
      > >
      > > >
      > >
      > > > Hope it helps somehow. Give us more feedback when
      > > you can
      > >
      > > > and we
      > >
      > > >
      > >
      > > > will try to help while it's still cold out and we
      > > are
      > >
      > > > too chicken to
      > >
      > > >
      > >
      > > > go outside and freeze our butts off.
      > >
      > > >
      > >
      > > >
      > >
      > > >
      > >
      > > > Timm
      > >
      > > >
      > >
      > > >
      > >
      > > >
      > >
      > > > --- In William-Optics@
      > >
      > > > yahoogroups. com, "Greg" <gbenecke1@
      > >
      > > > ..> wrote:
      > >
      > > >
      > >
      > > > >
      > >
      > > >
      > >
      > > > > I have both a Mergrez 90 and Zenithstar 66 SD
      > > that I
      > >
      > > > use both as
      > >
      > > >
      > >
      > > > imaging scopes and guide
      > >
      > > >
      > >
      > > > > scopes with each other and with a Vixen R200SS
      > > Newt.
      > >
      > > > I notice that
      > >
      > > >
      > >
      > > > when mated with the
      > >
      > > >
      > >
      > > > > R200SS as guide scopes there is an appreciable
      > > drift
      > >
      > > > in the guiding
      > >
      > > >
      > >
      > > > that I attribute to the WO
      > >
      > > >
      > >
      > > > > focusers. The R200SS has a Moonlite focuser on
      > > it
      > >
      > > > that is rock
      > >
      > > >
      > >
      > > > solid, so any drift I think is
      > >
      > > >
      > >
      > > > > from flexure in the focusers of the WO scopes.
      > >
      > > > Interestingly when
      > >
      > > >
      > >
      > > > the WO scopes are used
      > >
      > > >
      > >
      > > > > with each other the drift is almost nil. I think
      > > this
      > >
      > > > is because
      > >
      > > >
      > >
      > > > they sag in a similar fashion.
      > >
      > > >
      > >
      > > > >
      > >
      > > >
      > >
      > > > > Does anyone have any thoughts on how to fight
      > > the
      > >
      > > > battle of the sag?
      > >
      > > >
      > >
      > > > I have taken apart and
      > >
      > > >
      > >
      > > > > tightened and adjusted everything in the focusers
      > > and
      > >
      > > > made some
      > >
      > > >
      > >
      > > > improvement, but some
      > >
      > > >
      > >
      > > > > flex still remains. I have looked at replacing
      > > the
      > >
      > > > focusers with
      > >
      > > >
      > >
      > > > something like the Moonlite,
      > >
      > > >
      > >
      > > > > but one copy would cost as much or more than I
      > > paid
      > >
      > > > for the 66.
      > >
      > > >
      > >
      > > > >
      > >
      > > >
      > >
      > > > > I have been entertaining drilling and tapping
      > > holes
      > >
      > > > for set screws
      > >
      > > >
      > >
      > > > to lock down the tubes but
      > >
      > > >
      > >
      > > > > there goes the resale value if I ever want to
      > > sell
      > >
      > > > them. Any
      > >
      > > >
      > >
      > > > thoughts are appreciated.
      > >
      > > >
      > >
      > > > >
      > >
      > > >
      > >
      > > > > Greg
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