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Understanding the "technicalities" of galaxy observing

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#26 ERHAD

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Posted 07 April 2021 - 02:35 AM

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

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

 

Best,

JG

Thanks for the link, a lot of interesting info and read as a result of this thread, some of it will take some careful reading and a bit of time for it to soak in, but the knowledge acquired here is invaluable!

 

Thanks!

 

Erik


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#27 ERHAD

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Posted 07 April 2021 - 02:42 AM

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).

 

Thanks for your comment. A few things have really become apparent, and clearly depart from my current observing strategies:

 

Firstly, that much more magnification than I would have anticipated can be required/advisable for many objects, I have been far too conservative with magnification, always trying to maximise exit pupil, that I need to change.

 

Secondly, something I have never tried and hadn't occurred to me, trying to tease out detail from different parts of the object by using different magnifications. As a newcomer you try to observe the object as a unit and as a whole, it is starting to become clear to me that it is not the best approach. So much that I want to put into practice as soon as I can get under clear skies!

 

Thanks again for all the excellent input, I am really glad I started this thread, learning tons..

 

Erik


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#28 j.gardavsky

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Posted 07 April 2021 - 03:27 AM

Thanks for your comment. A few things have really become apparent, and clearly depart from my current observing strategies:

 

Firstly, that much more magnification than I would have anticipated can be required/advisable for many objects, I have been far too conservative with magnification, always trying to maximise exit pupil, that I need to change.

 

Secondly, something I have never tried and hadn't occurred to me, trying to tease out detail from different parts of the object by using different magnifications. As a newcomer you try to observe the object as a unit and as a whole, it is starting to become clear to me that it is not the best approach. So much that I want to put into practice as soon as I can get under clear skies!

 

Thanks again for all the excellent input, I am really glad I started this thread, learning tons..

 

Erik

Hello Erik,

 

very nicely summarized by yours!

 

In a nutshell, it is the complementary magnification and exit pupil, and then the sizing the field, on the primary side of application or choice of the telescope.

 

On the side of the accessories,

it is the use of the filters on the galaxies, still heavily underestimated:

- An UHC filter will reveal the HII star birth regions in some galaxies, like in the M33.

- The blue wide passband interference filters from the RGB(CCD) sets, may pull out the outer spiral arms, whenever these are dominated by the bluish OB stars associations, which is often the case, see the M81 with its wide spread spiral arms.

- The yellow filters may increase the visibility of the inner dark lanes by dimming the glow in a halo of the bluish stars (M31), or of the structures in the irregular galaxies (M82).

 

Wishing you a successfull hunting of the galaxies,

JG


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#29 CrazyPanda

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Posted 07 April 2021 - 08:33 AM

Thanks for your comment. A few things have really become apparent, and clearly depart from my current observing strategies:

 

Firstly, that much more magnification than I would have anticipated can be required/advisable for many objects, I have been far too conservative with magnification, always trying to maximise exit pupil, that I need to change.

 

Secondly, something I have never tried and hadn't occurred to me, trying to tease out detail from different parts of the object by using different magnifications. As a newcomer you try to observe the object as a unit and as a whole, it is starting to become clear to me that it is not the best approach. So much that I want to put into practice as soon as I can get under clear skies!

 

Thanks again for all the excellent input, I am really glad I started this thread, learning tons..

 

Erik

In general I would say that you should do observing of an object at a wide range of magnifications, and spend some time at each magnification to let your eyes/brain adjust. Objects may look aesthetically more pleasing at one magnification than another, or merely aesthetically pleasing in different ways at different magnifications, or show more details at higher (or lower) magnifications depending on the object being observed. But definitely don't be afraid to go higher with magnification even if it minimizes exit pupil. In many cases, the eye seems to crave magnification and can tolerate a reduction in apparent surface brightness from a smaller exit pupil.

 

And yes, try to look for details in objects and note what features you can see.


