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Equipment Discussions >> Reflectors

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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: star test inside of focus new [Re: Tristan]
      #6107451 - 09/29/13 01:01 PM

This series of tests may interest you. test There is a short movie of the test that you can play.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6108051 - 09/29/13 07:21 PM

OK, so here is another thought- I believe when the foucault and ronchi images above were taken the coating had not yet been applied. Is it possible the application of the coatings could have altered the figure to produce the patterns Im seeing when star testing?

Edited by Tristan (09/29/13 08:28 PM)


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Mirzam
Post Laureate
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: star test inside of focus new [Re: Tristan]
      #6108083 - 09/29/13 07:48 PM

No.

JimC


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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: star test inside of focus new [Re: Tristan]
      #6108160 - 09/29/13 08:33 PM

Yes, it's possible. It has happened to some Zambuto mirrors. He has a photo of one such mirror on his mirror forum. Does your mirror have an enhanced coating?, If so, there would more of a chance of it changing the figure. You could send it back to whoever refigured it and have it checked. Ring caused by bad coating You can advance the pictures using the arrow on the far right. Note the roughness in the last picture. All in the coating.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6108240 - 09/29/13 09:32 PM

Hi Paul. Can you try reposting the link? When I click it I get 400 bad request message. Thanks.

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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: star test inside of focus new [Re: Tristan]
      #6108250 - 09/29/13 09:38 PM

The link works on my computer and iPad. You may have to join the Zambuto mirror group to view it.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6108308 - 09/29/13 10:09 PM

OK, I finally got in to view it. Does make me wonder...I did not see a description of what was going on, but the image entitled tricolor is showing a similar purplish hue to what I've seen on my mirror from an angle at the edges. The hue is not visible straight on, but is visible at an angle. I believe the coatings that were applied are standard and not enhanced. Thanks again for posting the link.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Tristan]
      #6108318 - 09/29/13 10:15 PM

You know, thinking about it now, I wonder if the cell is to blame...the cell is the standard GSO 9-point cell. It seems to me I could test this by inserting a rigid plate of some kind between the mirror and the cell and see if the test improves. The idea being if the load is transferred uniformly to the plate instead of to the points of the mirror cell it would be better supported...maybe this is not the case, but as I think about it starts to make a little sense. If the cell isnt providing adequate edge support, I think it could cause the kind of results I am seeing. Does anyone else have any experience with a mirror cell causing issues with a mirror's figure?

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: star test inside of focus new [Re: Tristan]
      #6108512 - 09/30/13 12:35 AM

you can set up your own foucault test, not to take measurements, just to see the surface.

also mask off the outer 1/2" and see if your star test changes.

both simple tests.


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azure1961p
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Reged: 01/17/09

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Re: star test inside of focus new [Re: Pinbout]
      #6108521 - 09/30/13 12:48 AM

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete


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AlBoning
professor emeritus


Reged: 03/06/11

Re: star test inside of focus new [Re: azure1961p]
      #6108547 - 09/30/13 01:11 AM

I have no idea if doing this has any diagnostic value but it is interesting.

I created an aperture mask for my AD8 that converted it from being an obstructed 197 mm f/6.2 newt to an unobstructed 60 mm f/20 newt. The star test required using a much brighter star and it took a couple additional turns of the fine focus knob to get the same amount of defocus. The star test was stunning, near perfect, near identical sets of diffraction rings on both sides of focus. A refractor would have been envious with green CA.


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pstarr
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Reged: 09/17/04

Loc: NE Ohio
Re: star test inside of focus new [Re: azure1961p]
      #6108766 - 09/30/13 07:38 AM

Quote:

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete




Enhanced coatings are fine. I'm sure yours is. They are just more inclined to produce a clunker every great once in awhile. They have more layers then a standard coating.


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Starman81
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Re: star test inside of focus [Re: Tristan]
      #6109126 - 09/30/13 11:45 AM

Quote:

OK, I finally got in to view it. Does make me wonder...I did not see a description of what was going on, but the image entitled tricolor is showing a similar purplish hue to what I've seen on my mirror from an angle at the edges. The hue is not visible straight on, but is visible at an angle. I believe the coatings that were applied are standard and not enhanced. Thanks again for posting the link.




Quote:

You know, thinking about it now, I wonder if the cell is to blame...the cell is the standard GSO 9-point cell. It seems to me I could test this by inserting a rigid plate of some kind between the mirror and the cell and see if the test improves. The idea being if the load is transferred uniformly to the plate instead of to the points of the mirror cell it would be better supported...maybe this is not the case, but as I think about it starts to make a little sense. If the cell isnt providing adequate edge support, I think it could cause the kind of results I am seeing. Does anyone else have any experience with a mirror cell causing issues with a mirror's figure?




