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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: precaud]
      #5636520 - 01/22/13 09:57 AM Attachment (28 downloads)

I used a couple pieces of plastic and ran them around the circumference to center it as best I could, snugged up the holddowns, and marked through the hole with a fine red sharpie. (I had used a blue pen in round one).

As you can see, the point fell right on the inside edge of the donut, further out and 20* or so below the folded paper's mark (which is the blue mark inside the donut). As to all the dust on the mirror, well... welcome to New Mexico. Dust and dirt is our #1 export.

I then rotated the mirror so that the "error axis" from the factory's mark was directly in line with the focuser, which is aligned with a secondary AND primary adjust screw. That way I had adjustments that would directly affect the collimation in the exact direct of the error.

If I do this again, I'll cut some strips of plastic and wedge them between the mirror and cork in the nest, using it as a centering jig.

I think this technique is better than the folded paper circle.


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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: precaud]
      #5636816 - 01/22/13 12:38 PM

This is similar to what my star test looked like last night:
http://www.youtube.com/watch?v=FrpldHJFABo

Except,
: His starts and ends in a circular object that could be called a point of light; mine is a sparkler, and the spikes don't go away until the unfocussed donut size exceeds their reach.
: His bounces around a lot, mine is pretty stable.
: His shows signs of symmetry or concentric objects within the donut, mine does not.


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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: AD10 poor focus - help! new [Re: precaud]
      #5636909 - 01/22/13 01:25 PM

The blue mark on the upper left corner of the donut looks to real close to dead on.

Here's where the software I'm using shows the center to be.



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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: howard929]
      #5636934 - 01/22/13 01:41 PM

Hmmm... interesting. That's a mark I put down to help me line up the focuser just in case.

Is this calculation based on the shiney mirror part of the image?


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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: AD10 poor focus - help! new [Re: precaud]
      #5636949 - 01/22/13 01:48 PM

Yes. The orange lines represent the center of the shinny portion of that photo you posted, assuming it to be the polished portion of the blank.

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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: AD10 poor focus - help! new [Re: precaud]
      #5636959 - 01/22/13 01:54 PM

What none of this takes into account is orientation. So, somewhere on the outer edge of the donut lies the center of the mirror. Provided these photos have the same orientation, then the measurement I made would be useful.

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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: howard929]
      #5636979 - 01/22/13 02:05 PM

Yes. I kept track of orientation with a mark on the side of the mirror and on the nest. It also assumes a symmetrical bevel between mirror surface and blank, which it definitely does not have. There is quite aggressive and irregular grinding on the edge. Sloppy, even, I'd say.

But I'll look into it later this afternoon, thanks.


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precaud
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: precaud]
      #5637206 - 01/22/13 03:54 PM

Using a square with the ruler length set to the radius (the ruler has a cutout in the center of one end, making it easy to center on things), I'm probing the center area from different places around the edge. There is no way the software's calculated mark can work, relative to the outer dimensions of the glass. And from everything I've read, because of the way the surface is ground, the center
relative to the glass dimensions (not the mirrored portion) is what we want to find.

Interestingly, there is an axis on which the factory spot is well centered. But it is way off in the axis perpendicular to it. The second (red) mark, using the template made with a compass, happens to be right on that perpendicular axis, and the amount of offset (right at the edge of the donut) looks about right.

Given this, and the symmetrical star test, I think I'll stay with that spot until the replacement mirror comes.


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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: AD10 poor focus - help! new [Re: precaud]
      #5637228 - 01/22/13 04:05 PM

I beg to differ. The center of the polished area should be referenced, not the blank. What good would it do to reference the center of the blank if as in your case it's not in the center of the optical train and contributes nothing to it?

Edited by howard929 (01/22/13 04:12 PM)


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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: howard929]
      #5637252 - 01/22/13 04:16 PM

From what I've read, it has to do with the way the mirror is ground. But this is a pitch I can't swing at, and will leave it to more knowledgeable folks (of which you may be one) to decide what is correct.

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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: AD10 poor focus - help! new [Re: precaud]
      #5637284 - 01/22/13 04:26 PM

I went over this very subject a while back and according to Starman1 ( Don Pensack ) who I feel is VERY knowledgeable, the unground area of the blank isn't necessarily in the center of the optics and is left that way to resist chipping at the edges of the mirrored surface.

Edited by howard929 (01/22/13 04:29 PM)


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: AD10 poor focus - help! new [Re: howard929]
      #5637333 - 01/22/13 04:55 PM

One nice thing about the Catseye template is that it is transparent, allowing it to be centered on the edges of the optical surface rather than on the edges of the blank.

