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Equipment Discussions >> Reflectors

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Tristan
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Reged: 06/07/11

Loc: NC
star test inside of focus
      #6106665 - 09/28/13 10:18 PM

Hi Everyone. So I decided to star test one of my scopes because even though everything was well collimated and at equilibrium when I turned the scope towards Jupiter this morning the view was not very inspiring. I did a quick star test and outside of focus the rings were well defined and not much turbulence was visible. However, when I moved the focuser inside of focus the star test appeared MUCH more turbulent and spikey. At moments I could see rings through the constantly moving spikes and turbulence, but it never really settled down. Any idea what might have been causing this? The scope I used for the test is a solid tube, and has a 200mm fan blowing constantly on the rear of the mirror powered by a 12v 7ah battery. I tried turning the fan off and the view was no better. Any insight is greatly appreciated. Thanks.

-Tristan


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youngamateur42
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Re: star test inside of focus new [Re: Tristan]
      #6106673 - 09/28/13 10:23 PM

All that matters is the inside focus. Even if the outside is clear, what matters is infocus. If it's not well defined, the seeing isn't good. Just wait for a better night, and Jupiter should prove VERY inspiring!

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Jason D
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Re: star test inside of focus new [Re: youngamateur42]
      #6106694 - 09/28/13 10:45 PM

Quote:

All that matters is the inside focus.




Where are the basis of your claim? Both sides matter as much.

To the OP, if your scope and mirrors are cooled down properly then most likely your primary mirror is overcorrected or/and has a TDE.
TDE sounds like a good possibility based on your description.
Didn't you refigure your mirror recently? Was it this mirror or another?
Jason


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azure1961p
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Re: star test inside of focus new [Re: Jason D]
      #6106715 - 09/28/13 11:02 PM

Jason

He makes sense in that I personally see differing thermal effects on either side of focus. I don't think one matters and the other doesn't but they do show a difference in terms if seeing and thermals within the scope.

Pete

Edited by azure1961p (09/28/13 11:07 PM)


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pstarr
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Re: star test inside of focus new [Re: Tristan]
      #6106717 - 09/28/13 11:03 PM

Over correction will have we'll defined rings outside of focus with a bright outer ring. Inside of focus will look washed out as you have described. Keep in mind, a mirror that is cooling will show over correction until it cools. Test on several nights to be sure of what you seeing and that the mirror has reached equilibrium. Turned edge will also show a similar effect but will also have a defuse glow from scattered light surrounding the disk. You may want to have the mirror tested if the problem persists.

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youngamateur42
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Re: star test inside of focus new [Re: Jason D]
      #6106746 - 09/28/13 11:20 PM

Quote:

Quote:

All that matters is the inside focus.




Where are the basis of your claim? Both sides matter as much.

To the OP, if your scope and mirrors are cooled down properly then most likely your primary mirror is overcorrected or/and have a TDE.
TDE sounds like a good possibility based on your description.
Didn't you refigure your mirror recently? Was it this mirror or another?
Jason




I had always thought that it was the most important, I stand corrected


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: youngamateur42]
      #6106773 - 09/28/13 11:42 PM

Thanks everyone. This is the newly refigured 12" mirror. I left it cooling with the fan blowing for the past 3 hours. Things were slightly improved just now when I went out, but it was still noticeably different from the outside focus which did appear to have a well defined outer ring...inside focus the outer ring appeared spikey, but the inner rings showed much more turbulence then on outside of focus. Additionally, everytime I moved the scope to re-center the star it would get all crazed looking and take a few minutes to settle,

For comparisons sake I brought my 8 out and had it setup side-by-side...both sides of focus looked very similar and it did not exhibit the same inside focus pattern the 12 did. I'm not sure if this is an equilibrium issue...I suppose it could be since the temperature dropped from mid 60s to mid 50s in 3 hrs. Perhaps the mirror could not cool fast enough to keep pace with it. Then again, could this be a boundary layer issue?

If it is a TDE, is this something that still happens with premium refigures? Would I be justified in asking the problem be corrected? Thanks again everyone.

-Tristan


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6106801 - 09/29/13 12:14 AM

Hi Paul. when you say tested, do you mean interferometry? If so, do you know of anyone in the southeast who provides this service?

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azure1961p
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Re: star test inside of focus new [Re: Tristan]
      #6106803 - 09/29/13 12:16 AM

A premium with TDE? Then its not premium. If that's what it is you have a case to make to the fabricator. Honestly though, it sounds like a simple equilibrium issue.

