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Boundary layer vs tube currents vs atmosphere, vs...?

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#1 bokemon

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Posted 20 October 2020 - 07:44 AM

Hello folks,

I started using my Vixen R200ss telescope (760mm, F3.8) a few days ago and I am having trouble with seeing-related star blurring.  Here's what I see using my camera's live view mode (20-50 FPS):

  • In-focus star: besides the diffraction lines from spider, there are little radial spikes that dance around.  The entire star moves around a few pixels worth with both a high-frequency and low frequency component.
  • Bahtinov mask: I can see the center line move back and forth, which means the focus is changing quickly
  • De-focused image with secondary vanes clearly visible: When I wave my hand in front, I can see the heat waves, so I sort of have an idea what they look like.  The image has brightness fluctuations that quickly move around, a more overall shimmering distortion like it is under water, and finally there is an overall translation of the image.

I can't get near anything good enough to see the airy rings, but the spacing for those is not much more than my pixel size anyway, so...

 

I installed two fans on this telescope: One is about an inch above the primary mirror, and small enough to just hide inside the shadow of the secondary (I think...).  So this is supposed to "scrub" the boundary layer off the primary, but all the above mentioned effects are still present after 2+ hours.  Maybe a little better, but hard to tell.

There is another fan that is mounted to the back of the telescope, blowing air in.  There is only a narrow annular gap between the frame where the alignment screws are, and a thin sheet metal plate.  So in short, air can only go in with some resistance and does not hit the back side of the primary, but only creeps up the sides.  The airflow is only enough to lift a sheet of newspaper 1/2" off the front of the scope.  Should I drill some holes in this plate?

The fans didn't seem to cure the problem.  The only obvious difference is that the defocused heat waves now move quickly instead of slowly.

 

How can I tell what is the cause of the star movement?  Whether it is still due to primary mirror boundary layer, or tube currents, or heat wave directly in front of the telescope (I have a cooled CMOS camera there shooting out warm air, but I also temporarily put a dew shield / hood on the telescope with no difference), or maybe rising heat from the concrete the mount + tripod are on, or maybe just something high up in the air.  I suppose I can also compare with my 600mm camera lens, but I don't wanna have to dissemble the imaging train to put it back on the lens.  The images from the lens seemed to have smaller stars anyway.



#2 MalVeauX

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Posted 20 October 2020 - 08:02 AM

Hi,

 

Were you able to record any of it?

 

I think your issue may have been the atmoshperic seeing and/or local seeing conditions (environmental) and not just heat of the mirror itself or getting into the real minutia of air flow on such a small instrument's mirror before worrying too much, especially after a 2 hour acclimation. So my guess is that it was seeing conditions. Make sure you're not setup on pavement, stones, or anything that holds and radiates heat. Make sure you're not looking over a hot roof or hot road or something that also radiates heat. Setup on substrates that do not radiate heat readily like an oven and looking over sky areas that are not going through walls of radiating heat from hot surfaces on the ground. That's your "local" seeing that you need to eliminate and consider. Then worry about atmospheric seeing. It also helps to have your mirror higher up off the ground to avoid radiative heat there too. Cali sun can heat up sand, stone, rock, road, etc, to boiling under direct exposure. You don't want your mirror above that stuff, it will just wreck your local seeing.

 

Here's an airy disc on my 200mm F6 newtonian for example (the star is Arcturus). You can see heat radiation, but the important thing is the concentric rings.

 

Were you able to see something like this?

 

200mmF6_AiryDisc_Collimated_Arcturus_05242019.gif

 

Very best,


Edited by MalVeauX, 20 October 2020 - 08:03 AM.

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#3 Eddgie

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Posted 20 October 2020 - 08:14 AM

The boundary layer is on the face of the primary mirror so the boundary layer fans would have to blow across the face of the primary.  

 

Now this is very hard to accomplish without cutting holes in the side of the OTA.

 

As to how to check what you are seeing, it is not always easy to know.  The theory on seeing is that convection cells. Since the light coming from the sky would pass through numerous cells, the effect would be to see an irregular size and shape pulsing on an out of focus star.

 

Boundary layer disturbance would typically be characterized by wavering and rising smoke like lines on the out of focus image. It would be like smoke rising from fire and move in kind of slow current patterns. 

 

Also, you want to do the test on a bright star lower in the sky.  These are heat currents flowing across the primary and since heat rises, if the scope is pointed lower in the sky, you will see the thermals tend to move more or less in the direction from the bottom of the primary to the top of the primary though you may see curls in the currents as they flow.  But that is the key signature of boundary layer thermals... They look like slow, smokey whirls and eddys on the pattern. 

 

Here is a good article on thermal management. You would be looking for something in the pattern as shown in the three orange circles.  It also goes in to the an effective way to place the fans.

