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Another Collimation problem

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#1 AndyRN

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Posted 17 September 2021 - 09:46 AM

I have a year 2000 Nexstar 8 (grey tube) on and HEQ5

 

F6.3 reducer, 105mm back focus to a Canon T3i

 

Collimating with Vega which is near zenith for me.

 

Got this scope 2 months ago.

 

When I initially tried to collimate it, one of the screws was about as tight as it will go.  It just so happens that this screw corresponds to where the dark center ring during collimation is closest to edge

 

I have been trying to get this thing collimated, so last night I tried to reset everything and tightened all the screws (not with brute force just trying to get them all to a standard starting point) and then loosened them all a couple of turns.  The image below was before I did this adjustment.  In the image below it is Oblong.  I was able to get the circle center in the light field but the oblong shape never disappeared.  So then I tried to bring it into focus and as I did the oblong shape seemed to get worse or atleast didn't go away and the dark center circle started to drastically shift to one side of the light field of the star.  Once I get the scope as in focus as I can , the stars look like they do in the second photo.  The stars in the second photo are how they have always looked since I have had this scope.  I would describe it as oblong circle with the brightest point to one edge of the oblong circle.  I would say that the bright point on the oblong stars is where the dark center point during collimation is heading to.

 

The focuser on this is as smooth as butter, if that matters.

 

Any thoughts would be appreciated.  I haven't given up hope that it is just bad collimation but the oblong nature gives me some concern and also concerns me how the center shifts during focus.  Also, during this colimation I had it 3 turns out of focus, don't know if that matters, I've read to just do one turn out of focus and zoom in.

 

Crop of col1.jpg

 

crop.jpg

 

 



#2 ngc7319_20

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Posted 17 September 2021 - 10:14 AM

Take the reducer off there while you are collimating -- it just creates more variables and possible problems.

 

This process "tightened all the screws and then loosened them all a couple of turns" will not do anything to zero or reset the collimation.  I don't think there is any force or spring, etc., which will "null" the collimation when all the screws are loose.  Instead you need to get them all slightly tight, and then figure out which way it needs to go, and loosen the correct one a tiny bit, and tighten the other two equally. 

 

Any chance the secondary mirror assembly at the center of the corrector is loose and wobbling around?  I guess you would have noticed if it was.

 

Sometimes if these get dropped the center baffle tube gets tilted, and then it will be impossible to collimate.  Are there any dents on the back of the scope?



#3 AndyRN

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Posted 17 September 2021 - 10:28 AM

Should have mentioned I just put the reducer on last night for the first time, I've had issues with collimation prior to this, but a good idea to take it off anyway.


If it was dropped, it was not by me...no dents that I can see. Part of my concern, and what you are getting at, I think, is the oblong nature is due to an obstruction to the light path...

As far as tightening the secondary, it is my understanding is if the screws are too tight it can cause the secondary to warp and one of my screws was so tight it could not be tightened anymore to bring it into collimation and the other screws seemed very loose, which necessitated a more drastic approach of trying to get back to some starting point.

Edited by AndyRN, 17 September 2021 - 10:35 AM.


#4 cookjaiii

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Posted 17 September 2021 - 10:57 AM

Take the reducer off there while you are collimating -- it just creates more variables and possible problems.

 

This process "tightened all the screws and then loosened them all a couple of turns" will not do anything to zero or reset the collimation.  I don't think there is any force or spring, etc., which will "null" the collimation when all the screws are loose.  Instead you need to get them all slightly tight, and then figure out which way it needs to go, and loosen the correct one a tiny bit, and tighten the other two equally. 

 

Any chance the secondary mirror assembly at the center of the corrector is loose and wobbling around?  I guess you would have noticed if it was.

 

Sometimes if these get dropped the center baffle tube gets tilted, and then it will be impossible to collimate.  Are there any dents on the back of the scope?

I had this problem with an old 100mm Meade 2045 SCT.  There was no outward evidence of damage, but the primary mirror mount and baffle tube were not straight, i.e., they didn't point to the center of the secondary but off to the side instead. I hope I'm wrong, but I think that may be what's going on here.

 

The misalignment could be minimized by adjusting the secondary tilt, just as you have done, but perfect collimation could not be achieved. 



#5 KerryR

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Posted 17 September 2021 - 10:58 AM

As NGC (edit:and cookjaiii, while I was typing) mentioned above, I'd be suspicious of a shifted focus tube/baffle. Or perhaps a tilted secondary mirror/prism. I'd remove the diagonal and look up the rear of the scope with your eye centered on the visual back, ideally with a peep site-sight to center your eye perfectly. Under these conditions, the secondary and it's baffle should appear centered on the focus tube/baffle. If it's not perfectly centered, then the baffle or rear casting got shifted. Given your image, if this is the culprit, it should be very evident if things are out, I'd think. If everything looks good, repeat the process with the diagonal, just to verify it's not the culprit (which I doubt, but...). Er... the dew shield, if you have one, is on straight... right? (I'm sure it is, but verify anyway.)


