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Understanding the "technicalities" of galaxy observing

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#1 ERHAD

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Posted 01 April 2021 - 06:39 AM

First off, this is a long post, a bit on the geeky side. If like me, despite my limitations, you are interested in the ins and outs and the technical side of this hobby, you might find this interesting and it might spark a productive conversation. Else, you might want to stop reading now...

 

Having had a COVID related curfew (not allowed out after 10 PM)  for many, many months now, I am stuck in the city, only dreaming of finally being able to go to my dark skies (SQM 21.1, 75 min drive). As a "sad" substitute, I have embarked on a journey of learning and understanding, trying to grapple what really goes into observing the coveted really faint stuff.

 

I am interested in the visual observation of mainly galaxies, globulars, PN and all the elusive faint stuff. I purchased (and am really enjoying) "Galaxies and How to Observe Them", by Steinicke and Jackiel. What a wonderful book, specially the chapter on "Theory of Visual Observation", highly recommended. I have come to understand that many of the tings I thought I knew about observation were wrong, and that all my misconceptions had me chasing galaxies the wrong way (hence my poor-ish results so far).

 

There are a couple topics I am trying to get my head around, though. In the mentioned chapter, the authors go to say:

 

"The eye is very effective in detecting faint objects under low contrast conditions. Contrast is defined by C = Is/In. -where "Is" stands for object intensity, or "signal" and "In" for night sky intensity or "noise"-  A high value is necessary for visibility, but not sufficient. The very quantity is called “contrast reserve” ∆C. It is the difference between the contrast due to the object ( C ) and a “threshold contrast” (CT), which is the minimum contrast, needed for the eye to perceive a luminous area under the given sky conditions.

 

Now magnification enters the scene. What happens with a large, faint (i.e., low surface brightness) galaxy at higher magnification? Following the rules described previously, the exit pupil decreases and thus the apparent brightness (A) of both the object and the back- ground gets lower. Therefore the contrast remains constant. We might have won nothing – in theory. Fortunately, this is (again) not the whole story.

 

Remember, that the eye rewards a higher magnification in case of averted vision! Not only the amount of light detected by a single rod is important, but also the number of rods involved, i.e., the corresponding area of the retina covered by the light. With the aid of the ganglion cells, the eye–brain system is able to combine many rods to intensify the signal. Thus the perception depends on the “viewing angle” under which the object appears at the retina. Ideally this angle is 1°–2°. Most objects are not that large. For all smaller ones, simply increasing magnification will make it! Take for instance a faint detail in a galaxy, measuring 1′ on the sphere. A magnification of 60 –120 × is sufficient to blow it up to the required apparent size. Increasing the magnitude, some parts of the galaxy disappear, while other come out of the dark.

Not only the object’s area in the eyepiece is important, but also the rest, i.e., the back- ground. Its detection also depends on the area ratio. If the magnification is too low (small object, large background), the resulting area ratio (“signal difference”) is insufficient for the brain. The object is lost in the background noise. A higher magnification dims both the object and background (constant contrast), but the ratio of their individual sizes on the retina increases, the object appears. If magnification gets too high, the object fills most of the field of view and the signal difference decreases again. Thus there must be an “optimum detection magnification” (ODM) for extended objects."

 

 

Fascinating stuff! Now, it is the retina "viewing angle" that I'm trying to get my head around. If I understand all this correctly, having a bigger area of the retina covered by the object being observed might improve the "signal" and hence there is something to be gained by increasing magnification (within the limits of light gathering and exit pupil), and it is this fine balance between exit pupil and magnification that can give us the best results.

 

Now, in trying to understand the apparent angle of an object in the eye/eyepiece, the authors cite an angle of 1º-2º as ideal. The mentioned Mel Bartels , in his online contrast calculator, says the following:

 

"Very faint objects smaller than five degree apparent size can be impossible to see; the larger the apparent size the better as long as the apparent size does not exceed the eyepiece's apparent field of view."

 

I am trying to understand what the practical implications of all this are when at the eyepiece and would love to hear from you. If I have a desired goal of say, 2º apparent size for the object in the retina, how do I go about  calculating that?  Do I simply multiply the object's size (in arc minutes) by the magnification to know the apparent size? also, what do you make of the differences cited (1º-2º versus 5º in the case of Mr Bartels)?

 

The chapter on "Theory of Visual Observation", continues by saying:

 

Concerning the “optimum detection magnification” (ODM) we need to distinguish between two cases, one of which finally introduces the quantity “aperture.” For faint, small galaxies (moderate surface brightness) the ODM is high, thus we need a sufficient aperture. For faint, large galaxies (low surface brightness) the ODM is lower. We don’t need large telescopes in this case! Thus a small telescope can readily detect large low surface brightness galaxies of the Local Group, while a large aperture often reveals nothing.

 

To calculate the contrast difference (∆C) and the ODM the following quantities must be known: surface brightness of the night sky and the object (nominal value), and the telescope aperture. A positive value of ∆C promises visibility. A zero or negative value means simply: “next target!” Mel Bartels has developed a nice tool to calculate the relevant quantities. It demonstrates impressively, that in most cases the darkness of the night sky is more important than aperture"

 

Mr Bartels' online calculator can be found here:  https://www.bbastrod...nCalculator.htm

 

Using it I have learnt a number of things:

 

First, that at least in theory, there is a lot that is achievable with my scope (C9.25) at my local dar skies (SQM 21.1-21.3). I would love to get a larger scope, but for now I am determined to sharpen my skills and see what I can extract from my current setup.

 

Secondly, that the recommended magnifications for observing certain objects (where you are within the threshold of observability and at the desired ODM)  seem to be quite a bit higher that I thought or was expecting.

 

Let's have a look at a couple specific examples using all this info and the results of the recommended observing parameters from Mr. Bartels online calculator 

 

M101

Not the most challenging object out there, nor the easiest either. With an apparent magnitude of 7.86 and an apparent size of 28′.8 × 26′.9, it would result in (according to the calculators mentioned) an object surface brightness of 23.71. Plugging in all the data from my skies, optics and the object, the calculator spits out an "optimum" result of:

 

Magnification: 117x  

Exit pupil: 2mm  

Contrast of object+sky to sky: 9.09% (or a log contrast of 0.56). 