Pete

Edited by azure1961p (09/29/13 12:17 AM)


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christheman200
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Re: star test inside of focus new [Re: azure1961p]
      #6107000 - 09/29/13 06:54 AM

My guess is a boundary layer problem. I'm pretty certain that I'm having the same problem as you with my XX12i, albeit not so bad. I don't always have a lot of time to set the scope up/wait for it to cool so waiting for the boundary layer problems to go away is not really an option. Recently I've tried putting a fan in front of the scope and having it blow the air out, and while this has worked it seems to have resulted in causing the secondary to dew up faster. There's my input.

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pstarr
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Re: star test inside of focus new [Re: Tristan]
      #6107089 - 09/29/13 09:12 AM

Quote:

Hi Paul. when you say tested, do you mean interferometry? If so, do you know of anyone in the southeast who provides this service?




No, interferometry testing is quite expensive and not really necessary. Don't know of anyone in the southeast but there are several places around the country that will test it. It would be best to get a test from someone other than the person who refigured it before you go back to that person and say there's a problem. You could try a local club. Many have members that are quite good at evaluating mirrors. These guys will test it. tester tester tester tester tester Three of these will charge for the test, two do it free. I would use your mirror under some different conditions first to make sure it's the mirror and not a thermal problem.
I have a mirror that's on it's way to Steve Swayze right now for evaluation and possible refigure. It exhibits the same pattern you describe. It should be tested around mid week.


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Mirzam
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Re: star test inside of focus new [Re: christheman200]
      #6107092 - 09/29/13 09:18 AM

It is common for the inside focus to appear soft and spikey even in good mirrors. You need a night with exceptionally steady air to see an ideal star test.

When you are looking inside focus you are closer to focusing on the turbulent layer itself.

If you used a fan for three hours and continued to use it while observing then thermal problems with the mirror should not be the cause of what you observed.

JimC


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Eric63
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Reged: 06/16/12

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Re: star test inside of focus new [Re: Tristan]
      #6107123 - 09/29/13 09:56 AM

Tristan

I too looked at Jupiter that morning and found the view uninspiring, and the seeing was near perfect. In fact, I have had poor views of Jupiter this past month so perhaps viewing it would not be the best test of your system at this time (just a thought here because I find Jupiter so finicky right now). I turned to the moon right next to it and had the best view of it ever, keeping the magnification at 300X.

Eric


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Mirzam]
      #6107133 - 09/29/13 09:59 AM

I will continue to test the mirror for the next few observing esions. I think Im also going to try making a baffle iside the tube to direct the air from the fan to scrub acorss the surfac of the mirror and see if it makes a difference. The seeing yesterday morning was average and last night it was average to good, so maybe that was it, but if it was why would my 8" not experience the same problems? Here's the other kicker, I did not decide to pull the 8 out until a little later, and when I did it did not have a fan running and it showed a nearly matching star test both sides of focus? Perhaps its the larger mirror was chasing equilibrium for the 3 hours I had the fan on, but the thermometer showed it to be within 1-degree F of the air temperature...not sure how accurate it is though. Both scopes are very similar design solid tubes, and both are flocked. I did a ronchi test with an eyepiec tester and the ronchi tester showed what might be a slight turned edge, but it was definetly difficult to see. Im assuming the ronchi test would be affected by the same thermal issues the star test is. Thanks again for the help and feedback everyone.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Mirzam]
      #6107139 - 09/29/13 10:04 AM

Thanks Jim for sheding some light on this. What you say about the focuser being closer to the mirror and any boundary layers being present makes sense. The inside test in addition to being spikey did show much more turbulence across the whole airy disj, not just the outter ring. We'll see if the baffle to help scrub the boundary layer makes any difference.

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Eric63]
      #6107152 - 09/29/13 10:12 AM

Eric,

I did experience someting similar towards the very end of my observing session just before sun up yesterday morning. Jupiter was still looking blurry, and the moons Lo and Europa were pretty spikey (not from the spider, they were at an angle), but the moon looked really good at 507x magnification, which seemed really counterintuitive that I would be able to push it that far on the moon with good results and not on Jupiter. Perhaps if I do have boundary layer issues the effect is more pronounced on jupiter as it is farther away and is a smaller portion of the field of view...

Thanks for the feedback and helping to shed some light on this dilema.


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Tristan
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Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6107157 - 09/29/13 10:15 AM

Thanks for the links to the testers Paul. If things dont improve I'll contact one of them. I think I'm also going to go ahead and contact the optician who did the mirror refiure and see what his thought are on this.

