"Complete" description of seeing
Posted 03 November 2013 - 05:07 AM
Last night I had the maybe worst seeing conditions this year. I noted Pickering as ~3 with a huge halo but the same time the feeling that this is only the half story because air turbulences were not really the problem but rather the humidity or haze changing the diffraction pattern to a cotton ball.
After quitting my session in Del I had a look at Eps Lyr (remembering the nice description of "Rating Seeing" on the "Spirit of 33" Double Star Observing project http://www.carbonar....ting-seeing.htm) (link edited) and was amazed that even with full aperture of 120mm and magnification x180 I could not resolve even the easier Eps1 Lyr. At least Albireo was no problem and the colors were nice even with cotton balls. Also the Pleiades gave a nice sight at low magnification so open clusters are good targets even with this conditions.
But back to the topic: Degree of Light Pollution (in terms of NEML) and degree of air turbulences (in terms of Pickering, depending on used aperture) are certainly not enough to describe seeing conditions completely. The term "transparency" or "opacity" is also often used in this regard (good transparency positioned sometimes even as opposed to seeing quality) and I found also some scale definitions on the web (Saguaro Astronomy Club, The American Association of
Amateur Astronomers, Arkansas Sky Observatory, ...) but these definitions all refer to the Naked Eye Magnitude Limit before Light Pollution combining it somehow with atmospheric extinction. I have with this scales clearly the problem that I cannot eliminate light pollution to determine transpareny. I also have the feeling that a scale using the telescope like Pickering would be more adequate.
If Eps Lyr would always be up then the magnification needed to split at least one or both doubles would be a good indicator but this would then be intermixed with turbulence = Pickering. So this is also not such a good idea.
May be intensity and the size of the halo of a bright ~+2mag star in arcseconds would be a good indicator.
Are there any known concepts to determine transparency/opacity looking through the scope similar to Pickering for seeing without intermixing these two aspects?
Posted 03 November 2013 - 05:26 AM
I use Pickering and NELM, and sometimes try to determine TLM. Maybe it's a good idea to record the halo, as well, as an indication of scatter. After all, this does seem to be a major contributor to double star difficulty.
Posted 03 November 2013 - 09:01 AM
Checking also TML seems quite a good idea - I do this sometimes to sample TML data with reference to NEML to check if my RoT TML implemention is plausible. So far I have found more variation in this regard as I would like. As I do this usually not at zenit but in my field of view I am already aware that I have to include an average extinction of ~0.3mag here.
But still I have the impression that there is more variation as I would expect. The culprit might be different transparency as this seems to have a rather heavy impact on TML. Checking again Schaefers work with a multitude of TML influencing factors I found the factor transparency missing - this might be even the reason why Schaefers model did not fare this well when compared with empirical data. Haze might be also the culprit for my experience with faint companions getting dissolved in the grey background with higher magnifications. Last night for example with BU363 (HIP 101213) - 6.6" +6.18/10mag: With x45 I could barely detect a faint spot at 2 o'clock but already with x90 I lost resolution.
I think I found a new project for hazy nights: Checking the effects of low transparency on TML and in consequence on resolving faint components of doubles.
Posted 03 November 2013 - 07:13 PM
In my "darker " area at my condo its
Its optimistic by about 0.8v as my NELM typically here is 5.0.
I find its accurate to within a tenth for NELMS.
Here's tonight's reading;
Again a little optimistic but I have porch lights its not factoring in so knock off a mag almost. The visual description is spot on.
Check it out guys.
Posted 03 November 2013 - 08:01 PM
The first is the image itself, it's peak intensity on the focal plane. The more light in the disc, the easier to see the disc against any background that would tend to obscure it. The less light in the rings, the easier to see a faint companion.
Next would be seeing and the steadiness of both the disc and the ring structure. But, also an expanding image and a "closing" of the Raleigh gap resulting in bloated star images.
The next might be limiting magnitude, such as moonlight and light other forms of light pollution. It could also include things such as haze. But, limiting magnitude is also a function of dark adaption and whether one uses averted or direct vision to determine it.