 

M51

With an apparent magnitude of 8.4 and an apparent size of 11.2' × 6.9', it would result in an object surface brightness of 21.75. The recommended values from the calculator:

 

Magnification: 279x 
Exit pupil: 1mm 
Contrast of object+sky to sky: 55.02% (or a log contrast of 1.11).

 

NGC3628

To cite an example that is a bit more challenging than M51. With an apparent magnitude of 10.2 and an apparent size of 15' × 3.6', it would result in an object surface brightness of 23.16. The recommended values from the calculator:

 

Magnification: 78x
Exit pupil: 3mm
Contrast of object+sky to sky: 15% (or a log contrast of 0.4).

 

 

Let's look into this results. But first let's have a look at what Mr Bartels says with regards to interpreting these results:

 

"The chart plots the visibility of extended objects like nebulae and galaxies. The object is visible when the eye's perceived contrast -(log)- is greater than zero. However, because of the impreciseness of object magnitudes and object sizes, it is better to divide the log contrast into zones. Log contrasts greater than 0.5 are easy, contrasts down to 0.25 are visible, contrasts between -.25 and 0.25 are difficult and log contrasts under -0.25 are not visible."

 

He continues by adding:

 

"Experienced observers have no trouble viewing objects with 6% contrast and can observe objects with difficulty down to 3% or so as long as the object is at least of 3-5 degrees apparent size."

 

From all this it follows that there is a range of magnifications at which different objects work, that each particular object requires a different setting, and that if so, doing your homework before observing might pay off. With all this in mind, an object such as M51 should be "very easy" to observe.

 

Now, I have observed M51 a few times successfully, however, in a good night, If I am rested and relaxed,  I can barely see detail in the spiral arms with difficulty and can't clearly see the bridge between it and its companion, albeit, I have always observed at much Lower magnifications, trying to maximise exit pupil, so I am eager to get out there under the dark sky and try the higher magnifications suggested to see if there is an improvement. I must admit, that I have to improve my technique when it comes to fully dark-adapt my eyes too.

 

I would be delighted to hear from you. What do you make of all this? What is your experience when observing galaxies, what are your"goto" magnifications and exit pupils, and what do you think about some of the concepts here mentioned. Hopefully we can have a conversation where less experienced members like me can learn a thing or two from some of the more knowledgeable members here.

 

Thanks for reading.

 

Erik


Edited by ERHAD, 02 April 2021 - 03:23 AM.

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#2 Starman1

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Posted 01 April 2021 - 04:25 PM

You might like this DSO visibility calculator:

https://www.cloudyni...isibility-chart


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#3 Redbetter

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Posted 01 April 2021 - 05:09 PM

I haven't looked that closely at the ODM and contrast calculation methods and I don't have the book you reference by Steinicke and Jakiel.   I have seen various helpful things presented by them and have used such as a guide for some of my observations.  However, I am not that enamored of the ODM concepts and their real world application.   I don't buy the "area ratio" explanation at all.  About the only time I would consider area ratio is if the object was so large in the eyepiece that there was not enough free area around it to "frame it" and provide the necessary contrast with its own intensity gradient.

 

I prefer Mel Bartels' general approach although I have not dug into the size calculation aspect or his contrast calculations in detail.   Using "log contrast" makes it more abstract, less useful in the field to me, since I tend to do this in my head anyway.   I just use the object surface brightness versus the sky brightness as my contrast metric...much easier.   I expect to be able to detect objects that have a surface brightness 3 magnitude dimmer than the background sky (e.g. 21.5 MPSAS + 3 MPSAS = 24.5 MPSAS.)   I have a shot at 3.5 MPSAS dimmer (~4%) and perhaps even much as 4 (2.5%) but this latter is so marginal that one will tend to have a lot of self doubt about the observation.

 

What Mel says about 6% over brightening being visible matches my impression.  Lower than that is trickier and requires more apparent size and more careful study.  For galaxies, an apparent size of 2 degrees is toward the low end from what I have seen.  Mel's 5 degrees is probably closer, but it also depends on the nature of the object.  

 

The calculation for M101 looks way off with respect to surface brightness.  You have it listed as 20.93.  It would be closer to 23.8.  

 

As a general evaluation of visibility, surface brightness is quite useful.  However, I fear that many of the attempts to identify optimum magnifications will provide misleading results.  Surface brightnesses are both useful conceptually to explain what we see or don't see, and they are are also fraught with complication.     

  • They are quite approximate.  Are these visual, or photographic/B mags?  Are they for the full extent of the object or a portion of it?
  • Most objects are not close to uniform surface brightness.  So the mean surface brightness does not really apply to the bright clumps (galaxy hubs/cores).  Nor does it apply to lower surface brightness outer extents.  And there is a lot of "empty space" in some galaxies...which means the visible portions have higher surface brightness, but smaller scale.
  • It is easy to mix bases...V mag with B 25 MPSAS isophote dimensions.  Some galaxies have color indices (B-V) of 1 MPSAS.  So what does this mean for the actual contrast difference of a given size.
  • And various galaxies/types extend past the 25 MPSAS isophote.
  • Even the base visual magnitude of a galaxy is up for some debate, which is critical to determining surface brightness.

For the extreme example of the difference between mean surface brightness and an objects visibility, consider objects that have such low published surface brightness that they are at best marginal in pristine sky, Ursa Minor Dwarf for example.  Yet with a sufficiently large scope this object could be detected via resolving stars in the galaxy itself--panning to establish the over brightening of field stars.  Some globlulars are like this depending on the sky one is observing from and the aperture.  Palomar 4 and NGC 5053 are examples for different apertures.