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Tristan
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Reged: 06/07/11

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Re: star test inside of focus new [Re: Tristan]
      #6107204 - 09/29/13 10:45 AM

Here's a foucault of the mirror after refigure. I'm not an expert on interpreting these things, but it doesn't appear to show a turned edge. Here's a ronchi as well. I'm really starting to wonder if I have a boundary layer issue that is just hanging out on the surface and causing issues. Gordon has been very good to work with and my impression of him is that he is very meticulous, so I would be surprised to say the least if he did produce a TDE. That being said the fan is baffled from behind and moving in excess of 100-cfm up the tube, which I can definitely feel at the end of the tube. Not only that, it was running for 3 hours, so I would think any boundary layer would be dissipated, but then again, maybe it just sat right in the middle of the mirror and never moved...not sure.




Another thought that hasn't been broached yet- could a flawed secondary mirror cause something that appears to be overcorrection or a TDE in a star test?

Thanks again everyone for helping me get to the bottom of this!

Edited by Tristan (09/29/13 10:47 AM)


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Tristan]
      #6107225 - 09/29/13 11:01 AM

Perhaps what's happening is similar to this . However, the scope is kept in an unheated garage, and I would think would be trending closely with the air temperature already.

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Mirzam
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Re: star test inside of focus new [Re: Tristan]
      #6107293 - 09/29/13 11:39 AM

That looks like a very nice mirror to me. Smooth, zone-free and a good edge. Your problem is likely elsewhere. How good is the secondary? Although secondary problems generally manifest as astigmatism.

I find that by using a fan or fans blowing up the tube from the rear of the mirror there is no boundary layer problem. The improvement versus no fan usage is dramatic. (Talking about 10-12.5" scopes). The one time that I played around with an across the mirror surface fan on my 20" the images were made much worse.

One way to judge seeing if you are uncertain is to look at the edge of the Moon for waviness. If the edge is rock solid the seeing is very good.

Jupiter is a difficult target to see really well under most circumstances.

JimC


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

No.

JimC


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pstarr
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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

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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
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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
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Reged: 06/07/11

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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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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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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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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 new [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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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
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Reged: 06/07/11

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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
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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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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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azure1961p
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Re: star test inside of focus new [Re: Tristan]
      #6110363 - 09/30/13 11:48 PM

Quote:

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




Ahhh ok I dealt with this demon with my 8" f/9. The majority of the thermal work is handled by rear blowing fan but to complete the thorough equilibrium I also use a side blowing fan. The rear is a 5" fan the side is 3" or 80mm. Both have their own boxy 6 volt battery that seem to run forever.

Here's a hot hot tip...

Make sure your side blowing fan intercepts the side of the mirror. If its an 80mm or 100mm fan have about a half inch to a full inch of the fan aperture below the mirrors reflecting surface. That's right it's blowing on the side of the glass. What's key here is the blade wash is forced onto the boundary layer and it destroys it. It has to directly engage that air blanket.

What I did wrong the first time: I mounted my side fan so the bottom of the blades were just barely even with the mirrors reflecting surface. After all with the airstream that close it'll suck that boundary clean and blow it hither and yon.

It did nothing.

It was literally a zero result. When I dropped it down about .60 of an inch below the mirrors surface level - it was incredible. Yes there's that wasteful air that just blows on the side - but its a non issue the other air is doing the job in style.

Hot hot tip no. 2 - if you install a side blowing fan you MUST have exhaust ports opposite the airflow. It has to exit immediately. Without the exhaust ports (holes) the warm boundary air merely flows up the tube an across the light path. Without the exhaust ports the fan is just redistributing the heat. You ned to get rid of it in the shortest distance possible.

Another hot hot tip: I originally had both fans off one big 6v battery but they ran too slow. Both are rated for twelve bolts but the 6 volt battery each is perfect RPMs. I tried 12 volts - its nutty -way too high.

Final thoughts : a boundary layer is best for the lunar, planetary or doublestar observer whose bent on the last ounce of efficiency. I've found the rear fan about halves the flare and bloating while the side fan running at the same time halves that half. A deep sky observer who happens to like medium to low power could probably pass on the boundary fan but the observer if the formentioned will love it.