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#30 Starman1

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Posted 07 April 2021 - 04:37 PM

Thanks for your comment. A few things have really become apparent, and clearly depart from my current observing strategies:

 

Firstly, that much more magnification than I would have anticipated can be required/advisable for many objects, I have been far too conservative with magnification, always trying to maximise exit pupil, that I need to change.

 

Secondly, something I have never tried and hadn't occurred to me, trying to tease out detail from different parts of the object by using different magnifications. As a newcomer you try to observe the object as a unit and as a whole, it is starting to become clear to me that it is not the best approach. So much that I want to put into practice as soon as I can get under clear skies!

 

Thanks again for all the excellent input, I am really glad I started this thread, learning tons..

 

Erik

When Seeing supports it, I think of deep sky objects like galaxies and planetaries as I think about the Moon in my 12.5":

100x--whole object visible, not much detail, but impressive.  Shape easily seen.  Lanes and spiral arms on large galaxies seen and H-II regions as small bright knots.  Planetary nebulae outline shapes seen.

200x--whole object visible, lots of details seen, shape seen and some internal details.  Medium size galaxies (Virgo Cluster) look great, H-II clouds and star clusters seen in some local group galaxies.

300x--whole or partial object seen, lots of tiny details seen, object beginning to resemble a photograph.  Spiral arms seen in many galaxies, central stars in most planetaries and some outer shells.

400x--whole or partial object seen, lots of tiny details  seen, object resembles a photograph, small faint extensions and outer features seen.  Planetaries resemble photos, galaxies have internal details and small companions seen.

500x--whole or partial object seen, internal details incredible, photographic presentation, small faint extensions and knots and features seen.  Galaxies have become fainter, but planetaries are incredible, with shells, internal details, central stars and more.

 

If I were thinking about the Moon, those same magnifications would be for:

100x--whole Moon, maybe Earthshine.

200x--generally good view of entire moon and larger craters.  Details show up in maria.

300x--great view of craters, cracks, walls, fissures, rilles.  Best view over all, but smaller craters have details just beyond reach.

400x--crater details begin to resemble photos and small peaks begin to take on shapes, revealing their elevations.

500x--stupendous views, albeit getting dim.  Crater floors are incredible.  Plato has multiple craterlets, the Alpen Valley rille is seen, you're almost in orbit.  The view from 500 miles above the Moon.


Edited by Starman1, 07 April 2021 - 04:38 PM.

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#31 Asbytec

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Posted 07 April 2021 - 04:52 PM

In general I would say that you should do observing of an object at a wide range of magnifications, and spend some time at each magnification to let your eyes/brain adjust. Objects may look aesthetically more pleasing at one magnification than another, or merely aesthetically pleasing in different ways at different magnifications, or show more details at higher (or lower) magnifications depending on the object being observed. But definitely don't be afraid to go higher with magnification even if it minimizes exit pupil. In many cases, the eye seems to crave magnification and can tolerate a reduction in apparent surface brightness from a smaller exit pupil.

 

And yes, try to look for details in objects and note what features you can see.

This sums all the technicalities for me. I avoid the math and predictions of theory and just apply the concepts being discussed. The idea we want maximum image brightness and the largest exit pupil turned into a paradigm shift to try different magnifications even at the expense of image surface brightness (per square arc second). In my experience, that simple paradigm shift made all the difference in already dim galaxy viewing. I'd argue it's not only threshold objects that apply, but different (though subtle) details within the faint fuzzy. 


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#32 CrazyPanda

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Posted 08 April 2021 - 08:30 AM

This sums all the technicalities for me. I avoid the math and predictions of theory and just apply the concepts being discussed. The idea we want maximum image brightness and the largest exit pupil turned into a paradigm shift to try different magnifications even at the expense of image surface brightness (per square arc second). In my experience, that simple paradigm shift made all the difference in already dim galaxy viewing. I'd argue it's not only threshold objects that apply, but different (though subtle) details within the faint fuzzy. 

