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Cautious Tale for Anyone Respotting a Primary

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#1 FirstSight

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Posted 30 March 2013 - 04:08 PM

Fortunately, no serious disaster befell my primary mirror today when I replaced the center spot. I did inadvertently inflict some seemingly minor scratches in the immediate local area around my original (and replacement) Catseye Triangle centerspots, all of which are safely underneath the shadow of the secondary mirror, and invisible to the view. However, this does NOT mean the scratches are without substantial effect that will likely induce me to send the mirror off for recoating a couple of years before I probably would have otherwise done, and maybe twice as long before noticeably declining performance would have made it seem urgent. WHY? Because they do degrade the clarity of view of the Catseye triangle in both the Catseye cheshire and the autocollimator, not enough to make achieving good collimation impossible, but rather enough to make it needlessly more difficult. That defeats the whole purpose of deciding to replace the original triangle center spot, which was not because it was inaccurately placed, but because I had neglected to perforate its center before affixing it, and I had an extra (perforated) Catseye triangle available to replace it + spotting template. Since I was already going to take my primary out for a routine rinse with distilled water today, now seemed as good a time as any to replace the unperforated trinagle with a perforated one. Seemed like a great idea at the time!

REMOVING THE OLD center triangle started off easy enough, even though it would only peel off in incremental pieces rather than peeling off cleanly in once piece. However, somewhere a bit past the point of no return, the remaining portion of the spot got increasingly stubborn to remove. I wetted the area and gently, but firmly worked at it with a fingernail (and finally removed it entirely with a bit of help from some Acetone), but it took nearly fifteen minutes to work the old spot off the mirror. I at first thought the only very minor-looking scuffs I'd caused would be covered by the new spot, but this didn't turn out to be quite so. The problem is that even though they're confined to the immediate vicinity of the triangle, they degrade the clarity of seeng the points relative to the circumscribing circle in the Catseye cheshire, and the clarity of reflections of P1, P2, and P3 in the autocollimator. Though hardly prohibitive, they are a big annoyance to the Catseye collimation process, even while being a helpful boon to the Glatter collimation process (especially secondary position) switching to the perforated center-spot was intended to provide.

ANYHOW, the moral here is that removing an old center-spot is not as riskless a procedure as some prior threads have made it out to be, particularly with regard to the supposed safety of any nicks to the coating being confined to the secondary shadow. True, but beside the point for how nicks may interact with collimation accuracy. Also, as the photo shows, the nicks (which seem very minor at the time while working off the old center spot) can wind up being more substantially visible under collimation lighting conditions than you may at first assume (at least until you see them in very bright light as needed to take this picture).

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#2 John rombi

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Posted 30 March 2013 - 05:03 PM

Chris,
If you ever come across this situation again, I recommend that you Acetone to remove the spot. It won't hurt the mirror or coatings..CAUTION: Use it only in open areas, with plenty of ventilation and wear gloves. Keep it away from plastics. Rinse with Demineralised or distilled water.

#3 FirstSight

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Posted 30 March 2013 - 05:24 PM

Chris,
If you ever come across this situation again, I recommend that you Acetone to remove the spot. It won't hurt the mirror or coatings..CAUTION: Use it only in open areas, with plenty of ventilation and wear gloves. Keep it away from plastics. Rinse with Demineralised or distilled water.


I did wind up using Acetone, I just didn't resort to it quickly enough when the going got tough. The real problem is that it's easy to fail at first to recognize the extent to which you're scuffing the area around the spot being removed, rather than just the area immediately under it (which of course will be covered by the replacement spot).

Your advice is well-taken: Acetone first, then start removal.

#4 Jason D

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Posted 30 March 2013 - 05:46 PM

Chris, Acetone did not work for me that well. I did resort to using my fingernail to pry off the edge of the spot then used a pair of tweezers to finish it off. I did not leave any scratches and the removed spot did not disintegrate.

Why did you remove your old spot? Was it to reposition it or was it to change the spot shape?

Jason

#5 GeneT

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Posted 30 March 2013 - 05:50 PM

I missed something. How did you get the scratches?