Edited by KerryR, 17 September 2021 - 10:59 AM.

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#6 AndyRN

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Posted 17 September 2021 - 11:06 AM

So you guys might be right. When I put the adapter Tee for my canon onto the threaded portion of the center tube...that threaded portion has rotated when I have unscrewed the tee adapter. Could that have caused it to become shifted or maybe it was always loose...is this fixable?

#7 KerryR

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Posted 17 September 2021 - 11:08 AM

If the mirror baffle is tilted, it could be the rear casting, which is a comparatively easy fix because it's just bolted on, and can be loosened, re-seated, and re-tightened. If you're lucky, you won't even have to remove the corrector if the nuts on the inside grip enough to allow loosening and re-tightening. If the baffle got knocked out of square in the casting, that's a different beast, and is going to be much more invasive to fix... the latter can happen if the scope takes a heavy blow with the mirror well forward on the baffle. This is why it's usually a good idea to retract the mirror all the way rearward before packing the scope up.



#8 ngc7319_20

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Posted 17 September 2021 - 11:17 AM

So you guys might be right. When I put the adapter Tee for my canon onto the threaded portion of the center tube...that threaded portion has rotated when I have unscrewed the tee adapter. Could that have caused it to become shifted or maybe it was always loose...is this fixable?

Do you mean this camera adapter tube is loose on the back of the scope?  That is not a serious problem. Just tighten the ring pictured on the left.

 

71K+u9XOWIL._AC_SL1464_ x.jpg

 

This seems like a good video on collimation:

https://www.youtube....h?v=hqRVIDj4aZA

 

It may be easier to do the initial collimation (at least until you get it close to correct) indoors with an artificial star.  You can make an artificial star just by poking a needle size hole in a piece of aluminum foil, and placing it over a bright flashlight.  Put this artificial star 20 or 30 feet from the scope.  At least that way the stars are not moving across the sky as you are working, etc.


Edited by ngc7319_20, 17 September 2021 - 11:27 AM.


#9 AndyRN

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Posted 17 September 2021 - 02:30 PM

This is the part that rotates when I remove the T adapter.  The grey stuff is a small dab of antisieze.  

 

Any good directions on adjusting the mirror baffles or pointers...There are allen type bolts on the side of the rear casing but the back of the rear casting has no bolts except the three for the focuser.

 

 

 

back photo draw.jpg

 



#10 KerryR

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Posted 17 September 2021 - 03:46 PM

I'm thinking the thing you're pointing to is the culprit. On my C6 the baffle extends out the rear casting, and a threaded ring threads on to it, which locks the baffle to the tube. I believe, but don't know for certain, that the baffle threads into the casting, and the ring threads onto that, to lock the threads. The C6, of course, has a smaller baffle, but still has the same size visual back, and so the ring on mine is wide, and recessed into the casting. If the C8 is more or less the same configuration, then it'd need a thinner lock ring to do the same work. Consequently, I'm wondering if the ring you're pointing to could be tightened. This might mean the baffle has to first be spun tighter on the casting, which might involve removing the corrector (mark it for rotation and label/replace exactly any shims) and grabbing and spinning the baffle, and then tightening the ring you point to.

Before you do that, though, I wonder if you can reach into the rear port and deflect the baffle laterally. If it moves at all, you've found the problem... Could also pull the corrector and see if the baffle is loose by grabbing it and wiggling it...


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#11 AndyRN

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Posted 17 September 2021 - 04:38 PM

I'm thinking the thing you're pointing to is the culprit. On my C6 the baffle extends out the rear casting, and a threaded ring threads on to it, which locks the baffle to the tube. I believe, but don't know for certain, that the baffle threads into the casting, and the ring threads onto that, to lock the threads. The C6, of course, has a smaller baffle, but still has the same size visual back, and so the ring on mine is wide, and recessed into the casting. If the C8 is more or less the same configuration, then it'd need a thinner lock ring to do the same work. Consequently, I'm wondering if the ring you're pointing to could be tightened. This might mean the baffle has to first be spun tighter on the casting, which might involve removing the corrector (mark it for rotation and label/replace exactly any shims) and grabbing and spinning the baffle, and then tightening the ring you point to.

Before you do that, though, I wonder if you can reach into the rear port and deflect the baffle laterally. If it moves at all, you've found the problem... Could also pull the corrector and see if the baffle is loose by grabbing it and wiggling it...

 

I looked through the center baffle from the base towards the secondary mirror and I can't see anything out of place.  I tried to move it with my finger and there's nothing moving.