Absolutely. Star forming regions or dust rilles in galaxies 30-50 million light years away, are often not visible without high magnification. Bringing an object to a size appropriate for your low resolution rods to perceive it is critical.


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#33 Asbytec

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Posted 08 April 2021 - 09:00 AM

Anyone post Mel's calculator yet? Pretty cool. It does all the math for us. Wonder how good it is at predicting visibility?

https://www.bbastrod...nCalculator.htm

Edited by Asbytec, 08 April 2021 - 09:01 AM.


#34 Starman1

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Posted 08 April 2021 - 09:34 AM

Anyone post Mel's calculator yet? Pretty cool. It does all the math for us. Wonder how good it is at predicting visibility?

https://www.bbastrod...nCalculator.htm

I am not sure whether his predicted magnitude range is total integrated magnitude or what.

Because Total integrated magnitude is worthless as a predictor of visibility unless the object is very small.

It looks like he is predicting the best exit pupil to look at an object of a particular magnitude, and that simply doesn't work

if the magnitude is a total integrated magnitude.


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#35 Asbytec

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Posted 08 April 2021 - 05:22 PM

Don, it says his "limiting magnitude" range is for 7 to 2mm exit pupils, so I am not sure he's talking about galaxies. His apparent brightness variable is the integrated magnitude of the object. On predictability, however, we're still dealing with the variation in brightness across an extended object. I like his calculator, though, because he seems to include the relevant variables and provides some results. I've done the calculations piece meal using surface brightness calculators and scratching some math on paper. I have not been very successful. 



#36 Starman1

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Posted 08 April 2021 - 05:36 PM

Don, it says his "limiting magnitude" range is for 7 to 2mm exit pupils, so I am not sure he's talking about galaxies. His apparent brightness variable is the integrated magnitude of the object. On predictability, however, we're still dealing with the variation in brightness across an extended object. I like his calculator, though, because he seems to include the relevant variables and provides some results. I've done the calculations piece meal using surface brightness calculators and scratching some math on paper. I have not been very successful. 

Take two deep sky objects: both Total Integrated magnitude 16.0.

One is 1'x1', the other is 10'x10'.

The second one has a surface brightness 5 magnitudes lower than the first, magnitude 16.0 and 21.0 per square arc minute.

To get equal apparent brightness per square arc-minute, you'd have to use an exit pupil 10x wider on the larger object than on the smaller one.

 

The point is, total integrated magnitude is an important figure on very small objects, but it is not as important as the surface brightness magnitude on larger objects.

Any calculator that fails to take BOTH magnitudes into account is doomed to irrelevance as far as the visibility of the object is concerned.


Edited by Starman1, 08 April 2021 - 05:37 PM.

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#37 Redbetter

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Posted 08 April 2021 - 07:30 PM

When I looked at the calculator it was determining surface brightness based on size and magnitude.  From there it would use aperture and the contrast functions for illumination level and the eye's angular resolution at that level.


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#38 Asbytec

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Posted 08 April 2021 - 11:28 PM

Don, yes, I understand and agree with you. 



#39 Dave Mitsky

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Posted 09 April 2021 - 12:31 PM

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.

I've seen a few of the high surface brightness planetary nebulae such as NGC 3242 at magnifications of well over 1000x through large truss-tube Dobs with very interesting results.

 

https://observing.sk...r/NGC_3242.html


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#40 Asbytec

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Posted 09 April 2021 - 07:51 PM

A perfect example is NGC 2022. Looking for it at 100x it's pretty much stellar. It is fully resolved by the telescope, but not large enough on our eye. At 300x its annular form is easily visible. This is not the telescope seeing it, the annular structure is already resolved it on the focal plane. This is our eye resolving the telescopic image. Proper magnification is important, even if that is ludicrously high magnification sometimes. 




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