Another might be the halo of light around a star, or glare, being either optical or atmospheric. Even of clear nights with steady seeing, this is present to some extent and more so on brighter stars than dimmer. I'm thinking this is a good indicator of the conditions in that is includes things such as haze and glare from the disc and rings. But, it's also influenced by averted vision. But, it does seem the collective expanse of this halo is what makes stars visible or not (along with sky brightness and limiting magnitude.)
So, it seems someone should report seeing for sure. But, to be complete, one must report some amount of scatter or glare around a standard star. NELM might do the trick, but I think a halo might be all encompassing as it's visibility can overtake dimmer stars. Light pollution, natural or man made, is really an extended version of background glare that covers large swaths of the sky whereas visible halo or glare is more specific to the brightness of the star and the contrast against the sky background. But, this seems to be the factor in success or failure, even on tight nearly equal pairs as well as wider unequal pairs.
So, NELM might be a measure of sky brightness in general and a halo might be a reported factor encompassing atmospheric haze, including atmospheric haze and the glare form the star itself. I'm wondering is glare is a function of magnification, too. Does it fade or fall off in expanse with magnification, like an extended object might, while the dimmer star might not be as affected (as long as it's disc remains basically a point source?) As NELM increases, it displaces glare (apparently) but makes resolution, possibly, no less difficult as both affects might combine to retard resolution.
Man, there are so many complicated and variable factors involved. But, I like the halo reporting idea, it seems to get down to brass tacks as an indicator of resolution conditions near the primary star, along with seeing and NELM.
Pete, sounds like a great app, but not for android just yet.
Posted 03 November 2013 - 08:51 PM
Headin out for doubles now. I've got your recent ones on deck.
Posted 04 November 2013 - 11:38 AM
I can imagine a scope dependent transparency scale with following extremes:
1 = Halo for +2mag stars larger than 60". TML ~2mag less than theoretical scope limit allowing for the effects of light pollution and extinction. Diffraction pattern not visible - spurious disk and diffraction rings dissolved in a cotton ball. Equal bright doubles with separation less than 3" cannot be clearly resolved giving at best a fuzzy rod, unequal bright stars even more difficult
10 = No halo for ~+2mag stars. TML near the theoretical scope limit allowing for the effects of light pollution and extinction. Diffraction pattern perfect visible - spurious disk crisp and first diffraction ring solid allowing for the effects of seeing (air turbulences). Equal bright and moderate unequal doubles resolvable down to below Dawes and elongations down to 0.5 Rayleigh allowing for the effects of seeing (air turbulences).
Posted 04 November 2013 - 04:05 PM
if there were no atmosphere, and therefore zero scattering, then there would be no opacity in the sky, even with the sun directly overhead. so "light pollution" is an indirect measure of transparency. i'd argue there's only seeing and dispersion to evaluate.
diffusion can be a more severe imaging problem than seeing, as the linked article demonstrates. i use both naked eye limit magnitude (calculated using magnitude coded star charts) during clear windless weather to get a baseline light pollution level, but night to night i just examine the visible nimbus around a 3rd-6th mag. star: the "diffusion scale" is just the magnitude at which a noticeable nimbus first appears. (if you don't see a nimbus around sirius then your diffusion is zero!) the point of the standard eyepiece is that there is scatter in your optical system, too. don't blame the sky for that.
to me the most annoying part of diffusion is the frog in boiling water effect. it creeps up on you as optical dewing, and can progress pretty far before you realize there's a problem. the nimbus is a great warning signal to counteract that.
the visual assessment of seeing comes down to just three criteria -- at exit pupils below 1.0 in a magnitude 4-6 star: (1) whether you can discern a persistent airy disk, (2) whether you can discern the inner diffraction ring around the airy disk -- how far around the disk the innermost ring extends, how long it remains visible; (3) whether the whole image, disk and complete inner ring, is relatively stable -- for a second or more. (these criteria are separate from the arcsecond seeing criterion, which is better evaluated with instrumentation.)
separate from those criteria is assessment of image displacement or "jumping around". as low frequency increases the image jumps around more; high frequency will just sit in exactly the same place to torment your eye. (it's also often invisible in the "twinkling" of stars, which is most visible as low frequency turbulence.) it's important to assess how much the image jumps around, since the airy disk can punch through even pretty severe low frequency (near ground, in the tube, on the mirror) turbulence, while often even maximum magnification can't cut through the high frequency (jet stream) boiling ball of bees thing.
like wilfried, eps 1,2 LYR is a favorite test system, especially since the two stars are currently positioned at about right angles: you can often see that one is harder to resolve than the other because turbulence, as high wind, is directional. in general i find resolving "test" pairs like pi AQL to be a great first pass test.
