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precaud
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AD10 poor focus - help!
      #5632759 - 01/20/13 09:07 AM

I've been wrestling with this scope for a month now. First view in December was disappointing, good light gathering but poorly focused targets, much less clear and detailed than my XT6. Found that the focuser was faulty - went crooked when tensioner engaged. Got a replacement focuser and swapped it in yesterday - problem solved.

So finally I am able to properly collimate my AD10 - sight tube for 2ndary placement, laser for 2ndary tilt, and Cheshire for primary. With a collimation cap on, the view looks textbook, just like Jason D's graphics. Just for grins I tried the barlowed laser and it agreed well with the Cheshire (though the primary donut's shadow on the target wasn't a tidy circle, it was elongated at the bottom, I assume that's the laser target's fault.)

Took it outside at 5:30 to cool, and started viewing around 7pm. After checking/tweaking the collimation, I started viewing familiar targets - Jupiter, Orion Nebula, and the Trap to compare to what the XT6 can do. Saw pretty much what I saw the first time, maybe a little better; definitely more light gathering (could see stars E and F in the trap where the XT6 only showed E) but it was all due to the increased light and not from increased detail. I found myself once again constantly tweaking the focuser, trying to dial it in and get stars to be nice fine dots and not spikey mini-blobs. Double-checked the collimation, nothing had changed, it was still good. And this is how the hour went, no matter what eyepiece was in it, I still can not get it to focus down to anywhere near as sharp and detailed as the XT6 is. And with every eyepiece, all of which are supposed to perform well in an F/4.9 scope, stars turned into poorly focused blobs about halfway out from center.

After an hour of disappointing viewing, I realized I wasn't having fun, brought it in, took the XT6 out, and came inside to warm up for 1/2 hour while it cooled down. Checked/tweaked the collimation, and went to the same targets. Aaaahhh, much better. Sharp, detailed images, not as bright as the AD10 but MUCH sharper and more detailed. Yes, the XT6 is giving much better images than the AD10. This isn't how it is supposed to be...

What could possibly be going on? Am I doing something wrong? Is it a funky primary, a misplaced center mark? This is maddening. Any help would be appreciated.


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Jon Isaacs
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Re: AD10 poor focus - help! new [Re: precaud]
      #5632939 - 01/20/13 10:41 AM

Quote:

What could possibly be going on? Am I doing something wrong? Is it a funky primary, a misplaced center mark? This is maddening. Any help would be appreciated.




John:

Something doesn't seem right.

What do you see when you look at a magnitude 2-3 star at 200x? When you try to focus, as you run through focus, does the image seem to rotate 90 degrees, indicating possible astigmatism?

Have you removed the primary mirror for some reason? If so, it is likely that you tightened down the clips retaining the mirror, there should be a slight gap, otherwise the mirror will be pinched.

Probably the best thing would be if someone nearby could take a look, there maybe something obvious.

Jon


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precaud
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Re: AD10 poor focus - help! new [Re: Jon Isaacs]
      #5632983 - 01/20/13 11:05 AM

Jon, thanks, I didn't do that test, I only had the 7mm (178x) in briefly to see if maybe something was wrong with the barlowed 17mm Sterling... will do so tonight.

I bought this new in December, and haven't had the primary out.

I'm going to try to take a photo of the barlowed laser target, it only looked round on the upper half of the collimator's target face. Maybe that will show something?


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633104 - 01/20/13 12:25 PM Attachment (58 downloads)

Here's what the target on the barlowed laser looks like.

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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633149 - 01/20/13 12:55 PM Attachment (46 downloads)

And this is the view through the collimation cap (the cap's hole is slightly off center... it looks dead-center in the Cheshire)

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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633322 - 01/20/13 02:21 PM

Well I have the mirror out, checked the clips for pinching, they're ok. I went ahead and blackened that glaring bevelled edge. It can't hurt.

I just did some comparative measurements from the circumference to the edge of the center spot at several places. If the center mark is supposed to be centered on the outside diameter of the glass, than I can say that it most definitely is not. It's off center by some 2mm or so. That's not even close. How does one correct that?


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Jon Isaacs
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633348 - 01/20/13 02:38 PM

Quote:

That's not even close. How does one correct that?




There are a number of techniques. One is to carefully cut a piece of paper out the diameter of the mirror. Carefully fold it in quarters and that will be the center. Use that to mark the center.

Also, check your collimator by rotating it in the focuser while watching the center dot on the mirror. Often such collimators are significantly out of collimation.

