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Big dob for objects with structure and detail?

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#76 jpcannavo

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Posted 13 June 2013 - 10:01 PM

M51 and magnification! At RMSS in Colorado this past week I was getting great image scale with an 8mm Ethos on my F5 16 (250X). I actually think a 6mm would have been perfect!

Joe

#77 turtle86

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Posted 13 June 2013 - 10:26 PM

M51 and magnification! At RMSS in Colorado this past week I was getting great image scale with an 8mm Ethos on my F5 16 (250X). I actually think a 6mm would have been perfect!

Joe


Definitely worth trying! Seems that a lot of the brighter Messier galaxies can really take the higher mag--M33, M64, M65-66, M81-82 and M104 come to mind. Can't remember if I've tried the 6mm Ethos on M51 myself but agree that it looks magnificent with the 8mm Ethos. The 6mm Ethos is great for planetary nebulae and busting open globs.

#78 GlennLeDrew

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Posted 14 June 2013 - 05:58 AM

It's not necessarily the higher *integrated magnitude* objects which can take magnification. It's those objects with reasonably high *surface brightness* which bear high powers.

Example...

The California nebula has an intrinsic integrated brightness of about 7th magnitude. It cannot take high magnification because its surface brightness is *barely* brighter than the sky. (Intrinsically, it's about 25 MPSAS, but sky glow adds to this, resulting in an apprent total brightness of 4th mag or brighter.)

Many a 12th magnitude (and fainter) planetary can withstand quite high magnification due to the rather bright 14-15 MPSAS (over 100X brighter than the sky.)

High surface brightness makes for high contrast, and so smaller exit pupils can be utilized without the object becoming lost in visual system noise.

To drive home the point that integrated brightness becomes increasingly meaningless (useless) as object size increases, consider the sky itself. At a pristine site, where zenithal surface brightness is 22 MPSAS, the integrated brightness of the whole celestial dome (just sky glow, no stars) is easily -7 magnitude, or 10 times brighter than Venus.

I'd bet that most any sky observer, if told that the sky glow at the darkest site amounted to -7 magnitude, would think the bearer of the factoid insane. And so it is with large, low surface brightness nebulae; they have surprisingly bright integrated magnitudes. But these values give no idea whatsoever of the difficulty of detection.

#79 turtle86

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Posted 14 June 2013 - 12:44 PM

Sure seems that with low surface brightness objects, the quality of the sky conditions matters more than the aperture. I'd hazard to guess that from a "black" observing site, the California and Horsehead Nebulas don't need a big Dob to be seen. I do most of my observing at Chiefland, which is moderately dark but with a noticeable light dome to the north and a smaller one to the east. In my 18", the California and Horsehead Nebulas aren't too hard to see with a 31mm Nagler and h-beta filter, at least on a night when the humidity isn't too bad. But if the humidity goes over something like 80%, trying to pull these faint extended objects out of the muck can be a daunting task indeed.

#80 jpcannavo

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Posted 14 June 2013 - 01:57 PM

Glenn, Rob.
Yeah.
Surface brightness - i.e. luminance - is a key parameter (as of course are others). It would be neat to see some data quantifying various deep sky objects (galaxies and nebulae) along these lines. I actually think someone has, but can't remember where...?
Joe

#81 BillFerris

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Posted 15 June 2013 - 12:30 AM

The statement, "as you get closer to the picture in the dimmed room ... the brightness of the object does not change," may be correct for surface brightness but is incorrect when we consider apparent total brightness. This is something most everybody ignores--with one notable exception ;)--in these discussions. In order for object surface brightness to remain constant as apparent size increases, the object's apparent integrated magnitude must increase. In short, all objects become brighter as aperture increases.


Bill, just to be clear, and before you put yourself in a special category, I am perfectly clear on the relevant photometric distinctions here. ;)


Joe, good to see you acknowledge that objects do become brighter as aperture increases.

I would also be cautious with an analysis that attempts to explain the relevance of integrated brightness for detection in terms of "more information being delivered to the eye". While this model seems to work at one end of the contrast sensitivity function, where contrast sensitivity increases with image scale, it fails to predict that a maximum occurs (at some mid spatial frequency) with the other end of the function then falling off at lower frequencies.

CSF


Why caution? Light is information and increasing aperture does deliver more light to the eye. Regardless, the bottom line is that threshold contrast is lowered as aperture increases. Most 6-inch telescopes are capable of presenting thousands of galaxies beyond their reach at the necessary image scale. However, at a size that the dark adapted eye would be capable of detecting them, they're to low in contrast to be seen. By increasing aperture without increasing magnification, threshold contrast is reduced to a point where more and more galaxies can be observed.