Posted 04 November 2013 - 05:27 PM
Thanks also for mentioning the Bortle scale - this scale is certainly relevant for deep sky observing but of nearly zero relevance for double star observing. It is certainly nice to have a dark sky but light pollution has only a minor impact on resolving binaries in terms of a loss of up to ~10mm aperture in really bad cases of NEML ~3mag. At least this is what my set of limit observations shows clearly.
Posted 04 November 2013 - 08:16 PM
I think this is important and will be paying closer attention to it - the end effect of both atmosphere and optics - instead of just seeing and NELM.
...but night to night i just examine the visible nimbus around a 3rd-6th mag. star: the "diffusion scale" is just the magnitude at which a noticeable nimbus first appears. (if you don't see a nimbus around Sirius then your diffusion is zero!) the point of the standard eyepiece is that there is scatter in your optical system, too. don't blame the sky for that.
I like the idea of standardizing a report of scattering, such as using a standard candle and maybe at a 1mm exit pupil. And maybe using Sirius with no scatter as a baseline for zero. This nimbus is apparently very local to each observation where a faint companion is involved, where as seeing and NELM tend to be at larger scales. The latter can give a good impression of the sky while the former can offer some insight to a specific observation.
Surely the scale can change throughout the night and across the sky. Maybe the radius of the nimbus, scatter, or glow around the primary can be given in terms of radius seen in arc seconds and be specific for each observation. Brighter stars will have larger values, and dimmer stars might have nothing to see and reported as zero.
One question, though, I've asked before, is how does light below the visible threshold affect a faint companion that /should/ be well above that threshold? For example, in the first minimum there is zero diffraction energy and relatively dim stars should be seen. That they are not is probably due to scatter from the primary disc and maybe "back-scatter" from the first ring. This forms some sort of glow across the apparently dark first minimum making detection difficult even at relatively bright delta magnitude. Then, the same would happen for fainter companions further out as scatter, or nimbus, fell off and possibly below the visible threshold (or at least the scatter is not noticed and maybe processed out by the eye/brain.) Still, it does seem to affect dimmer companions.
Posted 05 November 2013 - 03:29 AM
One question, though, I've asked before, is how does light below the visible threshold affect a faint companion that /should/ be well above that threshold? ...
Ignoring the factor separation I guess similar to a tad of light pollution - the background may be less dark but the difference to a real black sky is too small to be visually noticed besides determine NEML by looking for the faintest star you can resolve. What seems drastic with naked eye gets harmless when using a scope - you may loose a few mm aperture but other factors are certainly more relevant.
Posted 05 November 2013 - 10:06 AM
So far I found Deneb of good use for this task: The +11.74mag companion is in a distance of ~75" giving a good reference for the size of the halo and there are several faint stars ~12mag nearby. I used the USCA4 catalog for the magnitudes of these stars in the star map. Should be of good use for ~150mm scopes.
Posted 05 November 2013 - 03:07 PM
So far I found Deneb of good use for this task: The +11.74 mag companion is in a distance of ~75" giving a good reference for the size of the halo and there are several faint stars ~12mag nearby.
there's no practical reason to use only double stars, because a standard eyepiece will control both the true field radius, the magnification (which determines the contrast available from the background), and the eyepiece scattering properties. an orthoscopic or RKE is ideal because the field diameter is relatively small (~45º) and there is minimal glass.
you can judge the size of the nimbus from the field radius alone, no calculation or lookup necessary. however, because diffusion scatters out to several degrees of sky, you are always judging the size of the nimbus against the background illumination, which is nimbus too. this is the point of using stellar magnitude rather than radius of nimbus as the estimator, since you can detect scatter more reliably than you can measure where it stops.
precision can be useful, but i don't usually look for it. keeping track of seeing and diffusion is useful to me as a context for what should or should not be visible, and what may need adjusting in the optical system by way of cooldown or dew remedies (or when it's time to close up for the night).
to norme's observations, if i understand them, unless there is a very local source of water vapor or particulates nearby, then diffusion should be fairly constant across the sky, accounting for the lateral extinction normal for the atmosphere. indeed, i often first notice changes in diffusion -- aka, an encroaching maritime humidity -- as a loss of visibility at high zenith angles or a significant increase in the visibility of urban light domes that are 10 or 20 miles away. (as i said, diffusion and light pollution are related.) in fact seeing is in my experience more variable across the sky, across time, and across zenith angles, than are the effects of diffusion.