 

https://skyandtelesc...NewtThermal.pdf


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#4 aa5te

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Posted 20 October 2020 - 08:41 AM

On my Hardin Optical 10" dob, I effectively sealed the bottom, behind the primary, and installed a $2 battery powered fan (Walmart closeout, runs on 2 D cells) that blows across the mirror back, up the sides, and then across the mirror. I got it to blow across the mirror by putting 1/2" or 3/4" (can't remember which) weather stripping foam about 1/2" above the primary mirror, which redirects the air flow out towards the center of the tube across the mirror. This effectively scrubs away the boundary layer on the primary and keeps the secondary from dewing up, and I notice no vibrations.

 

What I did notice from this is that the secondary seldom dews any more, and that my views went from being blurry/non-sharp on planets for 30 minutes to an hour after hauling the scope out to being able to achieve similar views within 5 minutes or so. If I turn the fan off for a while after thermal equilibrium (hours of being outside), wait a bit (30 minutes or so), then turn it back on, even for a few seconds, I can see an almost immediate improvement in the views.


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#5 TOMDEY

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Posted 20 October 2020 - 08:47 AM

Did you give your scope plenty of open-air time to stabilize? Other than that, above ground even five or ten feet usually helps hugely. And it's also location, location, location. Fans also generate their own heat, Catch-22. Ideally, you want a scope, observatory and location... all of which provide stable wavefront. All the fans in the world won't make up for those biggest drivers. Otherwise, it's like trying to filter out light pollution, whereas porting to darker skies is the best solution.    Tom


Edited by TOMDEY, 20 October 2020 - 09:26 AM.

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#6 bokemon

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Posted 20 October 2020 - 10:09 AM

Hi,

 

Were you able to record any of it?

 

I think your issue may have been the atmoshperic seeing and/or local seeing conditions (environmental) and not just heat of the mirror itself or getting into the real minutia of air flow on such a small instrument's mirror before worrying too much, especially after a 2 hour acclimation. So my guess is that it was seeing conditions. Make sure you're not setup on pavement, stones, or anything that holds and radiates heat. Make sure you're not looking over a hot roof or hot road or something that also radiates heat. Setup on substrates that do not radiate heat readily like an oven and looking over sky areas that are not going through walls of radiating heat from hot surfaces on the ground. That's your "local" seeing that you need to eliminate and consider. Then worry about atmospheric seeing. It also helps to have your mirror higher up off the ground to avoid radiative heat there too. Cali sun can heat up sand, stone, rock, road, etc, to boiling under direct exposure. You don't want your mirror above that stuff, it will just wreck your local seeing.

 

Here's an airy disc on my 200mm F6 newtonian for example (the star is Arcturus). You can see heat radiation, but the important thing is the concentric rings.

 

Were you able to see something like this?

 

attachicon.gif200mmF6_AiryDisc_Collimated_Arcturus_05242019.gif

 

Very best,

Yes, I have a video of it.  It looks a lot like yours, except that there is more overall image translation.

Unfortunately the telescope is set up on my driveway, on pavement, since that is my testing location.  I typically do imaging from midnight to 6 am.  If the driveway is still warm, can I cover it with some kind of a blanket in front of the scope?  I think I might really have to test out my lens again to see if it has the same jiggle, since it was set up in the exact same spot.  But I don't remember it had this same amount of "bad seeing".  BTW, the cleardarksky site says my seeing was supposed to be "good".

 

The boundary layer is on the face of the primary mirror so the boundary layer fans would have to blow across the face of the primary.  


Boundary layer disturbance would typically be characterized by wavering and rising smoke like lines on the out of focus image. It would be like smoke rising from fire and move in kind of slow current patterns. 

 

Also, you want to do the test on a bright star lower in the sky.  These are heat currents flowing across the primary and since heat rises, if the scope is pointed lower in the sky, you will see the thermals tend to move more or less in the direction from the bottom of the primary to the top of the primary though you may see curls in the currents as they flow.  But that is the key signature of boundary layer thermals... They look like slow, smokey whirls and eddys on the pattern. 

 

I think having the fan blow straight at the center of the primary should be ok since the air hits the mirror and spreads outwards in a radially symmetric pattern.  I checked with a fan against a plate and a small piece of tissue paper that this config does cause air to flow across the surface.  Also, if I turn off both fans, then I get the more slow swirling you describe.

 

Did you give your scope plenty of open-air time to stabilize? Other than that, above ground even five or ten feet usually helps hugely. And it's also location, location, location. Fans also generate their own heat, Catch-22. Ideally, you want a scope, observatory and location... all of which provide stable wavefront. All the fans in the world won't make up for those biggest drivers. Otherwise, it's like trying to filter out light pollution, whereas porting to darker skies is the best solution.    Tom

I typically do imaging from midnight to 6 am.  Currently I am using the tripod with the legs retracted for "more stability", but I guess I can try extending them all the way.  My driveway is my "testing spot" that's where I'll be until I get a bunch of other issues sorted out.
 