 

As objects become smaller and dimmer (higher magnitude), seeing becomes an increasing factor.  The main visible glow of threshold galaxies that are only 20 arc seconds (or less) across on their major axis are blurred to invisibility unless the seeing is relatively steady.   That is the different between being stuck at moderately high power versus the magnification needed for detection/confirmation. 


Edited by Redbetter, 01 April 2021 - 06:54 PM.

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#4 Asbytec

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Posted 01 April 2021 - 06:35 PM

The technicalities are great concept information. Because they generally apply to a uniformly bright surface and converting intrinsic brightness to surface brightness is an inexact science, I find variations of surface brightness make applying theory problematic in practice.

I agree with Red and usually attempt targets around 3 MPSAS, or slightly more, darker than the sky. I find the value in the concept lay with the idea a large exit pupil is not always the most productive. In fact, I tend to observe brighter NGC galaxies between 2mm and 1mm exit pupils and sometimes less than 1mm. Often I'll observe them with several exit pupils.

I boiled the concept down to starting at lower magnification, not being afraid to sacrifice some precious surface brightness to gain some image scale, and trying higher magnifications until the object becomes less productive. In the latter case, I drop down one notch to find my ODM. Doing so requires a little time at each magnification to get a feel whether any object detail is becoming more or less visible. It also saves time doing mental gymnastics in the field or during planning.

Observing also requires some effective techniques, such as using a hood and refreshing our eye, as well as some mental preparation, such as applying our acuity and paying attention to the dim image. I believe really paying attention, trusting what you see in a context that makes sense, recognizing what may be spurious, and knowing the difference (comes with experience) is important. I almost never know what to really expect to see. For example, a spiral arm prominent in images is often more difficult than it looks (NGC 772), but sometimes we'll detect some detail we might not expect to see. And, importantly, learn to recognize detail when you see it, then verify your observation as to whether something was actually there to be seen.

Observing galaxies, IMO, in fact observing itself is not easy. It requires we put in the time and effort, apply the above "technicalities" of the exit pupil, and use techniques that work for us. I pretty much gave up on galaxies until I (we) learned to observe them. Every time I sit down to observe a faint fuzzy, I wonder how will see anything at all, but after an hour I end with a page full of notes. I only stop observing when I'm sure I won't see anything else. All of that is fun and rewarding because we can actually make a little something of those dim objects so far away. Heck, a stellar nucleus is something. A elongated or mottled core is something. A faint halo is something. Sometimes a low contrast dark area or a sharp edge is something. All of the above is something else. :)

Interesting (to me) is the idea of placing less emphasis on the equipment and more emphasis on the observer. After all, the telescope observes nothing. We do. My signatures are derived from that experience.

Edited by Asbytec, 01 April 2021 - 06:39 PM.

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#5 Redbetter

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Posted 01 April 2021 - 11:50 PM

The problem I have with ODM and such trying to pick an "optimum" magnification for detection is that it still largely a guess.  And while a calculation might be right about magnification for detection of the object as a whole, I suspect it might not be the right choice to see various features within.  Instead, there is some reason to examine objects with more than a single magnification to reveal different aspects.  I use moderate exit pupil to see the extent of diffuse galaxies, but smaller or the smallest feasible exit pupil for the seeing in order to see other structures within (core, bright knots, etc.)

 

You gain experience by examining an object at several different magnifications.  This allows a person to discover what it takes to make a threshold observation, as well as how various aspects respond to scale vs. exit pupil depending on the characteristics.

 

For example, for large diffuse objects with little inherent contrast, I will start in the large exit pupil end, 6mm and up, especially with nebulae using filters, but also with some galaxies (such as the large very or ultra low surface brightness dwarf galaxies.)  Then I sill move up in increments depending on what I see or what I am after. 

 

For clusters of very small and very faint to extremely small/extremely faint galaxies with the 20" I might start with moderate magnification and exit pupil (3.2mm at 156x) to take in the surrounding area, and identify the more prominent members.  From there I often jump right to 278x w/ 1.8mm pupil if the seeing will support it.  [Sometimes I start my starhops at 278x, but that is typically because I have a good idea of what the size and brightness will be, and where to find something.]  This might not be the optimum, but it provides enough scale to identify the readily detected very small members and allows key characteristics to be identified.  If the seeing is good I will then go to 357x w/1.4mm pupil looking for more members, or seeking additional detail.  If the seeing is particularly good or if I am looking for some specific detail I might use 500x (1mm pupil) or beyond.  The key here:  better seeing allows more magnification and smaller exit pupil to detect smaller and dimmer objects against the background. 


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#6 Redbetter

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Posted 02 April 2021 - 12:38 AM

I don't want to discourage the OP from pursuing these sorts of tools and using them for a better personal understanding.  (Afterall, I am often one beating on the drum about object contrast vs. the night sky or light polluted sky.)  I am just trying to point out some of the unspoken and real world factors that impact the calculations and their utility. 

 

Magnification level itself is one of the last factors I consider.  Instead I use whatever seems most productive for the scope in the seeing conditions and with the size of the target in mind.

 

To screen whether an object is likely visible for a given scope I need a rough idea of the following:  magnitude of the object, surface brightness, overall size/profile, and how dark the sky is that I will be/am observing from.