And finally : I learned in Connecticut a boundary fan is seasonal. In the colder months (when you can see your breath) it is terrific. The gains are good. In the heat of summer , particularly if its stored without an AC the rear fan is all that's needed and the boundary layer can be virtually invisible like it never existed. Alas the glass isn't 40 degrees or more different than the outside air so that rear fan has easy work. I imagine midway into October will necessitate the boundary fan again.

Good luck.

Pete


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Tristan
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Re: star test inside of focus new [Re: Mike Spooner]
      #6110602 - 10/01/13 05:43 AM

Hi Mike. I'll be honest, I was a bit surprised to find this was an issue as the tube is about 14" and provides close to an inch of clearance around each side of the mirror. But based on my observations that is very likely what is happening. Someday I'd like to convert this scope to a strut or truss type dob, but I think for now I'm going to try and additional fan or baffle and see if that helps the thermal issues inside the tube.

-Tristan


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Tristan
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Reged: 06/07/11

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Re: star test inside of focus new [Re: azure1961p]
      #6110605 - 10/01/13 05:48 AM

Thanks for all the suggestions Pete. I am going to try the baffle first, and if I have to will put another fan on the side with some exhaust ports. Im just not too eager to drill holes in the side of tube. I've read and heard that for some people it has worked great, others it has made no difference. However, your point about positioning of the side fan is well taken. If the baffle doesn't do the job I will definitely resort to a side mounted fan and exhaust port on the side. Out of curiosity, how do you seal up the exhaust holes when not in use? The reason I ask is because my scope lives in the garage and I like to keep the tube sealed up as much as possible to prevent dust and insects from the making their way to the mirror. Thanks.

-Tristan


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AlBoning
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Reged: 03/06/11

Re: star test inside of focus new [Re: Tristan]
      #6112522 - 10/02/13 06:43 AM Attachment (29 downloads)

On any given night of observing I almost always do at least one star test if only to check the state of collimation. Usually late in the evening to ensure that both the scope and the sky have had a chance to steady down. On those occasions when I'm serious about actually trying to quantify some aberration I make up a cheat sheet with Aberattor and bring it out to the scope so I can reference it while observing the star test.

I've attached my Aberrator Cheat Sheet for spherical aberration (SA) in a newtonian. The first pair of diffraction patterns in the upper left is a perfect mirror and the amount of undercorrection increases by 0.05 wave in each subsequent example. Specifically this is for an 8" f/6 mirror, with a CO of 25%, at 240X, using 10 wavelengths of defocus, and the SA ranges from 0.0 to 0.25 wave in 0.05 steps.

Aberrator can be downloaded from http://aberrator.astronomy.net/


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Scanning4Comets
Markus
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Re: star test inside of focus new [Re: AlBoning]
      #6114490 - 10/03/13 02:22 AM

Overcorrection.

My star test always shows:

Outside of focus: Bright outer ring, with inner rings well defined and evenly spaced.

Inside of focus: fuzzy outer ring with hairy look to it and ill defined rings, but evenly spaced. I have made a 1/4" mask for the outer part of the mirror which did nothing.

I get super crisp views of planets and pinpoint stars when the seeing allows. I use a fan on the back of my mirror which has three speeds and leave it on at all times. My scope is well collimated and flocked / modded. I stopped star testing a long time ago, except when I wan to see how my collimation is. Enjoy your mirror and have fun with your scope!


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Mirzam
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Re: star test inside of focus new [Re: Scanning4Comets]
      #6114659 - 10/03/13 07:50 AM

Last night I was working very close to the resolution limit of my 10" f/6.5 scope while splitting a 0.5" separation double star at ~450x. Seeing was quite good and the rear fan was running. Every few moments the star would lock in and show the barest thin black line between the components. The diffraction pattern during these moments was very clean. Looking at the star test the pattern was similar on either side of focus, however, the in-focus side was a bit fuzzier and seemed to be less stable. The bottom line is that I know the scope is a good one, having made and tested the mirrors myself. The performance certainly bears out the expectations from the testing. And yet, the star test, because of its extreme sensitivity to seeing, does not always indicate a perfect optical system. Remember that when you star test you are looking for that momentary stability, just like when you are observing some critical small detail under variable seeing.

I agree that when a scope is giving excellent views, don't worry about what the star test looks like.

