Tube Currents: Cause and Effect
Cause of Tube Currents Aberrator Simulated Effects of Tube Currents on Star Test Aberrator Effects of Tube Currents on Stars Aberrator Simulated Effects on Doubles Aberrator Effects on Saturn Effects on Jupiter Reducing Cool Down Times An Experience with Tube Currents
Cause of Tube Currents
The incredibly short wavelength of light means that very small amounts of uncontrolled refraction (or reflection) can result in visible dis-improvement of image quality. This fact is pretty obvious when it comes to the atmosphere itself. Sighting on a low sky positioned Venus, for instance, easily reveals a great deal of chromatic aberration due to differences in the refractive index of the atmosphere from one side of the planet to the next!
In addition, heat rising from the earth, or from nearby structures, pavement etc. causes chaotic variances in refractive index as the air alternately expands, contracts, and otherwise differentiates the refractive index.
This same problem can surface within the telescope itself. Taking a warm scope into the cold air can create a "heat engine" in which warm air begins to rise from the bottom of the OTA to the top, there to cool then drop down again. This cycle can continue for many hours until the air inside the tube, and the tube itself achieves equilibrium.
Meanwhile, mirrors and lenses within the OTA also expand and contract wherever thermal deltas exist.
This first OTA problem - "the heat engine" is especially bad in scopes that are completely enclosed on both ends. The problem is worse therefore in catadioptics and large refractors than in Newtonians who have a convenient heat outlet - especially when the scope is pointed up toward the zenith!
The second problem - thermo-opto deformation - is especially bad where mirrors are used to shape the light cone. As such mirrors have twice the waveform deformation than lenses simply because any deviation on one part of the mirror is multiplied because the waveform hits the mirror too early or too late then is bounced back much too early or much too late!
To some degree or another, all scopes - regardless of type - suffer from both the thermo-circulatory and the termo-opto deformation effects. BUT the larger the aperture, or the finer the optics, the worse the impact on the observing eye.
Of all scope types, Maksutov's show these twin effects the worst. Why? Because Maks are isolated at both ends of the tube (trapping the warm air), have a mirrored primary (doubling the distortion caused by heat deformation of the mirror surface itself) and have natively superb optics.
So the problem is bad and E A S I L Y noticed!
Small netwonian reflectors and refractors however show such problems least. Newts because they lack full OTA containment, and refracts because they don't double the thermo-opto distortion.
SCTs have the same problem as Maks, but their optics are not usually as refined, and their apertures are often so large that other factors (seeing stability for instance), often mask cooldown issues.
All of the above is a backgrounder to an ongoing discussion of issues associated with thermo-circulatory (tube currents) and thermo-opto wavefront deformation. I invite all AstroTalk members to weigh in on these issues. Let's talk about how we can tell we are having tube current and allied cool-down problems, how long it takes things to stabilize - and under what conditions, and what steps can be taken to shorten the cooldown cycle. Let's also get a sense about what type scope is best for a person's observing conditions and scope storage limitations.
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Aberrator Simulated Effects of Tube Currents on Star Tests
The following series of images do a wonderful job of showing just how sensitive the afocal startest in revealing optical anomalies. Especially when you consider just how bad tube currents have to get at focus in order to reveal themselves plainly to the eye.
In putting together this series I, of course, used Cor Boerrevoet's Aberrator software. While doing so, I probably could have held things like magnification more constant. But such refinements can await another day.
One thing you may wish to keep in mind as you review the startest series below is how important that you wait until your scope is fully stabilized before attempting any collimation at the eyepiece. (Final collimation is generally done at the eyepiece and not using a laser collimator or other device.) If you look at some of the milder effects of tube currents depicted belwo you might easily imagine that the source of the distortion is coma caused by mis-alignment of siome optical or mechanical component.
| 150mm | Refractor | Newt | Mak Newt | Mak Cass |
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| .10 waves distortion |
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| .30 waves distortion |
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| .50 waves distortion |
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| 1.00 waves distortion |
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In reviewing this series (during its development), my own conclusion was that tube currents only really become perceptible to the eye when above .1 wavelengths of light in magnitude. And only become significant once in the .25 plus wave range. As we review the following in focus images of stars, double stars and planets let's see if this is really the case...