Jon


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precaud
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Re: AD10 poor focus - help! new [Re: Jon Isaacs]
      #5633426 - 01/20/13 03:32 PM

Hmmm, ok, I read an older post about that a few minutes ago. Makes me wonder how they find and mark the center spot at the factory, that it could be this far off.

Re: the laser, the first thing I did after buying it was make a V block... it stays very well centered on a spot on the wall 9 feet away. That should be good anough.


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633496 - 01/20/13 04:16 PM Attachment (33 downloads)

OK, I just spotted the center using the folded paper circle technique, and the mark is about 1.5mm off their center mark.

Edited by precaud (01/20/13 04:28 PM)


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633595 - 01/20/13 05:18 PM

OK, I've positioned the new center mark to be in line with the focuser, and recollimated the scope as best as I can using only crosshairs of the combo tool on that mark (not easy to do when you're far-sighted...). Then checked it with the laser. The dot on its target face is the smallest and cleanest I've seen on this scope, so that's a good sign. I'll star-test the scope tonight as per Jon's post and see if we're on the right track.

Edited by precaud (01/20/13 05:19 PM)


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coopman
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Re: AD10 poor focus - help! new [Re: precaud]
      #5633939 - 01/20/13 09:14 PM

I still don't think that this little bit of collimation error is causing the problem. You will see a lot more coma with the AD10 compared to the XT6 which has virtually none. Almost any contact from the mirror clips can have a detrimental effect on the image quality. If you cannot improve the situation, you may have to go back to Apertura and request a new set of optics, if not a new scope in its entirety.

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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5634048 - 01/20/13 10:26 PM

The paper mask you made to determine the center of the primary, was it the size of the mirror blank or the polished surface?

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precaud
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Re: AD10 poor focus - help! new [Re: Jon Isaacs]
      #5634093 - 01/20/13 10:53 PM

Quote:

What do you see when you look at a magnitude 2-3 star at 200x? When you try to focus, as you run through focus, does the image seem to rotate 90 degrees, indicating possible astigmatism?




Jon, I have both scopes outside now, and have been comparing them with same views and eyepiece(s). I don't see that rotation going through "focus" that you describe. (I put focus in quotes because it still never really gets there...)

It's difficult to quantify, but collimated for the "new" center spot, I would say it is improved somewhat, but still not right. But there is plenty of uncertainty in this: the careful collimation I did before taking the scope outside got thrown out as it took on the lower temps. I don't feel confidence in collimating with crosshairs, Cheshire, and flashlight in the dark and in the cold. It's an approximate collimation on an approximate new center spot.

coopman: As I understand it, coma is basically a focus deterioration toward the edge of the FOV. Yes, the XT6 shows little if any with most EP's. The AD10 definitely has more, though very EP-dependent. The 7MM X-cel LX had ALOT less of it than the Sterlings. But my concentration is and has been on focus in the center of field. That is where I can not get a sharp image.

So... though improved, the AD10 still doesn't focus stars down to sharp dots in the center. Compared to the XT6, images are more bright but less clear. The XT6 makes you feel like you're right there in it, almost a 3D feeling.

You know how Sirius has almost a "churning" quality to it, like it is a cauldron of ray emissions that don't allow you to see it as a well-defined shape? All stars have this same quality in this AD10. That's what I meant by "spikey". The image just isn't finite and convincing. It's a "likeness", not a "view". I love the improved light-to-dark dynamic range, but there's not enough detail to draw you in.

howard: the mask was a trace of the blank's circumference.


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pftarch
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Re: AD10 poor focus - help! new [Re: precaud]
      #5634117 - 01/20/13 11:05 PM

Was your magnification significantly different between the 6" and 10"? Often in miserable seeing I feel like my Z10 is not doing as well as my smaller scopes, but if I crank the smaller scopes up to the same magnification as my 10" their image falls apart too. Does your 10" put up crisp views at lower mags?

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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5634134 - 01/20/13 11:21 PM

Quote:

the mask was a trace of the blank's circumference.




Not saying they aren't on your mirror but there's no guarantee that the polished area is centered on the blank. Can you post a photo of the entire mirror, centering the camera as best you can from the open end?


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precaud
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Re: AD10 poor focus - help! new [Re: howard929]
      #5634178 - 01/20/13 11:51 PM

pftarch: Scope focal lengths are nearly identical (1200/1250mm) so mags are basically the same. I wouldn't use "crisp" to describe anything I've seen through it at lower mags. And at 178x, the AD10 is much less sharp. And other than the moon's presence, seeing is excellent tonight.

howard: By "open end" I'm guessing you mean camera directly above and centered on the mirror as best as possible? I'll try but the weekend is over so time is squeezed...

