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Equipment Discussions >> Reflectors

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precaud
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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! new [Re: 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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 04/01/08

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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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 01/02/11

Loc: Low End of High Ground
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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Loc: Lovettsville, VA
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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 12/05/12

Loc: north central New Mexico
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
It's All Good
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Reged: 01/20/08

Loc: Markham Ontario Canada
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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Reged: 01/02/11

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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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 01/02/11

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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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Reged: 12/05/12

Loc: north central New Mexico
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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Reged: 01/02/11

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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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Reged: 12/22/08

Loc: Grand Isle, VT
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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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! [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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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! [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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Reged: 12/05/12

Loc: north central New Mexico
Re: AD10 poor focus - help! [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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