by Nitpicker » Wed Dec 04, 2013 6:27 am
Chris Peterson wrote:Nitpicker wrote: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.
[quote="Chris Peterson"][quote="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. :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 [u]point source[/u] 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.[/quote]
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.[/quote]
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 [u]convincingly[/u] 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.