  • If I am screening for the 20" in dark sky (~21.5 MPSAS), I will consider galaxy targets down to 17.5g magnitude fair game and up to 18 B mag (in both cases if available or if it can be computed, the V mag is usually somewhat brighter than even the g mag, so more like 17+.)   That g mag limit for galaxies is only slightly worse than my stellar V mag limits in these conditions, because I rarely get seeing that allows a full 18 V mag for stars.
  • But this assumes that the galaxies appear to have good surface brightness--that is, closer to 22 MPSAS or better.  Because if they are instead closer to 23 or 24 MPSAS, the limiting detection magnitude will be incrementally worse.   By 24+ MPSAS the magnitude threshold is substantially handicapped even if the seeing and transparency are quite good.  So I will target something that is 24 to 25 MPSAS, but won't expect to detect it unless there is some cushion with regard to its actual magnitude.  A 15 mag galaxy with 24.5+ MPSAS surface brightness can be a real bear;  and a 13 mag with 25.5+ MPSAS surface brightness will be exceedingly difficult--even though I will be detecting tiny 17's with good surface brightness.
  • There are some special cases too, like when there are bright stars in the field, or if a galaxy is tiny and edge-on with high elongation and is approaching the magnitude threshold.  These very narrow ones can have good surface brightness, but be so thin that they are tough to recognize compared to an oval of the same magnitude and brightness.  Seeing is critical to them because less-than-stable seeing erases their contrast and prevents increasing magnification without further blurring contrast.  In images they look like they would be easy to detect in images, but in reality they are elusive, and when I find one it is seen as the faintest ethereal slash in averted vision.  These aren't ones I would detect without knowing their nature and exactly where to look.

That brings us around to the more important aspect:  once something is deemed as a likely threshold observable target, the difficulty is in actually finding it.  Understanding and identifying the field is of primary importance, particularly for the lower surface brightness targets.  Knowing the approximate size and surface brightness aid in identification/confirmation.

 

At last we are at magnification.  Once I know for certain I am in the right field, I will start progressing through magnifications looking for indications of the target.  If it is something small, I will go to high power as soon as I know that I am searching the correct area.  If it is somewhat bigger but more tenuous, I will ramp up more slowly, looking for that optimum the ODM calculations are about.


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#7 ERHAD

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Posted 02 April 2021 - 03:00 AM

You might like this DSO visibility calculator:

https://www.cloudyni...isibility-chart

Don, Thanks for taking the time to reply, and thanks so much for the resource, really interesting and helpful.



#8 ERHAD

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Posted 02 April 2021 - 03:12 AM

I haven't looked that closely at the ODM and contrast calculation methods and I don't have the book you reference by Steinicke and Jakiel.   I have seen various helpful things presented by them and have used such as a guide for some of my observations.  However, I am not that enamored of the ODM concepts and their real world application.   I don't buy the "area ratio" explanation at all.  About the only time I would consider area ratio is if the object was so large in the eyepiece that there was not enough free area around it to "frame it" and provide the necessary contrast with its own intensity gradient.

 

I prefer Mel Bartels' general approach although I have not dug into the size calculation aspect or his contrast calculations in detail.   Using "log contrast" makes it more abstract, less useful in the field to me, since I tend to do this in my head anyway.   I just use the object surface brightness versus the sky brightness as my contrast metric...much easier.   I expect to be able to detect objects that have a surface brightness 3 magnitude dimmer than the background sky (e.g. 21.5 MPSAS + 3 MPSAS = 24.5 MPSAS.)   I have a shot at 3.5 MPSAS dimmer (~4%) and perhaps even much as 4 (2.5%) but this latter is so marginal that one will tend to have a lot of self doubt about the observation.

 

What Mel says about 6% over brightening being visible matches my impression.  Lower than that is trickier and requires more apparent size and more careful study.  For galaxies, an apparent size of 2 degrees is toward the low end from what I have seen.  Mel's 5 degrees is probably closer, but it also depends on the nature of the object.  

 

The calculation for M101 looks way off with respect to surface brightness.  You have it listed as 20.93.  It would be closer to 23.8.  

 

 

 

Redbetter, thanks so much for taking the time to read my post and for the detailed response. This are all fairly new concepts for me and I am trying to slowly wrap my head around them. The whole size and ODM concept, while may not be gospel, has been an eye-opener for me with regards to not being afraid to try higher mags and explore the possibilities with smaller exit pupils. I have been mislead and believed that a bigger exit pupil is nearly always desired; clearly wrong. I have observed M51 several times, but at mags of about 50 to 70x. Also, the concept of using magnification to try to isolate/bring out certain parts of the object has been a revelation, as a newcomer you tend to go for the "big-picture", I really want to try new things the next time I'm under the sky.

Indeed, my calculation for M101, was wrong, I am correcting the original post, thanks for pointing that out.

 

The problem I have with ODM and such trying to pick an "optimum" magnification for detection is that it still largely a guess.  And while a calculation might be right about magnification for detection of the object as a whole, I suspect it might not be the right choice to see various features within.  Instead, there is some reason to examine objects with more than a single magnification to reveal different aspects.  I use moderate exit pupil to see the extent of diffuse galaxies, but smaller or the smallest feasible exit pupil for the seeing in order to see other structures within (core, bright knots, etc.)

 

You gain experience by examining an object at several different magnifications.  This allows a person to discover what it takes to make a threshold observation, as well as how various aspects respond to scale vs. exit pupil depending on the characteristics.

 

For example, for large diffuse objects with little inherent contrast, I will start in the large exit pupil end, 6mm and up, especially with nebulae using filters, but also with some galaxies (such as the large very or ultra low surface brightness dwarf galaxies.)  Then I sill move up in increments depending on what I see or what I am after. 

 

For clusters of very small and very faint to extremely small/extremely faint galaxies with the 20" I might start with moderate magnification and exit pupil (3.2mm at 156x) to take in the surrounding area, and identify the more prominent members.  From there I often jump right to 278x w/ 1.8mm pupil if the seeing will support it.  [Sometimes I start my starhops at 278x, but that is typically because I have a good idea of what the size and brightness will be, and where to find something.]  This might not be the optimum, but it provides enough scale to identify the readily detected very small members and allows key characteristics to be identified.  If the seeing is good I will then go to 357x w/1.4mm pupil looking for more members, or seeking additional detail.  If the seeing is particularly good or if I am looking for some specific detail I might use 500x (1mm pupil) or beyond.  The key here:  better seeing allows more magnification and smaller exit pupil to detect smaller and dimmer objects against the background. 

Fully understood, I clearly ned to send time under the stars and gain experience, see what works and what doesn't, but you insight has been extremely helpful. As previously mentioned, all of this has been a huge eye-opener.


Edited by ERHAD, 02 April 2021 - 03:22 AM.


