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Equipment Discussions >> Reflectors

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Big dob for objects with structure and detail?
      #5910329 - 06/08/13 08:21 PM

I know from the start that some or even many may argue that most all DSOs have discernable structure and detail and that is part of what is so neato about astronomy--staring long enough and hard enough and using averted vision to discern those details!

But my point of view is that a faint smudge of a galaxy that is literally a bluish smear does not qualify and even brighter little smudges (blue snowball) don't get me excited. I am interested in a list of objects with **magnificent** detail such as the whirlpool galaxy, the sombrero galaxy, many globular clusters, and of course the orion nebula--these sorts objects--that one can readily detect much structure and intricacy with a large size reflector (say 10" and up). I also like many open clusters because I think they are beautiful.

Does anyone have such a list?

My reasoning is #1, I would like to have such a list, if already available, for my own enjoyment as these are the objects I like to view.


#2, I am debating about my future in visual astronomy and what telescope I want to have. I currently have a 10" Orion and and 25" Obsession. I am considering getting a 24" or even 20" Starmaster and selling the Obsession. Both would be shorter and I would just go with the 20" for not needing a ladder at all, it fitting through doorways, it being easy for one person to mess with the mirror, etc., but I know the difference in views between a 20" and 24" can be pretty big and so I don't know if I want to downsize from the 25" to a 20".

However, I use my larger aperture reflector to try to see more of already very visible objects and not to find tiny smudges that can't be seen in smaller reflectors. All I care about is seeing more detail in already detailed structures like m51. So I'm wondering if 25" of aperture is overkill for me because I'm just not hardcore and like looking a 'pretty things'?

I'm pretty new to astronomy, so I don't have a great knowledge of the heavens, but if out of the 1000s of objects amateur astronomers look at there really aren't but a handful of m51-like objects, and most use big apertures to resolve super faint objects rather than look at the already interesting objects available at smaller apertures, I think I would feel okay with downsizing for comfort as I am only going to view that handful of objects I am interested in, and 25" is a lot of scope to mess with for that. Does this make sense?

Any advice here on the objects I like or the benefit (or lack of benefit) of huge aperture for what I am doing?

Thanks,

Rob


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910374 - 06/08/13 08:49 PM

Much of what facilitates the detection of detail in extended objects is surface brightness. For example, compare 0.7 arcminute Jupiter with a common 10 arcminute galaxy. Even though the latter has 200 times the surface area of the former, it presents but a *fraction* of the detail. In spite of the fact that the galaxy has much detail to similar contrast and scale. If we could magically bring the galaxy up to the surface brightness of Jupiter, the detail seen would be simply staggering!

It's our lousy resolving power at low light levels which is the hindrance. And no telescope can present an image having surface brightness higher than delivered by a smaller scope--and even the unaided eye. The bigger scope merely allows to magnify detail while preserving surface brightness.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910376 - 06/08/13 08:50 PM

Quote:

But my point of view is that a faint smudge of a galaxy that is literally a bluish smear does not qualify and even brighter little smudges (blue snowball) don't get me excited. I am interested in a list of objects with **magnificent** detail such as the whirlpool galaxy, the sombrero galaxy, many globular clusters, and of course the orion nebula--these sorts objects--that one can readily detect much structure and intricacy with a large size reflector (say 10" and up). I also like many open clusters because I think they are beautiful...

...
I'm pretty new to astronomy, so I don't have a great knowledge of the heavens, but if out of the 1000s of objects amateur astronomers look at there really aren't but a handful of m51-like objects, and most use big apertures to resolve super faint objects rather than look at the already interesting objects available at smaller apertures, I think I would feel okay with downsizing for comfort as I am only going to view that handful of objects I am interested in, and 25" is a lot of scope to mess with for that. Does this make sense?




Rob:

Such a list is short.. there are only so many showcase objects...

But objects like the Blue Snowball, the Eskimo Nebula, for me, I consider these to be relatively bright and with magnification can put on quite a show. At 100x, there is not so much to be seen but at 500x, really idling along for a 25 inch, the reveal their treasures. Of course, they also reveal their secrets in smaller scopes... a 25 inch scope is not needed for most objects I observe...

In my mind, downsizing makes a lot of sense. I have scopes ranging from 60mm to 25 inches, I enjoy them all. When you asked about upsizing to the 25 inch, I thought you would be wiser to spend more time learning and developing your skills before taking on a very large scope. Bigger scopes show more but what has made the the biggest difference for me is simply that over the years, even as my eyes have gotten worse, I have gotten better.. I see more now at 65 than I did at 45 because I have those years under my belt.

If I had one wish for you, it would be to slow down and just spend some time patiently enjoy the night sky with a telescope, any telescope, any size.. any quality.

There are nights when I do use my 25 inch, probably more nights when I use my 16 inch but there are also nights when I break out my Celestron Powerseeker 70 and just enjoy what it can show me...

Jon


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Jason D
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Reged: 10/21/06

Loc: California
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910382 - 06/08/13 08:54 PM

Rob, how does the view of M51 in your 25" Obsession compares to your 10" Orion reflector?
Jason


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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910384 - 06/08/13 08:57 PM

NGC 891 is a nice one when it gets in a nice viewing position, it's very nice in my 16" at a dark site.
[image]http://[/image]
[image]http://[/image]
[image]http://[/image]


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: Jason D]
      #5910422 - 06/08/13 09:20 PM

Quote:

Rob, how does the view of M51 in your 25" Obsession compares to your 10" Orion reflector?
Jason




M51 looks better in the Obsession than it does in the Orion. I can see more detail. Arms and dust lanes are plainly visible (although still faint and sort of hazy) in the 25" and, while there, I wouldn't call them 'plainly' visible in the 10" from what I recall from last time I used the 10" a couple of months ago.

I definitely 'get' the larger aperture making the objects I like to view look better. But I'm just thinking if there are only 30 objects that really fit the bill for me(I really have no idea how many there are, just a guess), is the big scope worth the hassle factor? I guess that is a question only I can answer. What I was really hoping for is for some one to tell me how many of these sort of targets there are: 20, 30, 50, 100, 200?

Adding to my troubles is I like to 'try' everything out. I have a 10" reflector and I wanted to know what a big boy was like so I got the 25" Obsession. We all know the story of my mirror and while it's okay, I still want to know what a superb mirror would be like in the same aperture and what sort of difference that makes (or doesn't). I'm then curious as well if a smaller aperture (20") scope with superb optics would be as good or close to as good as a larger aperture (24-25") with okay optics and, if so, I wouldn't give up good views and then I get the benefit of no ladder, lighter telescope components, movement through doorways, etc.

I may not find the 'magic' for me, I may just end up trying various things. The problem is its a hassle and expensive to "try" large dobs out (I know that a star party would allow me to look into different scopes but I've yet to attend one--I'm a loner astronomer Yes... I know... part of the problem you say.).

Maybe I should go for the smaller scope (well 20" really isn't too small is it) with superb optics and if I don't use it a ton because I don't look at but a handful of objects I'll still sleep well at night knowing I have an awesome scope that I can fit through door ways that doesn't take up the space of a small car and which I can remove the mirror from without asking my wife for help .

Rob


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910456 - 06/08/13 09:31 PM

Quote:

I still want to know what a superb mirror would be like in the same aperture and what sort of difference that makes




Whatever telescope you decide on, I recommend moving up to premium optics, and good quality eyepieces.


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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910473 - 06/08/13 09:38 PM

Quote:

I know from the start that some or even many may argue that most all DSOs have discernable structure and detail and that is part of what is so neato about astronomy--staring long enough and hard enough and using averted vision to discern those details!




No matter what size telescope you have, there will always be objects that appear as dim smudges. Its just that a dim smudge in a four incher will show detail in a 20 incher. I have owned a 20 and 18 inch Obsession, a 13.1 inch Coulter, a 12.5 inch Portaball, and a couple of 8 inch SCTs. I sold them all--except the 12.5 inch Portaball. For me, it is the perfect sized telescope. It is as easy to haul, set up, and take down as an 8 inch SCT. However, there is an additional wrinkle in the question you posed--dark skies. I have found that it does not matter much how large was my telescope if I was viewing in light polluted skies. Finding a good dark sky site is just as important in your quest as settling on what size telescope you should buy.


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bunyon
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Reged: 10/23/10

Loc: Winston-Salem, NC
Re: Big dob for objects with structure and detail? new [Re: GeneT]
      #5910499 - 06/08/13 10:08 PM

It depends what you mean by "magnificent" detail. If you mean M42 or M51. No, there aren't many objects like that. A dozen? Two at most.

But there is detail in many DSOs. It won't be knock your socks off but with an experienced eye, you will see it. That includes hundreds of objects in a 25 inch scope. But even the couple dozen blow you away objects aren't walk up to the scope from your living room, take a peek and see it all. Visual observing through a telescope requires dark skies, dark adapted eyes, a good scope, and, most important, experience and training and an appreciation for subtlety. No two ways about it, visual astronomy is a subtle art. The biggest, brightest most awesome object isn't going to blind you and you'll have to poke and prod with averted vision, nudging the scope, etc. to see everything there is to see. It takes patience and time to learn how to see.

I'm not trying to be self-righteous. I started observing when I was 12, many years ago. As Jon says, I see a lot more now with worse eyes (and skies). For me, for many visual observers, I think, it is that slow reveal that is appealing. It is seeing some small new thing every year, teasing out just a little more information from an object that to a newcomer looks like a grey smudge. That is, at its essence, what visual observing is: seeing just a tiny bit of the universe that you haven't seen before each time out. If you take the time, the universe will show herself to you. If you don't take the time, it won't matter how big or premium your scope is, she won't.