#6 FirstSight

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Posted 30 March 2013 - 06:32 PM

Why did you remove your old spot? Was it to reposition it or was it to change the spot shape?


No, the original spot was correctly positioned and I was replacing it with another triangle. I changed it because when I placed the original triangular center spot, I neglected to perforate a central circle in the middle of it, failing at the time to appreciate how helpful that could be to achieving the most accurate collimation with the Glatter laser and TuBlug (cause I didn't have either at the time). I already had an extra triangular center spot Jim Fly had sent me along with the original when I bought the Catseye tools and template.

I'll probably switch to a Catseye hotspot when I get around to having the mirror recoated, likely by Spectrum. At worst, this experience has nudged me to doing something within the next couple of months which I was contemplating doing in a couple of years anyway.

#7 FirstSight

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Posted 30 March 2013 - 06:36 PM

I missed something. How did you get the scratches?


Scraping the stubborn old spot off with my fingernail, in a gingerly careful manner or so I thought as I was doing it. GeezLouise, that old Catseye triangle was stubbornly adhered to the mirror and didn't come off without putting up frustraingly impressive resistance, even after resorting to a little help from Acetone.

#8 Jeff Porter

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Posted 30 March 2013 - 10:17 PM

Chris,

Sorry to hear about the trouble with the old spot coming off. I am sure that all of us have been there in one way or another at some point. I'm glad to hear that there was no serious problems.

I would be curious to hear a bit more about how long you let the old spot soak and what you used before trying to remove it. I find that I am respotting mirrors two or three times a year and would like to know if there are any lessons to be learned. I usually use acetone which sounds like it worked best in your situation.

Jeff P

#9 FirstSight

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Posted 31 March 2013 - 10:33 AM

ious to hear a bit more about how long you let the old spot soak and what you used before trying to remove it. I find that I am respotting mirrors two or three times a year and would like to know if there are any lessons to be learned. I usually use acetone which sounds like it worked best in your situation.

Jeff P


I started out simply wetting the central area of my mirror with distilled water, thinking the triangle would yield to gentle persistent prying with a fingernail. The scuffing was nearly entirely done during this initial period while I thought I was merely working just at the spot itself with my fingernail, not realizing how much the pressure flattened out the nail or inadvertently dragged a few mm across the immediately nearby surrounding surface while lifting it back away or approaching the remanant spot.

BOTTOM LINE: I simply underestimated how difficult the spot would be to remove without FIRST soaking it with acetone, and compounded it by rather stupidly failing to appreciate just how unwisely clumsy a tool even "careful" use of a fingernail is around a telescope mirror, even when confined to the supposedly "nonvisible" portion under the secondary shadow. If I remove and replace a center spot in the future, it's acetone and tweezers the whole way. Sometimes it's worth it when you blunder to fess up and analyze what went wrong on-forum to help prevent others from making similar blunders. Removing an old center spot is neither a task to be under-estimated nor one to take any impatient short-cuts to execute. DON'T be fooled by the fact that the area you're working on is confined to "nonvisible" areas under the shadow of the secondary; any scuffs inflicted can still potentially bite you by degrading your ability to clearly see the reflected outline of the center spot during collimation.

THE SAVING GRACE is that I had planned to recoat the mirror within the next couple of years anyway, and so I'll now just be doing it a bit sooner. Also, I have a new Feathertouch focuser and new sturdier mount on order for my NP-101 which are expected to arrive in another week or so, so I'll have new toys to play with while my mirror's away for three or four weeks being recoated. I've already ordered the hotspot and spotting template for application after the recoat.

BTW: obviously, the extra triangle was originally solid rather than one of the ones that comes pre-perforated, that's why the ragged interior edge around the home-done perforation. Even though the circular outlines for perfing it are included on the back of the solid triangles too, it's hard to cut a clean edge yourself, even using a sharp exacto knife. However, the home-perfed triangle is perfectly functional as intended for laser collimation use, and but/for the annoyance of how the scratches interfere with using the Catseye tools, I'd have been perfectly happy leaving it as is until I got around to recoating in another two years or so.