#9 ERHAD

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Posted 02 April 2021 - 03:16 AM

The technicalities are great concept information. Because they generally apply to a uniformly bright surface and converting intrinsic brightness to surface brightness is an inexact science, I find variations of surface brightness make applying theory problematic in practice.

I agree with Red and usually attempt targets around 3 MPSAS, or slightly more, darker than the sky. I find the value in the concept lay with the idea a large exit pupil is not always the most productive. In fact, I tend to observe brighter NGC galaxies between 2mm and 1mm exit pupils and sometimes less than 1mm. Often I'll observe them with several exit pupils.

I boiled the concept down to starting at lower magnification, not being afraid to sacrifice some precious surface brightness to gain some image scale, and trying higher magnifications until the object becomes less productive. In the latter case, I drop down one notch to find my ODM. Doing so requires a little time at each magnification to get a feel whether any object detail is becoming more or less visible. It also saves time doing mental gymnastics in the field or during planning.



Observing galaxies, IMO, in fact observing itself is not easy. It requires we put in the time and effort, apply the above "technicalities" of the exit pupil, and use techniques that work for us. I pretty much gave up on galaxies until I (we) learned to observe them. Every time I sit down to observe a faint fuzzy, I wonder how will see anything at all, but after an hour I end with a page full of notes. I only stop observing when I'm sure I won't see anything else. All of that is fun and rewarding because we can actually make a little something of those dim objects so far away. Heck, a stellar nucleus is something. A elongated or mottled core is something. A faint halo is something. Sometimes a low contrast dark area or a sharp edge is something. All of the above is something else. smile.gif

Interesting (to me) is the idea of placing less emphasis on the equipment and more emphasis on the observer. After all, the telescope observes nothing. We do. My signatures are derived from that experience.

Asbytec, thanks for your words. Indeed I need to work on improving my technique and I also need to not be afraid to try new things, higher mags and spend more time exploring an object and its components.

 

The one thing the online calculator has served for is to reassure how much is actually visible/achievable with my Curren setup, more so than I would have imagined. The weak link in my observing train right now is clearly the observer, so lust for bigger aperture as we all may, I really want to spend some time trying to see what I am capable of with my C9.25.



#10 ERHAD

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Posted 02 April 2021 - 03:20 AM

I don't want to discourage the OP from pursuing these sorts of tools and using them for a better personal understanding.  (Afterall, I am often one beating on the drum about object contrast vs. the night sky or light polluted sky.)  I am just trying to point out some of the unspoken and real world factors that impact the calculations and their utility. 

 

Magnification level itself is one of the last factors I consider.  Instead I use whatever seems most productive for the scope in the seeing conditions and with the size of the target in mind.

 

 

I am not discouraged at all, again, it has been extremely helpful. I must admit the the whole nomenclature used to describe all things astronomy is fairly new to me, and I am still trying to wrap my head around it: V mag, MPSAS, integrated mag, surface brightness, etc... I hope I'm not alone in thinking that it can be a little counter-intuitive at first. My little monkey brain is slowly making progress...smirk.gif


Edited by ERHAD, 02 April 2021 - 03:21 AM.

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#11 Asbytec

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Posted 02 April 2021 - 03:30 AM

The weak link in my observing train right now is clearly the observer, so lust for bigger aperture as we all may, I really want to spend some time trying to see what I am capable of with my C9.25.

It was the weak link for me, too, for many years. I avoided faint fuzzies because of it. I think your plan is a good one, use the scope you have to get the most our of yourself. After all, the theory is describing what you are capable of, it's based on our (average) physiology. Not the scope.

 

Yes, in my opinion, aperture fever is a result of wanting to see more but not knowing we already can. Of course an 18" Dob is a powerful scope, but a 9.25" is enough to please us when we learn to use it. It's a skill you can take with you with increasing apertures. A funny thing happens along the way, we actually learn to become satisfied by what we can see in the scope we have and stop worrying about what we cannot see. Aperture fever is replaced by more prudent decisions to get one rather than the remedy for all our dissatisfaction with the scope we have. 

 

The sketch below is not the view through an 18" Dob, but it is uniquely my view of it through my 8" f/6. I like it because I earned it. It's all I could make of it, and I was pleased to see only that much. At least I got something out of it. I credit it to the theory and concepts of exit pupils, image scale, and surface brightness. I still had to guess and try different magnifications. 

 

NGC 1365 (rev).png

 

The problem I have with ODM and such trying to pick an "optimum" magnification for detection is that it still largely a guess.

 

I don't want to discourage the OP from pursuing these sorts of tools and using them for a better personal understanding.  (After all, I am often one beating on the drum about object contrast vs. the night sky or light polluted sky.) 

 

Yep, by all means understand the concept being described in the theory. It's an important paradigm and applying the ideas communicated in theory can help you understand observing. Applying the theory in practice is a little different than the math. It's a "guess", but the concept is sound. In fact, so much so in my opinion, I select my eyepieces based on the exit pupils I am familiar with and use often for given object types. I really do not care so much what the actual magnification is, it's image scale and surface brightness I am after. ODM is a balance or range of both. I do not (really) fill "holes" in magnification, I wonder what exit pupil I am missing and try to fill that hole. The magnification will be what it will be. 


Edited by Asbytec, 02 April 2021 - 03:46 AM.

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#12 Voyager 3

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Posted 02 April 2021 - 07:57 AM

It was the weak link for me, too, for many years. I avoided faint fuzzies because of it. I think your plan is a good one, use the scope you have to get the most our of yourself. After all, the theory is describing what you are capable of, it's based on our (average) physiology. Not the scope.

 

Yes, in my opinion, aperture fever is a result of wanting to see more but not knowing we already can. Of course an 18" Dob is a powerful scope, but a 9.25" is enough to please us when we learn to use it. It's a skill you can take with you with increasing apertures. A funny thing happens along the way, we actually learn to become satisfied by what we can see in the scope we have and stop worrying about what we cannot see. Aperture fever is replaced by more prudent decisions to get one rather than the remedy for all our dissatisfaction with the scope we have. 