Again, I'm not trying to talk you out of visual astronomy at all. I would caution, as others have, to move slow, take your time and gain some experience. I would also add, don't imagine money can overcome the need for experience. Anyone will see more in a big premium scope but one who hasn't learned to observe won't see all that can be seen in any scope.


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Jason D
Postmaster
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Reged: 10/21/06

Loc: California
Re: Big dob for objects with structure and detail? new [Re: bunyon]
      #5910515 - 06/08/13 10:24 PM

Quote:

Again, I'm not trying to talk you out of visual astronomy at all. I would caution, as others have, to move slow, take your time and gain some experience. I would also add, don't imagine money can overcome the need for experience. Anyone will see more in a big premium scope but one who hasn't learned to observe won't see all that can be seen in any scope.




I completely agree.
Rob, take it easy. Do not sell or buy scopes. Get more mileage of what you already have first.
Jason


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Fred1
Carpal Tunnel
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Reged: 09/19/07

Loc: Somewhere in the Orion Spur
Re: Big dob for objects with structure and detail? new [Re: Jason D]
      #5910528 - 06/08/13 10:34 PM

Rob, one of the founders of my club puts together these videos every month. You'll find most of the galaxies you asked about as you move through the continuing series.
Galaxy Log After accessing the site click on Galaxy Log Videos on the right side.

Edited by Fred1 (06/08/13 10:39 PM)


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omahaastro
sage


Reged: 08/30/06

Loc: Omaha, NE
Re: Big dob for objects with structure and detail? new [Re: Jason D]
      #5910532 - 06/08/13 10:37 PM

There's a faction of amateurs who will never find what they're looking for... ie, the beautiful, even color, views of galaxies, nebulae, etc... they see in astrophotos. I think those people may need to simply pursue astrophotography to get their fix... that, or consider a video imaging system, like a Mallincam. Then, there are those of us who will continue to be thrilled, hunting down and looking for subtle details in these faint fuzzies.

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omahaastro
sage


Reged: 08/30/06

Loc: Omaha, NE
Re: Big dob for objects with structure and detail? new [Re: omahaastro]
      #5910556 - 06/08/13 10:53 PM

Yikes... reviewing the 'new to astronomy' gentleman's equipment list... it would seem he's already explored nearly every facet of the hobby, lol.

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: omahaastro]
      #5910577 - 06/08/13 11:09 PM

Quote:

Yikes... reviewing the 'new to astronomy' gentleman's equipment list... it would seem he's already explored nearly every facet of the hobby, lol.




Yes, I've started my adventure in astrophotography but still find something very special with visual. I get frustrated at the lack of 'wow' factor with many targets looked at visually, and so I'm looking for 'the list' of wonder targets that really scream awesome at you when you center them in the EP!

Rob

Edited by robininni (06/09/13 12:08 AM)


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: Fred1]
      #5910580 - 06/08/13 11:10 PM

Quote:

Rob, one of the founders of my club puts together these videos every month. You'll find most of the galaxies you asked about as you move through the continuing series.
Galaxy Log After accessing the site click on Galaxy Log Videos on the right side.




Thanks so much, i'm checking them out!

Rob


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auriga
Pooh-Bah


Reged: 03/02/06

Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5910643 - 06/08/13 11:55 PM

Quote:

I know from the start that some or even many may argue that most all DSOs have discernable structure and detail and that is part of what is so neato about astronomy--staring long enough and hard enough and using averted vision to discern those details!

But my point of view is that a faint smudge of a galaxy that is literally a bluish smear does not qualify and even brighter little smudges (blue snowball) don't get me excited. I am interested in a list of objects with **magnificent** detail such as the whirlpool galaxy, the sombrero galaxy, many globular clusters, and of course the orion nebula--these sorts objects--that one can readily detect much structure and intricacy with a large size reflector (say 10" and up). I also like many open clusters because I think they are beautiful.

Does anyone have such a list?

My reasoning is #1, I would like to have such a list, if already available, for my own enjoyment as these are the objects I like to view.


#2, I am debating about my future in visual astronomy and what telescope I want to have. I currently have a 10" Orion and and 25" Obsession. I am considering getting a 24" or even 20" Starmaster and selling the Obsession. Both would be shorter and I would just go with the 20" for not needing a ladder at all, it fitting through doorways, it being easy for one person to mess with the mirror, etc., but I know the difference in views between a 20" and 24" can be pretty big and so I don't know if I want to downsize from the 25" to a 20".

However, I use my larger aperture reflector to try to see more of already very visible objects and not to find tiny smudges that can't be seen in smaller reflectors. All I care about is seeing more detail in already detailed structures like m51. So I'm wondering if 25" of aperture is overkill for me because I'm just not hardcore and like looking a 'pretty things'?

I'm pretty new to astronomy, so I don't have a great knowledge of the heavens, but if out of the 1000s of objects amateur astronomers look at there really aren't but a handful of m51-like objects, and most use big apertures to resolve super faint objects rather than look at the already interesting objects available at smaller apertures, I think I would feel okay with downsizing for comfort as I am only going to view that handful of objects I am interested in, and 25" is a lot of scope to mess with for that. Does this make sense?

Any advice here on the objects I like or the benefit (or lack of benefit) of huge aperture for what I am doing?

Thanks,

Rob




Rob,

I don't think you are misguided at all and I don't think you need more experience to make your decision.

If your 25 inch is f/5, or f4.5, I think you will find a Starmaster 20" f3.3 very much smaller and much more convenient to use. Get it with a Lockwood mirror. You can sit down to observe. It will fit through doorways much more easily. Much easier to transport in a car or van.

Or you could get a jpastrocraft 16" f/4, which I have, with a Lockwood mirror. Extremely convenient and fun to use. Easy to set up and take down, integral wheels, which is a great help, seated observing for all objects.

A great many objects will be beautiful in either of these scopes. i too am a fan of showcase objects rather than dim hardly visible objects. Among my favorites are The Leo Trio, and the Veil. I estimate that there are at least 50 spectacular objects in large telescopes. All the bright globulars, for example, there are about 15 really bright ones, all the showpiece objects in Coma-Virgo, there are at least 20 of those, the Ring, The Helix, the Dumbell, the the Eskimo, the Blue Snowball, and M 8, 20, 43 among the bright nebulae, and lots of open star clusters, among them the Wild Duck and the three big clusters in Auriga.

With either of these scopes, you won't see quite as much detail but you will still see plenty of detail, you will see it a lot more often, and you will enjoy it more.

And Rick Singmaster, John Pratte, and Mike Lockwood are class acts, very good people to deal with.

Hope this helps,
Bill


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astrokwang2
sage


Reged: 05/06/04

Loc: Arizona
Re: Big dob for objects with structure and detail? new [Re: omahaastro]
      #5910716 - 06/09/13 01:00 AM

Quote:

... Then, there are those of us who will continue to be thrilled, hunting down and looking for subtle details in these faint fuzzies.




Yep.


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5910721 - 06/09/13 01:01 AM

Fundamentally, the realization that the visual impression of any low surface brightness 'fuzzy' can *never* come close to its photographic aspect must be hammered home.

One way to get there? Walk around your home at night when only exterior lights are illuminating the indoors. Compare the detail seen when doing the same during the daytime. Small body fonts in a newspaper article in the day are usually very much better seen than much larger headlines in a darkened room. The detail is there; it's just that under conditions of low light our resolving power sucks.


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5910777 - 06/09/13 02:03 AM

It seems you are well-funded. I think you are tired of the 25". If I were you, given what you have said, I would get a more recent (very fast focal ratio), more portable Dob in the 18"-20" range with Go-to drive built in from the maker. It's still going to require effort to use it, compared to your 10" or your CPC 1100.

If you need to sell the 25" to get the new one, then sell it. Otherwise, just hang onto it for a bit - put it in proper storage and let things develop. You may want to come back to it, either as is, or with new optics, or with a modified structure, or just to try again after you have some more experience before you sell it.

Then, given your other listed equipment, you can try the whole range of amateur astronomy at a high level and find what parts of it you want to pursue further, to focus on.

I don't remember if you listed any software. Skytools 3 won't show you images immediately like those at Hubble Heritage, but you can use its database to plug in what you know you like, and use the search filters to see how many similar objects there are in the entire sky.

I have a 12.5" Obsession. There are easily over 100 objects I find visually pleasing, just to look at them. Not all of them make me gasp, but I want to see them again and again. It sounds like you may be more selective, but with 18"-20", I think you can find 100 things too, but you're going to have to like (at least one type of) globular clusters, bright nebulae, and planetary nebulae. Maybe that's enough for you, maybe not.

I live in Burleson, not very far from you, and am not much of a joiner myself. Send me a PM if you care to.

Lee


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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: Big dob for objects with structure and detail? new [Re: derangedhermit]
      #5910860 - 06/09/13 04:11 AM

I'm a little the same way, wanting to see more than faint fuzzies. When I look at M51 I wish I had a brightness knob on my scope I could turn up, like on an old TV. All I can do is look for darker skies.
Google "galaxies" on the Internet, you'll find a list of the ten brightest. Plus other lists of galaxies. Look for the ones with the highest surface brightness.
There's one I want to see, low in the south about 9PM here right now, just above the giant globular cluster Omega Centauri. I think it's NGC 5128 a mag. 7 galaxy. Those are rare.
Check out all the nebulas in Sagittarius, they are magnificent!
Andy

Edited by acochran (06/09/13 05:34 PM)


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gatorengineer
Carpal Tunnel
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Reged: 02/28/05

Loc: Hellertown, PA
Re: Big dob for objects with structure and detail? new [Re: acochran]
      #5910937 - 06/09/13 07:11 AM

A couple of recommendations.