(No affiliation with Catseye products--just like-em).

Having said this, I do think that your method of locating the optical center is probably close enough to assess whether the mirror is a lemon.

JimC


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Jason D
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Reged: 10/21/06

Loc: California
Re: AD10 poor focus - help! new [Re: precaud]
      #5637336 - 01/22/13 04:56 PM

To the OP,
You might find the following thread useful
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5562160/page...
Jason


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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: Jason D]
      #5637471 - 01/22/13 06:05 PM

OK thanks, looks like I have some reading to do...

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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: precaud]
      #5644402 - 01/26/13 01:24 PM Attachment (12 downloads)

I read the above thread and most if not all of the caveats to using the blank's center referred to custom and low-volume parts.

An interesting confirmation of the accuracy of the compass-based mark: Here's a pic taken directly overhead with a camera known to have very little geometric distortion. The original was cropped to the OD (XnView is great for this), the result was 1807x1806 pixels, square within 1 pixel, giving about 183 pixels per inch resolution, or about .0005". The center was then marked using lines drawn constrained to XY movement.

The resulting spot is identical to the spot located by the compass method, right at the inside edge of the donut and in the same vector from the factory center.

I took similar photos at different rotations to find the center of the mirrored surface, and though they are all offset toward the same side as the red spot on the donut, in other axes they very greatly (from as high as the blue spot on the donut, to just as much below), due to the uneven beveling.

I can image there are many mirrors like this, where "deciding" the center of the mirrored surface is a complete judgement call and imprecise at best.

I like this photo technique of center-finding. The remaining challenge is to accurately put a mark down on it.


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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: AD10 poor focus - help! new [Re: precaud]
      #5644469 - 01/26/13 02:08 PM

Pre: I'm late to this party but I have read the entire thread with interest. Your first descriptions of the problem as seen at the eyepiece when comparing the 'sharp' 6" and the 'fuzzy' 10" sounds exactly like a description of poor seeing. On nights of average or somewhat poor seeing, the 6" scope will not be able to resolve the bad seeing and stars will seem to be more pin-point while the almost double resolving power of the 10" will show the boiling, spiky stars you describe.

Another point: At no time do you mention that the fuzz in the 10" scope is asymetrical around stars at the center of the field which would be a sign of poor collimation caused by a misplaced center spot.

Thirdly: at one point you said, " Seeing is not bad here tonight, maybe a tad worse than last night (the moon isn't helping matters) but not bad."
This statement concerns me in that the presence or absence of the moon has nothing whatsoever to do with seeing. I wonder if 'seeing' and 'transparency' are being mixed up here and, despite clear, transparent skies, you are experiencing bad seeing...... In winter, at mid northern latitudes, very clear skies after a cold front often have seriously degraded seeing for a couple of days...

To get pin-point, diffraction-pattern-visible views in a 10 inch telescope you'd need the seeing to be well below 1/2 arc second, which happens perhaps only one or two nights a year.....

Have a look here...

I'll wager that you have Pickering 6 to Pickering 10 seeing most of the time at your location. I know I do. For me to have Pickering 4 to Pickering 2 seeing I have to travel to the Florida Keys where they have seeing that good on a significant number of nights a year....

OTOH, if your scope is well collimated and cooled to ambient and you star test on a night of Pickering 5 seeing or better and still have the same problem then your mirror is the problem. A refigure might be in your future....

Dave


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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! [Re: Cotts]
      #5644760 - 01/26/13 05:14 PM

Dave, thanks for your detailed post. Since you're responding to the whole thread, there's a lot to unravel.

Quote:

At no time do you mention that the fuzz in the 10" scope is asymetrical around stars at the center of the field which would be a sign of poor collimation caused by a misplaced center spot.




That's because the first view was so bad, it was almost dizzying to look into, the coma was horrible, focus and coma were asymmetrical, it was pretty obvious something was grossly amiss, so I didn't do any star tests. Those only came after I got the centering was much closer.

Quote:

I wonder if 'seeing' and 'transparency' are being mixed up here




Very possible, if not likely. I'm only a few months into this hobby and don't feel in command of all of it's jargon yet. My tendency still is to translate what I'm seeing into analogous elements of fields where I am comfortable analytically (electronics and audio). Some translate well and seem to make sense. But there may not be a direct analogue for "seeing". I've tried to wrestle it into "noise floor" but it's not a good fit (as shown by my comment about the moon, which clearly limits my ability to perceive gradations in the lower levels of brightness, i.e. raises the background noise). So "seeing" is a work in progress.