 

The sketch below is not the view through an 18" Dob, but it is uniquely my view of it through my 8" f/6. I like it because I earned it. It's all I could make of it, and I was pleased to see only that much. At least I got something out of it. I credit it to the theory and concepts of exit pupils, image scale, and surface brightness. I still had to guess and try different magnifications. 

 

attachicon.gifNGC 1365 (rev).png

 

Yep, by all means understand the concept being described in the theory. It's an important paradigm and applying the ideas communicated in theory can help you understand observing. Applying the theory in practice is a little different than the math. It's a "guess", but the concept is sound. In fact, so much so in my opinion, I select my eyepieces based on the exit pupils I am familiar with and use often for given object types. I really do not care so much what the actual magnification is, it's image scale and surface brightness I am after. ODM is a balance or range of both. I do not (really) fill "holes" in magnification, I wonder what exit pupil I am missing and try to fill that hole. The magnification will be what it will be. 

Well done Norme . You've got the central bar of this classic barred spiral . Have you ever sketched NGC 1300 ? Must be pretty high for you in Philippines comparitively  . ( As does for me , unfortunately, I have a 6 lane highway to my Southern horizon ).



#13 Asbytec

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Posted 02 April 2021 - 08:16 AM

Have you ever sketched NGC 1300? Must be pretty high for you in Philippines comparitively.

Yes. Thank you. In fact, NGC 1300 motivated me to attempt NGC 1365. The latter was more difficult (for me). When I get back on my PC, I'll add a sketch of NGC 1300.

Actually, NGC 1300 is a good example where I reinforced something I learned observing NGC 772 and it's prominent arm. That is how to "detect" spiral arms. Not to really see them as distinct spiraling arms, however, rather just what's left of the fading halo outside the difficult to see, low contrast dark interspace near the core. That remaining glow almost has to be where the spiral arms are like NGC 1365 above. I could not resolve them, but I could see where they are. NGC 1300's elongated bar is very apparent. It's surrounded by a faint halo fading into darkness and inlaid with a low contrast dark interspace near the core. Some dim stars nearby.

Don't want to stray too far from topic. So to segue back, I think the theory being discussed helps. I read Glenn LeDrew's article Don linked above.

Edited by Asbytec, 02 April 2021 - 08:28 AM.

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#14 Starman1

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Posted 02 April 2021 - 09:48 AM

One good example of where magnification helps is with star clusters.

Stars are essentially points in the scope, so they do not dim with increased magnification

[in theory, they do, because the Airy discs have a size, but to our eye they do not].

As a result, the higher the magnification, the darker the background sky and the greater the contrast between stars and sky.

This is why the faintest stars are seen at the highest powers.

 

Seeing, of course, will tend to distort, enlarge, and blur the stars above some particular point, so the very highest power you can

produce with your scope will not likely be the magnification that reveals the faintest stars, but some other high power will be.

So looking for faint stars in a star cluster at low power is not productive.

 

A good example of this is a globular cluster like M13.

M13's brightest members are magnitude 11.9 or a little brighter.  That's easy in a very small scope.

But its horizontal branch (where the number of stars increases very dramatically) is around magnitude 15.0-15.1.

Stars of that magnitude are within reach of an 8" scope, but only at high power (say 240-250x in an 8"), so the appearance of M13

alters incredibly as the magnification is increased--from a somewhat nebulous object with stars in the periphery and a few scattered here and there

to a pile of stars like a mound of grains of sugar at high power.  An 8" scope doesn't fully resolve the cluster (no normal-size scope really does, because the number of stars continues

to increase down to magnitude 20 and below), but can provide a view that is wall-to-wall stars from one side to the other.

 

So stars benefit from a higher magnification.  Do other objects?

Yes, planetary nebulae are a good example.

For years I observed them at low-medium powers in order to make them more visible.  It's only been in the last couple decades I started experimenting with high powers

on them.  Today, 400x-500x is more of an average for me to view them because internal details, multiple shells, even central stars, are more visible at high powers than they are at

low powers.  They have a lower apparent surface brightness at high magnifications than at lower powers, of course, but the question I'd ask is whether you want to just notice the planetary

is there, or whether you'd like to see details within it.  For me, I'd rather see details.  I'd seen the planetary NGC7009 (the Saturn Nebula) many times over the years (it's visible in a very small scope),

but I can't say I'd ever seen it until one night I tried 500x and was amazed at the internal detail, central star, outer shell, and even the extensions on either side that became visible.

 

Since then, I've tried the same high magnifications of many other planetaries and been amazed at the differences with my notes from earlier years when I used lower powers.

 

Galaxies are faint.  What about galaxies?

Here, a good example is Stephan's Quintet (note: there are 6 galaxies here).  I's visible at 100x and looks like a faint glow covering an area near the bright galaxy NGC7331.

At 180x, it begins to look like multiple objects.  But at 300x, there are multiple galaxies here with brighter cores and fainter outer sections and even the orientations of those galaxies can be noted.

Flip back to NGC7331 and the tilt of the spiral arms can be seen and the 4 small companion galaxies in the field now not only can be seen, but also the angles of their orientations.

 

So even faint galaxies can be better seen at higher powers. 

 

I want to emphasize that that doesn't mean every object is best at high powers.  Many objects are best at low or medium powers.  If there is anything to take away from my comments it's this:

Don't be afraid to try higher powers on any object, to either see it better or to see details within it better.  Don't only use low powers thinking the object will be more visible that way.

Every night you observe, test the atmosphere to see how high you can go before things get too blurred, and use the magnifications you have to get up to that point.

It may only be 200x, or it may be 1000x, but don't be afraid to use high powers on deep sky objects just because they are not as bright as the planets or the Moon.

 

You will have to be dark-adapted to do so, however, and that means at least 30-45 minutes outside, away from all lights.


Edited by Starman1, 02 April 2021 - 09:51 AM.