I really like the night sky observers guides by will bell... Also Deep Sky wonders by Sue French (a little more on the widefield side but there are goodies in their for dobs)

I would also strongly recommend getting sky safair if you have an apple or android device and download their lists.... They have the texas star party observing lists etc...

To get the most out of your scope you have to get the most out of your eyes, I love my dark skies apparel hooded observing vest (but I am pretty light polluted here)...

Lastly if you dont already have one, a loaded astrocrub filter slide, with a DGM NPB, a Lumicon Oxygen, and a good HBeta would be a recommendation.....

For a big dob under dark skies the veil is hard to beat...


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Big dob for objects with structure and detail? new [Re: gatorengineer]
      #5911075 - 06/09/13 09:58 AM

I created this list some years ago. You'll need dark skies for some, steady seeing for others, good filters and eyepieces will help, and finally, the patience to spend time at the eyepiece really observing--not just hit-and-run target shooting--and the willingness to go back again and revisit these objects, because there's often something more to see when you thought you had already seen it all--I just recently proved this to myself (and to Stephen O'Meara).

I've been observing for many years. I'm sure I've looked at M42 a thousand times and I still enjoy each opportunity I revisit this amazing object, it's like owning a Van Gogh. These objects make up my gallery, regularly open for a pleasant stroll, an occasion to share with others, or an opportunity for intense scrutiny with all I can throw at it.

By all means--have fun!


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: Vic Menard]
      #5911101 - 06/09/13 10:25 AM

GatorEngineer:

I do have Sky Safari on my iPhone and iPad. I also have a filter slide and the only filter I don't have is H-beta as I have been waiting for months for the 1000 Oaks 2" to be in stock.

Vic:

Thanks for sharing! That's perfect. I'll print it and put it to use.

Rob


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demiles
professor emeritus
*****

Reged: 11/07/06

Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5911249 - 06/09/13 12:36 PM

Sky conditions will vary night to night and so will the detail you see. I highly recommend getting to the darkest skies that's practical for you, spend time with the bright Messier objects and don't be afraid to really crank up the power. Tracking on a large scope can really take your observing to a higher level.

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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Big dob for objects with structure and detail? new [Re: gatorengineer]
      #5911620 - 06/09/13 04:29 PM

Quote:

For a big dob under dark skies the veil is hard to beat...



Amen, +1, can't agree more. Both sides. And the middle part. And some other parts. It's perfectly placed high in the sky.

M51 and M81 are also favorites, when high up.


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astrokwang2
sage


Reged: 05/06/04

Loc: Arizona
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5911622 - 06/09/13 04:32 PM

If you aren't wowed by objects in the 25, then it's an issue of experience. You can have all the cool observing guides/lists and the darkest skies on the planet, but if the granulated dust lane in NGC 891 or the hint of spiral structure in NGC 1003 doesn't impress, then more time needs to be spent getting to know the night sky.

I was thinking of the many experienced folks I've observed with. The common point of experience seems to be that at one time, we all owned smaller scopes (4,6, or 8 inch) which we used to observe the brightest targets (usually the Messiers). When you observe M51 a couple hundred times in a 8 inch, you notice a lot of variance. On some nights, there'll be distinct spiral structure seen in the main galaxy with a hint of a bridge to its companion. On other nights, it will seem like a barely visible oval smudge. Over time, you develop a definite visual memory of M51. So, on great nights of seeing, you can really appreciate the increased detail and notice tantalizing hints of other
structural details visible in larger scopes.

And when you get to observe in a significantly larger scope, those "hints" in the smaller scope (like H2 regions in the spiral arms) become visible. This is usually a wow moment! Or you may be lucky to see the "trihorn" structure flaring off the companion galaxy. The impressiveness of these details come after logging hours of NOT seeing these details in a smaller instrument or under poor skies.

In addition, these impressive details are often low-contrast. Appreciating low-contrast detail seems to be a "skill". I sometimes observe with a friend who has an engineering background. He appreciates all the scientific contexts of a nebula or galaxy's size, distance, composition, etc. But oftentimes, he doesn't see low-contrast detail. We were observing the Leo Trio (M65,M66,N3628) in his 8 inch recently, and he and another gentleman only barely saw "something" where NGC 3628 was located. Had I not pointed it out, they would have missed it. The central dust lane was not visible, but you could barely detect how one edge was flaring out. He was relatively new to amateur astronomy and had only used the scope a handful of times for night time observing.
I'm sure if he keeps observing on a regular basis, objects like NGC 3628 (aka "the Hamburger") will be routine.

As far as observing guides for visual observers, I believe guides that include sketches to be very useful. I think sketches better recreate the view through an eyepiece compared to a fancy processed CCD image. The Night Sky Observer's Guides are really useful in this regard as they often include a mixture of sketches and photos. The descriptions are certainly geared toward the visual observer.

On a personal note, I would say that I got into hunting down galaxies because I COULDN'T SEE 95% of the little ovals from the Peterson Guide's star charts. It was frustrating at first, but it became a challenge for me.
I remember stupidly questioning the accuracy of the charts, doubting my *BLEEP* SCT, blaming the cloudy skies of Seattle. Fast-forward 25 years and one night in Chiefland, FL, when I was working on my Herschel observing list, I decided to track down as many galaxies as were listed on chart 1 of the SkyAtlas 2000. I found I could see about 90% of the listed galaxies in my home-built 10" f/4.5 reflector. What had changed? I had a slightly larger scope and better skies. And I was also better prepared for what I would see: small, faint fuzzy shapes -- sometimes circular, sometimes oval -- distant light which had travelled millions of light years to reach my eye.


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bunyon
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Re: Big dob for objects with structure and detail? new [Re: acochran]
      #5912053 - 06/09/13 09:08 PM

5128 rocks if you have a good, clear, non-light-domed southern sky. It's getting a little late in the year for North America but I've seen this from TSP a few times and it never fails. Fairly easy to find as it's plainly obvious in a 50mm finder. Well worth the look.

Quote:

I'm a little the same way, wanting to see more than faint fuzzies. When I look at M51 I wish I had a brightness knob on my scope I could turn up, like on an old TV. All I can do is look for darker skies.
Google "galaxies" on the Internet, you'll find a list of the ten brightest. Plus other lists of galaxies. Look for the ones with the highest surface brightness.
There's one I want to see, low in the south about 9PM here right now, just above the giant globular cluster Omega Centauri. I think it's NGC 5128 a mag. 7 galaxy. Those are rare.
Check out all the nebulas in Sagittarius, they are magnificent!
Andy




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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5912418 - 06/10/13 03:14 AM

The "Wow!" factor isn't nearly so much about the object as it is about the observer. Many deep-sky enthusiasts get genuinely excited about detecting an object at the threshold of visibility--not because it shows magnificent detail--because they enjoy pushing their equipment and themselves to the limit. For some, it's chasing extremely distant objects that gives a thrill. For others, it's observing obscure objects which may have been visually observed only by a handful of people on the planet. Some deep-sky hounds enjoy hunting specific types of objects; just planetaries or globulars or galaxy clusters. There are as many reasons to enjoy deep-sky observing as there are deep-sky observers. The trick is to understand what excites you. Then, equip yourself to pursue it.

Bill in Flag


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auriga
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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5912621 - 06/10/13 09:07 AM

Quote:

The "Wow!" factor isn't nearly so much about the object as it is about the observer. Many deep-sky enthusiasts get genuinely excited about detecting an object at the threshold of visibility--not because it shows magnificent detail--because they enjoy pushing their equipment and themselves to the limit. For some, it's chasing extremely distant objects that gives a thrill. For others, it's observing obscure objects which may have been visually observed only by a handful of people on the planet. Some deep-sky hounds enjoy hunting specific types of objects; just planetaries or globulars or galaxy clusters. There are as many reasons to enjoy deep-sky observing as there are deep-sky observers. The trick is to understand what excites you. Then, equip yourself to pursue it.

Bill in Flag




Bill,
This is a truly perceptive post and reflects your impressive experience as an observer. It really does describe the various aspects of deep sky observing that appeal to different observers.

For me, faint objects that I can barely see don't appeal to me as much as showpiece objects, very distant objects, and globular clusters of all kinds.

I have friends who take delight in seeing objects or features that I can barely see and which I wonder if I am really seeing. I like to be sure I am really seeing the object, not just imagining it. But everyone is different.

It must be thrilling to be one of only a handful of people on the planet who have seen a specific object.

The meaning of what I am seeing is important to me also. Looking through
Baade's Window at 6522 and 6528 close to the center of our galaxy is inspiring.

Bill


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Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5912652 - 06/10/13 09:28 AM

M8, "Lagoon Nebula" is a really nice view, especailly with an OIII filter....

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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5912675 - 06/10/13 09:44 AM

All I care about is seeing more detail in already detailed structures like m51. So I'm wondering if 25" of aperture is overkill for me because I'm just not hardcore and like looking a 'pretty things'?







Rob

You show a Mallincam Video Camera in your signature. Any reason you are not using the Mallincam with, say, a C8 or C11 to see the fine detail in deep sky objects that you crave but can’t see visually with even a 25-inch scope?

If the object isn’t showing enough detail for you in the Mallincam after 15-50 seconds, your next step is acquiring time on the Hubble.

