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Observing >> Deep Sky Observing

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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Apparent magnatude conundrum
      #5880486 - 05/23/13 03:15 PM

Last night I was in the back yard and was observing some objects, it was a nice cool night in the high desert with not too bad of wind thought I would see what I could get with my camera. By the time I got it all aligned with the camera mounted (seems to work better if I align with the cam mounted) I thought I would see a few items. I got a couple of moon pics and decided to try some other objects, Iíve never tried to take a pic of the cat eye, so I thought what the heck. I got some good images I need to process and stack the frames on today when I get home, but I figured I would try to see other things. Now this is where I get a little unsure about the reasoning behind it.
Catís eye and Ghost of Jupiter, two objects Iíve gotten pictures of have apparent magnitudes of 9, both of which have been able to photograph rather well, granted the gain control is turned up to over Ĺ (my camera has settings I can see it at 32x-256x on gain control. Ideally I took photos at 128X on the settings.) now moving to galaxies, the pinwheel is 7.8, Sombrero at 8.98, and whirlpool at 8.4 I couldnít get the objects to show up in the camera no matter the setting. I think I saw the core, not sure. I recorded any way and will see what turns up if anything at all.
While the galaxies are faint, is the magnitude that low that it just canít be seen? In the Ghost of Jupiter I can photo the star in the center of the nebula, for detail along with wonderful color, however I canít seem to get a lick of the galaxies, unless the dot I saw on the camera was the core.
So are these objects to fait for me to photograph, granted the moon was out and will defiantly try again when there is no moon. . . .curse you moon! Or is it possible the moon light was enough to make it hard to see, like is that mag of 9 the threshold for my scope maybe or should I be taking a different approach? I can see them with my eye, except pinwheel for some reason; I can see it in the hills, but none of them with the camera. The community here has always helped me out in the past and I trust the collective wisdom.
(if equipment is a factor, Iím using a 6SE celstron, an orion star shoot deep space II, with and without a barlow, registax 6, touch up in photoshop to pull the data out.)


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Apparent magnatude conundrum new [Re: Tyranthrax]
      #5880617 - 05/23/13 04:06 PM

It's all about surface brightness. Those planetaries have surface brightness of perhaps 15 mag/arcsec^2. The very cores of most galaxies are not quite this bright, and rapidly fade away from there. The average surface brightness of your typical galaxy might be 20-21 MPSAS, which is fully 100X fainter than the bright planetaries. This is why PNs can punch through light pollution, while for more than the core regions galaxies require rather dark skies.

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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Re: Apparent magnatude conundrum new [Re: GlennLeDrew]
      #5880631 - 05/23/13 04:11 PM

So if I get nothing when the moon is not out and I have good skies I am guessing this means I need to convince my wife I need a bigger apature. . .(I've been looking for a good excuse to get the 10-12"

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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Apparent magnatude conundrum new [Re: Tyranthrax]
      #5880637 - 05/23/13 04:14 PM

You need to take the size of the objects into consideration as well. The two planetary nebulae are small and compact for their given apparent magnitude, which gives them a high surface brightness. The galaxies mentioned are much larger, which means their surface brightness is much lower, since their total amount of light is spread over a much larger area.

Think of a bucket of sand. If you pour it out in a nice little pile on the lawn, it's very easy to see, even if the lawn is very large, even from some distance away. Spread the bucket of sand over the entire lawn and it's impossible to see the sand, even if you step on it. That's the basics behind surface brightness. Compact object with bright apparent magnitude = easy to see and can take high magnification. Large object, even if techically of bright apparent magnitude = much harder to see.


Clear skies!
Thomas, Denmark


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Astrojensen
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Reged: 10/05/08

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Re: Apparent magnatude conundrum new [Re: Astrojensen]
      #5880639 - 05/23/13 04:15 PM

Ah, beat me to it, Glenn.


Clear skies!
Thomas, Denmark


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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Re: Apparent magnatude conundrum new [Re: Astrojensen]
      #5880655 - 05/23/13 04:21 PM

Ah Thank you guys! It would appear I am pushing the limits of the 6" apature. The upside is there are plenty of Planitaries and things to see until I get the bigger, as well as I can keep trying,

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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Re: Apparent magnatude conundrum new [Re: Tyranthrax]
      #5880669 - 05/23/13 04:27 PM

was thinking about this, if I cansee them with my eye but not the camera even at the increased gain, which lets me see stars I can't see, shouldn't my camera be picking them up?

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BillFerris
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Reged: 07/17/04

Loc: Flagstaff, Arizona, USA
Re: Apparent magnatude conundrum new [Re: Tyranthrax]
      #5880709 - 05/23/13 04:45 PM

Hello and welcome to the wonderful world of deep-sky. If you haven't done so, already, I recommend you post your question in the "Astrophotography and Sketching" forum. Digital imaging of the deep-sky can yield amazing results provided you are using good technique with your equipment. The astroimagers in that forum should be able to help you get much better results your next time out with the scope and camera. I would encourage you to be as specific as possible with the information you provide about your imaging set up, including: the scope brand and model, aperture and focal ratio, any eyepieces, eyepiece accessories, camera and settings, length of exposure and sky conditions. The more information you are able to provide, the easier it will be for someone to identify equipment or technique changes that should lead to better results.