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#15 Asbytec

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Posted 02 April 2021 - 10:43 AM

Don, great reply. I've tried to find NGC 660, but failed. It is not too dim, intrinsically (total integrated magnitude for the OP), but it's pretty small. A few arc minutes. I understand bringing it to a critical size for detection (higher magnification) may have dimmed it's surface brightness below my ability to detect it. Contrast was insufficient. I think this is a function of having a too small an aperture. It just could not gather enough light to spread over its surface being large enough to detect. I understand this is why it requires a larger aperture. I can't rule out getting lost star hopping, but I think I was in the right place.

Edited by Asbytec, 02 April 2021 - 10:45 AM.


#16 Astro-Master

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Posted 02 April 2021 - 02:12 PM

You might find a good zoom eyepiece like the Baader Mark IV Hyperion 8-24 Zoom helpful in determining the best power for the sky conditions when observing DSO's, and then switch to a wider field eyepiece if needed.

 

I have many high quality wide field eyepieces, but still find the BH Zoom helpful for determining the highest power for seeing conditions quickly, or for keeping a bright star out of the FOV with the narrower FOV of the zoom.


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#17 ERHAD

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Posted 03 April 2021 - 06:52 AM

It was the weak link for me, too, for many years. I avoided faint fuzzies because of it. I think your plan is a good one, use the scope you have to get the most our of yourself. After all, the theory is describing what you are capable of, it's based on our (average) physiology. Not the scope.

 

Yes, in my opinion, aperture fever is a result of wanting to see more but not knowing we already can. Of course an 18" Dob is a powerful scope, but a 9.25" is enough to please us when we learn to use it. It's a skill you can take with you with increasing apertures. A funny thing happens along the way, we actually learn to become satisfied by what we can see in the scope we have and stop worrying about what we cannot see. Aperture fever is replaced by more prudent decisions to get one rather than the remedy for all our dissatisfaction with the scope we have. 

 

The sketch below is not the view through an 18" Dob, but it is uniquely my view of it through my 8" f/6. I like it because I earned it. It's all I could make of it, and I was pleased to see only that much. At least I got something out of it. I credit it to the theory and concepts of exit pupils, image scale, and surface brightness. I still had to guess and try different magnifications. 

 

attachicon.gifNGC 1365 (rev).png

 

Yep, by all means understand the concept being described in the theory. It's an important paradigm and applying the ideas communicated in theory can help you understand observing. Applying the theory in practice is a little different than the math. It's a "guess", but the concept is sound. In fact, so much so in my opinion, I select my eyepieces based on the exit pupils I am familiar with and use often for given object types. I really do not care so much what the actual magnification is, it's image scale and surface brightness I am after. ODM is a balance or range of both. I do not (really) fill "holes" in magnification, I wonder what exit pupil I am missing and try to fill that hole. The magnification will be what it will be. 

That is a wonderful sketch. It never ceases to amaze me the sketches some of you can produce. I couldn't make one to save my life... Not only is it great, but it is also great help, it gives a very useful idea of what can be achieved at the eyepiece, what to expect and what to strive for... Thanks!


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#18 ERHAD

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Posted 03 April 2021 - 06:55 AM

You might find a good zoom eyepiece like the Baader Mark IV Hyperion 8-24 Zoom helpful in determining the best power for the sky conditions when observing DSO's, and then switch to a wider field eyepiece if needed.

 

I have many high quality wide field eyepieces, but still find the BH Zoom helpful for determining the highest power for seeing conditions quickly, or for keeping a bright star out of the FOV with the narrower FOV of the zoom.

I don't own a zoom eyepiece, but I do own the next best thing: one of Denkmeier's powerswitch diagonals. Reduced, nominal and barlowed magnifications with just one eyepiece with the flick of the switch (so to speak). I can usually go for a whole night with just two eyepieces, three tops...


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#19 Asbytec

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Posted 03 April 2021 - 07:58 AM

That is a wonderful sketch. It never ceases to amaze me the sketches some of you can produce. I couldn't make one to save my life... Not only is it great, but it is also great help, it gives a very useful idea of what can be achieved at the eyepiece, what to expect and what to strive for... Thanks!

Thank you for your very kind words. Please understand the sketch is embellished. It almost has to be because the visual image is more dynamic. A realistic view would require an animated gif with detail coming and going, and sometimes with the object not even visible. A sketch is static, it's a collection of views over an hour compiled into one static view. The object is also sometimes difficult to see visually, so the surface brightness and contrast in the sketch is made a little easier so the onlooker does not have to work as hard or wait as long. Observing is fun, but I don't want onlookers to observe my sketch for an hour. Too much work.  smile.gif

 

I found, trying a range of magnifications, to be the most productive around 100x (2mm exit pupil). It is a common rule of thumb that images are best around 2mm exit pupils. However, the sketch of NGC 1300 (below as promised) is at 1.3mm exit pupil (Dmm/Magnification) resulting in 150x in a 200mm aperture. More to the OP, I believe the surface brightness is a little brighter than NGC 1765 or possibly the sky was a little darker that night. Both result in higher contrast, at least in the brighter bar and core. It was easy to detect at lower magnification, so it was plenty big and bright. However, it was more productive in terms of detail at a higher magnification and, counterintuitively, at a slightly dimmer image. The eye likes a bright image, but it also likes a large image. Trade a little of one for the other. That's the guess we make. 

 

NGC 1300 Rev.png

 

In this sketch, again, I never saw (resolved) the spiral arms. However, at times there was a barely detectable low contrast dark interspace between the brighter bar and the rapidly fading dim halo where the spiral arms "are". Someone asked me if I could see the ends of the arms (they do not touch the bar), but I had to confess not resolving the dark space where they end. Just a faint, indistinct halo fading into darkness. Wish I could have, but I didn't. But, hey, I am not going to argue with seeing a slightly brighter patch on the bar to the NW (left). That was more than I expected. Again, I was happy with what I could see and not worried about what I cannot. 