Bob


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RAKing
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Re: Big dob for objects with structure and detail? new [Re: astrokwang2]
      #5912703 - 06/10/13 10:02 AM

For me, it hasn't been the equipment as much as it has been the experience. I have owned everything from 15 inches down to 3 inches and even though my equipment has improved over the years, my eyes and expectations have improved even more.

Every object looks different every night - and that's one reason I keep going out.

Cheers,

Ron


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robininni
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Re: Big dob for objects with structure and detail? new [Re: bobhen]
      #5912705 - 06/10/13 10:04 AM

Quote:

All I care about is seeing more detail in already detailed structures like m51. So I'm wondering if 25" of aperture is overkill for me because I'm just not hardcore and like looking a 'pretty things'?







Rob

You show a Mallincam Video Camera in your signature. Any reason you are not using the Mallincam with, say, a C8 or C11 to see the fine detail in deep sky objects that you crave but can’t see visually with even a 25-inch scope?

If the object isn’t showing enough detail for you in the Mallincam after 15-50 seconds, your next step is acquiring time on the Hubble.

Bob




Bob,

Yes the reason I don't use my Mallincam currently is I have my c-11 setup for astrophotography and, more importantly, I have found that each time I have used the Mallincam, it is luke warm---giving a somewhat bland performance compared to astrophotography or visual alone. Let me explain:

In my mind, astrophotography with even minimal processing blows away the Mallincam in clarity, color, detail, etc. At the other end of the spectrum, visual has a very special 'oneness' associated with it that when removed from the eyepiece I lose. It also is especially clear and crisp, just not at the detail level I would like usually. So with a Mallincam, I get something in the middle of both which is unappealing to me. I'll probably sell the Mallincam, I just haven't gotten around to doing so.

Rob


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alexvh
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5913326 - 06/10/13 03:11 PM

Quote:

Much of what facilitates the detection of detail in extended objects is surface brightness. For example, compare 0.7 arcminute Jupiter with a common 10 arcminute galaxy. Even though the latter has 200 times the surface area of the former, it presents but a *fraction* of the detail. In spite of the fact that the galaxy has much detail to similar contrast and scale. If we could magically bring the galaxy up to the surface brightness of Jupiter, the detail seen would be simply staggering!

It's our lousy resolving power at low light levels which is the hindrance. And no telescope can present an image having surface brightness higher than delivered by a smaller scope--and even the unaided eye. The bigger scope merely allows to magnify detail while preserving surface brightness.




So at the same magnification M31 will look the same regardless of aperture?


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Jarad
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Re: Big dob for objects with structure and detail? new [Re: alexvh]
      #5913362 - 06/10/13 03:24 PM

No, it will look brighter at the same mag. But it will probably look better by increasing the mag and keeping the brightness about the same.

Jarad


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Re: Big dob for objects with structure and detail? new [Re: alexvh]
      #5913403 - 06/10/13 03:42 PM

Quote:

So at the same magnification M31 will look the same regardless of aperture?




No. At BRIGHTEST (i.e. using an eyepiece that gives you a 7mm exit pupil), M31 will be the same brightness per square degree as your naked eye view. The difference is, naked-eye, M31 covers perhaps six square degrees. At 10x, it covers six HUNDRED square degrees. At 20x, it covers twenty four HUNDRED square degrees and fills your field of view.

Using a bigger scope allows you to create a larger image at the same surface brightness. The bigger the scope, the bigger the image can be at your eye.


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csrlice12
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Re: Big dob for objects with structure and detail? new [Re: Thomas Karpf]
      #5913505 - 06/10/13 04:22 PM

Just realize that at a certain point, the FOV of a dob might be too small to "see" the entire object. Like M31, a fast short wide-field refractor(i.e. f5 like the ST80) with a low power eyepiece would be better then a big powerful dob. Sometimes raw power isn't enough......and I'm a real dob fan....

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Thomas Karpf
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Re: Big dob for objects with structure and detail? new [Re: csrlice12]
      #5913564 - 06/10/13 04:48 PM

The maximum field of view can be visualized by imagining a long skinny triangle with it's base as the clear aperture of the focuser and the length as the focal length of the scope.

With a 2" focyser, if the scope's focal length > 920mm (about 36", or a 6" f/6), then M31 will not fit in the field of view.


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: Thomas Karpf]
      #5913761 - 06/10/13 06:45 PM

To amplify on the visual aspect of an extended object (M31 was specified) at *constant magnification* while changing aperture...

As we decrease aperture from that which provides the largest useable exit pupil, the view decreases in surface brightness. Our visual system's resolving power decreases, and its noise increases. Details formerly visible (such as the pair of prominent dust lanes on the NW side) become progressively more difficult to distinguish. Then the fainter outer portions of the galaxy become lost in the noise. Then the moderately bright intermediate regions of the disk fade out. Continuing to reduce aperture (and hence exit pupil diameter) will continue to diminish the visible extent of the bright central bulge, until only the tiny core is all that remains.

This illustrates the fact that small apertures cannot do at high power what a large aperture can do at what for it is low power. This aspect of optical performance is most pronounced in the low brightness regime, where the eye's own resolving power is dictated very strongly by scene surface brightness and contrast.

Let's explore matters at constant aperture.

Where objects are bright enough for our eyes to work in the photopic regime at even small exit pupils, the detail we see scales pretty much linearly with magnification (until diffraction becomes resolved.)

But for most extended DSOs, detail perceived does not scale linearly with magnification. As power is increased, the dimmer view causes a loss in our eye's resolving power. And so detail improves at a smaller rate than expected. As the exit pupil continues to shrink, the rate of improvement in detail perceived ever more rapidly worsens. Eventually a turnover point is reached where the amount of detail actually *decreases* with increasing magnification. At just what point this occurs depends critically on the scene contrast. A very low contrast object such as the Cave nebula does not bear magnification well. But high surface brightness planetary nebula NGC 6543 in Draco can withstand 'crazy high' magnification by comparison.


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aatt
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5914171 - 06/10/13 10:18 PM

I have had a 15' for about a year now and although it was a little anticlimatic at first, I am now really appreciating the benefits of increased aperture.It is great to see so many galaxies that my 6" could not even capture-the celestial smorgasbord is now there for the taking. Teasing out those galaxy details is definitely a work in progress. I am much better now than I was a year ago and I have found that prolonged study of a single object is the best way to hone those detail skills, rather than hopping from one faint puff to another.I just got the scope out to a green zone for the first time last week and although the seeing was poor, the visual transformation of some of the brighter galaxies was tremendous.Patient study and darker skies are part of the ticket. The rest is passion and perseverance-and all the little techniques folks here talk about. The payoffs keep coming if you keep at it.

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BillFerris
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Re: Big dob for objects with structure and detail? new [Re: alexvh]
      #5914400 - 06/11/13 01:07 AM

Quote:

So at the same magnification M31 will look the same regardless of aperture?




What you'll notice if you have an opportunity to observe a galaxy like M51 (the Andromeda Galaxy isn't a good example because it's extremely large and nearly edge-on) through a range of apertures under a dark sky, is that objects and details appear more obvious to the eye as aperture increases. Also, subtle details which are at or beyond the threshold of visibility in a smaller scope will be visible at the eyepiece of a larger scope. By delivering a more intense light packet to the eye, a larger aperture allows you to discern more subtle contrasts. In other words, increasing aperture lowers the threshold contrast at which an object will be visible to the eye. The bottom line is this translates to improved views.

That said, many observers can get more out of the scope they have by investing in the time it takes to drive to darker skies to observer, by spending more time at the eyepiece to study an object and by learning to use a range of magnifications to tease every bit of detail from an object. A person often doesn't need to buy a larger scope to see more. Developing their technique and spending more time under a dark sky can deliver more of everything.

Bill in Flag


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5914463 - 06/11/13 02:30 AM

Bill,
You said, "By delivering a more intense light packet to the eye, a larger aperture allows you to discern more subtle contrasts."

Just to be clear...

For extended objects, a larger aperture by itself does not deliver a more intense light packet.

At given exit pupil, all apertures deliver equally intense light packets.

At given aperture, a lower magnification (larger exit pupil) delivers a more intense light packet.

If a more intense light packet is what's required to see subtler contrasts, then one would tend to go to lower magnification/larger exit pupil. Of course, as we know, this is not generally the case, as object size on the retina is also very important, and so a higher power is often preferred to a lower power.

Threshold contrast is controlled fundamentally by the exit pupil; larger pupils allow to detect subtler brightness variations, because visual system noise is reduced.

Threshold detail is fundamentally controlled by the aperture. A larger aperture provides a larger image at given exit pupil/surface brightness.

To say that a larger aperture lowers threshold contrast is really saying that a larger aperture reveals more detail. Which is axiomatic.

Suppose we have two galaxies which are intrinsically identical, but one is 10X more distant. The nearer one is 20' across, and the farther is 2' across. both have identical surface brightness, but in integrated light the farther one is 1/100 as bright, or 5 magnitudes fainter.

A 2" scope is trained on the near galaxy, and a 20" scope on the distant galaxy. At the same exit pupil, both views will be identical; the 10X bigger scope exactly compensates for the 10X larger distance. The larger scope is *not* lowering the threshold contrast; the perceived surface brightness, brightness profile and visible extent are the same. The larger scope is merely presenting detail commensurate with its aperture.

If the larger aperture in this example lowered the threshold contrast, we should expect to see more detail and a larger extent in the galaxy it's trained upon.

My questions to you, bearing in mind the specific experiment outlined...