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Aberrator Simulated Effects of Tube Currents on Stars
To really get a decent look at the remaining images in this series you will really need to click on one or more selected images and review each at full scale. As you do so, try and get a sense at what point the tube current effect could easily confuse the eye into thinking that a particular star is "elongated" (due to distortions of the airy disk) or double (brightening on the first diffraction ring). Then of course next time your out splitting some hairy binary, be sure to make sure that you are seeing a companion - and not an optical chimera...
| 150mm | Refractor | Newt | Mak Newt | Mak Cass |
| no distortion |
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| .10 waves distortion |
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To my eye, some airy disk elongation begins to become apparent at about .5 waves of tube current induced distortion. Due to the Mak Cass brigher first ring (as a result of large linear obstruction) you can even begin to notice a faint disparate secondary as well.
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Aberrator Simulated Effects of Tube Currents on Doubles
It used to be that amateur astros would get very excited if their scopes could resolve matched brightness doubles at or near Dawes limit. (I certainly felt this way early on!) The reality however is that even "poorly figured" scopes can resolve such pairs reasonably well - as long as they have no glaring issues with collimation or surface roughness. The new standard for "performance testing" a new scope is to tackle disparate pairs (doubles whose primary is greater than 1.5 magnitudes brighter than their secondarys.) In the following Aberrator exercise I input two stars separated by 1 arc second of magnitudes 6.0 and 7.5. A truly diffraction limited scope of 5 inches aperture or more should have little trouble revealing the fainter secondary inside the primary's one and only diffaction.
| 150mm | Refractor | Newt | Mak Newt | Mak Cass |
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| .10 waves distortion |
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| 1.00 waves distortion |
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As with single stars, once tube currents induce .5 waves of optical distortion, issues surface with distinguishing the induced diffraction ring brightening from an actual faint secondary...
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Aberrator Simulated Effects of Tube Currents on Saturn
As you look over the set of images associated with Saturn you might choose to pay attention to how increasing tube current induced distortion effects Cassini's Division and the southern frontier of the planet's South Equatorial Belt. Cassini of course, is a high contrast feature in the ring while the SEB's southern frontier is a low contrast feature in the planet's atmosphere.
| 150mm | Refractor | Newt | Mak Newt | Mak Cass |
| no distortion |
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| .10 waves distortion |
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My own sense of the SEB/Cassini question is that somewhere in the .5 wave distortion range, the SEB's low contrast frontier is lost. Meanwhile, the Cassini Division is still easy enough to pick out - dispite appearing a bit ragged on the fringes...
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Aberrator Simulated Effects of Tube Currents on Jupiter
Jupiter's low surface contrast cloud top details are the cat's meow when testing optics - and seeing conditions. As we have already noted, Saturn's northern SEB frontier turns to mush in the half-wave distortion range. One might suspect something similar in terms of Jupiter...
| 150mm | Refractor | Newt | Mak Newt | Mak Cass |
| no distortion |
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| .10 waves distortion |
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Although the Great (not-so) Red Spot is visible in all the above images, it should be pointed out that any quality scope of half the aperture should reveal the spot - when anywhere near the planet's central meridian. Reviewing the image as a whole, it's apparent to my eye that Jupiter's presentation is detectably effected as .1 waves of distortion is introduced. Keep in mind that this distortion compound's with the .15 wave's distortion already present in the optical assembly itself. The result: 1/4 wave performance - right on the fringes of "diffraction-limited" performance.
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Reducing Cool Down Times
There are a few things that experienced amateurs do to reduce the impact of tube currents on observation. Of greatest importance, is to select a scope type and aperture that is "harmonious" with viewing habits and conditions. Refractors are more immune to this sort of thing than reflectors. This as mentioned earlier is due to the fact that the "doubling effect" of bouncing waves off distorted surfaces is eliminated...