I've relied some on regional weather maps showing frontal boundaries and the like, assuming that when I'm located nearer to the center of pressure systems, seeing will be better. And that was the case for the couple of nights when I wrote. I also look at the good work of the CMC, and their data showed seeing for my locale starting at average (3/5) in the early evening and transitioning to excellent (5/5) in the early morning.

Quote:

Your first descriptions of the problem as seen at the eyepiece when comparing the 'sharp' 6" and the 'fuzzy' 10" sounds exactly like a description of poor seeing.




Could very well be, but as just mentioned, the 10" was collimated to a misplaced center mark and I think it's best to throw out the first evening's observations. Once the centering was corrected, then comparisons were more on an even footing, despite the uncertainty of whether the 10" is exactly centered or not. Hence the ongoing experiments to gain some confidence in the spotting.

Quote:

On nights of average or somewhat poor seeing, the 6" scope will not be able to resolve the bad seeing and stars will seem to be more pin-point while the almost double resolving power of the 10" will show the boiling, spiky stars you describe.




Provocative thought, one that occurred to me also. But I gave it less weight on the following evening after doing star tests at 350x on the AD10 and XT6. Both showed the same kind of turbulence in the defocused donut, but the XT6 had signs of symmetrical figures and concentric rings that were disturbed by the turbulence; the AD10 didn't - it was all turbulence. This, and when comparing view of things like M42, the resolution nod goes to the 6", which shouldn't be the case. Even Polaris' double was clearer in the XT6. Slightly less bright, but much better defined.

Quote:

In winter, at mid northern latitudes, very clear skies after a cold front often have seriously degraded seeing for a couple of days...




Interesting point, I'll watch out for that, thanks.

Quote:

To get pin-point, diffraction-pattern-visible views in a 10 inch telescope you'd need the seeing to be well below 1/2 arc second, which happens perhaps only one or two nights a year.....




Hmmm... interesting...

Quote:

I'll wager that you have Pickering 6 to Pickering 10 seeing most of the time at your location. I know I do. For me to have Pickering 4 to Pickering 2 seeing I have to travel to the Florida Keys where they have seeing that good on a significant number of nights a year....




Thanks for the link. I'm guessing you reversed the numbers referring to his scale, where 1 is worst and 10 is best?

Quote:

OTOH, if your scope is well collimated and cooled to ambient and you star test on a night of Pickering 5 seeing or better and still have the same problem then your mirror is the problem.




I'm pretty comfortable and confident with collimation. However, Howard made a point that cooling may have played a role in this too. Despite both scopes having been out for 90 minutes, comparing a 6" to a 10" while ambient temps are dropping, the 10" will be at a disadvantage...

Quote:

A refigure might be in your future....




I plan on setting up a Paypal account to take contributions for exactly that purpose!

Summing up: Sorting out the relative contributions of all factors at play is difficult, especially without measuring instruments to apply to it. There was enough evidence that this may be a lemon mirror, and a replacement is on the way. So my task has changed. Soon I'll have two mirrors and my job will be to analyze and choose the better of them. I'll have to suss out it's center spot accuracy, too. Challenging, but fun. And this forum is invaluable. There's no better place to ramp into an accelerated learning curve on this stuff!


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dpwoos
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Reged: 10/18/06

Loc: United States
Re: AD10 poor focus - help! new [Re: precaud]
      #5644834 - 01/26/13 06:04 PM

Quote:

And this forum is invaluable. There's no better place to ramp into an accelerated learning curve on this stuff!




I agree that this forum is valuable, but strongly disagree that your local astro club isn't very likely to be a FAR better source of information. I think you should observe with your club, where in one evening you will likely have a (near) complete understanding of what is going on. Observing with our club, you would be able to compare your scope's performance side by side with others of known quality. We would help you to check your collimation, and make sure that your scope wasn't suffering from thermal issues. We even have a club member who can do an interferometric analysis of your mirror. I am not saying that you can't get good information here, but there is a reason why one goes to the doctor's office as opposed to simply having a chat via email.


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precaud
Pooh-Bah
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: dpwoos]
      #5644853 - 01/26/13 06:20 PM

There is no local astro club anymore. The nearest active one is 60 miles away. And I question the value of bring that into this thread,.

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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: AD10 poor focus - help! new [Re: precaud]
      #5644924 - 01/26/13 07:05 PM

Alas, I did reverse the pickering numbers - brain cramp.....

I would second the advice that observing in any sort of group environment, be it a club event or a Star Party, would be an excellent strategy. Folks would have their scopes, of known quality, and you would be able to compare views yourself to see if your scope is faulty.... Might be worth the 60 mile drive...

Dave


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