Will the views be the same or different?

If different, why?

If not different, how then can it be said that the larger aperture lowers threshold contrast?


I don't like the phrase "lowering threshold contrast" because it too easily connotes the magical and incorrect notion that contrast has increased. Particularly for the less knowledgeable.

If through a small aperture I can discriminate a patch of nebulosity only when it is 10% or more brighter than the sky, can I expect to see through a larger scope a nebula that is 9% brighter than the sky, and with an even bigger scope, a nebula 8% brighter than the sky?

No. No matter the aperture, the same contrast limits apply. And so threshold contrast is not altered by aperture.


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jpcannavo
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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5914726 - 06/11/13 09:48 AM

Rob
I also enjoy the more detailed "showcase" objects. One thing to realize, though, is that the list gets longer with larger aperture - assuming observing conditions are sufficiently good.

Another thing to appreciate is that it doesn't take a long list of deep sky objects to keep you busy. I just got back from a one night 6 hour stretch under pristine Rocky Mountain skies at the RMSS. I must tell you that I spent that 6 hours with just a few objects. You mention M51 - I spent nearly half the night on M51 alone! But not all in one stretch, but rather going back and forth between M51 and just a few other objects. Each time back to M51 I would try to see more than I had just seen a 1/2 hour ago. Later in the evening I did the same with the M17 and M20. Oh, BTW do add NGC 4565 to your list, its right up at the top - it will seem almost photographic in detail with your aperture under dark skies with good seeing.

But, that having been said, this sort of experience is not everyones cup of tea. To illustrate, birding comes to mind. For me, it is remarkably similar to the endeavor we are discussing here. Telescopes (granted smaller) are used, illusive targets are hunted down and, often, remote/highly selected observing locations are sought. And, it also involves glimpsing and seeing stuff that you can see more clearly and conveniently in a magazine. Yet - it doesn't grab me! Why, I don't know, it just ain't my cup. It's particular "poetry" doesn't speak to me, perhaps in the way Haydn doesn't but Mozart does.

Getting deeply gratified by deep sky observing is a combination - for me - of several things. Part of it is straining to catch more detail that I already know is there, I know the photons are reaching me, if I can just see them! But clearly thats not enough! If it were I might as well sit in a darkened room straining to look at familiar photographs! So for me, it is also the unique flavor of the experience. The dark, the spooky night, the remote location, the complete forgetting about everything else, the quiet, the knowledge that I am peering far beyond this terrestrial realm, the finickiness of my spaceship (the scope!), and the morning and many, many days after; revisiting and savoring through memories of the views while anticipating the next visit to the same object; "Did I rally see the connecting bridge to M51 as clearly as I could? next time I'll push the mag more, or shut off tracking and let it drift into view..."

There is a gratification to this endeavor that few other things give me. And, I often find myself in a meta mode of sorts, reflecting on just what that gratification is - what it is like. So, for example, several nights ago while peering into the dark at M51, straining to see how much detail I was actually seeing, I actually asked myself: In what sense am I enjoying this? Is this more intensely pleasurable then say really good meal when very hungry? Without digressing endlessly into some phenomenological account and philosophical analysis on the nature of pleasures, I will say this. In some way the "peak" hedonic value of the M51 experience is less intense. But while the "rush" of the meal leaves me with little once it is over, the "M51" pleasure (as stated above) stays with me for weeks, months, years as I savor the memory of the entire experience and plan, prepare and look forward to the next.

My advice is to slow down, slowly draw up your list of fav objects. Start an observing notebook. Keep returning to these objects - as you would some good friends. Try to get know them well, and better and better. In the process you will discover if the poetry of the deep sky really does speak to you, or if your cup of tea lies elsewhere.

Joe


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robininni
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5914761 - 06/11/13 10:14 AM

Joe,

Very deep! Thanks for such a revealing post of your observing habits and philosophy. I agree with everything you said. And the notebook is a great idea for me. I should have already started that.

Thanks,

Rob


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BillFerris
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5915064 - 06/11/13 01:23 PM

Quote:

Bill,
You said, "By delivering a more intense light packet to the eye, a larger aperture allows you to discern more subtle contrasts."

Just to be clear...

For extended objects, a larger aperture by itself does not deliver a more intense light packet.




Glenn, you're redefining my statement to mean something I did not say. Quite simply, a larger aperture delivers more light from an extended object to the eye than a smaller aperture will. This is a simple, undeniable fact. If you point a telescope of a given aperture at a galaxy, the galaxy will have an apparent magnitude at the eyepiece. If you point a second telescope with 60% larger aperture at the same galaxy, the galaxy will have an apparent magnitude one full magnitude brighter. This real benefit of increased aperture--delivering more light from an extended object to the eye than a smaller aperture delivers--is what I'm referring to when I write that a larger aperture delivers a more intense light packet to the eye. I neither said nor implied that increasing aperture produces an increased surface brightness. Why you would respond as though I had is beyond understanding.

Bill


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robininni
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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5915075 - 06/11/13 01:32 PM

Quote:


If you point a telescope of a given aperture at a galaxy, the galaxy will have an apparent magnitude at the eyepiece. If you point a second telescope with 60% larger aperture at the same galaxy, the galaxy will have an apparent magnitude one full magnitude brighter.

Bill




As long as the magnification remains constant that statement is true, right?

Rob


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BillFerris
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5915076 - 06/11/13 01:33 PM

Quote:

I don't like the phrase "lowering threshold contrast" because it too easily connotes the magical and incorrect notion that contrast has increased. Particularly for the less knowledgeable.




The concept of threshold contrast and aperture's role in determining threshold contrast are perfectly understandable to those who bring an open mind to the table. And since threshold contrast is central to the activity of observing and detecting faint, extended objects, it's important to expose new observers to the concept. It would be easier if I didn't have to reply to posts that skew my words into something I never wrote.

Bill in Flag


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auriga
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5915087 - 06/11/13 01:39 PM

Joe,
Another very worthwhile post. It gives a sense of the wonder of observing and the aesthetic and philosophical meaning of observing the skies. Poetic. Thanks for posting.
And I agreei in general with your sentiments. Except about birds. I am not a "birder" but from what I hear, birds are not "illusive, they really exist.
But for some deep sky objects there is perhaps some doubt, as in the famous "AintNo List."
Bill


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Galicapernistein
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5915285 - 06/11/13 03:40 PM

Quote:

Rob


But, that having been said, this sort of experience is not everyones cup of tea. To illustrate, birding comes to mind. For me, it is remarkably similar to the endeavor we are discussing here. Telescopes (granted smaller) are used, illusive targets are hunted down and, often, remote/highly selected observing locations are sought. And, it also involves glimpsing and seeing stuff that you can see more clearly and conveniently in a magazine. Yet - it doesn't grab me! Why, I don't know, it just ain't my cup. It's particular "poetry" doesn't speak to me, perhaps in the way Haydn doesn't but Mozart does.


Joe




For me, birding and astronomy reinforce one another. Looking for the only surviving members of the dinosauria clade as they return from the other side of the earth is as much fun to me as looking for a distant galaxy. 230 million years ago small, upright reptiles emerged, grew big, and eventually took over the world. All we have left from those times are tough little feathered remnants. There are only a few galaxies a quarter of a billion light years away that I can see in my scope, but I can see the descendants of those ancient reptiles every day. But then, I listen almost exclusively to Bach, so to each his own.


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robininni
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Re: Big dob for objects with structure and detail? new [Re: Galicapernistein]
      #5915341 - 06/11/13 03:59 PM

Quote:



For me, birding and astronomy reinforce one another. Looking for the only surviving members of the dinosauria clade as they return from the other side of the earth is as much fun to me as looking for a distant galaxy. 230 million years ago small, upright reptiles emerged, grew big, and eventually took over the world. All we have left from those times are tough little feathered remnants. There are only a few galaxies a quarter of a billion light years away that I can see in my scope, but I can see the descendants of those ancient reptiles every day.




I'm not into birding, but I can appreciate the similarity with some aspects of astronomy. I find it fascinating to stare at galaxies of sizes and at distances we can assign numbers to, but we really can't humanly comprehend. Then to think God created the immense universe and everything in it, which is also, beyond all comprehension, and that what we occupy in this reality is immeasurably small just fills me with awe.

Rob

Edited by robininni (06/11/13 04:13 PM)


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csrlice12
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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5915387 - 06/11/13 04:26 PM

+1 on taking your time. Last Saturday I was out from 8pm - 1am and only viewed Saturn and the Lagoon Nebula. Saturn was an eyepiece swapout exercise. A person there was considering buying an Intelliscope and had just purchaed a 5mm Radian. We used the Radian in the dob on Saturn for about 30-40 minutes, then pulled back to the 11T1 (Gawd what a sharp image). Finally used the 7T1 and was completely blown away....the view was razor sharp and the Cassini division was a nice dark black, probably the best view of Saturn I've had. We tried the 5mm radian and my 5mm Pentax XW, but the seeing just wasn't up to that for either eyepiece, the views were good, you could still see the Cassini division, but it was a little blurry. Alone, I broke out the 30mm Pentax and the 24mm ES82(nonwaterproof) and the OIII filter and spent at least an hour or so viewing with these two eyepieces with and without the OIII. I was never a big filter fan; but this OIII has convinced me there really is something to them.....the background stars disappeared, but the nebula's dark lanes and glowing whisps just popped out....I'll definitley be using the OIII a lot more.