Should you choose to buy a refractor another factor is eliminated - aperture. The smaller the scope, the more quickly it will lose heat and achieve thermo-equilibrium. By purchasing a high quality refractor, you automatically limit aperture size. This due to their great cost and lack of portability!
Of the remaining scope types, newtonian reflectors are less effected by tube currents than Maksutovs and SCTs. As noted earlier, catadioptic designs have enclosed optical tube assemblies. Because of this such scopes are reduced to cool down by irradiation and surface conduction - unaugmented by air exchange.
Air exchange, of course, is not necessarily a good thing in itself. Dust particles and water vapor can deposit on a Newtonians near-inaccessible primary mirror. The performance hit here can exceed that of tube currents - especially on deepsky observation.
Of course, an SCT, MCT, or Mak Newt does can allow for some heat loss using air exchange. This can be done by removing the diagonal or eyepiece and tilting the scope to allow heat-laden air to escape. But under such circumstances the scope is unusable. And the possibility of internal contamination exists. So be sure to place a lint free cloth over the opening to prevent material from entering the tube...
Enclosed scopes larger than 6 inches - especially Maksutov's - can be purchased with internal fan filtration units (FFU's). These can speed the loss of heat from the tube through ventilation. The advantage to FFU's is that they can be used while the scope is pressed to service. SCT's rarely need such units since their corrector lens is made of ordinary glass and is quite thin. So in the balance, SCTs recover from thermal dis-equilibrium more quickly than MCT's of similar apertures.
Two final possibilities exist to minimize tube current effects: Store the scope outside at ambient temperature. (But take steps to avoid "sun baking" of the enclosure during the day.) Or setup the scope at sunset to allow it to "track" against conditions as the temperature falls.
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An Experience with Tube Currents
Yesterday was very warm - maybe even 80 degrees - here in the Santa Cruz Mountains. Argo was left in a closed room whose temperature stored all that heat, even as the temperature plummeted to about 40 degrees by the time I got home from the Sunday gig at the Lodge. I'd been looking forward to going after a few double stars after the gig. When I got home and set Argo up, the double star chase proved rather abortive. Despite decent seeing, stars of any magnitude whatsoever flared to all directions.
Interestingly, that first view of Jupiter was quite good - easily 6/10 stability. A galilean had exited the SEB (probably Io) and its shadow was easily detected trailing well behind it. had I continued observing Jupiter I would have noticed something interesting. After a few minutes the view would have begun to visibly degrade. The planets edge would soften and blur. Only the two equatorial belts would have been detectable. The Galileans would have begun to flare in all directions. All this as the "thermo-circulatory engine" would begin to kick in.
But I din't stay with the planet. I shifted to Selene instead and watched as the view steadily degraded until I couldn't bear to look upon her once comely form any longer.
Thence to track down a lowering Theta Auriga. Resolvable still! Castor - easy but nothing special. On to Iota Leonis - a no go AND very difficult to turn up positively due to the Moon's station in early Leo. It is here that I began experimenting with the "drift method" in aligning the mount (Polaris is not visible from Backyard Boulder Creek's southern observing locale.)
As I played with the drift method (using the fine RA adjust on the mount to re-center Iota as it drifted acoss the field), I watched as the stars image got worse and worse, then slowly started to improve.
After a half hour of this or so, I relocated Argo to the north observing station. At this point I observed easch of the Big Dipper stars in turn. Only the dimmest (3.4 magnitude Delta) was able to show anykind of airy disk at 210x. But really no diffraction ring...
Things began to improve. Turned Argo on disparate double Iota UMA at 210x. Caught a faint blue star to the north(?) west. This is the first glimpse I've ever had of this faint 10.5 magnitude 7 arcsec secondary!
Returned to view the other Iota - in Leo. By this time, the star was placed above a neighbor's home. Now it was structural atmospheric distortion that prevented resolution. (I was convinced I had resolved it earlier before relocating to the north. 6 mag secondary almost due east of primary...)
It was now two hours later, and only now was I getting anything that resembled "decent" (6+/10) seeing stability. Two hours to recover from a 40 degrees delta in scope- to air temps.
Nope, wouldn't want to use a Mak in North Dakota!