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Is there a way to fix SCT Mirror shift

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#201 PowellAstro

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Posted 27 August 2014 - 12:07 AM

I thought you still had enough threads behind the mirror to leave everything as is and just screw on an additional retainer ring that has the slots machined in it for the bearings. I dont think you need any focus position that requires your mirror to be pulled all the way to the back of the OTA.



#202 Whichwayisnorth

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Posted 21 April 2015 - 07:15 PM

 

Forgive me, maybe I'm being dense, but what does the mount have to do with it? They both point all over the sky.



(snip) I would be interested though to hear how this experiment turns out in 12 months. Good luck.

 

 

Time is almost up!



#203 jhayes_tucson

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Posted 22 April 2015 - 09:43 AM

The good news is that I haven't forgotten about it.  The bad news is that I can't say any more about it.  So, you'll just have to stay tuned.

 

John


Edited by jhayes_tucson, 22 April 2015 - 09:45 AM.

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#204 junomike

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Posted 22 April 2015 - 01:35 PM

The good news is that I haven't forgotten about it.  The bad news is that I can't say any more about it.  So, you'll just have to stay tuned.

 

John

Can you at least enlighten us to how long we'll need to stay tuned?

 

Mike



#205 jhayes_tucson

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Posted 22 April 2015 - 03:01 PM

No, I can't say.  I personally hope that it's not long...

 

John



#206 Kokatha man

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Posted 08 September 2015 - 06:30 AM

...thought I'd drag this thread out of the vault to see what happens..!?! ;)

 

Actually, speaking specifically about C14's atm, - as I commented earlier for folks who don't need to shift the primary mirror often...using longer setscrews to replace the "transport bolts" which thread through the rear 1/4" threaded holes in the back-casting & into similar thread-holes in the 2 unused arms of the "triple-tree" at the back of the primary will lock the mirror rigidly - the 3rd arm of the triple-tree being the focuser actuating arm.

 

Someone commented about their C14 having larger, unthreaded holes in the rear casting of their particular model for the transport bolt arrangements: no matter, just the need for a slightly different arrangement...actually easier & able to accommodate primary mirror adjustments as well! :)

 

As a tester for ZWO's planetary cameras I found myself switching back & forth from a smaller-pixel-sized sensor to a larger-pixelled sensor & then back again within the space of less than 6 months this year with 3 different cameras to evaluate (from the ASI120MM/S to the ASI174MM & then the ASI224MC) so my fixed primary set-up became a p.i.t.a...

 

I've just finished installing a new version utilising M. Jousset's design & although it has been around for quite a long while thought that if it hasn't been linked to before I'd do so for anyone interested...with the simple loosening & tightening of 2 pinch-screws on the rear of the scope the primary can be adjusted at will & I anticipate the mirror will be perfectly stable once locked: https://translate.go...htm&prev=search

 

"Mea culpa" - full disclosure!" :rollingeyes:  :grin:  Not normally one to take the lazy, "backyard butcher" route but tbh I didn't want to take the primary mirror out again to drill out the holes in the rear-casting & re-tap them to 10mm as per the mechanism shown in the link ...I forwent the threaded section on the ends of the aluminium billets that the locking rods slide in & merely JB Weld epoxied these guide billets into position...I can report that I got everything perfectly aligned however with a little care & my ability to create an imitation silk purse from a sow's ear has the scope still looking reasonably schmick - & certainly infinitely better than crushing mirror bushings or inserting screws into them imho..! :lol:


Edited by Kokatha man, 08 September 2015 - 06:31 AM.


#207 jhayes_tucson

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Posted 08 September 2015 - 06:04 PM

The good news is that I haven't forgotten about it.  The bad news is that I can't say any more about it.  So, you'll just have to stay tuned.

 

John

 

I'd love to reopen this discussion but unfortunately my earlier comment still applies, so stay tuned.

 

John



#208 rolo

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Posted 09 September 2015 - 09:52 PM

I can't see why the engineers at Celestron can't resolve this problem that's been plaguing them from the get go. My Intes Micro 10" Maksutov had a simple helical type threaded  system that worked great. I don't recall any image shift. See this link and scroll down.

 

 

 

http://www.photoinfo...ter-M603-03.htm



#209 Kokatha man

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Posted 09 September 2015 - 11:30 PM

...leaving aside the more abstruse possibilities & addressing the thread's basic question "Is there a way to fix SCT Mirror shift" I am almost ready to test the modifications I have put in place...just washed the primary in situ about half an hour ago using my large sheet of mylar/acetate & only need to wash the corrector after some "scope drying time" & re-install...PLUS wait for some clear skies & reasonable seeing! :lol:

 

I say "test" but I already know that any mirror shift will be minimal - this is a design I took from M. Jousset but is in effect merely a much more sophisticated version of the setscrews/bolts I employed previously but with the ability to now adjust the primary mirror's position without the major faffle those long bolts necessitated!