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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5915426 - 06/11/13 04:52 PM

Quote:

Quote:


If you point a telescope of a given aperture at a galaxy, the galaxy will have an apparent magnitude at the eyepiece. If you point a second telescope with 60% larger aperture at the same galaxy, the galaxy will have an apparent magnitude one full magnitude brighter.

Bill




As long as the magnification remains constant that statement is true, right?

Rob




Rob, I think what you want is the same mag for the number of inches of aperture. e.g Comparing an 18" F/4.3 to a 25" F/4. Figuring 15x per inch, then the 25" would be at 375x and my 18" would be at 270x. I believe this will give you best comparison for contrast and detail retrieval for such objects. This is just for illustration purposes only and not figuring in Paracorr use. If I'm off base, I'm sure someone will chime in.


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Re: Big dob for objects with structure and detail? new [Re: Fred1]
      #5915848 - 06/11/13 09:13 PM

To alter threshold contrast is to change the brightness ratio which can be perceived. This is exit pupil dependent, not aperture dependent. A small scope at a larger exit pupil permits to discriminate lower contrast than a big scope at a smaller exit pupil.

And so it is completely true to say that a *small* scope does lower threshold contrast as long as the exit pupil is larger.

This is fundamentally why I object to the simplistic statement that a larger aperture lowers threshold contrast.

The context in which the concept of threshold contrast is usually invoked centers around the visibility of a *specific object*, and at the *threshold* of perception of detail. This is really no more than stating that a larger aperture delivers a brighter and/or larger image, which permits to perceive more detail. The completely expected result; no surprises here.

But consider such an object as the Califiornia nebula, which is large and resolved at very low power (certainly 10X, if not less.) Which unfiltered scope do you suppose makes the nebula stand out more prominently against the sky?

A 3" at a 6mm exit pupil, or a 12" at a 6mm exit pupil? Equal image surface brightness makes for equal ease (or difficulty) in detection. Both instruments deliver images having the same contrast ratio. Being already well resolved in the small scope, the bigger scope (at the same exit pupil) offers mo real advantage for detection.

How about a 3" at a 6mm exit pupil and a 12" at a 3mm exit pupil? The dimmer view in the latter makes for a more difficult detection. The larger aperture *by itself* has not lowered threshold contrast. The smaller exit pupil has *raised* threshold contrast. In this instance, the large scope is a liability.

To say that increasing aperture lowers threshold contrast in an absolute sense can be true only when the exit pupil is increased. Otherwise, when restricting to describing the aspect of a given object, we're really restating--in a potentially misleading way--the well known fact that increasing aperture increases detail seen.


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5915879 - 06/11/13 09:32 PM

I have always looked at and understood contrast alot differently than what is stated above...

A junk mirror (in this case rough) will have poor contrast

A perfect mirror in a lousy structure will have lousy contrast. (stray light leakage)

A perfect mirror with a poor eyepiece will have lousy contrast.

A perfect mirror in an urban setting will have lousy contrast. (relative to that scope in dark skies)

A perfect mirror in an uncollimated scope will have lousy contrast.

(Larger CO also means that there are photons not getting to where they should but that is a slightly differnt story).

A perfect mirror in poor seeing will have lousy contrast. (Trap E and Trap F, dont come and go night to night.....)

Simple explanation is photons getting to where they arent supposed to be. This is the cause of poor contrast, and many inexperienced observers get lost in the sauce, but simply put this is it.

Space is black because there is no scattering of photons.

So any talk about contrast needs to start with an assumption of a sound structure, a smooth primary, a quality multicoated eyepiece (using the birder analogy, Swarovski, and Leica coatings are what make the difference), goodd seeing and a well collimated (and cooled) system.

Optics after that are a straight signal to noise ratio. the larger the antenna (mirror) the more signal is gathered relative to noise. Meter class scopes you can see color in DSO's, why because the signal is higher and the cones can pick it up....

Why use avert vision because there are more rods off of the center axis of the eye....

For extended objects the object will always look brighter in a larger scope, at the lowest useable magnification for that scope than any smaller scope (Period). This is the cause of aperature fever, which is known to beset many in the hobby.


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5915913 - 06/11/13 09:56 PM

Quote:

Quote:


If you point a telescope of a given aperture at a galaxy, the galaxy will have an apparent magnitude at the eyepiece. If you point a second telescope with 60% larger aperture at the same galaxy, the galaxy will have an apparent magnitude one full magnitude brighter.

Bill




As long as the magnification remains constant that statement is true, right?

Rob




It's true for any magnification producing a true field large enough to present the whole galaxy. It's true for all celestial objects...stars, clusters, nebulae, etc. Of course for point sources like stars, there is no magnification limit.

Bill in Flag


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5916016 - 06/11/13 10:45 PM

Quote:

To alter threshold contrast is to change the brightness ratio which can be perceived. This is exit pupil dependent, not aperture dependent.




Increasing aperture does lower threshold contrast and this is easily demonstrated. Simply observe the same galaxy cluster on the same night under the same dark sky with at least two scopes of different aperture. This can be done at any dark sky dark party. Observers will see more galaxies in the larger aperture. Some of those galaxies will be the same size as galaxies seen in smaller apertures but they will have fainter integrated magnitudes than were visible in the smaller scope. These fainter magnitude galaxies have lower surface brightnesses than their brighter counterparts. By definition, they will be lower contrast objects that were made visible by increasing aperture.

Increasing aperture reduces the threshold contrast at which objects become visible to the eye. Folks interested in learning more about the role of aperture in the visibility of faint extended objects can read the article in my signature.

Bill in Flag


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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5916079 - 06/11/13 11:30 PM

The example of bringing small objects formerly below the detection threshold into visibility by application of a larger aperture is more analogous to the case for stars.

How about the case where an object is already large and resolved, such as the huge California nebula? One can enjoy a more obvious detection in a finder or RFT than through a light bucket.


As to space being black and not scattering photons... Deep images show the galactic disk region to be suffused with clouds and general 'cirrus' which does scatter light. It's not important to visual observers because natural airglow and zodiacal light vastly dominates.

As to the gain in signal to noise afforded by a larger aperture... The gain in signal does not go to increasing contrast, for both the (extended) object and the sky are brightened equally. The signal gain goes to increased detail.

A meter class scope only reveals color in some DSOs more readily because the larger image it provides covers more retina at given exit pupil. The surface brightness and contrast in that large scope is no higher than that for a smaller scope (at the same exit pupil.)


It is not required to limit discussions of contrast to 'ideal' conditions. As long as the variables, such as sky and instrument quality, are the same, the results are meaningful and valid.


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5916174 - 06/12/13 12:34 AM

I hate retyping: an earlier post is somewhat informative here: Aperture and extended detail

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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5916373 - 06/12/13 05:46 AM

Joe,
I read that post of yours at the time, and am in full agreement. Your idea of examining a photo from different distances in dim light--I did that years ago.

My argument against the *universal* statement that an aperture increase lowers threshold contrast might be summed up this way.

Imagine a rather larger aperture working at a somewhat smaller exit pupil. Magnification has increased, and the view has somewhat lower surface brightness. Therefore the larger scope has a *higher* (worse) threshold contrast because of the dimmer view. Yet in spite of this, it still reveals more detail simply by virtue of the increased image scale. The improvement afforded by magnification has increased faster than the worsening of threshold contrast.

And just what is threshold contrast? An example...

At a 7mm exit pupil, an object might be detected when about 4 magnitudes fainter than the sky. This is fairly low threshold contrast. Reduce the exit pupil to 1mm, and now an object must be no fainter than the sky by about 1 magnitude in order to be glimpsed. This is fairly high threshold contrast. The foregoing would apply equally for all apertures.

The only way a larger aperture lowers threshold contrast, then, is when the exit pupil is increased. If all observers only used ever larger exit pupils when increasing aperture, the the generalization that an aperture increase lowers threshold contrast would hold. But observers tend to employ the same range of exit pupil in all their instruments, and so it cannot be universally stated that an aperture increase lowers threshold contrast.

My seeming belabouring of the point might come across as arguing semantics. But it goes to a *fundamental* understanding of all the variables. I am taking great pains to quell any misapprehension that increasing aperture by itself improves contrast.


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robininni
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5916556 - 06/12/13 09:16 AM

Glenn,

For what it's worth coming from a relatively inexperienced guy, I totally follow you and it makes complete sense that improving the threshold contrast (I'd just call it making an object seem brighter) by increasing aperture is actually all about having increased the exit pupil for the same or possibly even higher magnification.

Rob


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5916676 - 06/12/13 10:40 AM

I think there are some muddles here.

Contrast sensitivity, or its inverse: contrast threshold (1/contrast sensitivity), is a function of the various properties of the real image on the retina and the retina itself. Now, wrt the issue here, the relationship between CS (i.e. contrast sensitivity) and spatial frequency (where high spatial frequencies = small angular visual detail, and low spatial frequencies =large angular visual detail ) yields a function.

As it turns out, the CS function reaches a maximum at about 6 cycles per degree. In other words, the ability to detect small low contrast features increases with increasing apparent angular target size - or perhaps more precisely, real image angular size on the retina - up to about 10 minutes of arc or so.

Now this function is determined by various neurophysiological processing (spatial summation etc) by the retina and nervous system. And, subject to that function, CS for a potential visual detail is determined by its angular extent at the retina (or equivalently apparent visual angular extent).

Now realize that the image on the retina "knows nothing" (if you will) about the aperture, exit pupil or other manner of its origin. To say, then, that exit pupil or aperture changes contrast threshold is, I think, misleading. I prefer to think of the issue here as follows, and as a consequence see no mystery or confusion.

