APOD: Plasma Jets from Radio Galaxy Hercules A (2012 Dec 05)

[Nitpicker]

The galaxy is also known as PGC 59117, and is of visual magnitude 17.8. It appears to be the largest galaxy in a dense cluster. Although this is within reach of amateur astrophotographers, it would be tiny on their CCD chips, so most amateurs leave such distant objects alone. So, the Hubble view is shown. (Of course, the radio jets would not show up in CCD images.)

According to this image:

Click to view full size image

the optical signal of Hercules A would appear with roughly the same angular size as many of the Solar System planets routinely photographed by amateurs. But at vmag 17.8, which is at least 5 orders dimmer than anything in the Messier and Caldwell catalogues (and spread out over its apparent size) it would definitely be a struggle to detect. Besides, elliptical galaxies aren't the pretty ones anyway. You can take overexposed photos of the Moon or Venus and obtain results that look like elliptical galaxies. Trust me, I know.

I have a modest SCT scope (1500mm FL, f/10) and a DSLR capable at high ISO, and fairly dark, but still suburban skies, and I cannot convincingly detect a point source dimmer than vmag 15. It is even more difficult when I slow down the focal ratio beyond prime focus to reduce the FOV, as I do when imaging planets.

[geckzilla]

Beings in elliptical galaxies probably have a lot clearer view of the cosmos without all that dust, though. I wonder if they are full of planetary nebulas, as well?

[Chris Peterson]

But at vmag 17.8, which is at least 5 orders dimmer than anything in the Messier and Caldwell catalogues (and spread out over its apparent size) it would definitely be a struggle to detect. Besides, elliptical galaxies aren't the pretty ones anyway. You can take overexposed photos of the Moon or Venus and obtain results that look like elliptical galaxies. Trust me, I know. :ssmile:

I have a modest SCT scope (1500mm FL, f/10) and a DSLR capable at high ISO, and fairly dark, but still suburban skies, and I cannot convincingly detect a point source dimmer than vmag 15. It is even more difficult when I slow down the focal ratio beyond prime focus to reduce the FOV, as I do when imaging planets.

That seems curious. With my 12" SCT here, I can easily image small, mag 18 extended objects with a few minute exposure. Even when I lived in the middle of Southern California light pollution, I only needed 10 seconds to capture the Ring Nebula, including its mag 15 central star, with an 8" SCT.

Certainly, a DSLR isn't ideal for astronomical imaging, but it isn't that bad! Also, aperture is what matters. Focal ratio has no impact on sensitivity except for very short exposures, where readout noise dominates.

[Nitpicker]

But at vmag 17.8, which is at least 5 orders dimmer than anything in the Messier and Caldwell catalogues (and spread out over its apparent size) it would definitely be a struggle to detect. Besides, elliptical galaxies aren't the pretty ones anyway. You can take overexposed photos of the Moon or Venus and obtain results that look like elliptical galaxies. Trust me, I know.

I have a modest SCT scope (1500mm FL, f/10) and a DSLR capable at high ISO, and fairly dark, but still suburban skies, and I cannot convincingly detect a point source dimmer than vmag 15. It is even more difficult when I slow down the focal ratio beyond prime focus to reduce the FOV, as I do when imaging planets.

That seems curious. With my 12" SCT here, I can easily image small, mag 18 extended objects with a few minute exposure. Even when I lived in the middle of Southern California light pollution, I only needed 10 seconds to capture the Ring Nebula, including its mag 15 central star, with an 8" SCT.

Certainly, a DSLR isn't ideal for astronomical imaging, but it isn't that bad! Also, aperture is what matters. Focal ratio has no impact on sensitivity except for very short exposures, where readout noise dominates.

I'm certainly not saying my technique is optimised. I'm sure I could squeeze a bit more out of my scope, but the law of diminishing returns tends to make me lazy. I've been reluctant to get into image stacking and processing in any serious way -- I probably need to address that.

But with my 6" SCT (1500mm/10) at prime focus on my DSLR, with a high ISO 6400 setting, I need a good 30 seconds of exposure to convincingly detect a point source of vmag 15. As you say, aperture is what matters.

My understanding of slowing down the focal ratio (via eyepiece projection, say) is that you reduce the FOV, thereby spreading the desired target across more pixels, so each pixel gets less photons per unit time. So a slower focal ratio requires a longer exposure. Happy to be corrected here. I always appreciate your comments, Chris.

Also, based only on my experience in the field, I have simply concluded that a diffuse object like a galaxy or nebula is always more difficult to detect than a point source of the same vmag.

[Chris Peterson]

But with my 6" SCT (1500mm/10) at prime focus on my DSLR, with a high ISO 6400 setting, I need a good 30 seconds of exposure to convincingly detect a point source of vmag 15. As you say, aperture is what matters.

Okay. I'll note there is a substantial difference between that clarification and your original statement, " I cannot convincingly detect a point source dimmer than vmag 15."

My understanding of slowing down the focal ratio (via eyepiece projection, say) is that you reduce the FOV, thereby spreading the desired target across more pixels, so each pixel gets less photons per unit time. So a slower focal ratio requires a longer exposure.