 

Those bolts had to be removed with the scope sitting face-down on the ground for thread-alignment purposes...& as they had to go through 2 sets of threaded holes (the ones on the scope's rear casting & those on each of the 2 unused arms of the mirror's "triple-tree" element) I was always a tad concerned about slight mirror mis-alignment.

 

This can of course be corrected out for nearly all intents & purposes during collimation but I did wish for an arrangement where whatever axial concentricity Celestron had applied was most faithfully adhered to - down the track when I work out a suitable set-up & necessary gear I will try & get the whole system (OTA, primary, secondary & corrector) into the optimal possibility but for the moment I believe that this arrangement will work at least as well as my prior system with much greater flexibility...

 

For the record again we (my partner & I) are high-resolution planetary imagers where mirror stability is integral to optimal collimation...which along with thermal equilibrium, razor-sharp focus & of course good seeing offers the highest resolution outcomes! (see our website in the link below for examples of our work)

 

So, here's a specific, "nuts & bolts" & "right now"  example of: "There is a way to fix SCT Mirror shift"  :lol:

 

C14MirrorLocks.jpg


Edited by Kokatha man, 09 September 2015 - 11:34 PM.


#210 Alph

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Posted 10 September 2015 - 01:09 AM

So, here's a specific, "nuts & bolts" & "right now"  example of: "There is a way to fix SCT Mirror shift"  :lol:

I am not sure what to think about it. Have you seen this http://www.optcorp.c...p-stoppers.html

#211 Kokatha man

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Posted 10 September 2015 - 04:44 AM

There ya go!!!

Looks like Ken has taken the basic principle & turned it into a commercial, easy to assemble product...

As I've said I took the design off of M.Jousset's website mod but I wouldn't want to infer that his was the original idea, the design is really just an innovation from the old transporter bolt situation which I used formally...

The only thoughts I have since you posted this link to Ken's product - apart from "there's nothing new under the Sun" ;) - is (a) my version took me 3 days (part-time) to construct & assemble & cost me around $US30 & (b) I made the 316 stainless guide rods 5mm diameter: Ken's must be considerably thinner to accommodate sliding inside the threaded sleeves he has which have the 1/4" O.D. threads for the rear-casting entrances...see my "Mea culpa" comment! ;)

Jousset's method was to re-tap the rear-casting to 10mm meaning thicker rods again...but I'm confident my 5mm rods won't flex over the (roughly) 15mm-20mm* distance they are un-supported...as is Ken obviously with his thinner rods - although his instructions do warn about bending his rods! :)

In my humble opinion (& with some personal humour) much of this thread seems to be predicated upon either re-inventing the wheel or somewhat abstruse ideas...with a sprinkling of "watch this space!" thrown in for good measure :) but as I said (& you've quoted) I firmly believe this is a practical & effective means of addressing the question posited in the thread title... ;)

* Just for the record & those interested in such things, my C14, Chinese, 2 rear vents the same as the Edge models & about 4 (or is that 5?) years old has a total primary mirror travel of 28mm...

EDIT: thinking a bit more & looking at Ken's assembly instructions & piks the s.s. rods his product uses must be very thin indeed from my appraisal...but I imagine it would still be sufficient...

Edited by Kokatha man, 10 September 2015 - 05:20 AM.


#212 Kokatha man

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Posted 10 September 2015 - 07:27 PM

...apologies - I think I was having an "oldies' moment" with that "15mm-20mm" un-supported rod distance in the post above: it's more like 35mm-60mm depending upon the position of the primary...Ken's rods aren't much thicker than toothpicks, but being a hard stainless I suppose they'd still provide sufficient rigidity...but I'm much happier with my 5mm rods! ;)

#213 drollere

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Posted 11 September 2015 - 09:23 AM

external focuser + mirror lock.

 

choose the right suite of eyepieces and you'll never need to adjust the primary/secondary focal distance.



#214 jhayes_tucson

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Posted 12 September 2015 - 12:42 AM

I've used Ken's Flop Stoppers and they work as well as the Celestron mirror locks.  I'd call it "pretty good."   The last time I talked with him, he wasn't selling them anymore; though, you might find some still in stock at a supplier like OPT.