 

 

I think I found the problem, at least part of it.  It looks like the center baffle is off center or the cork has shifted.  Not sure which...assuming the center baffle but I don't really see how that can move from one side to the other because of how solid it is.  So could the cork on the mirror cause the oblong shape????.  

 

 

base photo edit.jpg



#12 Old Speckled Hen

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Posted 17 September 2021 - 06:55 PM

If it is a farster scope you can remove the secondary and examine the tilt of the secondary mirror for gross misalignment before doing anything else more drastic..



#13 ngc7319_20

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Posted 17 September 2021 - 08:12 PM

This is the part that rotates when I remove the T adapter.  The grey stuff is a small dab of antisieze.  

 

Any good directions on adjusting the mirror baffles or pointers...There are allen type bolts on the side of the rear casing but the back of the rear casting has no bolts except the three for the focuser.

 

I don't recall any part like that on the rear cell which would rotate.  Is it some stray adapter?   Can you just unthread and remove it?



#14 ngc7319_20

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Posted 17 September 2021 - 08:17 PM

 

I think I found the problem, at least part of it.  It looks like the center baffle is off center or the cork has shifted.  Not sure which...assuming the center baffle but I don't really see how that can move from one side to the other because of how solid it is.  So could the cork on the mirror cause the oblong shape????.  

 

I agree the cork doesn't look right.  That cork and the spanner nut just in front of it are supposed to hold the primary mirror on the "slider" tube that rides on the center baffle.  It is possible the primary mirror is loose?   Or the primary has been disassembled and reassembled incorrectly (tilted)?



#15 AndyRN

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Posted 17 September 2021 - 08:18 PM

I don't recall any part like that on the rear cell which would rotate.  Is it some stray adapter?   Can you just unthread and remove it?

I guess it's possible that it didn't rotate and it felt like it did...but when I look at that threaded part that sticks out the back...well it's hard to describe but looks as if it could be threaded into the center baffle.  It's definitely not an adapter though.



#16 AndyRN

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Posted 17 September 2021 - 08:28 PM

I agree the cork doesn't look right.  That cork and the spanner nut just in front of it are supposed to hold the primary mirror on the "slider" tube that rides on the center baffle.  It is possible the primary mirror is loose?   Or the primary has been disassembled and reassembled incorrectly (tilted)?

It is possible it has been removed and reassembled.  How tight do you think that spanner nut is?  I've been looking at that nut, sweating,  and imagining my needle nose pliers slipping out of the detents and destroying the mirror..lol.  

 

That cork would not be causing an imaging problems...correct?  It's so close to center, and if I remember back to my Newtonian days the center of the mirror has little effect on the image.

 

It's not a fastsar model.

 

I think I might pull the corrector off and look at secondary...if it looks fine I will probably put it back and maybe give collimation another shot...or go for the gusto and pull the primary out as well...what do you guys/gals think.

 

 

Thanks for all the help!!!!



#17 ngc7319_20

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Posted 17 September 2021 - 09:53 PM

It is possible it has been removed and reassembled.  How tight do you think that spanner nut is?  I've been looking at that nut, sweating,  and imagining my needle nose pliers slipping out of the detents and destroying the mirror..lol.  

 

That cork would not be causing an imaging problems...correct?  It's so close to center, and if I remember back to my Newtonian days the center of the mirror has little effect on the image.

 

I think I might pull the corrector off and look at secondary...if it looks fine I will probably put it back and maybe give collimation another shot...or go for the gusto and pull the primary out as well...what do you guys/gals think.

 

Correct -- the exposed cork itself won't cause any problems.  But it is supposed to be cushioning and clamping the primary mirror... so it raises some questions about why the cork moved sideways and whether the primary might be loose, etc.  Or maybe it got messed with and the primary is tilted now.

 

Yeah, if you pull the corrector, mark the orientation first.  Be careful.   Sometimes there is a sticky paper ring "gluing" the corrector in place -- might need to warm it slightly to loosen the glue.  Some models have grub screws centering the corrector -- check if there are any -- but maybe its just the larger / newer models that have these.  Sometimes there are little bits of cardboard centering the corrector -- keep an eye out for these.

 

Once the corrector is out, maybe put some clean nitrile gloves on and see if the primary feels loose or wobbly around the edges.

 

Ugh... needle nose pliers...  if you need to tighten the primary nut, first make yourself a custom wrench.   Find some PVC plumbing part at the local Home Depot that just fits over the slider tube.   Drill two holes in the edge of the PVC part and add two small nails or pins to engage the two holes on the nut.  That should let you tighten the nut it while minimizing accidents.

 

I don't think the nut needs to be very tight.  Just needs to be snug -- to compress the cork a bit.


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#18 AndyRN

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Posted 18 September 2021 - 05:02 AM

Thanks, I'll report back with my findings.




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