#10 amicus sidera

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Posted 31 March 2013 - 06:02 PM

I would suggest that, rather than using a sticker of any kind, a return to what has been used by amateurs since at least the 1960's to mark the center of their mirrors is in order:

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#11 amicus sidera

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Posted 31 March 2013 - 06:05 PM

A template is created to assure accuracy of the mark, which will last through several cleanings. When it fades, it's a simple matter to re-mark it again. This will entirely mitigate the hazards inherent in applying a sticker to the delicate mirror surface, much less trying to remove one.

Accurate collimation of a reflector is an inherently easy task; it's when one strives for perfection that the soufflè falls.

#12 precaud

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Posted 31 March 2013 - 07:02 PM

Chris, I'd be surprised if, that close to the center, those scratches detracted from the image quality at all. They'd be well under the secondary's shadow.

I accidentally made similar scratches when I respotted my XT6, with no ill effects. The sharper image afforded by the more accurately-placed center spot was noticeable, though.

#13 FirstSight

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Posted 31 March 2013 - 07:08 PM

A template is created to assure accuracy of the mark, which will last through several cleanings. When it fades, it's a simple matter to re-mark it again. This will entirely mitigate the hazards inherent in applying a sticker to the delicate mirror surface, much less trying to remove one.

Accurate collimation of a reflector is an inherently easy task; it's when one strives for perfection that the soufflè falls.


The Catseye center spots (triangular or hotspot) are vastly more functional than a simple black-market dot; they are reflective for easy visibility under red-light conditions, they are calibrated in size to be a perfect match to exactly fit within the circular outline seen in the Catseye chesire, and their shape is designed to give far more intelligible, precise feedback on how well the focuser and primary axes are aligned together than a simple black magic marker would.

The whole point of the Catseye collimation system is that it makes perfect collimation of fast reflectors not only possible, but easy, especially once the initial mystery of using the autocollimator is dispelled and one realizes how simple and elegant even that tool is.

Catseye even sells an inexpensive transparent reusable template that makes easy work of extremely accurate placement of either the triangle or the hotspot, which once placed won't need replacing until the next time the mirror is recoated. I only wanted to replace mine because I wanted to try using a perforated rather than solid spot.

More importantly, I only messed up because I wasn't patient enough to undertake the removal project in a properly prudent way by preemptively dousing the spot in acetone. Simple stupid plowing ahead with "let's get this simple job done, how can it possibly go wrong (it's under the secondary shadow anyway)?" kind of attitude. Starting out the right way really wouldn't have been significantly harder than trotting out to the garage to fetch the acetone *before* proceeding. (The scuffs were mostly done the first two or three minutes before I retrieved the acetone).

#14 amicus sidera

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Posted 31 March 2013 - 07:36 PM

With all due respect, I consider that method overkill, even with a very fast instrument; I'm of the opinion that "perfect" is not the enemy of "excellent" when collimation is being discussed, regardless what the marketplace may claim. Feel free to ignore my suggestions, as I in no way wish to denigrate your use of a sticker and ancillary equipment over an inkspot... if you feel the former is more accurate, and it enhances your observing, so be it.

In any event, as a poster stated above, the fine scratches in the mirror's center should not impair its performance in any way detectable to human vision, so no worries.

#15 johnnyha

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Posted 31 March 2013 - 08:19 PM

I had a similar problem, when I removed mine once I not only left soem scratches but the area around the old triangle was rendered "milky" and lost its reflectance. So I suffered the same problem - the area around the triangle needs to be nice and reflective or you cannot see the reflections very well and it is difficult to line up the triangle. The next time I cleaned the mirror I was able to use alcohol to get rid of the milky residue.

Good luck!

#16 johnnyha

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Posted 31 March 2013 - 08:20 PM

I had a similar problem, when I removed mine once I not only left some similar scratches but also, the area around the old triangle was rendered "milky" and lost its reflectance. So I suffered the same problem - the area around the triangle needs to be nice and reflective or you cannot see the reflections very well and it is difficult to line up the triangle. The next time I cleaned the mirror I was able to use alcohol to get rid of the milky residue but the scratches still made it a little more difficult than it would be otherwise.