 

John



#215 Kokatha man

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Posted 12 September 2015 - 05:53 AM

I've used Ken's Flop Stoppers and they work as well as the Celestron mirror locks.  I'd call it "pretty good."   The last time I talked with him, he wasn't selling them anymore; though, you might find some still in stock at a supplier like OPT. 
John


...yeah, but I think mine will perform much better! ;)

As I said above, regardless of what sort of material you employ those are tooth-pick thin & would inevitably have some flexure, even over small distances - & I would not want to employ my habit of a final anti-clockwise (pushing mirror forwards) habit with such slender support: this tactic is a "must-do" if only to remove the slack in the focuser thread mechanism!

Re your <"I've used Ken's Flop Stoppers and they work as well as the Celestron mirror locks"> I'm a bit puzzled - no mirror locks that work only on the focuser screw could ever be as stable/supportive as a 3-point system...it's simple mechanics - perhaps (if you are correct) it only reinforces my concerns about trying to use such slender rods..? ;)

EDIT: John, it occurs to me that you might be referring to the later (post my 2012?) model C14's which do have mirror locks...another version of the mechanism I've engineered using knob-tightening to clamp the rods in a locked position. (from a cursory look at a schematic here on CN)

I made certain that the rods on my device were extremely fine tolerances within the guide blocks which required the care when setting them up I referred to in my original post here...some folks also report mirror movement with the Celestron mechanisms via focus shift...I have nfi why this should be but we image at 8000mm-10,000mm & simply would not dream of focusing & thinking this will not shift over an imaging session...every 6 minutes is the maximum time before re-checking focus - but DSO aficionados might have somewhat different expectations with their much, much shorter f/l's... ;)

Edited by Kokatha man, 12 September 2015 - 08:34 AM.


#216 jhayes_tucson

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Posted 12 September 2015 - 08:45 AM

The Celestron system uses two posts that lock plus the focus screw to constrain the mirror position.  The rods in both the Celestron and Flop-Stopper system are thin so that they don't bind the system laterally (since the system is over constrained and since it relies on tight tolerances.)  They are very stiff along the axis of the telescope and serve as mirror locks only--they were never intended to fix the "focus shift" issue.  

 

I will be interested to hear how well your system works but unless I am misunderstanding something, it looks like it will only help to reduce mirror flop.  I hope that it works but it looks to me like your approach just a beefier version of the Celestron system, so it is also mechanically over-constrained.  I doubt that it will do anything to address the underlying cause of focus shift, which is due to tilt in the primary mirror.  Because of the optical magnification, the requirement to completely reduce focus shift below a noticeable value is very tight--particularly on a C14.  I certainly wish you well, but I will be surprised if it does much (if anything) to reduce focus shift.  One way to document the performance is to make a movie demonstrating the before and after performance at high magnification.

 

I've done enough work on this problem to know that it is not easy to fix--particularly on the C14.

 

John



#217 RandyC

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Posted 12 September 2015 - 11:37 AM

Hi, I recently got a C14 XLT and the collimation appears good based on the defocused star test. When I use the Celestron collimation eyepiece it also looks good. My dealer told me not to use the mirror lock knobs because they are only for shipping. The primary on this one looks pretty well collimated. I have a perfectly parallel T-mount between the camera and scope which threads into the visual back. Based on this image (upper left to lower right) from a full frame Sony A7s, is this mirror shift, collimation or something else?

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#218 PowellAstro

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Posted 12 September 2015 - 12:02 PM

Looks like some coma and field curvature.

#219 jhayes_tucson

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Posted 12 September 2015 - 02:05 PM

Hi, I recently got a C14 XLT and the collimation appears good based on the defocused star test. When I use the Celestron collimation eyepiece it also looks good. My dealer told me not to use the mirror lock knobs because they are only for shipping. The primary on this one looks pretty well collimated. I have a perfectly parallel T-mount between the camera and scope which threads into the visual back. Based on this image (upper left to lower right) from a full frame Sony A7s, is this mirror shift, collimation or something else?

 