Posted Image

As for all the other elaboration, I think we are all more or less converging on the same explanation, but perhaps with varying degrees of clarity.


I'm skeptical on that point. There's a lot of resistance within the amateur astronomy community to accepting the role of threshold contrast in visual observing of faint extended objects.

Bill in Flag

#82 GlennLeDrew

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Posted 15 June 2013 - 02:47 AM

Bill,
Any resistance probably stems from an incomplete understanding of just what threshold contrast is, or how it's defined in this context. I'm still not sure how you're defining it.

As a staunch proponent, could you provide an unambiguous definition?

#83 alexvh

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Posted 15 June 2013 - 04:58 AM

I do wish i could compare a 14" and 16" dob next to each other.
I am facing an agonising decision on this!

#84 GlennLeDrew

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Posted 15 June 2013 - 10:52 AM

The 16" has 14% better linear resolution than a 14", and 31% (0.29 magnitude) more light grasp. The difference would not be so easy to see, really. If the bulk and mass are not the issue, go with the 16"; at least you'll not be plagued with second thoughts. :grin:

#85 jpcannavo

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Posted 18 June 2013 - 07:41 AM

Bill
This needs a bit of tightening.

The statement, "as you get closer to the picture in the dimmed room ... the brightness of the object does not change," may be correct for surface brightness but is incorrect when we consider apparent total brightness. This is something most everybody ignores--with one notable exception ;)--in these discussions. In order for object surface brightness to remain constant as apparent size increases, the object's apparent integrated magnitude must increase. In short, all objects become brighter as aperture increases.


Bill, just to be clear, and before you put yourself in a special category, I am perfectly clear on the relevant photometric distinctions here. ;)


Joe, good to see you acknowledge that objects do become brighter as aperture increases.


To be clear on a basic distinction in optics –image vs. object - you surely don’t mean that the intrinsic luminosity of the objects (as in M51 itself) we observe increases with aperture. I assume you mean that the apparent integrated brightness of the virtual image of the object increases with aperture. i.e that the luminous flux through the exit pupil, due to that object, increases with aperture.

Trained on a given object, telescopic aperture receives a greater luminous flux from an object than the naked eye alone – i.e. telescopes gather light. As a consequence the luminous flux, from that object, entering the physiologic pupil (the eye) increase as well. But only up to magnifications that contain the entirety of the objects image within the field of view. Beyond that magnification, said flux decreases as a smaller and smaller portion of object yields an image in the telescopic view.

I would also be cautious with an analysis that attempts to explain the relevance of integrated brightness for detection in terms of "more information being delivered to the eye". While this model seems to work at one end of the contrast sensitivity function, where contrast sensitivity increases with image scale, it fails to predict that a maximum occurs (at some mid spatial frequency) with the other end of the function then falling off at lower frequencies.

CSF


Why caution? Light is information and increasing aperture does deliver more light to the eye.


Note sure what you mean by “information” here: “Information” in the epistemic sense, the information theoretic sense, the systems theoretic sense, the physicist’s sense…etc? I assume you must mean information as sensory input, where said input leads to a conscious perception - in this case visual perception.

This then establishes the relevance of an analysis in terms sensory physiology, and specifically the transduction of light energy into visual perception. As such, we run up against the retina/visual system and its intrinsic properties.

Unfortunately a model that naively concludes “more being perceived” from “more going in” fails in an explanatory sense – it doesn’t tell us why “more” leads to “more”. And it fails in a predictive sense – it is empirically falsified by the occurrence of maxima (as I elaborated above) in the contrast sensitivity function (herein “CSF”), which is widely described.

http://www.google.co...ast sensitiv...


Regardless, the bottom line is that threshold contrast is lowered as aperture increases. Most 6-inch telescopes are capable of presenting thousands of galaxies beyond their reach at the necessary image scale. However, at a size that the dark adapted eye would be capable of detecting them, they're to low in contrast to be seen. By increasing aperture without increasing magnification, threshold contrast is reduced to a point where more and more galaxies can be observed.


The assertion “Regardless, the bottom line is”, smacks of dogmatism. Moreover, this is all too vague: Aperture lowers the contrast threshold (herein “CT”) - or equivalently increases contrast sensitivity (herein “CS”) - of and for what, and how? (Certainly not by changing the CSF of the retina/visual system for that is an intrinsic physiologic property)

Let’s reexamine all of this and in the process hopefully address the “mystery” referred to in the last paragraph of “Lowering The Threshold”.

We will consider two fundamental categories: 1) increasing aperture with constant magnification, and 2) increasing aperture with constant exit pupil – i.e. magnification increasing in proportion to aperture.