I wish I knew someone with a known-good 10" dob so I had a better idea what to expect.


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pftarch
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Reged: 09/21/07

Re: AD10 poor focus - help! new [Re: precaud]
      #5634244 - 01/21/13 12:55 AM

Can you return and replace the scope? (I had to send back a Z10 due to an astigmatic mirror, my replacement was much better.)

Sometimes you just get a dud mirror.


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precaud
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Re: AD10 poor focus - help! new [Re: pftarch]
      #5634647 - 01/21/13 10:25 AM

Yes, I can return it, and Dave at Opticsmart has already mentioned it. For me, with so much time invested in getting this thing right so far, it's a last resort. And... he doesn't have another one in stock to replace it with. So I'm trying to eliminate all other possibilities that can be corrected by me before.

What would really help right now is a distant daytime light pointsource that I could use to do high-mag star tests and tweaking to. Even with good seeing, doing this with temps in the 20's is no fun...


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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5634882 - 01/21/13 12:38 PM

From another thread currently going on, a +1mm error in placing the center spot will, even if other errors are corrected, ruin high power views with your telescope.

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Mirzam
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Re: AD10 poor focus - help! new [Re: howard929]
      #5634931 - 01/21/13 01:04 PM

Bad seeing can cause blobby stars, and the effect is certainly more pronounced with larger apertures. But to me it sounds like your problem is spherical aberration of the primary. This assumes that you don't see oval shapes when stars are slightly defocused.

You are allowing the scope to cool for an hour or so before use?

JimC


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precaud
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Re: AD10 poor focus - help! new [Re: howard929]
      #5634936 - 01/21/13 01:05 PM

howard, which thread is that?

edit - never mind, I found it...

Edited by precaud (01/21/13 01:10 PM)


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635017 - 01/21/13 01:50 PM

Hi Jim,
Yes, scopes were put out 90 minutes prior to viewing. I wrote that in the first post but skipped it in later ones, didn't want to sound needlessly repetitive...

Seeing here is excellent lately, and if the forecast is to be trusted, will continue for another 3 days... Tonight I'm just going to do star tests, I've done enough comparatives.

But - news flash - I've just learned the dealer has a new primary mirror for it in stock and has offered to send it! I think we've eliminated all other reasonably possible causes at this point. I'm hoping he can check/verify the center mark accuracy before sending it.


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635105 - 01/21/13 02:37 PM

Is there a link that correlates what adjustments to make for specific visual abberations when doing high-magnification star tests?

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Mirzam
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635307 - 01/21/13 04:34 PM

Well, if the problem is spherical aberration there is no corrective adjustment. You will have mushy focus because the light from different zones of the mirror is not coming to a common focal point. It's just a poorly made mirror.

In contrast, one of the easiest indicators of a good mirror is a nice crisp "snap" to focus.

JimC


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precaud
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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5635371 - 01/21/13 05:11 PM

Quote:

Well, if the problem is spherical aberration there is no corrective adjustment. You will have mushy focus because the light from different zones of the mirror is not coming to a common focal point. It's just a poorly made mirror.




That makes complete sense. And thus far, signs are pointing to that possibly being the problem with this one.

I've just come up with a different way to locate the center with better precision than the folded paper, it looks very promising. I took pics but don't have time to detail it right now, gotta go to the gym and work out the kinks from all this "stationary activity". The "new new" center spot is a bit over 2mm off the factory, right at the inside edge of the donut, and about 20* below the previous "new". I'll recollimate to the "new new" before viewing tonight and report back on it.


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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635398 - 01/21/13 05:23 PM

You're on a slippery slope with the problems your having. It could be poor optics. It could also be the center spot location coupled with your collimation methods and assessments of them. I'm inclined to think your collimation method with a barlowed laser is adequate but since you found that the center spot was off and have replaced it, it's possible more care was needed. With a photo as discussed If you so choose, i can apply it to some software and manage since it won't be from a removed mirror, to get a very good idea of how close to center the spot is.

With my 8" f/6 primary, I was advised to keep it at no more then .75mm from the center of the mirror and your faster mirror would require a tighter tolerance for high powered views that your not getting.


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Mirzam
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Re: AD10 poor focus - help! new [Re: howard929]
      #5635418 - 01/21/13 05:39 PM

Having built a fair number of telescopes the best method of center spotting that I've found is the Catseye center spotting template.

http://www.catseyecollimation.com/

JimC


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precaud
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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5635622 - 01/21/13 07:59 PM

Sure, but I don't have one here, and I need something NOW that has better accuracy than folded paper. Context...