Posted 05 November 2013 - 08:44 PM
Many people say light pollution does not affect double star detection. Surely it doesn't with brighter pairs, but it seems to affect limiting magnitude and probably a brighter companion close enough to a brighter primary, especially if both conditions (general light pollution and local glare - or nimbus) are extreme.
I guess my question above was whether either effect could hamper resolution if they were below the visible threshold. For example, a fainter companion of an 8th magnitude primary might reside outside any visible nimbus but still be difficult even under dark sky conditions. In fact, on a 8th magnitude star, there may be no visible glare at all yet a companion is still difficult. (I realize we're flirting with eye response with very dim stars, and sky background it apparently darker at higher magnification. But just to illustrate what I mean.) Surely glare plays a role, but even if it's below the visible threshold - and even at magnitudes where the FOV itself is apparently dark?
Anyway, I am going to be paying attention to it. Most of the primary stars I target are brighter than 8th magnitude, delta mag of about 3 or less, and separations of 1.5" arc or less.
Below the sketches are somewhat exaggerated, with a nimbus visible under dark skies (left) and pretty much the same under light polluted skies (right.) Each has 4 dimmer companions off to the right at varying distances, the first is bright and on the ring.
Posted 06 November 2013 - 04:20 AM
So far I have considered transparency a nice to have enhancement when looking through a scope if seeing is good enough and made some effort to determine seeing quality on the Pickering scale and noted less good transparency more as a side comment. Reading the article Bruce indicated above "Experimental comparison of turbulence modulation transfer function and aerosol modulation transfer function through the open atmosphere" from Dror and Kopeika was an eye opener for me.
Some quotes and conclusions:
- The effects of turbulence MTF and aerosol MTF on overall quality of seeing conditions are basically multiplicative
- Despite this the effect of aerosol MTF is the dominating one on overall quality of seeing conditions ("Often turbulence has been assumed to be the primary or even the only source of atmospheric blur. In this paper ... Aerosol MTF is observed to be always significant and often more dominant than turbulence MTF ...")
- The effects of turbulence MTF can be balanced by adaptive optics but not the effects of aerosol MTF
- The effects of aerosol MTF can be "repaired" to some degree by digital image processing
- The time dynamics of turbulences (= bad seeing) offer the chance of a crisp image for at least for fractions of seconds (enabling thus lucky imaging) - so at least some chance for visual resolution
- The time consistent effects of aerosol scattered light result in a consistent blur - so constant bad conditions for visual resolution
- No surprise: High humidity is a dominant factor for aerosol caused light diffusion so relative humidity is an important information when reading the weather reports for time and location of planned sessions.
So contrary to Bruce I think precision is required for determing the degree of aerosol causing blur as this is more important than the degree of tubulence causing more or less bad seeing.
So contrary to Bruce I think a good entry point like Deneb makes this easier as you have in one field of view all necessary information (including TML) to determine seeing and transparency.
Regarding light pollution:
Contrary to Bruce I think diffusion and light pollution are not related but only that diffusion enhances light pollution.
And contrary to Bruce and Norme I think that light pollution has only minor effects on resolving doubles and this opinion is backed by statistical evidence of several hundred limit observations.
May be it is not necessary do develop a scope dependent scale of transparency with 10 levels like Pickering's seeing scale but at least a clearly defined scale with 5 levels for very bad, bad, fair, good and excellent would be of good use.
And it seems necessary to use such a scale on a regular base as transparency is at least similar if not more important than seeing.
Posted 06 November 2013 - 08:54 PM
I used your method of observing that 12th magnitude star near Deneb and found the nimbus to fall just short of that star. I am not sure if that is good or bad or average.