Good luck!

#17 Starman1

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Posted 31 March 2013 - 10:22 PM

With all due respect, I consider that method overkill, even with a very fast instrument; I'm of the opinion that "perfect" is not the enemy of "excellent" when collimation is being discussed, regardless what the marketplace may claim. Feel free to ignore my suggestions, as I in no way wish to denigrate your use of a sticker and ancillary equipment over an inkspot... if you feel the former is more accurate, and it enhances your observing, so be it.


Striving to hit the center of the 'good collimation' envelope is a practical, not obsessive, thing. The reason is simple: mechanical and structural flexures occur when the scope is in use. If collimation is in the center of the 'envelope', those flexures won't be sufficient to pull the scope outside of the envelope of good collimation.
On the other hand, a scope that is collimated to just inside the envelope may stray out of it during the night as the scope is used.
It is because I DON'T want to waste one second of observing time checking collimation during the night that I strive for perfection in collimation before the night begins.

#18 Jason D

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Posted 31 March 2013 - 11:03 PM

Accurate collimation of a reflector is an inherently easy task; it's when one strives for perfection that the soufflè falls.

I agree with the first part of your statement but I would add that quality collimation tools, good collimation knowledge and practice will make striving for perfection is as easy.
Jason

#19 Nils Olof Carlin

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Posted 01 April 2013 - 02:35 AM

Chris,

in order to avoid this complication, I always (=not very often) cover one thin segment of the sticky side of the spot with a thin piece of tape, leaving a potential flap to lift and grab if/when I need to peel it off.

Nils Olof

#20 FirstSight

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Posted 01 April 2013 - 09:05 AM

Striving to hit the center of the 'good collimation' envelope is a practical, not obsessive, thing. The reason is simple: mechanical and structural flexures occur when the scope is in use. If collimation is in the center of the 'envelope', those flexures won't be sufficient to pull the scope outside of the envelope of good collimation.


The degree to which the thumbscrew controlling the compression ring is tightened around the collimation tool (or eyepiece) significantly affects whether the reflections of the center mark appear aligned or not in the autocollimator, even though the focuser in question is a high-quality Moonlite CR2 focuser and the tool is a 2" Catseye autocollimator. (i.e. the phenomena is entirely one of the inherent mechanical flex in the compression ring mechanism, and not one of loose registration in the focuser or imprecision of the collimation tool). However, to my thinking that's MORE not less reason to use high-quality collimation tools, which do give me extremely accurate feedback regarding the extent to which various types of mechanical flexures in my scope's structure affect optical alignment of the axes, allowing me to intelligently choose which compromises or compensations I should make to achieve best alignment. Furthermore, I can get perfect alignment on even on a poor night for using stars, e.g. thin clouds filling much of the sky, but when planets and the moon are perfectly visible. I've never, ever experienced a time when my quality collimation tools indicated optimal alignment when that disagreed with the quality of the view through the eyepiece, or when the diffraction pattern of stars gave me more useful or precise information than the Catseye tools do. The Catseye tools are vastly easier to work with; they stay put while making adjustments, but the stars don't, nor are they subject to interference from the night's seeing conditions.

#21 Jeff Porter

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Posted 01 April 2013 - 10:19 AM

Chris,

Thanks for the update. Most of my experience has been with mirrors whose coatings were only a few years old or so. I was curious to see if there was some other issue with older mirrors.

Congrats on the new focuser for your NP-101. That is a great scope from what I have read.

Jeff P

#22 Howie Glatter

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Posted 01 April 2013 - 02:11 PM

". . the phenomena is entirely one of the inherent mechanical flex in the compression ring mechanism, and not one of loose registration in the focuser or imprecision of the collimation tool. ."