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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635783 - 01/21/13 09:45 PM

OK, after collimating on the "new new" spot, I'm doing star tests on Polaris at 350x (7mm + 2x barlow). With best focus, the star is bright, very spikey, ragged edge, it looks like a lit sparkler a couple hundred feet away. There is a four-point star formed by the spider vanes. I can see Polaris' double, though it is somewhat obscured by a spider diffraction spike. It is faint, but larger than I would expect.

As I defocus (both in and out), the pattern is very symmetrical but still spikey, and soon a very well defined dark spot develops. Continuing to defocus, the spikiness on the edge eventually goes away and turns into a churning donut image. Continuing to defocus, at no point do I see well-defined rings as I have seen in some photos.

This is all consistent with what I've been seeing to this point. A sky full of tiny sparklers.

Thoughts?


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panhard
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635859 - 01/21/13 10:32 PM

I do believe that the spikeyness is caused by turbulence in the atmosphere. It isn't your scope. I think if the stars are twinkling the seeing is poor. That is what you are describing. I always get mixed up between seeing and transparency.

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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635925 - 01/21/13 11:12 PM

At the end of Astro Babys Guide to Collimation are some examples of views when star testing. Do any of them match what you're seeing?

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precaud
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Re: AD10 poor focus - help! new [Re: howard929]
      #5635979 - 01/21/13 11:39 PM

Thanks Howard. It's closest to his "turbulence" and "damaged", but worse. There is no evidence of concentric rings. Very symmetrical donut, but no detail.

pan, I can't say turbulence isn't playing a part in this, but it's not the major part. Seeing is not bad here tonight, maybe a tad worse than last night (the moon isn't helping matters) but not bad. Stars in general are pretty steady... a high pressure dome is building over the region, the fast air currents have moved on to the north and east. Plus, being at 7000 feet does have some benefits... I'm always amazed when I go to Albuquerque (just 60 mi. south) how dirty the atmosphere is... not to mention the horrible LP.


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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5635981 - 01/21/13 11:40 PM

I noticed the other day and should have mentioned it, the horizontal spider vanes aren't straight. The lower one needs to be tightened to make it so and the vanes don't separate evenly into equal sized 4ths. The secondary holder looks shifted a bit to the right side. The vanes should be equal length, tight and hold the secondary holder concentric with the OTA. I remember my GSO dob needed the vanes to be tweaked when it was new so no surprise here that yours look like they need some as well.

BTW - just to keep the peace, AstroBaby is a HER not a his.

Edited by howard929 (01/21/13 11:44 PM)


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precaud
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Re: AD10 poor focus - help! new [Re: howard929]
      #5636006 - 01/21/13 11:59 PM

After the star testing, I did some general viewing of familiar targets, and collimation on the "new new" center point is a definite improvement. Coma has been cut significantly, by half or better. Overall the views are much more convincing, especially low-mag views. Detail in the center hasn't improved, but this is a step in the right direction.

This lends credence to the assertion that even a 1mm center spot error has a significant impact.

I'll describe the center mark technique tomorrow. It's late and I'm cold and tired.


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howard929
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Re: AD10 poor focus - help! new [Re: precaud]
      #5636041 - 01/22/13 12:39 AM

Here's what I did. I used your photo of the focuser view (through the collimation cap) to try to determine the center spot locating relative to the center of the primary. Not being sure it was a valid view for this I just sat on it. I just went searching for similar views through threads and went back to the AstoBaby site and found one there. I applied the same technique there and found the center spot to be well centered to much less then 1mm. I don't know when your photo was taken, before or after you re-spotted the mirror but it's a full 3mm off center. For your mirror, it needs to be less then .75mm off true center, likely closer to .6mm for high powered views.

The mirror itself may be poorly figured as that does seem to happen. Getting the center spot much closer to the center then it is in the photo as well as properly setting up everything else might solve this issue and it might not but it wouldn't be a waste of time going forward with Newts.

Edited by howard929 (01/22/13 12:42 AM)


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Mirzam
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Re: AD10 poor focus - help! new [Re: howard929]
      #5636344 - 01/22/13 07:41 AM

FWIW, another way to judge seeing is to look at the lunar limb. If it is rippling like crazy you have rotten seeing and should expect to see blobby, spikey stars. If the limb just quivers a little bit every few seconds you have decent seeing. If it is rock solid you have great seeing.

JimC


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BSJ
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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5636364 - 01/22/13 08:11 AM

Only way to know for sure is to get the mirror tested. Does your star test look like the simulated views seen here? http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/5083275/...