Without knowing more about how you guided, it's a little hard to give you an answer with a high level of certainty.  You definitely have some coma which is a normal field aberration in the XLT but it can be amplified by secondary misalignment.  You also have a bit of field curvature, which is normal in the XLT.  There also appears to be the possibility of a bit of image motion.  That can come from a lot of things, including "mirror flop."  Mirror flop is what happens when the primary mirror in a Cassegrain type system (which includes SCTs) physically tilts relative to the optical axis by a small amount.  That can cause big problems during a long exposure.  It is most commonly solved by guiding through the telescope using an OAG or ONAG optical system to guide.  If the amount of "flop" is small and smooth (as it usually is,) the guiding system can compensate for it.  Mirror flop in the Celestron systems is caused by the way the primary mirror is mounted.  There is a small bit of mechanical clearance in the focusing mechanism that leave a bit of room for the mirror to tilt.  You can evaluate your system by simply looking at how much focus shift you have.  Point your scope at a star, use a moderately high magnification eyepiece, and see how much image motion you have when you change direction of the focus knob as you focus.  That is what I refer to as "focus shift."  The source of focus shift is very closely related to mirror flop (though there are some issues in the design that amplify the problem a bit.)    In general, the amount of focus shift that you see in the eyepiece is probably going to be around 1.5x - 2.5x the amount that the image will shift due to the mirror changing position during guiding due to pure "mirror flop" but the magnitude depends a bit on the scope and the circumstances.

 

John



#220 RandyC

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Posted 12 September 2015 - 02:32 PM

Hi John and PowellAstro, thanks for your help. I am going to try getting slightly longer bolts for the mirror locks so I can get to the second set of threads. I guided using the Celestron 80mm guidescope @600mm focal length and QHY5L-II mono camera. No focal reduction. Do you think one of the higher end collimators is worth it. http://www.ebay.com/...=item20dff8dca6



#221 PowellAstro

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Posted 12 September 2015 - 03:23 PM

I don't see a collimation issue just some coma and field curvature. These are normal and not bad at all for a full frame sensor. You may want to try a field flattener/coma corrector next.

#222 Kokatha man

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Posted 12 September 2015 - 07:23 PM

The Celestron system uses two posts that lock plus the focus screw to constrain the mirror position.  The rods in both the Celestron and Flop-Stopper system are thin so that they don't bind the system laterally (since the system is over constrained and since it relies on tight tolerances.)  They are very stiff along the axis of the telescope and serve as mirror locks only--they were never intended to fix the "focus shift" issue.  
 
I will be interested to hear how well your system works but unless I am misunderstanding something, it looks like it will only help to reduce mirror flop.  I hope that it works but it looks to me like your approach just a beefier version of the Celestron system, so it is also mechanically over-constrained.  I doubt that it will do anything to address the underlying cause of focus shift, which is due to tilt in the primary mirror.  Because of the optical magnification, the requirement to completely reduce focus shift below a noticeable value is very tight--particularly on a C14.  I certainly wish you well, but I will be surprised if it does much (if anything) to reduce focus shift.  One way to document the performance is to make a movie demonstrating the before and after performance at high magnification.
 
I've done enough work on this problem to know that it is not easy to fix--particularly on the C14.
 
John

 
 

Hi, I recently got a C14 XLT and the collimation appears good based on the defocused star test. When I use the Celestron collimation eyepiece it also looks good. My dealer told me not to use the mirror lock knobs because they are only for shipping. The primary on this one looks pretty well collimated. I have a perfectly parallel T-mount between the camera and scope which threads into the visual back. Based on this image (upper left to lower right) from a full frame Sony A7s, is this mirror shift, collimation or something else?

 
Without knowing more about how you guided, it's a little hard to give you an answer with a high level of certainty.  You definitely have some coma which is a normal field aberration in the XLT but it can be amplified by secondary misalignment.  You also have a bit of field curvature, which is normal in the XLT.  There also appears to be the possibility of a bit of image motion.  That can come from a lot of things, including "mirror flop."  Mirror flop is what happens when the primary mirror in a Cassegrain type system (which includes SCTs) physically tilts relative to the optical axis by a small amount.  That can cause big problems during a long exposure.  It is most commonly solved by guiding through the telescope using an OAG or ONAG optical system to guide.  If the amount of "flop" is small and smooth (as it usually is,) the guiding system can compensate for it.  Mirror flop in the Celestron systems is caused by the way the primary mirror is mounted.  There is a small bit of mechanical clearance in the focusing mechanism that leave a bit of room for the mirror to tilt.  You can evaluate your system by simply looking at how much focus shift you have.  Point your scope at a star, use a moderately high magnification eyepiece, and see how much image motion you have when you change direction of the focus knob as you focus.  That is what I refer to as "focus shift."  The source of focus shift is very closely related to mirror flop (though there are some issues in the design that amplify the problem a bit.)    In general, the amount of focus shift that you see in the eyepiece is probably going to be around 1.5x - 2.5x the amount that the image will shift due to the mirror changing position during guiding due to pure "mirror flop" but the magnitude depends a bit on the scope and the circumstances.
 