Consider two scopes one 2" one 20" both observing the same galaxy, and both at 10x per inch. i.e. 20x and 200x respectively. Now the luminance (surface brightness) of the various features of the real image of the galaxy on the retina is identical in each telescope. And, the luminance of the retinal real image of the background sky is identical for both scopes. So contrast parameters of both retinal real images are identical. But, the image provided by the 20" scope is 10x larger. Now realize that aperture has not changed the above CS function at the retina (nor would exit pupil). The CS function is physiological fixed. Moreover, aperture has not changed the CS at any given region on the retina. The CS of any given retinal region is still a function of the spatial frequency of detail on that region. (And the same would hold if we varied exit pupil, CS would still only be a function of spatial frequency on that region). This is why I feel speaking of aperture or exit pupil changing CS is misleading.

Now if we assume that the actual visual target - the galaxy - has a relatively smooth gradation of (nearly) ever increasing fine detail, as any good astrophoto shows it does, then the 20" scopes increase of image scale by a factor of 10 has now brought detail 10 times as small into the visual detection range, where detection here is a function of contrast sensitivity. And for a given area of the 2 dimensional extended object (the galaxy) we could argue that 10x10 =100 times more detail is visible in the 20" scope. (for those more familiar with the topic at hand, i.e. HR Blackwell 1946, I am admittedly ignoring here the falloff for lower spatial frequencies)

This is how I understand aperture increasing visual detail. As for mystery, the only one here is the CS vs. spatial frequency function itself. And the answers to that mystery take us far afield, and into the realm of visual neurophysiology.

Hope this helps.

Joe


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917237 - 06/12/13 03:41 PM

Hi,
Recently I saw a post in this forum that states the issue as follows (paraphrased by me, hopefully accurately):

For extended objects: the brightness of an image of an extended object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, more will be seen in the larger aperture, since the magnification is higher, and the eye sees larger images better, even though the brightness of image of the object remains the same since the exit pupil remains the same.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope, and so the image of the object will be brighter.

At least that is how I understand it so far. Let me know if this is off base.

Bill


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robininni
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5917274 - 06/12/13 04:09 PM

Quote:

I think there are some muddles here.

Consider two scopes one 2" one 20" both observing the same galaxy, and both at 10x per inch. i.e. 20x and 200x respectively. Now the luminance (surface brightness) of the various features of the real image of the galaxy on the retina is identical in each telescope. And, the luminance of the retinal real image of the background sky is identical for both scopes. So contrast parameters of both retinal real images are identical. But, the image provided by the 20" scope is 10x larger. Now realize that aperture has not changed the above CS function at the retina (nor would exit pupil). The CS function is physiological fixed. Moreover, aperture has not changed the CS at any given region on the retina. The CS of any given retinal region is still a function of the spatial frequency of detail on that region. (And the same would hold if we varied exit pupil, CS would still only be a function of spatial frequency on that region). This is why I feel speaking of aperture or exit pupil changing CS is misleading.

Now if we assume that the actual visual target - the galaxy - has a relatively smooth gradation of (nearly) ever increasing fine detail, as any good astrophoto shows it does, then the 20" scopes increase of image scale by a factor of 10 has now brought detail 10 times as small onto the rising (positive slope) portion of the CS vs image scale function. In other words, fine detail 10 times as small now reaches the visual detection threshold as limited by contrast differences. And for a given area of the 2 dimensional extended object (the galaxy) we could argue that 10x10 =100 times more detail is visible in the 20" scope. (for those more familiar with the topic at hand, i.e. HR Blackwell 1946, I am admittedly ignoring here the falloff for lower spatial frequencies)

This is how I understand aperture increasing visual detail. As for mystery, the only one here is the CS vs. spatial frequency function itself. And the answers to that mystery take us far afield, and into the realm of visual neurophysiology.

Hope this helps.

Joe




Joe,

I like reading your posts and until now I haven't disagreed with anything you've written, but the statement I bolded above doesn't seem right to me. And if it isn't right, the logic that follows isn't either.

If the luminance of the image of the galaxy on the retina is identical in each telescope, it should also be unchanged using only the naked eye. If this were so, you could look at the sun through a telescope and suffer no ill effects given you only look for a few seconds, just like you can do so with your naked eye. Who is willing to back up this theorom by trying this???

Rob


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Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5917279 - 06/12/13 04:12 PM

Quote:

Hi,
I have never understood these explanations. My initial assumption was that my failure to understand it is because I am stupid, a belief no doubt widely shared by many.

Recently another possibility has occurred to me: the explanations are correct but are stated in such a way as to be clear only to those who already understand the issues.

As a writer I can tell you that explaining something complex in a simple and lucid way can be very difficult and often requires an unusual turn of mind that can isolate the main issue.

Recently I saw a post in this forum by a Jon Isaacs who does have this turn of mind.

Jon states the issue as follows (paraphrased by me, hopefully accurately): the brightness of an image of an object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, in a larger aperture the brightness of the object will say the same as in a smaller aperture at that exit pupil, but more will be seen in the larger aperture since the magnification is higher and the eye sees larger images better.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope.

At least that is how I understand it so far. Let me know if this is off base.

Bill




Bill, what John said is exactly true. This is also exactly what Glenn stated earlier in this thread (although not word for word). This is the only correct explanation of this subject. End of story.

Rob


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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5917363 - 06/12/13 05:08 PM

Quote:

I hate retyping: an earlier post is somewhat informative here: Aperture and extended detail




Here's an excerpt from the above which can serve as a good jumping off point for discussion...

Quote:

as you get closer to the picture in the dimmed room (for us older folks we assume well corrected vision!), the brightness of the object does not change, yet detectable visual detail clearly increases! (one can argue in fact that the amount of visible detail increases with the square of aperture). If you try this, you will appreciate what aperture does for deep sky observing. You can also simulate the expected gain with different aperture jumps. Of course, you can just get out there and look through a bunch of scopes! But experiments like this are fun nonetheless.




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.

Since so many amateurs use the terms, surface brightness and brightness interchangeably, I find it helpful to clearly identify this increase in brightness as an increase in apparent integrated--or apparent total--magnitude. This change in brightness is directly the result of the larger or more intense light packet being delivered to the eye by the larger aperture. If one thinks of light as information, you can describe this as more information being delivered to the eye. If you think about it in terms of human visual performance, that larger light packet allows the observer to detect more subtle contrasts.

This is critical to understanding visual observing. If one defines object contrast as the ratio of surface brightness of an extended object to the surface brightness of the surrounding field, then object contrast remains fixed in all apertures and at all magnifications. As a result, object contrast remains constant. Increasing aperture cannot improve object contrast. This can be discouraging when one considers the reality that the dark adapted eye is primarily a detector of contrasts. Its resolving power is atrocious. In a dark enough environment, color is impossible to detect. Our biggest asset when using a telescope at high power under a dark sky is our ability to detect faint light sources. The fully dark adapted eye is thousands of times more sensitive to light. As deep sky observers, we need to leverage this to detect subtle changes in contrast.

We cannot use aperture or magnification to change the contrast of an object versus the surrounding sky. However, by increasing aperture, we do become better at detecting subtle contrasts. In other words, threshold contrast is lowered. Pick any size for a galaxy...1', 5', 10', whatever. The faintest galaxy of that size visible in an 8 inch scope will be brighter than the faintest galaxy of the same size visible in a 16 inch scope. And since the two galaxies are of the same size, the galaxy with the fainter integrated magnitude will also have a lower surface brightness and, by extension, be a lower contrast object. Threshold contrast has been lowered. This is just one of many scenarios in which one can apply the concept of threshold contrast to better understand why we see what we see. This relationship between aperture and threshold contrast not only explains how we're able to see fainter galaxies but also why the same galaxy observed in two different apertures appears more obvious and impressive in the larger scope. Again, that galaxy's contrast versus the surrounding sky will be identical in both scopes. However, it will be farther above the threshold of visibility in the larger aperture. In this manner without changing object contrast, we can explain and understand how a faint galaxy emerges to the eye and how bright galaxy's appear more impressive.

It also provides a framework for understanding the mistaken perception embraced by visual observers for many years that increasing aperture literally improved object contrast. Prior generations of observers made the very natural and straight-line inference that, if an object looks more contrasty or obvious to the eye, then it must be more contrasty. Of course, that was a mistaken conclusion. The galaxy doesn't display greater contrast versus the surrounding sky when viewed in a bigger scope. However, it does have greater separation from the threshold of visibility and this creates an impression that contrast has improved.

It's been said that discussions of threshold contrast and the role it plays in the observation of faint extended objects does not take into account the improved resolution offered by a larger aperture. On the contrary, I would say that the tangible gains of presenting objects as brighter (integrated magnitude) and more detailed (improved resolution) combine to result in a lowered threshold contrast for detection. It has also been said that discussions of threshold contrast can give the mistaken impression that increasing aperture improves object contrast. I don't share this view. However, if some people do come away with that impression, it is not because of some problem inherent in the concept. It's because we need to do a better job of presenting, explaining and talking about it. The response shouldn't be to not talk about threshold contrast. The response should be to find more innovative and creative ways to communicate with and educate the amateur astronomy community about this important concept.

Bill in Flag


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917440 - 06/12/13 05:56 PM

Joe,
Part of what I was trying to impress, as you explicitly stated, is that the eye doesn't 'know' a priori what the aperture or exit pupil are.