My real star test looked just like the simulations...

Refiguring very much improved the views.


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precaud
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Re: AD10 poor focus - help! new [Re: BSJ]
      #5636384 - 01/22/13 08:30 AM

howard: thanks for looking at it. That pic was based on the factory center mark. I'm not sure how your technique determines the circumference that it's finding the center for, I blackened the edges and the mirrored surface is definitely irregular on the edges. But your 3mm figure jives well with my "new new" spot. I have not moved their donut yet.

Jim: Yes, I've seen this exact same thing in my XT6. It's pretty predictably disturbed at dusk, when air layers are in flux, and calms down gradually from there, unless other factors are playing.

BSJ: Them's discouraging results... I'll read through your thread, thanks.


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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5636499 - 01/22/13 09:43 AM Attachment (11 downloads)

A brief description of my centering technique. I was looking for something that would improve on the folded paper circle approach. I wanted to try something that used a compass to draw the shape, since the center is the reference point and not derived, and realized I had the tools on hand to do it. (I have a bigger compass but couldn't find it)

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precaud
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Re: AD10 poor focus - help! new [Re: precaud]
      #5636503 - 01/22/13 09:46 AM Attachment (12 downloads)

I used the large outside calipers to find the mirror diameter, which was very consistent around the mirror. It measured 9.81". I set the compass for half that, drew some test radii to confirm the measurement, and then drew the circle using very light pressure, so as not to move the center point. As the circle closed, the ends met nicely, and I needed a magnifier to find the teeny hole in the paper left by the compass' point. I marked it and then enlarged it just enough to fit a pen point through. I then carefully cut out the circumference, raised the mirror holddown clips just a bit, and placed it on the mirror. The rubber "clips" were helpful in holding the paper in place.

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precaud
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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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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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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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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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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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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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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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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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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
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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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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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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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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
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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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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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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
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Re: AD10 poor focus - help! new [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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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
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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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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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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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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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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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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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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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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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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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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
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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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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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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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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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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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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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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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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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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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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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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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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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Re: AD10 poor focus - help! new [Re: precaud]
      #5652500 - 01/30/13 02:51 PM

Precaud-

I use this site to gauge expected seeing conditions, by which I mean high atmosphere wind velocity.

unisys north american model at 300 mb

Areas in the "pink" are low wind velocity, and if you get such a forecast over your area, it at least means that seeing has a possibility of being good. This happens infrequently where I live, especially during the winter.

I think I may have mentioned earlier that one of the easiest tests for a decent optic is "snap-to-focus".

If you can get a very distinct focus point, even for just a moment, it at least demonstrates that the mirror is capable of bringing everything to focus. It is not a perfect test, for example, you could still have a turned edge, but at least the parabolic curve is reasonably accurate.

JimC


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Re: AD10 poor focus - help! new [Re: precaud]
      #5652978 - 01/30/13 06:37 PM

Quote:

...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?



Star testing tests the entire optical system, collimation, thermal effects/seeing, and your eye. Ideally, you should evaluate the primary by itself with a Foucault or Ronchi test. If you intend to test the primary mirror in the telescope on a star, you need to minimize all the other possible contributors.

Quote:

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!



Suiter recommends this procedure--but I haven't had great results, mostly due to ground thermals.

Quote:

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



There may be some detail you could use as a metric, but you'll need to determine that yourself.

Quote:

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...



That's parallel for all practical purposes.


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Re: AD10 poor focus - help! new [Re: Vic Menard]
      #5653791 - 01/31/13 08:29 AM

Jim: I like that site, thanks for the link. It's the same thing I mentioned earlier; the wind speeds aloft are greater in areas where pressure systems interact. I've just been watching our local news' weather segments to see it but this is better, thanks.

As for the mirror testing, I got a call last night from my experienced friend, he's back in town and can help me with it over the weekend. He doesn't have any instruments to test them with but his eyes are definitely better than mine...

I did some crude side-by-side visual comparisons last night, and there is definitely a difference between the two mirrors. There is a subtle difference in the "color" of their reflected image. That was a surprise. Does it suggest a difference in their coatings?


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Re: AD10 poor focus - help! new [Re: precaud]
      #5654688 - 01/31/13 04:57 PM

Boy, what a difference a mirror can make!

Thankfully, there's no need for long, scrutinizing analysis of these two mirrors. The replacement is so FAR superior in clarity and detail, the difference is immediately obvious even on terrestrial targets.