I think the problem is inherent in the cylindrical slip-fit system with inward radial clamping that we use. Because there's space in between that allows movement between the cylinders, the inner member can easily slide sideways a little at the line of contact where the surfaces are asymptotically flat. The maximum skew can be dramatically reduced by decreasing the space between cylinders to about 0.001" with tighter manufacturing tolerances.
So called "compression rings" make the problem worse. Because of the groove machined in the drawtube, there is no drawtube surface directly opposite (180 degrees from) the end of the clamp screw – there is only empty space. This increases the tendency for the accessory to tip when the clamp screw is tightened. The reason for this is that a machined hole is almost always slightly larger in diameter just at the opening.The only good thing compression rings do is to protect the accessory barrel from marks, but this can also be done with a well-designed plastic tip on the clamp screw.

#23 Starman1

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Posted 01 April 2013 - 04:23 PM

". . the phenomena is entirely one of the inherent mechanical flex in the compression ring mechanism, and not one of loose registration in the focuser or imprecision of the collimation tool. ."

I think the problem is inherent in the cylindrical slip-fit system with inward radial clamping that we use. Because there's space in between that allows movement between the cylinders, the inner member can easily slide sideways a little at the line of contact where the surfaces are asymptotically flat. The maximum skew can be dramatically reduced by decreasing the space between cylinders to about 0.001" with tighter manufacturing tolerances.
So called "compression rings" make the problem worse. Because of the groove machined in the drawtube, there is no drawtube surface directly opposite (180 degrees from) the end of the clamp screw – there is only empty space. This increases the tendency for the accessory to tip when the clamp screw is tightened. The reason for this is that a machined hole is almost always slightly larger in diameter just at the opening.The only good thing compression rings do is to protect the accessory barrel from marks, but this can also be done with a well-designed plastic tip on the clamp screw.

:bow:
From your lips to the ears of focuser and adapter makers.
I had a Moonlight focuser without a brass protection ring (I call it that since it isn't really a compression ring, or collet) and the fit was so snug I had to rotate my collimation tools into place in the focuser. And the 1/4" aluminum thumbscrew had a tip wide enough not to tip the eyepieces with "safety grooves".

Now I have a Moonlite focuser with a brass ring and though the fit is tighter than most I've encountered, it's still sloppier than the prior focuser's drawtube without one, and for just the reasons you mention.
Fortunately, I use a Paracorr in that scope, and it fits snugly, along several inches of its length, below the machined out brass ring housing.

I'm glad to know there are those who value tight tolerances and good machining over gimmicks like brass rings and "safety" grooves.

#24 bob irvin

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Posted 02 April 2013 - 02:28 PM

Good thread & good "heads up" to those of us on the road to doing this but
haven't yet.

thanks Chris.

bob

#25 FirstSight

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Posted 06 May 2013 - 02:36 PM

GOOD NEWS: Today, my 12" Primary was delivered back to me from recoating, and the result indicated that the scratches near the center were only in the old (now replaced) coating, and not into the mirror surface itself. Had the scratches been permanently etched into the surface, true they would have been under the shadow of the secondary but nevertheless would have annoyingly interfered with accurate collimation using the Catseye tools.

ALSO ON THE UPSIDE is how obviously apparent the difference is between the old replaced coatings and the new ones: formerly, many sleeks and pits were apparent (completely aside from any dust) on the old coatings, and there were some definite pinholes beginning to show when seen from behind with a bright light shining on the surface. The new coating is...well, pristine, and no pinholes whatever appear when seen from behind with a bright light shining on the surface. This could potentially result in reduced light scatter and improved contrast, though I won't be able to tell whether there's any visible improvement until I get the mirror back out under the stars.

I re-spotted the mirror this time with a Catseye Hotspot (and Catseye hotspot-adapted template), reinstalled it in the cell and OTA, and re-collimated it. I will vouch that the hotspot is indeed significantly easier to precisely center using either Glatter or Catseye tools (I use both) than even the Catseye red triangle was. The only downside is that although P itself is easily visible in the autocollimator, the three other reflections are marginally less easy to see than they were with the red triangle, but not so much as to be truly at all problematic, and may be mainly due to the changeover in color from red to white.

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