 

Sketching is a great (I argue the best) exercise to learn observing, Or simply taking notes of everything you see. Doing so really makes us pay attention to what the telescopic image is "trying" to show us. And, back to the OP, finding the right balance between image scale and surface brightness, both a function of the exit pupil, for a given object contrast against the sky can be productive. It's counterintuitive, to reiterate, but we can see objects dimmer than the sky because the light from both the image and the sky combine to increase the contrast of the object against the sky alone. 

 

All of this is applying the principles you are discussing. As Red explains, with a lot of guess work. smile.gif


Edited by Asbytec, 03 April 2021 - 08:09 AM.

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#20 Redbetter

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Posted 03 April 2021 - 03:34 PM

I don't own a zoom eyepiece, but I do own the next best thing: one of Denkmeier's powerswitch diagonals. Reduced, nominal and barlowed magnifications with just one eyepiece with the flick of the switch (so to speak). I can usually go for a whole night with just two eyepieces, three tops...

The problem is that using a binoviewer means a dimmer (lower surface brightness) image, so the optimums likely change.  Generally, folks report that binoviewers make it harder to detect DSO's at the threshold.  



#21 ERHAD

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Posted 03 April 2021 - 06:17 PM

The problem is that using a binoviewer means a dimmer (lower surface brightness) image, so the optimums likely change.  Generally, folks report that binoviewers make it harder to detect DSO's at the threshold.  

 I do use binoviewers for solar system targets, but single eyepieces for DSO. The powerswitch diagonal can also be used in single eyepiece configuration.



#22 stargzr66207

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Posted 05 April 2021 - 03:41 PM

Another interesting source on this subject is Visual Astronomy of the Deep Sky by Roger N. Clark. He delves into the magnification/contrast question at length. He agrees that increasing the magnification to enlarge the angular size of faint objects enhances their visibility to the naked eye.  It's good reading. Haven't checked recently but I got my copy from Willman-Bell.

 

Ron Abbott


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#23 ERHAD

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Posted 05 April 2021 - 05:24 PM

Another interesting source on this subject is Visual Astronomy of the Deep Sky by Roger N. Clark. He delves into the magnification/contrast question at length. He agrees that increasing the magnification to enlarge the angular size of faint objects enhances their visibility to the naked eye.  It's good reading. Haven't checked recently but I got my copy from Willman-Bell.

 

Ron Abbott

Thanks for pointing that out. There are a couple very interesting articles in his website. This one in particular addresses the topic of  OMVA (optimum magnified visual angle):

 

https://clarkvision....mva1/index.html

 

Great read.

 

Erik.


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#24 j.gardavsky

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Posted 06 April 2021 - 06:34 AM

A good coverage on the contrast resolution vs spatial frequency is alo here,

http://www.mkrgeo-bl...f-human-vision/

 

Best,

JG


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#25 CrazyPanda

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Posted 07 April 2021 - 01:30 AM

I would be delighted to hear from you. What do you make of all this? What is your experience when observing galaxies, what are your"goto" magnifications and exit pupils, and what do you think about some of the concepts here mentioned. Hopefully we can have a conversation where less experienced members like me can learn a thing or two from some of the more knowledgeable members here.

 

I haven't tried doing much threshold observing with my scope. Most Messier and NGC galaxies are obvious in my 15" and ~21.0-21.3 skies.

 

To that end, the first eyepiece in my focuser when galaxy observing is the 10 Ethos. With my dob coma corrected to F/5.2, that yields just under a 2mm exit pupil and 197x magnification. There is almost never a galaxy I can't at least detect at that exit pupil and magnification. Anything that I cant is typically too faint for my light pollution levels anyway. For a target like M51, it is optimal to my eyes. I have focal lengths of 12.5, 11, 10, 9, and 8. Some would call me crazy for having such tight spacing, but there is no question that 10mm on M51 is optimal for that target for my scope and skies. Meanwhile it's poor on M101. The 12.5 is better suited for that. For M33, I always see spiral arm definition best in the 11mm (which is a DeLite), despite the narrow field of view. 12.5 is better on M81. But M82 soaks up magnification like crazy, and I'll use either the 6 or 3.7 ethos on it (and would probably find the 4.7 Ethos optimal if I had one).

 

For galaxy groups like Stephen's Quintet, higher magnification wins. 8mm or 6mm ethos, despite how dim they are.

 

Objects like the Needle Galaxy benefit from different magnifications and exit pupils in different ways. The 10 Ethos seems to reveal its arms the best, but the 6mm shows a stellar nucleus rising above its dust lane in a way that the 10 does not.

 

Tonight I did observe IC 1101, which is a billion light years distant. This *was* a threshold detection challenge, and the 8 Ethos proved to be the optimal balance of brightness and magnification to see it in averted vision.

 

Also, if you have a zoom eyepiece, try it on M31 some time. You can see how there's a point where the dust lanes in just sort of "pop". Some optimal threshold of size and brightness creates the perception of best contrast, and they really stand out. I think a zoom is such an essential tool for galaxy observing I'm considering getting a good one (either the APM when it comes out, or just splurging on the Leica).

 

As far as this paragraph goes:

 

 

 

Concerning the “optimum detection magnification” (ODM) we need to distinguish between two cases, one of which finally introduces the quantity “aperture.” For faint, small galaxies (moderate surface brightness) the ODM is high, thus we need a sufficient aperture. For faint, large galaxies (low surface brightness) the ODM is lower. We don’t need large telescopes in this case! Thus a small telescope can readily detect large low surface brightness galaxies of the Local Group, while a large aperture often reveals nothing.

 

I totally believe it. I see M101 and M33 just as easily in my 60mm finder at 10x as I do in the 15" at any magnification. Of course, I can see *detail* in those galaxies a bit better with the 15", but I don't need the 15" aperture to actually detect them.

 

But for most other galaxies, I absolutely need the bigger aperture and higher magnification to detect them. They are invisible in the 60mm.


Edited by CrazyPanda, 07 April 2021 - 01:30 AM.

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