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AD10 poor focus - help!

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#1 precaud

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Posted 20 January 2013 - 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.

#2 Jon Isaacs

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Posted 20 January 2013 - 10:41 AM

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

#3 precaud

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Posted 20 January 2013 - 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?

#4 precaud

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Posted 20 January 2013 - 12:25 PM

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

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#5 precaud

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Posted 20 January 2013 - 12:55 PM

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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#6 precaud

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Posted 20 January 2013 - 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?

#7 Jon Isaacs

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Posted 20 January 2013 - 02:38 PM

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

#8 precaud

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Posted 20 January 2013 - 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.

#9 precaud

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Posted 20 January 2013 - 04:16 PM

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

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#10 precaud

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Posted 20 January 2013 - 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.

#11 coopman

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Posted 20 January 2013 - 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.

#12 howard929

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Posted 20 January 2013 - 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?

#13 precaud

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Posted 20 January 2013 - 10:53 PM

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.

#14 pftarch

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Posted 20 January 2013 - 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?

#15 howard929

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Posted 20 January 2013 - 11:21 PM

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?

#16 precaud

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Posted 20 January 2013 - 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.

#17 pftarch

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Posted 21 January 2013 - 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.

#18 precaud

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Posted 21 January 2013 - 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...

#19 howard929

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Posted 21 January 2013 - 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.

#20 Mirzam

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Posted 21 January 2013 - 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?

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#21 precaud

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Posted 21 January 2013 - 01:05 PM

howard, which thread is that?

edit - never mind, I found it...

#22 precaud

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Posted 21 January 2013 - 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.

#23 precaud

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Posted 21 January 2013 - 02:37 PM

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

#24 Mirzam

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Posted 21 January 2013 - 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

#25 precaud

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Posted 21 January 2013 - 05:11 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.


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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