In your previous post you dealt with the case of a specific object at the same exit pupil through different apertures. This is the most straightforward of all the possibilities to grapple with. The conclusion is as valid for a dim object as it is for a bright one, and is completely expected by (most) anyone:
- the image surface brightness is the same
- the contrast sensitivity is the same
- the linear detail perceived scales as the aperture

One 'muddying' factor here is clearly contrast sensitivity, which has a direct bearing on threshold contrast, and which in turn is dependent on image surface brightness. If we progressively stop down a scope, the image dims and contrast sensitivity worsens. Where at full aperture we might perceive a brightness difference as small as 10% above sky brightness, when well stopped down this might be as bad as 50%, or even 100% above sky brightness.

Another 'muddying' factor is the role of image scale, at least when we restrict to dealing with a single object. Altering the image size makes deconstructing the contribution of the various elements difficult to assess, on an intuitive level at any rate.

To aid in *deconstructing* the various contributions was why I presented the case of two differing apertures taking in otherwise identical galaxies, each lying at a distance (scaling with aperture) which results in their both subtending the same angle on the retina at given exit pupil. The view in each scope is indistinguishable. Contrast sensitivity and threshold contrast are unchanged, no matter the aperture.

And so the concept of threshold contrast as seems to be espoused would seem to be inextricably entwined with the role of image scale, as well as instrumental resolving power. All the more so when every example employed deals exclusively with what happens at the threshold of detection of detail for a *specific* object. The result is then really the statement of the fundamental principle that a bigger aperture employed on any given object delivers a more detailed image. In this context, contrast sensitivity and threshold context are completely redundant.

To fully understand the effects of all the variables is most difficult when the variables are not isolated and dealt with in turn. An example of a common myth resulting from an under-appreciation of the variables involved.

If the observer is unaware of the fact that as the image dims his resolving power worsens due to reduced contrast sensitivity, what does he conclude after bumping up the magnification on a small galaxy and more detail appears? The contrast has improved because the sky has darkened! (This myth is pernicious in the extreme; it's still bandied about to a surprising extent, and I despair of ever seeing it relegated to the status of polio.)

Later on he learns that it's really the case that the increase in image scale has afforded the perception of detail faster than the rate at which image dimming reduces contrast sensitivity.


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5917481 - 06/12/13 06:20 PM

Oops... Removed this post due to a misinterpretation... Sorry!

Edited by GlennLeDrew (06/12/13 06:23 PM)


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917968 - 06/12/13 11:13 PM

Rob
As strange as it seems, the luminance (i.e. surface brightness or light intensity per unit area) of the real image of the sun on the retina is no greater when looking through a scope than when looking with the naked eye. In fact, depending on exit pupil, it may be smaller!! (for the few second or two your eye would last!). BUT the total integrated brightness - to use bill's terminology - will be much much greater on the retina with the scope, since the real image is much much larger. As such, far more heating power (wattage) is being delivered and your eye cooks very quickly. There is no need to do the experiment, the photometry gives the answer.

This concept - the optical invariant, conservation of etendue etc. - that aperture cannot increase luminance is very counter intuitive, but it is a critical concept in optical engineering.

Let me end with a very very rough analogy. Imagine being hit with one drop of boiling water (analogous to glancing at the sun naked eye). Not much damage, probably no band-aid needed. Now imagine an entire kettle of boiling water spilling on you (like glancing at the sun with a scope). Now the water in the kettle is no hotter (very roughly analogous here to luminance) than the drop, but the kettle load (far more total heat) will land you hospitalized in a burn unit in critical condition. (admittedly, this is so rough an analogy as to verge on metaphor, but it has heuristic value nonetheless).
Hope this helps.
Joe


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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5918074 - 06/13/13 12:47 AM Attachment (8 downloads)

Quote:

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.

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

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. Speaking of clarity, I do like the above referenced explanation by Jon Isaacs. He kind of cuts right to the chase. informed by the directness of his account, I edited my prior post a bit.

Dark and steady skies!


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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5918101 - 06/13/13 01:16 AM

Joe,
I enjoyed reading the Wiki article on CSF. It did seem to confine to the photopic regime, where 5-7 cycles/degree are optimally detected. But at low light levels, as I understand it, this can decrease to a fraction of a cycle/degree (i.e., several degrees/cycle.) Unless I'm conflating minimum size for detection (which can be several degrees) with a cyclical variation in a much larger field.

This is another 'confounding' factor when dealing with the matter of DSO observation.


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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5918184 - 06/13/13 03:36 AM

Quote:

.... I know the difference in views between a 20" and 24" can be pretty big and so I don't know if I want to downsize from the 25" to a 20".

Rob




Hi Rob.
I won't get into the discussion of contrasts, brightness etc. but will just say that in my experience the difference in views between two scopes is much more evident when changing down as opposed to changing up. When I built my 16" I was a little disappointed at the gain over my existing 12". I decided to keep both for a while before deciding which one to sell. I ended up using the 16" only (after the many nights of setting both up for side by side comparisons) for a month or two and when I eventually used the 12" again it was clear the 16" was the keeper. Same with the 16" - 20".

Dave


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5918421 - 06/13/13 09:33 AM

Glenn
Interesting stuff! Look around, there is a deep literature base here. The classic work is HR Blackwell 1946 !

Its neat how this topic dovetails with photometry and visual deep sky observing.

One point I was trying to nail down (perhaps with limited success) was the importance of appreciating the Contrast Sensitivity Function (herein CSF) and how it provides the explanation, in this context of this discussion, of Jon Isaacs straight forward (if somewhat understandably simplified) statement above that "The eye discerns larger images better than smaller images". But look at the function and note that this is only true up to a point!

The other point I was trying to emphasize (again with limited success!) is the fact that the CSF is only a property of the retina/visual system itself - extra ocular optics do not add additional variables to this function.

Speaking of aperture exit pupil changing contrast sensitivity/threshold, is a somewhat different matter, and I am somewhat hesitant - but will give this more thought. Note however that it is critical here to distinguish between contrast sensitivity/threshold, and the contrast sensitivity/threshold function! (i.e. the distinction between a function f and the value of f somewhere in its domain).

On the other hand, the visual detectability of a given target is very much a function of aperture, exit pupil - and one might add sky darkness, transparency, seeing etc. - as well as retinal/visual system properties. (we can speak of a multi-valued visual detectability function!) But again, none of these extra retinal/visual system variables change the CSF function. They instead determine the point that an object falls on that 2-dimensional plot of that function , thereby determing whether and object becomes visible or not.


There are many rich digressions here. For example the final phenomenologic visual representation, and how that leads to judgments about contrast in the virtual image at the EP. This all makes me wish I had more time. Bt every time my wife catches me at the computer she looks at me and I imagine her thinking: "did he finish doing such and such for our move out of NYC ?"

Dark and steady!

Joe


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Re: Big dob for objects with structure and detail? new [Re: dave brock]
      #5918534 - 06/13/13 11:05 AM

Dave
At RMSS this past week I was setup next to an excellent lockwood 12.5". The differences with my 16 initially seemed subtle, but then became more pronounced as the back and forth views continued through the night. Some day I look to make the 16 to 20 jump, but only after I get more dark western sky observing under my belt. (can you tell I am psyched about moving to Colorado!)
Joe


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Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5918833 - 06/13/13 01:47 PM

Quote:

Hi,
Recently I saw a post in this forum that states the issue as follows (paraphrased by me, hopefully accurately):

For extended objects: the brightness of an image of an extended object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, more will be seen in the larger aperture, since the magnification is higher, and the eye sees larger images better, even though the brightness of image of the object remains the same since the exit pupil remains the same.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope, and so the image of the object will be brighter.

At least that is how I understand it so far. Let me know if this is off base.

Bill




That's basically my understanding too. Roger Clark's Visual Astronomy of the Deep Sky does a great job explaining how the eye in night vision mode discerns larger images better than smaller images. Larger images mean greater detail, but since greater detail requires greater mag and smaller exit pupil in a given scope, larger images come at the cost of lower contrast. For me, part of the fun and challenge of visual astronomy is finding that "sweet spot" where I can see the most detail in a given deep sky object before it melts into the background due to loss of contrast. M51 is a favorite object and it really is astonishing just how much mag it can take in my 18" scope. I always encourage people I observe with to try several different mags on a object. There's nothing to lose, and you never know just what you might see.


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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5919258 - 06/13/13 06:06 PM

On the matter of brightness and contrast as regards minimum size for detection... In my Gallery (link in my sig) is a chart I made up, based on material in the RASC Observer's Handbook, which in turn is largely based on Bkackwell's 1946 study. I also provide a fairly lengthy 'user guide', if you need help getting to sleep.

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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5919733 - 06/13/13 11:01 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


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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5919762 - 06/13/13 11:26 PM

Quote:

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.


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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5920068 - 06/14/13 06:58 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.


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5920622 - 06/14/13 01:44 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.

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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5920752 - 06/14/13 02:57 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


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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5921588 - 06/15/13 01:30 AM

Quote:

Quote:

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.

Quote:

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.



Quote:

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


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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5921689 - 06/15/13 03:47 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?


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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5921741 - 06/15/13 05:58 AM

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


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Re: Big dob for objects with structure and detail? new [Re: alexvh]
      #5922139 - 06/15/13 11:52 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.

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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5927026 - 06/18/13 08:41 AM Attachment (2 downloads)

Bill
This needs a bit of tightening.

Quote:

Quote:

Quote:

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.

Quote:

Quote:

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.com/search?client=safari&rls=en&q=contrast+sensitiv...


Quote:

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.

Edited by jpcannavo (06/18/13 09:02 PM)


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