Living in the tropics, humidity is prevalent as are aerosols. And it makes sense the aerosol MTF can be persistent inhibitor to observing unequal pairs, more so that seeing MTF. All MTF conditions are multiplicative or additive, including aberration and CO. (See Suiter, I cannot remember how they combine to form a final MTF.) But, it would seem light pollution would be minor in comparison, but would add to the glare in some way. Both are enhanced by aerosols, though, to some extent - if not extincted by them with distance from a light dome.
Aerosol blurring is more sever on brighter stars and falls off on dimmer stars. But, does that mean we can see closer unequal pairs on dimmer stars? For example, does Sirius and the Pup resolution scale with brightness of that nimbus at a constant delta magnitude? Can the Pup be seen much closer if Sirius were several magnitudes fainter with a smaller visible nimbus radius - below the visual threshold at the Pup's separation? Or does deltaM ~10 require about 10" arc in every case?
Posted 07 November 2013 - 04:31 PM
I think glare is different from halo/nimbus caused by aerosols but things are probably interacting here.
Sirius is a special case in my opinion and halo seems not to be the problem here as one of the used tricks to resolve the pup is to have Sirius just outside the field of view of the used eyepiece - this way you avoid the glare but you could this way not avoid the halo.
Regarding the size of the halo of Deneb with ~60" indicating some higher degree of humidity - I don't know either if this is good or bad or average but like you in your Aries session I had this myself once also combined with rather good seeing of Pickering ~7 and got in total reasonable good results in resolving unequal doubles. I did not give at this opportunity enough attention to the question of the degree of crispness or blur of the diffractions pattern but retrospect I assume this level of transparency could be considered as "fair".
Further observations will show if the size of the halo of a reference star like Deneb is a significant indicator for the degree of aerosol diffusion or if we have to concentrate solely on the degree of blur of the diffraction pattern like on the degree of turbulence in the diffraction pattern to determine seeing.
PS: Trying to get a complete picture of the effects of light pollution on resolving unequal doubles we have to consider also very faint doubles near the telescope magnitude limit - here the situation is certainly serious as LP reduces the TML up to ~0.6mag for a 150mm scope for NEML below ~3mag and extinction due to altitude may also "cost" ~0.3mag. Thus the resolving power of a 150mm scope will be even for very wide doubles with 10" separation or more be limited to at least ~1mag less than the theoretical TML
Posted 07 November 2013 - 07:58 PM
It is pretty humid here in the tropics, add some aerosols and the nimbus (not unlike a high thin cloud) can scatter light around stars. It also scatters artificial lighting - back toward the objective. On dry nights light pollution should be minimal as would be any nimbus scatter leaving mainly optical veiling glare and diffraction. But the sum total of all scatter can be seen using a bright star.
Maybe some definitions are in order, at least for me.
Nimbus sounds to me like the scatter caused by aerosols in the atmosphere. It's essentially a very thin cloud. As such it is part of the image formed by the optic. Artificial light also appears to be part of the image (across the FOV) and appears to be similar to nimbus in that it is formed external to the optic. However, the source is ground lighting (as opposed to incoming star light) and tends to be broader in extent (as opposed to local to a brighter star.) So, I see them as two of the same effect of atmospheric scatter whose extent depends on conditions. It does appear local nimbus is a stronger source, but both combine to reduce contrast to some extent.
Glare, then, might be scatter formed by the optic (coatings, unclean, various surfaces, and internal reflections, etc.) I am not sure if tricks can alter it's extent. Maybe, but it can be minimized. Diffraction differs in that is it comprised of the bright ring structure, and of course, is inevitable in the nature of light. It can be altered or minimized using various obstructions, including none.
n. pl. nim·bi (- or nim·bus·es
1. A cloudy radiance said to surround a classical deity when on earth.
2. A radiant light that appears usually in the form of a circle or halo about or over the head in the representation of a god, demigod, saint, or sacred person such as a king or an emperor.
3. A splendid atmosphere or aura, as of glamour, that surrounds a person or thing.
4. A rain cloud, especially a low dark layer of clouds such as a nimbostratus.
loss of contrast due to light scattering within a lens system, as in a fluoroscopic image intensifier.