You mention imaging two planetary nebulae (Cat's Eye and Ghost of Jupiter) and three galaxies (Pinwheel, Sombrero and Whirlpool). Here, are their respective physical characteristics:

NGC 6543 "Cat's Eye": 8.1 magnitude, 56" (~1'.0) in diameter
NGC 3242 "Ghost of Jupiter": 7.3 magnitude, 87"x81" (~1'.4) in diameter
NGC 598=M33 "Pinwheel Galaxy": 5.7 magnitude, 71'x42' in size
NGC 4594=M104 "Sombrero Galaxy": 8.0 magnitude, 9'x5' in size
NGC 5194=M51A "Whirlpool Galaxy": 8.4 magnitude, 11'x9' in size

The first thing that jumps out to me is the relative sizes of these objects. The two planetary nebulae are only about an arcminute in size while the galaxies range from being about 9X as large to being about 70X as large. So, although these objects are reasonably similar in apparent total magnitude, the planetaries--being much smaller--have a much higher surface brightness than the galaxies. Whereas the galaxies range in surface brightness from 20 to 23 magnitudes per square arcsecond, the planetaries fall in at about 16.5 magnitudes per square arcsecond. In other words, they range from 25X to 400X brighter per unit area than the galaxies. From an imaging standpoint, if you were to use the same ISO setting and exposure time for M33 (Pinwheel Galaxy) and NGC 6543 (Cat's Eye nebula), you would need to reduce your focal ratio by nearly 9 stops to record an equivalent level of detail in M33 as you would record in the Cat's Eye.

The bottom line take away is that celestial deep-sky objects are often inherently faint and of low surface brightness. Successful imaging depends on being able to achieve a good balance between fast focal ratio, image scale, sensor sensitivity, exposure time and tracking. It is not a trivial matter by any stretch but, when you start to get good results, the time and effort invested is rewarded with exquisite images.

Bill in Flag


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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Re: Apparent magnatude conundrum new [Re: BillFerris]
      #5880865 - 05/23/13 06:05 PM

I love this hobby becuase of many woderful reasons, in the top 4 is that I learn things every day, if I'm not looking up the objects I plan on seeing or ones I saw so I know what I'm looking at, how it was formed etc, I am learning alot about gerneral aspects. The best thing about learning these things is that its not just information thrown out there, a question arises and the question usually needs an answer. It really clicked on the explination of the surface area, the smaller the object the brighter those few pixles are, like an led light versus an 8 foot florecent. Couple that with teh fact my eye is a bit more sensative than the imager and it makes complete sence. Now the challenge is to get those images later on down the road. Thank you all!

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Apparent magnatude conundrum new [Re: Tyranthrax]
      #5880988 - 05/23/13 07:57 PM

And do note that in this case, it's not the aperture itself which is at fault. You're having no trouble capturing those rather tiny PNs. The galaxies, being larger, would be captured with an even *smaller* aperture.

As far as extended objects are concerned, a larger aperture essentially allows to capture smaller objects/finer details. That's it.

A 2" f/5 and a 20" f/5 will record any extended (resolved) object to the same brightness level in their images. The bigger scope is merely resolving to a 10X better degree of resolution. But the sky and the object will have the same surface brightness in each image.

High surface brightness objects like PNs have very high contrast against most any sky (Rock Mallin even captured M57 with the Sun above the horizon, after combining numerous images so as to improve signal to noise). And so they record with quite short exposures, and stand out like the proverbial sore thumb.

But galaxies--the outer parts anyway--are about equal to the brightness of the sky itself, which results in low contrast. To bring them out, your exposure must be increased so as to record the sky glow to a brightness well above the camera's noise level. A short exposure yielding a 'black' sky is insufficient for objects not much brighter than the sky glow.


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BillFerris
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Reged: 07/17/04

Loc: Flagstaff, Arizona, USA
Re: Apparent magnatude conundrum new [Re: GlennLeDrew]
      #5881261 - 05/23/13 10:34 PM

Quote:

As far as extended objects are concerned, a larger aperture essentially allows to capture smaller objects/finer details. That's it.




Within the context of imaging, increasing aperture while maintaining a fixed focal length reduces your focal ratio or f-stop. With a faster f-stop, you're able to make the same image with a shorter exposure as compared to an imaging system with the same focal length but a smaller aperture. Shortening the needed exposure time is a serious benefit for the digital imager or photographer.

Bill in Flag


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Apparent magnatude conundrum new [Re: BillFerris]
      #5881459 - 05/24/13 12:46 AM

Quite true, Bill, I was generalizing to a more-or-less 'constant' f/ratio as aperture varies.

*However*, given that aperture controls the light admitted, if we normalize to a given surface brightness at the focal surface, aperture and resolving power do scale in step. And so the seemingly over-simplified generalization that aperture and resolution/image scale holds true.


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Tyranthrax
sage


Reged: 04/22/13

Loc: Reno, NV
Re: Apparent magnatude conundrum new [Re: GlennLeDrew]
      #5882171 - 05/24/13 12:18 PM

bah every time I try to find the F ratio and how to use it it givesme camera stuff.

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