Odd I hadn't noticed it before, but there is a power pole 1/2 a block down which has several insulators which reflect the sun nicely (even a couple of those multiple-stacked ones which give a row of glints, just like a 7-star linear cluster). Visually I have a clear shot to the pole through the side door. So I rearranged the dining room, put the scope up on the dining table (there are same advantages to not having an SO to ask permission for such things ), blocked the lower 2/3 of the doorway to stop the cold air from pouring in, and voila, a nice test target while the scope (and me) stays inside.

I won't go on and on about this, but now the AD10 is performing as I would have thought it would. It is now definitely sharper and clearer than the XT6. Yes, I could see significant aberrations caused by thermals and wind while viewing the insulator reflections and wood grain patterns on the pole cross ends. But when it was calm, the scope focussed them down better than the XT6 did, and you could then watch the wind, etc. perturb the point, defocus and spread it out, and then settle back to a nice sharp point. It was like looking at a sharper, better-focussed photo, at all magnifications.

Boy, what an ordeal this has been.

A couple points of interest about the old mirror. I measured the thickness, and it is tilted, it varied 0.065" from max to min. Now I don't know if that is enough to qualify as "wedge"... but what was also interesting, the "wedge" axis is exactly perpendicular to the axis of greatest deviation from the factory center mark to the correct center. Pretty strong evidence that the mirror's disfiguring is coincident with that axis.

Anyway, this long, frustrating road is now come to an end... I've learned a lot from it. A big Thank You to everyone who helped me out along the way.


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Re: AD10 poor focus - help! new [Re: precaud]
      #5654743 - 01/31/13 05:28 PM

It would still be a good idea to check the new mirror under the stars for possible astigmatism. This is a defect that may only be apparent at higher powers, where it will degrade detail. Look for oval out-of-focus diffraction patterns that rotate 90 degrees on either side of focus. This is one star test that is very easy to perform and not dependent on good seeing.

Hopefully all will be well.

JimC


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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5654842 - 01/31/13 06:24 PM

Will definitely do that, Jim, most likely tomorrow night, it's already too windy for tonight. Most of my tests today on the reflections were between 125x and 250x, and I saw no signs of astig going through focus, the donut was nice and symmetrical.

On a calmer day, those insulator reflections on the power pole are going to come in very handy for fine-tuning and collimation tweaking. I especially like the light-gray ceramic insulators that have 7 donuts, giving 7 points of light in a row. On one, the donuts are all the same diameter, an array which should be good for quantifying coma. On another, the donuts decrease in size, giving 7 different light-point sizes in a neat row. Very versatile test light sources!


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Re: AD10 poor focus - help! new [Re: precaud]
      #5654875 - 01/31/13 06:42 PM

I also want to say a very public Thank You to David at OpticsMart. Situations like this can test the nerve of dealer and customer alike, and I couldn't have asked for a more helpful partner on the other end than he.

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Re: AD10 poor focus - help! new [Re: precaud]
      #5662776 - 02/05/13 12:45 AM

We had a good night for viewing - cloudless, mostly still, transparent, not the best seeing but not bad, all indicators solidly in the blue.

With both scopes collimated to their new center spot, I took them out to cool and compare views again. I ran the AD10 mirror fan the whole time, and did 250x star tests at the start and about midway through the session. They showed a very symmetrical donut on both scopes.

The AD10 is definitely better with this new mirror. Images are nice and bright. More detailed than before. There is definitely some astigmatism, it seems to be worse with lower-power EP's. (Is that normal?)

Comparing the same views with the same eyepieces, I still give the nod to the XT6 for realism. It correlates well with what I saw in my daytime viewing. For example, viewing M47, everything in the cluster is just plain sharper, more defined, more realistic. The AD10 is brighter but less engaging, less convincing.

So I'm still not sold on this scope. Tomorrow night is supposed to be even better conditions, I'll give them another go.


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Re: AD10 poor focus - help! new [Re: precaud]
      #5662984 - 02/05/13 07:35 AM

Astigmatism has several possible causes that you need to evaluate.

1. It may be in your own eyes. A symptom of this is that the astigmatism is more evident at low magnification--in this case the exit pupil of the telescope is relatively large and more of the defective eye lens is used. See if the oval pattern rotates when you rotate your head. You can also check your eyeglass prescription and see if there is a "cylinder" correction indicated.

2. The astigmatism may be in the secondary mirror. Convex or concave secondaries, when tilted, cause apparent astigmatism. To evaluate this, you will need to rotate the primary mirror by about 1/8-1/4 turn. If the astigmatic oval does not rotate the secondary is to blame. If it does rotate, the problem is with the primary.