#7 TOMDEY

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Posted 20 October 2020 - 10:38 AM

Yep. I guess you already know that blacktop and concrete are notorious for thermals? Just scooting over to the grass might help.    Tom



#8 MalVeauX

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Posted 20 October 2020 - 11:01 AM

Setting up in your drive way, it may not seem warm, but that stone is holding heat and it lets it go all night long. If your mirror is not significantly high up above it so that the heat moves with wind, it will rise into your OTA and cause local seeing to degrade. Think about how you look down the road and the air above the road is shimmering and moving like water. That's happening, to a lesser degree, under your scope. Some independent measurements show that it takes about 3 feet above the ground to see a significant difference. So if your mirror is higher up than that, it will help. If it's less than that, it will be effected and take your local seeing conditions down.

 

Setup on soil/grass. Spray down the area with water to cool it down in advance. Evaporation carries heat with it rapidly. If you simply cannot do anything other than setup on pavement, soak it with water a while before you try to setup on it to cool it down. Setup a fan near by your setup to blow surface heat away from your imaging location as it rises from the stone (think gentle breeze). If you need more help with this, a wood sheet like some ply and/or some reflectix material are good heat barriers that you can put under your setup so that it acts as a thermal barrier to rising heat plumes.

 

Very best,


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#9 Asbytec

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Posted 21 October 2020 - 12:32 AM

Hi,

<snip>

 

Here's an airy disc on my 200mm F6 newtonian for example (the star is Arcturus). You can see heat radiation, but the important thing is the concentric rings.

 

Were you able to see something like this?

 

attachicon.gif200mmF6_AiryDisc_Collimated_Arcturus_05242019.gif

 

Very best,

In you image, I see high speed atmospheric effects racing across the image. We can also see some slow moving bright caustic lines "dancing" across the surface (offset by darker patches) suggesting some thermal boundary layer still exists. Also some slow moving thermal distortion rising across the defocused disc are probably thermal in nature. I believe the primary cause of seeing spikes the OP sees dancing beyond the perimeter of the diffraction artifact is a thermal stability phenomenon. I see them when my scope is cooling. 


Edited by Asbytec, 21 October 2020 - 12:33 AM.


#10 Oberon

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Posted 21 October 2020 - 04:13 AM

Basically you have to think of seeing as a bundle of, say, 10 variables, all of which have to be great to get great images, And worse, you only get to control or influence a few of them. Like mirror temp (influence). Telescope location (?). Tube currents (control). There are all hard but somewhat doable. Unlike professionals, you also get to choose the nights you observe, aka, select the weather  conditions you subject yourself to, because you have absolutely zilch control over the atmosphere, and it has many layers of tricks up its sleeve to frustrate your ambitions.
 

But when it all lines up....when all comes good...thats when lifetime memories happen.



#11 Asbytec

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Posted 21 October 2020 - 05:54 AM

 

But when it all lines up....when all comes good...that's when lifetime memories happen.

That's a fact. 



#12 Eddgie

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Posted 21 October 2020 - 08:05 AM

 

 

I think having the fan blow straight at the center of the primary should be ok since the air hits the mirror and spreads outwards in a radially symmetric pattern.  I checked with a fan against a plate and a small piece of tissue paper that this config does cause air to flow across the surface.  Also, if I turn off both fans, then I get the more slow swirling you describe.

 

 

Well, if you are confident that your fan is effective, them it must be seeing. Still, I think your location is not optimal. If you noted in the article above, the fan was on one side of the OTA and there was a series of vent holes on the other. The idea is to scrub the currents off and push them out of the tube. Even if you are blowing the disturbed air radially out and it does not have anywhere to go, it is just going to curl around back over the front of the primary.  Have you ever seen images of wingtip vortices where the air curls around? I would think that your fan at the center of the primary would do more or less the same thing.  But hey, if you think the location is good, I won't argue the point further. 

 

 

 

If you trust your design then it is probably seeing and perhaps local air currents external to your scope. Heat rising from the ground or rooftops can cause plume like effects.

 

Whatever, I hope you are able to sort it out.  Effective thermal management can offer greatly improved performance. 



#13 bokemon

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Posted 21 October 2020 - 09:10 AM

This is why I asked if I should drill out the back plate so that the rear fan will have more flow and push this air out.  It already does to a small extent, but I am not sure if it is enough.



#14 bokemon

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Posted 21 October 2020 - 09:11 AM

Also, I am trying to get my videos converted over into a small gif so I can post it.



#15 TOMDEY

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Posted 22 October 2020 - 02:39 AM

Just recalled... I assume most of you have done this. Get on a bright star and cut a knife or similar into it so you have a Schlieren display of your pupil? That way you actually see the wavefront (in phase contrast), which is profoundly telling. What I use is an opaque dot about the size of the Airy Disc in a thin transparent window. I automated that at work for pass-fail on optical systems. That would be a great tool for empirically optimizing your blowers, suckers, etc. etc. What you see is what you get... independent of what the theory is telling you it should be. Brute force, and the fastest way to optimum build and field deployment.

 

Ideally, you build this into your field-deployed telescope... so you can just press a button and evaluate the operational wavefront upon demand. And a minute later you're right back into imaging! The greater context is what I called Quantitative Knife Edge (QKE). The entire device is about the size of a... pill bottle, smallest size pill bottle.  Tom

 

~click on~ >>>

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