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Equipment Discussions >> Reflectors

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precaud
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Re: AD10 poor focus - help! new [Re: Cotts]
      #5644990 - 01/26/13 07:39 PM

Guys, please, it's great advice, but an unavailable option, not one that I can act on, so please let's keep the thread on-topic. I look forward to viewing with folks like that some day.

I know one person in town like that (I bought my XT6 from him), I give him master-status (great eyes, 40+ years experience), but he too is unavailable. So I will sort this out as best I can by mid-week, and I'll be as ready as I can be when the new mirror arrives.


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sslcm56
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Re: AD10 poor focus - help! new [Re: precaud]
      #5647928 - 01/28/13 11:49 AM

The knowledge and experiance on these forums is just unreal!
Sooo....what I'm getting here is that when I get my Catseye tools I should just measure from the edge of the blank since my mirror has a very uneven bevel around the edge. Is that right?


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precaud
scholastic sledgehammer
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Re: AD10 poor focus - help! new [Re: sslcm56]
      #5648495 - 01/28/13 03:30 PM

Quote:

what I'm getting here is that when I get my Catseye tools I should just measure from the edge of the blank since my mirror has a very uneven bevel around the edge. Is that right?




I haven't seen a strong consensus of opinion even among the experts that post here. Best I can tell, it depends on whether you think the paraboloid shape is defined by the final polishing and edge treatment, or by the main grinding. With limited knowledge about how mass-produced mirrors are made, I lean toward the latter - that the main shape is put down with the mirror spinning on it's physical center. And in that case the blank's O.D. is the reference.

But I could be all wrong.


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Mirzam
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Re: AD10 poor focus - help! new [Re: precaud]
      #5648734 - 01/28/13 05:12 PM

The bevel is put on the edge of the mirror blank at the beginning of grinding and is maintained as it wears away in order to prevent chipping of a sharp edge. During the later stages of fine grinding and during polishing there is not much glass being removed so the bevel no longer has to be touched up. The fine grinding/polishing process creates a "figure of revolution", which is a curve that is symmetrical across all diameters of the ground surface. This means that the inner edge of the bevel should be very close to a perfect circle even if the outer edge is not. I would therefore use the inner edge of the bevel as the reference circle for determining the optical center.

All this being said,it seems to me that it would be pretty difficult to make an "off-centered" optical surface on a mirror blank when working by hand. However, I can imagine that it would be possible to get this with a machine mounted blank if the mounting was not done perfectly.

JimC


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precaud
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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5649272 - 01/28/13 09:11 PM

Thanks for the explanation, Jim.

Quote:

The fine grinding/polishing process creates a "figure of revolution", which is a curve that is symmetrical across all diameters of the ground surface.




Assuming "ground surface" = "mirrored surface", this then raises the question; what if, as I see on my AD10 mirror, some "diameters" don't have equal lengths and don't have end points that are equidistant from the center (that mythical point we're searching for)? This mirror has some pretty aggressive asymmetric grinding in three places.

Quote:

This means that the inner edge of the bevel should be very close to a perfect circle even if the outer edge is not. I would therefore use the inner edge of the bevel as the reference circle for determining the optical center.




OK. If the "outer edge" = "the blank's periphery", the question again is, which inner bevel? The outermost? The innermost? A "best fit" circle on the mirrored surface, favoring the outermost?

Every answer seems to raise more questions...


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sslcm56
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Re: AD10 poor focus - help! new [Re: precaud]
      #5649354 - 01/28/13 10:07 PM

From haveing operated a metal lathe for a while I know thet if you spin something w/o it's being centered you get an UGLY vibration. If you cut something(metal or glass) off center you get that same vibration due to the diff in weight distrabution.
So it seems to me that(and I am absolutely guessing here) the outside of the blank would have to be the starting point for getting dead center.


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Mirzam
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Re: AD10 poor focus - help! new [Re: precaud]
      #5649867 - 01/29/13 08:50 AM

John--Your mirror situation does sound challenging. Based on your description I think that I would try to find the average position of the inner bevel. I can visualize how to do this with a clear acetate template having a scribed circle, but using direct measurements sounds hard to do.