3. If only a small amount of astigmatism is present in the primary the mirror can still be useable. A small amount would be indicated by slightly oval out-of-focus images at high powers. Your upper magnification range will be limited but for most purposes the scope will perform okay.

JimC


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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5662993 - 02/05/13 07:42 AM

I should have mentioned that you will only see the effects of astigmatism in your own eyes when you are NOT wearing glasses. So another way to check is to examine the star images with and without your glasses on. This assumes that your glasses correct for astigmatism and that your prescription is up-to-date.

JimC


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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5663116 - 02/05/13 09:13 AM

Jim, thanks for your input, well-written and on-point as always. I was farsighted astigmatic as a child, it gave me horrible headaches, I wore glasses, but the astig seemed to pass with the teens and I tossed them. I don't wear glasses when viewing, only wear "readers" when reading, and I haven't had my eyes checked in decades (unless you count the driver's license renewal test ).

The AD10 star tests at 250x last night looked good - very symmetrical going both in and out. It's mostly at lower powers where I see the astig. I'll try rotating the primary 90*, if the 2ndary can be eliminated as a possibility, that will narrow down the possible causes. Thanks for that suggestion.

What bothers me most about the AD10 so far is that, it just doesn't draw me in. On the XT6, I'm drawn in to the view... I feel like I'm looking "through" space... but on the AD10, I feel like I'm looking "at" it, or at an artist's rendering of it. I am still not seeing anything that I would label as "increased resolution" with it.


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Re: AD10 poor focus - help! new [Re: precaud]
      #5663629 - 02/05/13 02:01 PM

If you don't see astigmatism at high power then there is no need to rotate the mirror. The low power astigmatism is evidently in your own eyes.

For many years I did not use my glasses when viewing. Eventually I found that low power views were not very sharp anymore due to astigmatism. Later, even the higher power views became a problem. The solution is to use either a dioptrix (Televue astigmatism corrector) matched to one's eyesight, or eyepieces that provide long enough eye relief to allow their use with eyeglasses. I went with a partial set of Pentax XW eyepieces and have been having good success with them.

JimC


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Re: AD10 poor focus - help! new [Re: Mirzam]
      #5663709 - 02/05/13 02:49 PM

Yeah, it didn't make sense that it was not present at high mags... time to get the eyes tested, I guess.

The guy I bought the XT6 from is back in town, I'm bringing the AD10 over to his place tonight and get his input (40+ years experience). It will also give me a chance to look through his Discovery dob, Naglers, etc. ... and see what I've been missing! I'll report back after.

PS - that Unisys site's seeing forecast has been quite good so far... thanks again for that.

Edited by precaud (02/05/13 02:51 PM)


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Re: AD10 poor focus - help! new [Re: precaud]
      #5663746 - 02/05/13 03:12 PM

also maybe try a paracorr in the AD10, this might help clean up some of the Coma and maybe a little of the astigmatisim......maybe the guy you bought it from has one you can try next time you see him.

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Re: AD10 poor focus - help! new [Re: csrlice12]
      #5664985 - 02/06/13 10:58 AM

Had a fun and informative evening viewing with my "mentor" last night. Seeing was really good, the best yet this year - for the first time I saw actual symmetrical rings on star tests. He spent quite some time checking out the AD10's optics, and we did a lot of comparative viewing between it and his 8" Discovery. What a collection of Televue and other gems he has!

I won't bore you with all the details, but I left with some key takeaways/conclusions:

: My expectations for "increased resolution" from a 10" Newt were perhaps unrealistic. For the sharpness I'm looking for, I probably need to get a refractor.

: In his opinion, the AD10's optics are "good enough" - no significant TDE, astig, or other major distortions. Coma was normal. The only negative was, sharpness falls off a little sooner than expected at high megnifications (175x and above), but below that it's not bad. "About normal for a mass-produced F/5 Newt" he said.

: My collimations are good. Many thanks again to the CN collimation gurus!

: Some EP's are more finicky than others about keeping your eye well-centered on them. I was confusing some of the effects of that with astig.

: The optics on the XT6 I bought from him are indeed excellent (he said so when he sold it to me, but I had no idea what that meant at the time). From what I saw last night, it holds its own quite well against his F/7 Discovery in the detail department.

: My 5mm Celestron X-cel LX is questionable. It looked mushy in both scopes. I'm returning it for exchange.

: Beyond brand name, there is definitely "synergy" between some EP's and the scope. He had a "cheapie" 32mm Plossl that was really excellent in the AD10.

: After looking through several 30-55mm wide-angle Televues, the views are really impressive. But peering around the eyestop edge like a porthole is not my thing. I found myself preferring FOVs in the 60+ range.