Chiming in before the experts do here... The tricolor is an artifact of coating that I think happens with Majestic Coatings coated mirrors (and maybe some others). I am guessing yours was coated by Majestic. AFAIK, it is only visible at very shallow angles and doesn't have any impact on optical performance.

On the second point, I would think the 9-point cell is far superior than just a 1-point back plate like you describe. I am new to PLOP but I ran it with your scope/mirror combo (assuming 1.5" mirror thickness and 2.1" secondary) and the 9-point came out looking MUCH better than the 3-point cell (no option for a 1-point cell, which would do much worse I think than the 3-point).


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Starman81]
      #6109733 - 09/30/13 06:04 PM

Thanks again everyone. So, based on the responses I am getting if the figure appears good in Foucault, and the mirror cell isn't the culprit, and the coatings are assumed to be OK, the are most of you thinking this is an issue related to seeing more than anything else?

-Tristan


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: star test inside of focus new [Re: Tristan]
      #6109819 - 09/30/13 06:59 PM

I think seeing is the most likely explanation, although to be clear, I cannot assess the correction by eyeballing the foucault and the ronchi images. It looks about right but you really need a quantitative measurement (or an autocollimator) to be sure. Still Gordon Waite knows what he is doing and I think your mirror is probably fine.

Problems with the cell typically would cause astigmatism, not an apparent error in correction or turned down edge.

JimC


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Starman81]
      #6109873 - 09/30/13 07:47 PM

Thanks Starman. The coatings were from Majestic. As for the mirror, I guess I was thinking if the mirror was uniformly supported and in continuous contact it would be better than having individual points of stress...then again if that were really true we would probably see aluminum platters with heatsinks cast right in to dissipate the heat from the mirror and help it equilibrate faster.

-Tristan


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Tristan]
      #6109878 - 09/30/13 07:48 PM

As an update- the scope has been out cooling in the shade since about 6:30 with the fan running full blast. Seeing is supposed to be better tonight so I will give it another shot and see what happens. I'll report on my findings a little later- crossing my fingers its just seeing or a boundary layer issue.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Tristan]
      #6110171 - 09/30/13 10:18 PM

OK, so I just wrapped up some more observing. Seeing was about good. I did make another change to the scope though for tonight's session- I added some foam weather stripping over the temperature probe for the thermometer on the mirror so the fan was not blowing on it directly. the night started out without a promising outlook, as the inside test continued to look spikey and MUCH more turbulent than outside of focus where rings were well defined and clear with little turbulence. Towards the 9:30 the mirror was showing within 0.2-degrees F of the outside air temperature and occasionally I would get glimpses of some rings. I started thinking if there was a boundary layer trapped in the middle of the mirror what could I do to disrupt it? I found the answer, a can of compressed air. I shot some air down the tube from the front and watched the star test spiral about with the air currents, as it settled the inside focus patter began to clear, and resemble more of the outside focus pattern. I did it a few more times and things continued to improve. the rings were still not as bright or distinct as outside focus, but they were definitely there, and not nearly as spikey as previous or as turbulent. I am really starting to think this is a boundary layer issue coupled with seeing.

I think I am going to create a circular baffle out of black foam core board and located it just above the mirror clips so the air that would be moving up the side of the tubes is instead directed across the front and see if that improves the images farther...If seeing permits I will attempt this coming weekend and report my results.

Thanks again everyone for helping me to troubleshoot this issue, I really appreciate it!

-Tristan


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Mike Spooner
Vendor (mirrors)


Reged: 08/06/10

Re: star test inside of focus new [Re: Tristan]
      #6110261 - 09/30/13 11:01 PM

How tight is the tube around the mirror? I once had a 6" mirror in a 7" tube and it looked like it had a turned edge until I pointed it straight up so the cool air would fall evenly on the mirror (and the heat could escape). I would point it at a star at 45 degrees and in less than a minute or two the star test would look like TDE (soft spiky inside and sharp outside). I put it in an 8" tube and rarely have a problem pushing 500x in good seeing. I suspect it was a boundary layer or stratification along the tube when it was used at an angle and the larger tube was enough to keep it from impinging the kight path.
--Mike Spooner


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: star test inside of focus new [Re: pstarr]
      #6110320 - 09/30/13 11:28 PM

Quote:

Quote:

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete




Enhanced coatings are fine. I'm sure yours is. They are just more inclined to produce a clunker every great once in awhile. They have more layers then a standard coating.




If imagine the larger thin mirrors might be at risk here. I've read dielectric coatings actually squeeze the glass like a vice hence the reason only small pieces of glass tolerate it well like diagonals.

Pete


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