Veiling glare is stray light in lenses and optical systems caused by reflections between surfaces of lens elements and the inside barrel of the lens...that can degrade image quality in the presence of bright light sources in or near the field of view. It occurs in every optical system, including the human eye.
Anyway, I want to get into the habit of reporting overall scatter conditions along with seeing and limiting magnitude as it seems to be the major inhibitor of unequal and dim pairs. NELM has always been easier to determine and report.
Posted 08 November 2013 - 11:11 AM
The blurring only appears to be more severe on brighter stars. For fainter objects, the integration time of the eye increases (to a max. of ~0.1 sec) & the effect of this is that scintillation decreases ... but the spreading out of the light by atmospheric turbulenece remains the same. Also the angular resolution of the eye worsens as objects fade towards limiting magnitude & this can appear to make the image sharper in the same way as reducing the magnification. The optics of the system remain unaffected & it's the optics that ultimately govern resolvability.
Aerosol blurring is more sever on brighter stars and falls off on dimmer stars. But, does that mean we can see closer unequal pairs on dimmer stars?
When doubles have very unequal components with small seperations, aerosol diffusion in the atmosphere together with dust on the optics will spread the light from the theoretical point image of the primary over an area & this diffused (scattered) light can mask the faint secondary embedded within it. A good reason for objectives & eyepieces to be kept reasonably clean.
Posted 08 November 2013 - 12:34 PM
Posted 08 November 2013 - 01:43 PM
The way I read this:
The concept I am trying to get my head around is the decrease in blur visibility (falling off below the visible threshold) still affects a dimmer companion that would otherwise be visible...bright enough to peek above a dark sky background but not a background that 'appears' dark.
If you have for instance a double star with 5 mags difference between primary & secondary, the contrast in the image will be the same irrespective of the brightness of the primary.
The issue with vision is that the smallest contrast that can be discerned decreases as the brightness falls towards the limit of vision (the same effect that makes low contrast planetary details harder to see when the magnification is pushed too high, even when the seeing is "perfect"), so brightening the image (without affecting anything else) should theoretically make the secondary easier to see, despite the glare.
Posted 08 November 2013 - 02:02 PM
Halo was again about ~60°, may be a tad more and the +11.76mag companion was rather hard to resolve. I then had Deneb wandering out of my field of view and suddenly the companion "jumped out of the dark" - so this seemingly halo was more glare than haze. So transparency was rather good and I had a reasonable good session then in Peg. The most difficult object seemed HO480 0.9" +8.72/10.03mag - no resolution and because of the faint fuzzy impression my conclusion was that some haze even if low was enough to make the resolution impossible. My RoT showed for HO480 anyway 179mm so also here no real chance with 140mm. Later I found out that the current WDS separation is 0.3" - something must have happened here to WDS as in the version 12.05 the notes read "AB. First: 1892|225deg|0.7. Last: 2009|2deg|0.9. #Obs: 29" and in 13.08 suddenly "RA PM:17, Dec PM:-6, AB. First: 1892|225deg|0.7. Last: 1993|270deg|0.3. #Obs: 28". This would then require an aperture larger 400mm.
Thus are the adventures of the double star observer with catalogs.
Tonight I have the chance of some more haze and will try to get a better grip on the assumed relevance of the halo size for resolving doubles and also use a handicrafted half cover for the eyepiece to easier eliminate Deneb glare.
Posted 08 November 2013 - 07:44 PM
Wilfred, I tried the occulting technique trying to observe the Pup as Sirius left the FOV. No luck with that, but should have paid closer attention to the extent of the halo size as the primary left the FOV. Will try that again. Gonna have clear skies tonight, if the seeing holds with a Typhoon just passing south of us.
Posted 09 November 2013 - 05:26 AM
...The jury is still out for me as to how much light pollution reduces limiting magnitude...
Norme, I think Schaefer has done a good job here. There is even an online TML calculator on the Web (http://fisherka.csol...eferLMCalc.html) based on his work - the missing factor here is certainly the influence of transparency but the values for TML reduction due to light pollution and extinction seem very realistic.
Posted 09 November 2013 - 08:51 AM