JimC


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pstarr
Carpal Tunnel
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Re: star test inside of focus new [Re: Mirzam]
      #6114688 - 10/03/13 08:19 AM

Quote:

Last night I was working very close to the resolution limit of my 10" f/6.5 scope while splitting a 0.5" separation double star at ~450x. Seeing was quite good and the rear fan was running. Every few moments the star would lock in and show the barest thin black line between the components. The diffraction pattern during these moments was very clean. Looking at the star test the pattern was similar on either side of focus, however, the in-focus side was a bit fuzzier and seemed to be less stable. The bottom line is that I know the scope is a good one, having made and tested the mirrors myself. The performance certainly bears out the expectations from the testing. And yet, the star test, because of its extreme sensitivity to seeing, does not always indicate a perfect optical system. Remember that when you star test you are looking for that momentary stability, just like when you are observing some critical small detail under variable seeing.

I agree that when a scope is giving excellent views, don't worry about what the star test looks like.

JimC




Splitting close double stars is not really a good test of optical quality. A lesser optic may actually do better than one of higher quality. web page


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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: pstarr]
      #6115108 - 10/03/13 12:51 PM

Thanks for posting this Paul. But I think the author may have miscommunicated something. In once sentence he states that a large central obstruction can decrease the size of the airy disk making it easier to resolve close doubles. My understanding is that if you have a lesser quality optic that more of the light is pushed out of the central portion of the airy disk which would cause it to bloat, right? If that was the case a lesser quality scope with the same size central obstruction, focal, length, and aperture should not be able to better resolve close doubles than a higher quality optic, correct?

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Tristan
sage


Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: Scanning4Comets]
      #6115117 - 10/03/13 12:54 PM

Thanks for the posts Jim and Marcus. I am starting to think after my experiment the other night that I am obsessing too much about atmospheric and local temperature issue too much. I plan on trying a baffle ring inside the scope to redirect airflow over the primary and will try again on Jupiter if seeing is good Saturday morning. I'll let you know how it goes. Your points about seeing being too finicky to get a reliable star test are well taken.

-Tristan


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Tristan
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Reged: 06/07/11

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Re: star test inside of focus new [Re: Tristan]
      #6118578 - 10/05/13 07:35 AM

OK- So I got up at 5 AM this morning after installing the internal baffle last night, setup the scopes let the fan run for about 30-45 mins and the mirror showed the same temp as ambient at 60.8 F. When I turned the mirror on Jupiter I was greeted with the best view I have ever had! It was like looking at a 3D picture, it was noticeably spherical with hundreds of tiny tendril like cloud bands. I was able to count 7 larger dark bands on the surface, and saw MANY more small thin ones but could not count them as the planet was travelling through the eyepiece a fast clip (507x). Being very pleased with the view I went ahead and did some star testing. The pattern, while showing less contrast inside of focuse matched VERY closely both sides of focus. My only complaint is that the fan causes noticeable vibration at the eyepiece now with the baffle installed....even at low RPM. I think its because the air coming around the mirror cell is turblent and it smacks into the baffle causing the tube to vibrate...a lot. So, I have to run it for awhile, then I can switch it off and observe.

At the end of the day the views are MUCH improved and what I expected from this mirror. Was the problem solved ENTIRELY by the baffle? I dont think so. I think the baffled made a significant contribution to what I saw, but the seeing was also nticeably better thismorning as well.

Thanks again to everyone who helped me troubleshoot this issue. The scope performed beautifully this morning and I cant wait until my next observing session!


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Mirzam
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Re: star test inside of focus new [Re: Tristan]
      #6118597 - 10/05/13 08:05 AM

Glad to hear about your observing results!

You may want to try another type of fan. I use one called a noiseblocker. It does not move a whole lot of air but is vibration free.

Noiseblocker fan

JimC


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precaud
Pooh-Bah
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Re: star test inside of focus new [Re: Mirzam]
      #6118749 - 10/05/13 10:24 AM

Congrats on the results, Tristan! Success in troubleshooting is VERY satisfying.

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Tristan
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Reged: 06/07/11

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Re: star test inside of focus new [Re: precaud]
      #6119259 - 10/05/13 04:15 PM

Thanks John. That was one of the better observing sessions Ive had in quite awhile, If I wasnt so exhausted I'd get up early again tomorrow and try it. I need to sleep in at least one day this week.

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Tristan
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Re: star test inside of focus new [Re: Mirzam]
      #6119591 - 10/05/13 08:32 PM

Thanks for the suggestion Jim. I've never seen these fans before. they look like they are very well made.

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precaud
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Re: star test inside of focus new [Re: Tristan]
      #6122274 - 10/07/13 11:59 AM

Just thinking about this trapped "boundary layer" bubble of warm air. If it is actually warmer than the surrounding air, it will rise unless something is there to stop it. So what is stopping it?

