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AD10 poor focus - help!

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#76 howard929

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Posted 30 January 2013 - 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.

#77 Vic Menard

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Posted 30 January 2013 - 10:52 AM

...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...

#78 precaud

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Posted 30 January 2013 - 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.

#79 Vic Menard

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Posted 30 January 2013 - 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...

#80 precaud

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Posted 30 January 2013 - 01:21 PM

Hi Vic, thanks for your interest.

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?

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...

#81 Mirzam

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Posted 30 January 2013 - 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

#82 Vic Menard

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Posted 30 January 2013 - 06:37 PM

...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.

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.

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.

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.

#83 precaud

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Posted 31 January 2013 - 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?

#84 precaud

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Posted 31 January 2013 - 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.

#85 Mirzam

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Posted 31 January 2013 - 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

#86 precaud

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Posted 31 January 2013 - 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!

#87 precaud

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Posted 31 January 2013 - 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.

#88 precaud

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Posted 05 February 2013 - 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.


#89 Mirzam

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Posted 05 February 2013 - 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

#90 Mirzam

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Posted 05 February 2013 - 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

#91 precaud

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Posted 05 February 2013 - 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.

#92 Mirzam

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Posted 05 February 2013 - 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

#93 precaud

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Posted 05 February 2013 - 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.

#94 csrlice12

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Posted 05 February 2013 - 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.

#95 precaud

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Posted 06 February 2013 - 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! :bow:

: 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.

#96 csrlice12

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Posted 06 February 2013 - 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......

#97 precaud

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Posted 06 February 2013 - 07:36 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.


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).

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. :confused:

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.

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.

#98 precaud

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Posted 02 March 2013 - 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.cloudynig...eflectors/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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