As to how the mirror got this way, it seems like the possibilities are 1) the blank is not round, or 2) the blank was mounted slightly off center on the grinding support. Whether excessive vibration would occur, or cause anyone to care, is hard to say.

JimC


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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5649913 - 01/29/13 09:36 AM

Quote:


I haven't seen a strong consensus of opinion even among the experts that post here. Best I can tell, it depends on whether you think the paraboloid shape is defined by the final polishing and edge treatment, or by the main grinding. With limited knowledge about how mass-produced mirrors are made, I lean toward the latter - that the main shape is put down with the mirror spinning on it's physical center. And in that case the blank's O.D. is the reference.

But I could be all wrong.




John,

Even if the blank is perfectly centered on a spinning disc or plate for grinding and polishing so it won't wobble, it says nothing about where on that blank the actual grinding/polishing equipment is positioned relative to that center point.

Dealing with center spot placement where .5mm makes a difference, I can't see how referencing the polished/mirrored portion of the blank can be other then the most relevant portion of the blank to poll for center.


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precaud
scholastic sledgehammer
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Re: AD10 poor focus - help! new [Re: howard929]
      #5649988 - 01/29/13 10:21 AM

Hi Howard,
Quote:

Even if the blank is perfectly centered on a spinning disc or plate for grinding and polishing so it won't wobble, it says nothing about where on that blank the actual grinding/polishing equipment is positioned relative to that center point.




Yes, but no matter what shape you put down on the surface of a spinning mirror, the resulting figure will be symmetrical to the rotational center.

Quote:

Dealing with center spot placement where .5mm makes a difference, I can't see how referencing the polished/mirrored portion of the blank can be other then the most relevant portion of the blank to poll for center.




If the bevel width is consistent, then yes. But then, you could just use the blank O.D!. Look at the photo I sent and you have an example where it isn't so clear. It's pretty obvious that someone ground off mirrored surface from three areas. (Whether that was intentional, to remove some kind of defect, or just sloppy workmanship, we can only guess). But the rest of the mirror has a more consistent bevel width.

So conceptually, I am leaning toward either:
1. Using the blank's OD, or
2. a combination of your idea and Jim's averaging; a circumference based on a "best-fit to the outermost average portion of the mirrored surface". This basically throws out the areas with a shorter radius as "anomalies" and goes with an average of the longer radiuses (radii?).

And I think a very accurate way to see and find that circle is what you (and I) have been doing:
a) computer-generated circles superimposed onto high-resolution photos, or
b) choosing a mirror orientation that reflects the "best-fit" and frame it down to XY coordinates, and then
c) calculate the center of the circle (a) or square (b) and mark it on the mirror.

As you know, this can be done with a decent digicam and photo editing program that has basic drawing functions. Scale the DPI to the pixel resolution of the photo and you could measure the spot. Or better, just make a reference mark on the mirror before the photo is taken and mark the calculated center relative to it. That would be much easier.

This method can't be any less accurate than using a physical template, and is likely more accurate.

Edited by precaud (01/29/13 10:30 AM)


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sslcm56
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Re: AD10 poor focus - help! new [Re: precaud]
      #5651297 - 01/29/13 10:23 PM

All this is makeing me CRAZY!!!! When I get the Catseye tools I will just spot the mirror by the OD and if that don't give a good view I will respot it by the ground surface. Problem solved

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rlmxracer
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Re: AD10 poor focus - help! new [Re: sslcm56]
      #5651371 - 01/29/13 11:22 PM

I'm also having a similar issue with my GSO dob (higher the mag softer the view). I found my factory center spot off center by 1.5mm or so which I've fixed. I have the same 1.25" combo tool as the OP which I am only using to center and adjust the secondary. I'm using a 2" Farpoint cheshire to adjust the primary. I'll be recieving the Catseye XLK auto collimator this week to fine tune. I am thinking my primary just has a so so figure, but I'm am no expert.