Similarly, methinks one would not need a ring around the entire mirror, just one on the bottom say 1/3.


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Starman1
Vendor (EyepiecesEtc.com)
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Re: star test inside of focus new [Re: precaud]
      #6123329 - 10/07/13 06:35 PM

Quote:

Just thinking about this trapped "boundary layer" bubble of warm air. If it is actually warmer than the surrounding air, it will rise unless something is there to stop it. So what is stopping it?

Similarly, methinks one would not need a ring around the entire mirror, just one on the bottom say 1/3.




Conduction: air is a poor conductor, so warm air can sit next to cooler air and take a while to equilibrate.

Convection: it doesn't take much to blow away the boundary layer. I've found that simply having some air blow up toward the top of the tube by moving from the back and around the edge of the mirror is often sufficient.
It creates a circular vortex which, because it's moving (and therefore at lower pressure), moves the air from the center of the tube to its edge and keeps it out of the light path as it proceeds up the tube (assuming you have sufficient clearance here, of course).

Movement of warm air up and away from the mirror: this occurs, of course, but slowly, and if the mirror is several degrees warmer than the ambient air, too slowly to dissipate the layer of warm air in front of the glass. This warm layer dissipates when the mirror is no longer losing heat, which occurs as the mirror nears ambient (say <2 degrees F). If the mirror starts out 20F warmer than the ambient (common), simple dissipation can take hours.


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Pinbout
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Re: star test inside of focus new [Re: precaud]
      #6123490 - 10/07/13 07:53 PM

Quote:

If it is actually warmer than the surrounding air, it will rise unless something is there to stop it.




in the s&t article with bryan greer's pics from awhile back he demonstrated the boundry layer was at its worst when the mirror's face was pointed straight up and less worse when angled like 45 but still not so good alot.


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Mirzam
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Re: star test inside of focus new [Re: Pinbout]
      #6123518 - 10/07/13 08:09 PM

I think that Don's description above is quite to the point. If a mirror is very warm you will not be able to get rid of the boundary layer. Indeed the mirror will cause gross "tube currents" until it more nearly approaches thermal equilibrium.

After a mirror has cooled for an hour or so, there is still a boundary layer because the mirror never perfectly reaches ambient temps. (Well, I suppose it is theoretically possible if the ambient temperatures are rising). This thermal mass effect is actually helpful for dew control. But importantly, at this stage it takes only a very small amount of airflow to disrupt the boundary layer that is there. I've used fans of different strengths--some that are quite strong and some that provide just a little poof of air. The weak ones still get the job done and are less prone to causing vibration.

JimC


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azure1961p
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Re: star test inside of focus new [Re: Mirzam]
      #6123570 - 10/07/13 08:34 PM

Even with a boundary layer fans and a rear surface blowing fan I can still see residual thermal effects in the light path - GREATLY reduced but a trace evident. I shrug we don't live in a vacuum afterall. One freaky night this past summer though - having had the scope in an unairconditioned condo as I worked all day - then taking it out into the hot damp summer air I was STUNNED to find both sides of focus literally crystalline. I wasn't even running the boundary fan - why mess it up???? It just turned out to be perfect glass temp for air temp and - oh man . It was uncanny. Even weird.

Split Epsilon Lyrae at 70x!!!! Sometimes the thermal gods smile.

Pete


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garret
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Reged: 07/26/09

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Re: star test inside of focus new [Re: Pinbout]
      #6134547 - 10/13/13 09:18 AM

Quote:

in the s&t article with bryan greer's pics from awhile back he demonstrated the boundry layer was at its worst when the mirror's face was pointed straight up and less worse when angled like 45 but still not so good alot.




You find this article in S&T september 2000.
According to this article, a steady boundery heat layer above the mirror is not a huge problem, a turbulent boundary layer is a real problem what you must lose by a vent or a fan.
And, fast moving spikes in a star test is seeing effect, a slow moving or steady spike in a star test is heat above the mirror (the boundary layer), or heat in the tube.


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Tristan
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Reged: 06/07/11

Loc: NC
Re: star test inside of focus new [Re: garret]
      #6137812 - 10/14/13 08:36 PM

Thanks Garrett. I think that was my issue is that I just had a bubbling boundary layer rolling around on the primary. Now that I have added the internal baffle to disrupt the layer my views have been noticeably improved!

Now if the clouds would just clear off so I can do some more observing!

-Tristan


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