For a long exposure, the S/N is determined primarily by the number of photons collected, which is a function of the aperture and exposure time. With a slower focal ratio, each pixel sees fewer electrons, and therefore has higher noise, but the object as a whole doesn't have more noise than you'd have if you used the same scope at a shorter focal length. You can test this quite easily by just shrinking the image digitally to what you'd have with the faster scope. Aesthetically, we tend to like high surface brightness images, which means you'll need a longer exposure time. But the same information is present in the shorter exposure image made at a long focal length.

This is why the best imaging strategy is to select your focal length based on the camera you are using, and then go for as much aperture as you can afford.

Also, based only on my experience in the field, I have simply concluded that a diffuse object like a galaxy or nebula is always more difficult to detect than a point source of the same vmag.

Visually, that is certainly true. Our eyes aren't pure photon counters- they respond to surface brightness as well.

[neufer]

But with my 6" SCT (1500mm/10) at prime focus on my DSLR, with a high ISO 6400 setting,
I need a good 30 seconds of exposure to convincingly detect a point source of vmag 15.

A Naked Eye Nova Erupts in Centaurus
by David Dickinson, Universe Today, December 4, 2013

<<If you live in the southern hemisphere, the southern sky constellation of Centaurus may look a little different to you tonight, as a bright nova has been identified in the region early this week.

An animation showing a comparison between the constellation Centaurus before and after a nova eruption. Credit and copyright: Ernesto Guido, Nick Howes and Martino Nicolini/Remanzacco Observatory.

The initial discovery of Nova Centauri 2013 (Nova Cen 2013) was made by observer John Seach based out of Chatsworth Island in New South Wales Australia. The preliminary discovery magnitude for Nova Cen 2013 was magnitude +5.5, just above naked eye visibility from a good dark sky site. Estimates by observers over the past 24 hours place Nova Cen 2013 between magnitudes +4 and +5 “with a bullet,” meaning this one may get brighter still as the week progresses.>>

[Nitpicker]

Okay. I'll note there is a substantial difference between that clarification and your original statement, " I cannot convincingly detect a point source dimmer than vmag 15."

Quite right Chris. I now wish I had never mentioned my own case, as I am limited to subs of ~30s due to field rotation, due to my (poor) choice of alt-az mount for (not very) deep sky photography. Initially, I thought that by setting the ISO so high, I was bringing the image to the limit of sky glow faster. I now know (thanks mainly to the education you've given me over the last couple of months) that while I am detecting more objects this way, I am also increasing the readout noise and reducing the S/N ratio. I'm probably still a fair way from the limit of my sky glow.

My original point -- however poorly expressed originally -- was that very few amateur deep sky photographers, shooting through the atmosphere, seem to produce narrow-field images, no doubt due to the extra time and money it requires. It is hard to compete with technology like the HST.

It would have been better if my original statement said something like "the vast majority of amateurs would struggle to produce a satisfactory narrow-field image of Hercules A (or more interesting targets comparable in apparent size and magnitude)". This is slightly different from Roger's implication (as I perceived it) that amateurs leave objects like this alone because they can only achieve wide-field images.

Aesthetically, we tend to like high surface brightness images, which means you'll need a longer exposure time. But the same information is present in the shorter exposure image made at a long focal length.

I take your point, but if my eyes can't detect an object in an image, I would argue (perhaps semantically) it is more than just a lack of aesthetics. I assume you mean that "the same information is present in the shorter exposure image made at a long focal length" [or short focal length].

...

My original point could just as easily have not been made, as it wasn't a particularly important point. But I'm glad I made it, and that I had the opportunity to clarify it, because it opened up a sideline of discussion from which I have learnt a few things. Many thanks.

[Nitpicker]

But with my 6" SCT (1500mm/10) at prime focus on my DSLR, with a high ISO 6400 setting,
I need a good 30 seconds of exposure to convincingly detect a point source of vmag 15.

A Naked Eye Nova Erupts in Centaurus
by David Dickinson, Universe Today, December 4, 2013

<<If you live in the southern hemisphere, the southern sky constellation of Centaurus may look a little different to you tonight, as a bright nova has been identified in the region early this week.

An animation showing a comparison between the constellation Centaurus before and after a nova eruption. Credit and copyright: Ernesto Guido, Nick Howes and Martino Nicolini/Remanzacco Observatory.

The initial discovery of Nova Centauri 2013 (Nova Cen 2013) was made by observer John Seach based out of Chatsworth Island in New South Wales Australia. The preliminary discovery magnitude for Nova Cen 2013 was magnitude +5.5, just above naked eye visibility from a good dark sky site. Estimates by observers over the past 24 hours place Nova Cen 2013 between magnitudes +4 and +5 “with a bullet,” meaning this one may get brighter still as the week progresses.>>

Thanks neufer. Currently, looks like I'd be able to observe Nova Cen 2013 between about 2 and 4am from here. Hmmm, I might look at the photos I took of Nova Del 2013 earlier this year, and go to bed early instead.

[jacksmom60]

Hi guys, is it me or do those jets look like they are turning counter clockwise? The jet on the left coming towards us and the right one travelling away.

I agree with you on this....I see the same thing. They do look as though they are turning counter clockwise with the one on the left coming forward and the right moving away.