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Vic Menard
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Re: AD10 poor focus - help! new [Re: sslcm56]
      #5651902 - 01/30/13 09:40 AM

Quote:

...So it seems to me that(and I am absolutely guessing here) the outside of the blank would have to be the starting point for getting dead center.




Whether a spindle is attached, or a frame, the outer edge of the blank is the reference.


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Vic Menard
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Re: AD10 poor focus - help! new [Re: howard929]
      #5651943 - 01/30/13 10:02 AM

Quote:

...Even if the blank is perfectly centered on a spinning disc or plate for grinding and polishing so it won't wobble, it says nothing about where on that blank the actual grinding/polishing equipment is positioned relative to that center point.



Interestingly enough, it is the randomness of the motions over the spinning disk that guarantee a smooth, spherical surface. If you take away the randomness, the machine polished surface has a tendency toward centered, stepped zones. At least, that's my experience from my ATM days. Of course, a spherical surface has no real "center"--parabolizing changes that.

Quote:

Dealing with center spot placement where .5mm makes a difference, I can't see how referencing the polished/mirrored portion of the blank can be other then the most relevant portion of the blank to poll for center.



A center spot placement error of 0.5mm (two hundredths of an inch) contributes 0.25mm error (one hundredth of an inch) to an otherwise "perfect" primary mirror alignment. This shouldn't be significant at f/5, likely not visible in the image at f/4, and something to ponder over a glass of wine (and preferably on a cloudy night) at f/3...


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howard929
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Re: AD10 poor focus - help! new [Re: Vic Menard]
      #5651950 - 01/30/13 10:07 AM

Quote:

Quote:

...So it seems to me that(and I am absolutely guessing here) the outside of the blank would have to be the starting point for getting dead center.




Whether a spindle is attached, or a frame, the outer edge of the blank is the reference.




Vic,

Is that correct even if the polished portion is shifted a full .5mm towards one side of the blank in one direction and .7mm in the other?

I may be going overboard with this but errors like that seem to be common.

Howard


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Vic Menard
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Re: AD10 poor focus - help! new [Re: howard929]
      #5651984 - 01/30/13 10:29 AM

Quote:

Quote:

Whether a spindle is attached, or a frame, the outer edge of the blank is the reference.




Vic,

Is that correct even if the polished portion is shifted a full .5mm towards one side of the blank in one direction and .7mm in the other?

I may be going overboard with this but errors like that seem to be common.

Howard



It depends on whether the shift is the result of the bevel/chamfer or wedge. If the side of the mirror blank is higher on one side than the side opposite, the higher side will grind/polish more quickly, shifting the surface. Most modern glass is machined or molded flat, but I've heard of significant wedge anecdotally in these forums.

A more likely cause of the variation you describe is chamfering, especially when the final polished (and often sharp and fragile) edge is a random series of tiny chips that the mirror maker then tried to "erase" while preserving the maximum clear aperture.

The other possibility I can think of is that the glass was inexpensive float glass. In this case, the surface and back are probably reasonably parallel, but there are random highs and lows (perhaps +/- one or two hundredths) that cause the mirror surface to grind/polish more quickly in random areas that ultimately are revealed at the edge (the glass removed toward the center clears the effect). The end result should be the same as a random chamfer.

Edited by Vic Menard (01/30/13 10:35 AM)


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howard929
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Re: AD10 poor focus - help! new [Re: Vic Menard]
      #5651997 - 01/30/13 10:35 AM

We were typing at the same time so I see that you answered my question before it was posted.

The way I see this, this isn't my hobby, it's yours. I'm way too new to any of this for it to be mine.

A .5mm error in spot placement is well within acceptable limits at f/4? I believe it if you say it.

I'm done.


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Vic Menard
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Re: AD10 poor focus - help! new [Re: howard929]
      #5652023 - 01/30/13 10:52 AM

Quote:

...A .5mm error in spot placement is well within acceptable limits at f/4? I believe it if you say it.

I'm done.



At f/4, the critical primary mirror axial alignment error tolerance is +/-0.35mm. This is the high performance (25X to 50X per inch of aperture) tolerance. The broad range is dependent on the application, the seeing, and the user's expertise. For less demanding, moderate magnification applications in average seeing, the tolerance may be relaxed to 0.5 or even 0.7mm (remember, the coma "free" field diameter is only 1.4mm).

