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Equipment Discussions >> Reflectors

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Tristan
sage


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
Carpal Tunnel
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Reged: 11/21/12

Loc: La Verne, CA
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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Reged: 10/21/06

Loc: California
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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Reged: 01/17/09

Loc: USA
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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Reged: 09/17/04

Loc: NE Ohio
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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Reged: 11/21/12

Loc: La Verne, CA
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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Reged: 01/17/09

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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
sage


Reged: 06/12/13

Loc: Toronto, Canada
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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Reged: 09/17/04

Loc: NE Ohio
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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Reged: 04/01/08

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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
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
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
sage


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
sage


Reged: 06/07/11

Loc: NC
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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Reged: 04/01/08

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Re: star test inside of focus [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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