Anyway, it looks like I'll be keeping the AD10 and plugging away with it. I have a little work to do in the EP department. (Budget requests will be made and denied, I'm sure...) And maybe keep my eyes open for a 5" refractor to satisfy the sharpness craving.


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Re: AD10 poor focus - help! new [Re: precaud]
      #5665455 - 02/06/13 02:58 PM

If the seeing was "good" (and you are in NM, which is about as close to viewing heaven as you can get) you should have been able to push that 10" WELL above 175X. Are you sure the seeing and transparancy were good, because it sounds like you are describing an "average" night for what your scope should do. Keep at it though, the AD10 is a great scope. And some night, the viewing really will be "good" and then you'll see what that scope can really do...

I'd still check the mirrors though for pitting, etc...it could just be its and older coating and is coming up on the time for recoating......


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Re: AD10 poor focus - help! new [Re: csrlice12]
      #5665903 - 02/06/13 07:36 PM

Quote:

If the seeing was "good" (and you are in NM, which is about as close to viewing heaven as you can get) you should have been able to push that 10" WELL above 175X.




Should have been, yes. And could... but with noticeable loss of sharpness. Splitting Castor with a 7mm Nagler with both scopes showed the AD10 sharpness starting to degrade at that point. Not totally falling apart, but definitely softening. And even more so at 5mm (250x).

Quote:

Are you sure the seeing and transparancy were good, because it sounds like you are describing an "average" night for what your scope should do.




No, I'm just saying that. I try to make a point of exaggerating actual conditions.

Quote:

Keep at it though, the AD10 is a great scope. And some night, the viewing really will be "good" and then you'll see what that scope can really do...




I appreciate your thoughts, but it was a great night of seeing, the best yet this year. And not all AD10's are created alike.

Quote:

I'd still check the mirrors though for pitting, etc...it could just be its and older coating and is coming up on the time for recoating......




You probably haven't followed this tread from the start, (can't blame ya), it's a new scope, this is the 2nd primary mirror. It's definitely better than the first one. But it appears to be an average performer.


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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]
      #5708487 - 03/02/13 10:27 AM

Here's a progress report of my ongoing wrestling match with the AD10. In many respects this really could be called a "first light" report, because the scope is, for the first time, living up to its promise. As I detailed in this thread,
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Nu...
the final battle was getting an accurate center spot and accurately collimating to it. All the ingredients are finally in place. So was it worth it?

I did some daytime viewing on Thursday afternoon and saw for the first time that the AD10 actually outperformed my XT6 at magnifications above 120x in terms of clarity and detail. I could take it right up to 277x, which is far as I can go with EP's and barlow on hand, with no sacrifice in clarity. So my hopes were up for the first view under a night sky.

Last night transparency and seeing were better than average, but not superb. A star test on Polaris showed more and better defined rings than I have ever seen. It is now looking like what the textbooks say it should. And in focus, Polaris was sharp and its double was a very clearly defined dot, a far cry from the indistinct smudge I had seen up until now.

Up into the Perseus double, the views of the clusters were superb. And what's this around the edges? Coma feathers clearly visible. Previously I was unable to distinguish between coma and any other aberration. Now its shape can be clearly seen. Again, textbook.

I won't get into a long viewing report, but I spent the next three hours exploring both new and familiar sites and loving every minute, WoW factor in abundance. Near the end of the evening Leo was high enough in the sky, and I got my first view of M65 and M66. Holy shiite. Two clearly-defined galaxies within the fov! Astounding. There aren't enough adjectives to describe how captivating it was.

So the AD10 has arrived. Finally. The struggle is over, and I can enjoy this thing.

Now my analytical part kicks in, and I would say that the degrading impact of miscollimation on an F/5 has been, if anything, understated. The fact is, the most significant difference between my previous account, where the views were OK but substandard to a good 6", and now, where they are clearly superior to it, is that the center spot has been more accurately identified and placed, and the scope collimated to it with greater accuracy. We're talking about a 1.2mm change. And the difference is not just at high magnifications: wide-field views are much more convincing and "draw me in" in a way they did not before.

So if anything, I would say that the (.005 * F/ratio cubed) PAE needs to be taken VERY seriously. Working at twice that amount was enough to degrade an otherwise-good F/5 scope into something really unpleasant to use. And without a verifiably accurate center spot, the chance of getting there is no better than a roll of the dice.

So I'm really looking forward to someone else with a fast scope to take up the challenge; use the photographic technique to verify and if necessary correct your center spot placement, and report back with results.


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