An attempt at the Leo Triplet. 20 x 10 minute subs with a QHY8 on a GSO RC 8" scope and an EQ6 mount. Darks, flats, etc applied. The biggest challenge was getting the three galaxies in the tight field of view at the same time as getting a reasonable guide star on the OAG chip!
Still a bit of noise/lack of detail here on this brightish set of targets?? Not sure if it's just dodgy processing. But an (eventual) planned upgrade to a mono camera may help with getting more light/less noise? Comments appreciated.
Ignoring all the good things about your shot like composition and colour, and just addressing your noise question:
(1) The noise level is perhaps not too grim for 3 hours with your kit. (This huge rant doesn't mean I think your image is especially noisy.)
(2) Twelve hours would halve the noise. Most of Trish's and my shots are at least that long. We've thrown up to 50 hours at the Helix for example. But it is diminishing returns.
(3) Consider longer subs. This greatly reduces noise and actually reduces effort! With an EQ6, a not so huge focal length, and off-axis guiding, you should easily be able to manage half an hour. (I used to manage 1 hour with an EQ6 and a 10"). That would get three times as much signal past the read-out transistors for each dose of read-out noise. The disadvantages of longer subs are: satellite trails, aircraft, cosmic ray hits, wind buffet, all more likely but harder to reject with a small stack, and field rotation can be a problem. If you don't have an off-axis guider, forget it.
(Tips for practicing really long subs: Choose a still night without wind buffet or risk of intermittent cloud. When doing polar alignment, get the polar axis as close as you can to the right latitude, but a tiny half-skerrick west of the pole. If you do the opposite: exactly south but say too high, you will get both nasty field rotation and your guiding will suffer from hunting in the dec axis gears backlash.)
(4) Another long shot possible thing to keep in mind is that putting the three galaxies (for very best of reasons!) right in the corners of the image is that they will be getting vignetted. You could look at your flats. A wild guess is that the vignetting will only be trivial, say 10-20% and it doesn't matter. But if it was 50%, then it would be working against you.
(5) Stating the obvious, be at the darkest site you can, with no haze, no moon glow, and image high in the sky. Photographing below say 15 degrees will usually only make the image worse because of sky glow. Bit tricky with Leo.
(6) Run the camera as cold as you can down to say -30C. After that it doesn't matter so much. Autumn is upon us. Every 6 degrees will halve the dark current, and since the noise is Poisson, the dark current noise should halve too.
(7) Advanced technique that I quite like: Take some unbinned shots (you've already done that bit) to get the bright sharp detail. Then on another night take some 2x2 binned shots to get the faintest regions with less detail but much less noise. Combine the two using a suitable mask to take the bright detail from the binned shot, the faint stuff from the binned shot, and pro rata in-between. You can make the mask from a blurred copy of the image.
Nice Galaxy details Payl, have controlled the central brightness on all three very well too
Noise, ? interesting, Im not sure, but Ive had smoother with 14 x 10 min subs Qhy10 no darks or flats - might be a processing thing ?
I think it's quite a nice image, Paul, but a little noisy as you said. You could improve that with some noise reduction in processing but there are limits to how much NR you can do before losing sharpness.
As Mike said, the ideal solution is to get more data and maybe longer subs. What you want is subs long enough that read noise from the camera is not significant. How long that is depends on your camera parameters and sky brightness. It's not that hard to determine from a couple of example subs whether you're read noise limited or not. Have a look at the method proposed here:
(1) The noise level is perhaps not too grim for 3 hours with your kit. (This huge rant doesn't mean I think your image is especially noisy.)
(2) Twelve hours would halve the noise. Most of Trish's and my shots are at least that long. We've thrown up to 50 hours at the Helix for example. But it is diminishing returns.
(3) Consider longer subs. This greatly reduces noise and actually reduces effort! With an EQ6, a not so huge focal length, and off-axis guiding, you should easily be able to manage half an hour. (I used to manage 1 hour with an EQ6 and a 10"). That would get three times as much signal past the read-out transistors for each dose of read-out noise. The disadvantages of longer subs are: satellite trails, aircraft, cosmic ray hits, wind buffet, all more likely but harder to reject with a small stack, and field rotation can be a problem. If you don't have an off-axis guider, forget it.
(Tips for practicing really long subs: Choose a still night without wind buffet or risk of intermittent cloud. When doing polar alignment, get the polar axis as close as you can to the right latitude, but a tiny half-skerrick west of the pole. If you do the opposite: exactly south but say too high, you will get both nasty field rotation and your guiding will suffer from hunting in the dec axis gears backlash.)
Some great info. Thank you. Will try that.
Quote:
Originally Posted by Placidus
(4) Another long shot possible thing to keep in mind is that putting the three galaxies (for very best of reasons!) right in the corners of the image is that they will be getting vignetted. You could look at your flats. A wild guess is that the vignetting will only be trivial, say 10-20% and it doesn't matter. But if it was 50%, then it would be working against you.
Actually, my vignetting is not too bad. Corners are about 80% of the centre.
Quote:
Originally Posted by Placidus
(5) Stating the obvious, be at the darkest site you can, with no haze, no moon glow, and image high in the sky. Photographing below say 15 degrees will usually only make the image worse because of sky glow. Bit tricky with Leo.
In the suburbs I'm afraid. Still, the moon was gone by the time that was taken. Tried to concentrate the imaging either side of the meridian to minimise the air mass.
Quote:
Originally Posted by Placidus
(6) Run the camera as cold as you can down to say -30C. After that it doesn't matter so much. Autumn is upon us. Every 6 degrees will halve the dark current, and since the noise is Poisson, the dark current noise should halve too.
Interestingly, there seems to be very little dark current in the QHY8. The 10 minute darks are showing almost nothing above the biases (besides the dodgy pixels of course).
Quote:
Originally Posted by Placidus
(7) Advanced technique that I quite like: Take some unbinned shots (you've already done that bit) to get the bright sharp detail. Then on another night take some 2x2 binned shots to get the faintest regions with less detail but much less noise. Combine the two using a suitable mask to take the bright detail from the binned shot, the faint stuff from the binned shot, and pro rata in-between. You can make the mask from a blurred copy of the image.
Sounds interesting. Do you have a link to more on this technique. I tend to use PixInsight, but do have some access to Photoshop too.
Thanks again for all your tips. The time you've taken to reply and help is much appreciated.
Quote:
Originally Posted by astronobob
Nice Galaxy details Payl, have controlled the central brightness on all three very well too
Noise, ? interesting, Im not sure, but Ive had smoother with 14 x 10 min subs Qhy10 no darks or flats - might be a processing thing ?
Its a great result all the same, plenty of details
Thanks. And that shot with the QHY10 is really nice. Very smooth.
Quote:
Originally Posted by RickS
I think it's quite a nice image, Paul, but a little noisy as you said. You could improve that with some noise reduction in processing but there are limits to how much NR you can do before losing sharpness.
As Mike said, the ideal solution is to get more data and maybe longer subs. What you want is subs long enough that read noise from the camera is not significant. How long that is depends on your camera parameters and sky brightness. It's not that hard to determine from a couple of example subs whether you're read noise limited or not. Have a look at the method proposed here:
Thanks Rick. Yes, tried to reduce noise a bit more in the processing, but started to lose detail, as you suggest. That's a great link, thanks. Looks like I should be pushing for longer exposures, which is in line with Mike's suggestion too.
Lots of good things going in this pic Paul--nice tight round stars all the way from corner to corner, good colour and lots of detail, especially in M66.
Geoff