John


Highly entertaining John! ;)

#223 jhayes_tucson

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Posted 12 September 2015 - 07:24 PM

Hi John and PowellAstro, thanks for your help. I am going to try getting slightly longer bolts for the mirror locks so I can get to the second set of threads. I guided using the Celestron 80mm guidescope @600mm focal length and QHY5L-II mono camera. No focal reduction. Do you think one of the higher end collimators is worth it. http://www.ebay.com/...=item20dff8dca6

 

1)  Uh, it sounds a bit like you are tying to thread a screw into the mirror shipping locks for something other than shipping.  I strongly recommend against doing this.  First, it is possible to send a screw right though the back of the support plate into the glass--and that would be bad.  Second, if you simply try to tighten the screws, you will lose focus and you'll put a lot of stress on the baffle tube.  It is really hard to fix anything with those screws.  My advice is not to put anything into those holes.

 

2)  The Celestron 80 mm guidescope has a lot of mechanical stability problems all on it's own--particularly at the focuser end.  It should work fine but only if you really stabilize the focuser.  That's too complicated for me to describe here and now.

 

3)  I personally don't think that you need any type of alignment tool other than a bright star to get the system well aligned.  Others may feel differently, but I suggest researching how to align your secondary just using a high powered eyepiece.  It's relatively quick and easy once you figure it out.

 

John



#224 Kokatha man

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Posted 12 September 2015 - 08:24 PM

Hi John and PowellAstro, thanks for your help. I am going to try getting slightly longer bolts for the mirror locks so I can get to the second set of threads. I guided using the Celestron 80mm guidescope @600mm focal length and QHY5L-II mono camera. No focal reduction. Do you think one of the higher end collimators is worth it. http://www.ebay.com/...=item20dff8dca6


Hi Randy - you can use longer bolts as you suggest, this is a technique I used on my C14 previously to ensure a stable primary mirror which basically addresses any movement issues in this optical element...but of course once you do this you will not be able to shift the primary mirror!!! :(

For this reason I don't recommend using this method - why I've made the modifications I just have above - but it will certainly work well: most of the planetary imaging on my site was done with the additional 2 bolts in situ on my C14 - the only caveats I'd place is (1) the aforementioned (2) hi-resolution planetary imaging is done with constant focus-checking & employs only a small region at the centre of the FOV...but it will hold both collimation & focus very well once set, it's just that when you image at around 10 metres "focus" is a completely different kettle of fish to DSO imaging!

It is quite difficult to insert these additional bolts through the rear-casting threads PLUS the threads in the 2 unused arms of the triple-tree. (the 3-armed unit behind the primary which has the threads you refer to)

This is because (as anyone with any engineering knowledge should appreciate) you need very accurate alignment between the 2 sets of threads to do so...but don't worry about screwing them right through & hitting the primary, that likelihood is pretty far-fetched!

You will need to place the scope face-down on a clean floor, this lets gravity "settle" the primary into the best alignment ie, for those 2 sets of thread-holes.

Screw both bolts into the rear-casting first & continue screwing until they touch those 2nd set of thread-holes...it helps if you have a scope which has the vents on the rear-casting because removing these covers allows you to see into these areas inside the scope...I'd also recommend having the primary as far back (clockwise on the knob) as possible when doing this - it makes it difficult to see these part through the vent-holes but it does shorten the distance between the sets of threads which makes things easier from an alignment perspective...

You then carefully (making sure the bolt is properly engaging the thread) screw one bolt in no more than a turn & a bit & then do like wise with the other bolt - doing this ensure you are not pulling these elements out of alignment marginally, which would make the operation difficult & risk cross-threading.

Repeat this, alternating from one bolt to the other until they are fully-engaged - if you can't look inside because your model doesn't have the vents to remove just mark the bolt thread with a marker pen when you reach the 2nd set of threads (this is easily felt) & work on another 5mm of engagement.

That's it! :)

There are collimators specifically made for SCT's such as those using projection screens but you can't use any generic types: I always recommend people use the tried & true star-test at very high magnifications...as planetary imagers we have the advantage of being able to do this onscreen & thus take into account the entire imaging train when we do so, as well as look at the screen while we adjust...but I still think you're better off with an ep rather than any proprietary SCT collimator...but then again as far as I'm concerned collimation is something you do for each & every imaging session! ;)

#225 RandyC

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Posted 12 September 2015 - 09:13 PM

I agree, probably a little better secondary collimation and the rest is field curvature. The greatest aberration seems to be in upper left. The lower right is almost fine. Perhaps the focus knob is in lower right and the loose lock screws are on left. My C11 (from before the time of mirror locks..) with the exact same A7s setup had almost no aberration. I guess bigger mirrors come with bigger problems.  C11:

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Edited by RandyC, 12 September 2015 - 09:13 PM.



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