Is a 0.5mm placement error that contributes a 0.25mm primary mirror axial error well within acceptable limits at f/4? As I stated earlier, it's likely it will not be visible in the image...


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precaud
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Re: AD10 poor focus - help! new [Re: Vic Menard]
      #5652052 - 01/30/13 11:04 AM

Thanks for weighing in, Vic.

I got the new mirror yesterday, the bevel definitely looks cleaner and more consistent, though offset slightly on the blank. I'll do some thickness measurements to see if that's the reason.

Based on the "best-fit" concept, the center mark of this mirror is just under 2mm off-center; about 1mm using the blank's O.D.

Edited by precaud (01/30/13 11:12 AM)


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Vic Menard
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Re: AD10 poor focus - help! new [Re: precaud]
      #5652184 - 01/30/13 12:09 PM

Hi John,
I came in kind of late to this thread, so I went back and started at the beginning.
You're comparing the stellar performance of a 10-inch f/4.9 Dob to a 6-inch f/8 Dob. There are "big" differences between these two scopes.

First, there's aperture. The 10-inch collects almost 3X more light than the 6-inch, which means the 6-inch effectively filters the glare of brighter stars. Where aperture is great when searching for faint objects, the glare from brighter objects can overwhelm fine detail (and a stellar point is fine detail). The 10-incher also resolves almost 2X more detail than the 6-inch. This means the star "dots" (Airy disks) are almost 2X smaller than the same stars in the 6-incher. And along with resolving smaller stars, the 10-inch also resolves smaller turbulence (the cylinder of light entering the front of the OTA has 3X the area of the 6-inch cylinder).

That ought to be enough, but there's more. The f/4.9 focal ratio of the 10-incher requires the (high performance) primary mirror axial alignment to be corrected to better than +/-0.65mm, while the 6-inch f/8 tolerance is more than 4X more generous, +/-2.8mm. And while the depth of focus (1/4 wave) at F/5 is 0.03mm, at f/8 it's more than double, 0.07mm.

Finally, your 10-inch primary mirror probably has at least 3X the mass of your 6-incher, so it's going to take longer to reach equilibrium with the air surrounding it.

It probably makes you wonder why Newtonian enthusiasts seem to always be planning their next, bigger scope. For some, it's all about "brute force", low to moderate magnification, light gathering machines. For others, it's about analyzing and correcting each complication to finesse the exquisite detail only available to larger apertures. If you're serious about it, at 10-inches of aperture, your journey has just begun...


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precaud
scholastic sledgehammer
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Re: AD10 poor focus - help! new [Re: Vic Menard]
      #5652312 - 01/30/13 01:21 PM

Hi Vic, thanks for your interest.

Quote:

You're comparing the stellar performance of a 10-inch f/4.9 Dob to a 6-inch f/8 Dob.




Or non-stellar, as the case may be...

And thanks for putting it all in context, you've given me alot to chew on, it does help to appreciate the complexity of this dance.

Since the vendor has sent me another mirror, my job for the moment is to determine which is the better of the two. Besides star tests and other nighttime viewing comparisons, are there other things I can do to evaluate them? Perhaps even in daylight?

One thought was to put up a reflective object at a distance to do star tests during the day, but I haven't been able to locate a suitable place on my neighbor's property to put it!

I have a direct view to installations on a nearby mountain top. Will views of something like that be useful in comparing these mirrors?

Quote:

For some, it's all about "brute force", low to moderate magnification, light gathering machines. For others, it's about analyzing and correcting each complication to finesse the exquisite detail only available to larger apertures. If you're serious about it, at 10-inches of aperture, your journey has just begun...




I'll take what's behind door #2... let the journey begin!

Cheers.

PS - I mic'd the new mirror's thickness and it's within .010" to the edge of the mirrored surface all around. Squaring inconsistencies of the tool to the bottom of the blank could easily be the cause of that...

Edited by precaud (01/30/13 01:29 PM)


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