Case 1) A galaxy (object) with low contrast against the background sky is being observed telescopically. We now increase aperture while maintaining constant magnification, but without exceeding maximum exit pupil. The surface brightness (herein “luminance”) of the eyepiece virtual image of that galaxy will consequently increase as image scale –i.e. apparent image size – is held constant. Also, the image of the background sky will undergo a similar and proportional increase in luminance. Result: the luminance of galaxy and sky images increase without sacrificing contrast.

I say: “without sacrificing contrast”, so as to avoid (only being an amateur!) sorting through the complexities of defining visual contrast (Weber contrast, Michelson contrast, etc., see also the Weber Fechner Law). I am quite sure, however, that invariant with these various definitions is the fact that contrast has not decreased. And, clearly, luminance has increased.

Now see the diagram of the CSF of the retina below. This depicts the change in the CS of the retina with two variables: image scale and retinal illuminance. So what exactly then is CS?

Roughly, the CS of the retina/visual system is that physiological property that allows a visual perception of an object by virtue of the contrast, against the background, of its real image on the retina. And, the higher the CS, the lower said contrast can be with the object still being perceived.

Contrast threshold (herein CT) is an equivalent specification of the above property that establishes the lowest contrast that the real image on the retina may have for the visual system to achieve a visual perception of the object. These then are inversely related to each other, where CS = 1/CT (with appropriate choice of units).

In short, a given low contrast object will be visually detectable if CS is high enough or, equivalently, if CT is low enough.


Returning to the CSF diagram below, note the axes. The vertical axis is actually labeled as modulation threshold e-1, i.e. 1/modulation threshold. Realize, however, that modulation threshold is equivalent here to CT. And, since 1/CT = CS, it follows that 1/modulation threshold = CS. Thus the vertical axis can be understood as quantifying CS.

The horizontal axis is labeled as spatial frequency, which is inversely proportional to image scale/size. Therefore increased spatial frequency is proportional to decreased image scale/size. So we can equate high/low spatial frequencies with a small/large images respectively.


Continuing to examining the diagram, there are multiple curves. Each of these plot CS vs. spatial frequency for various levels of retinal “illuminance”. Retinal illuminance can be understood as the surface brightness of the retinal real image. It is apparent then that CS varies with illuminance, which itself varies with the luminance of the virtual image at the eyepiece.

Now return to the observation of our object, the galaxy, while noting the curves plotted higher on the vertical axis in the diagram. As we increase aperture, while holing magnification constant (but not exceeding maximum exit pupil), the illuminance of the real image on the retina increases and “plots” on these higher curves. (in reality there are an infinite number of such curves since illuminance is a continuous variable) Consequently CS will increase for the retinal image at a given scale (spatial frequency) as aperture increases.

So in short, for case 1): Greater aperture with constant magnification yields an image with greater luminance, which yields a retinal image with greater illuminance resulting in greater CS – or equivalently a lower CT – of the retina for the real image of that object. The final result being greater visual detectability of the galaxy!

Case 2) The same galaxy is being observed telescopically with increasing aperture, but now with proportionally increasing magnification, i.e. constant exit pupil. This increases virtual image size - resulting in a greater retinal real image scale - while holding luminance at the eyepiece, and illuminance of the retinal real images of galaxy and sky, constant. Observe the CSF diagram again. Now note that for the majority of plotted points, CS increases as we move left on the horizontal axis and image scale increases (i.e. spatial frequency decreases).

For completeness sake we must note that on the higher illuminance curves maxima emerges, beyond which increasing image scale reduces CS. (i.e. failure of “more” yields “more”). But I think we can assert that the majority of deep sky detail relevant for this discussion falls on the portion of these curves where CS varies positively with image scale.

So in short, for case 2): Greater aperture with constant exit pupil yields a larger eyepiece image while holding luminance constant. This yields a larger retinal image, which (for the deep sky detail relevant here) moves us left on the horizontal axis of the CSF to regions of larger image scale. This yields higher CS (or equivalently lower CT) of the retina for the real image of that object. The final result again being greater visual detectability of the galaxy!

Now of course, we can stipulate a 3rd case that combines cases 1 and 2 above, ie increasing aperture with simultaneous changes in luminance and image scale. I think fleshing that out easily follows from an understanding of the above two cases.

In summary I think we can informally say that aperture lowers CT/increases CS, but as long as we understand what that means. Specifically, that aperture lowers CT/increases CS of the retina for the real image of an object, given appropriate changes in the illuminance and image scale of that object due to the increase in aperture.

Also realize that this account was illustrated for a galaxy against the background sky. It generalizes to other deep sky details against their background, such as a knot framed against the background of galactic spiral arm.


This is about as good as I can do with the spare time I have. I have tried to keep the few photometric concepts clear – hopefully without too many blunders.

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