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
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:
Exit pupil: 2mm
Contrast of object+sky to sky: 9.09% (or a log contrast of 0.56).
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:
Exit pupil: 1mm
Contrast of object+sky to sky: 55.02% (or a log contrast of 1.11).
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:
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.
Edited by ERHAD, 02 April 2021 - 03:23 AM.