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Old 28-08-2019, 02:01 AM
Xeteth (David)
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Assistance with image problems

I've spend the past few nights in Melbourne battling the cold and getting familiar with how to setup my mount, polar align, guiding etc.

I'm pretty damn happy with the progress I've made - going from never having done any of this before to being able to be polar aligned and plate solved in under 10 minutes (thanks Sharpcap PA tool and Astrophotography Tool Plate Solve!). Setting up EQMOD was a nightmare, but I got there in the end.

Anyway, time to start shooting images. So I go ahead, slew to the Helix Nebula and start taking 60 second subs at ISO 400. I'm still learning what the best ISO etc to use is, but I'm starting to get the hang of it all.

I'm having a major problem though - I believe it's either hot pixels or a busted camera chip. I'm using a Canon 650D DSLR which I'm borrowing from a mate, there seems to be some rather nasty stuff showing up on the images I'm taking.

Here's 20 subs (and 2 darks) stacked and terribly processed in Photoshop (I wanted to get more but the damn clouds moved in!). I'm only just learning how to process the data, lots of learning to do. If you zoom in you'll notice quite a few red pixels (I'm guessing these are hot?) and a whole bunch of purple ones also (which I have no idea what they are?).

Any critique/advice would be much appreciated (bear in mind this is my first ever set of subs and I'm in a fairly steep learning curve at the moment ).
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Old 28-08-2019, 02:12 AM
AnakChan (Sean)
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They do look like hot pixels. Do you see them in your dark shots too (in the same location)?

I’m not too familiar of how to use Photoshop to subtract the dark frames. I didn’t know Photoshop could. Subtracting from dark frames should remove the hot pixels, unless you’re stacking everything together (your 20 subs and 2 darks) on Photoshop in which case you could merely just be adding the hot pixels.
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Old 28-08-2019, 02:19 AM
Xeteth (David)
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Quote:
Originally Posted by AnakChan View Post
They do look like hot pixels. Do you see them in your dark shots too (in the same location)?

I’m not too familiar of how to use Photoshop to subtract the dark frames. I didn’t know Photoshop could. Subtracting from dark frames should remove the hot pixels, unless you’re stacking everything together (your 20 subs and 2 darks) on Photoshop in which case you could merely just be adding the hot pixels.
My mistake, I should have said I'm stacking in DeepSkyStacker then doing some processing in Photoshop afterwards. Some of them can be seen in the darks but they're not as prominent... I think I need to go back and check out how exactly to stack properly to remove hots. If they're in the darks then they should be removed in the stacking process, right?

Also that RGB tinge that can be seen all around in the background where it should be essentially black - what is causing that?
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Old 29-08-2019, 09:57 AM
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sil (Steve)
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Yep I can see lots of hot pixels there, the "purple" ones are actually blue pixels and there are also green hot pixels too throughout. the colour of the image is off and the green hot pixels look like sharp stars in your image.
take more blacks and use bias frames too. dont rush things, set outside one night and just shoot any part of the night sky really, take a minimum of 20 light subs, 10 darks, 10 bias and 10 flats. That will give you a basic set of subs to learn the processing, forget about trying to capture a target in particular. That will distract and shape your processing. But combining the captured subs is identical regardless of the starscape, also in how you post process after combining really. the rgb tinge you refer to is sensor noise and some thermal probably. its always in subs really manifesting as an even amount of random noise in the three colour channels. this is in practice the main "Noise" in Signal to Noise ratio and to reduce it you take more subs as the strong signal (ie the picture you want) is present in all frames so when averaged it result in close to 100% of the actual value while the noise which is in three colour channels separately at much lower levels gets averaged down to a lower amount making it much less noticable and thus increasing the Signal to Noise ratio. Its part of why you should never push your background in processing to pure black values as it shouldn't be. The other reason is its not actually pure black ever. Every empty patch of space a telescope has peered closely at contains stars and galaxies no matter how faint. so the blackness of space is really the very very very dark grey of space. Plus photoshop only gives you 8 bits of control and that single bit gap between 0,0,0 (pure black) and 1,1,1 (almost black) often contains a heap of signal which you lose forever in pursuit of eradicating noise (which you cant really do technically).

A common misunderstanding of imaging learners is that more subs means a better brighter picture. This is not the case. It gives you a more accurate signal and increases SNR. Increasing SNR gives you more room to stretch the combined data to then make a better brighter image. You still have to put in effort to to make it happen. But taking those subs (lights darks bias flats) all play their part in giving you a better end SNR but once combined and display on the screen you usually only see a black image because thats whats there UNTIL you start stretching. As you stretch you are amplifying both the Signal as well as the Noise! So combining everything just lets you be able to stretch further before the noise becomes too noticable again. Yes you can get away with making images from only lights but as you progress and encounter limitations you start to appreciate adding darks, bias and flats much more and the more you take of each during your session the better the potential results of your final image will be. Just taking a couple of the others wont do much for your image that you can see initially but later on you will. Especially when you are taking the time to set up and hunt down a target I suggest you also dedicate the time to take about two dozen each of dark, bias and flat frames too. So in a years time you can return to the set you took and be able to make better use of all the data you captured. Start off by considering it an essential part of the capture process instead of only capturing your lights as its a good habit that will help in the long run.

right now your on a good start, more subs will even out that rgb noise background and taking more darks and bias will help those hot pixels to be killed off.
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Old 29-08-2019, 11:08 AM
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The_bluester (Paul)
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The biggest thing I learned when I started with a DLSR (And older Canon) was that they will produce loads of hot pixels, a 60 second dark frame looked like a colourful starfield.

The best bet is to shoot heaps of darks in whatever configuration you are going to use to image with. So if you will take 60 second shots at ISO 400 with no break between, shoot at least 40 darks in the same way (And at a similar air temperature) to create a master dark in whatever software you use for stacking. Likewise for any bias frame, shoot a heap and combine them for a good master bias.

If you are using a guider, dither between subs as that helps the stacking software reject any hot or cold pixels or fixed noise not removed by a master dark frame.
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Old 30-08-2019, 02:53 PM
Xeteth (David)
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Thanks for the advice and tips Paul & Steve! Tonight is looking like a brilliant night in terms of weather so I'll be out there again. I'm going to take a lot more darks, flats and bias as you've suggested.

I'll also setup dithering in PHD2 which should help as well. Thanks again for the advice!
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Old 30-08-2019, 05:44 PM
AnakChan (Sean)
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I wonder when you stretch your darks if you see the hot pixels in the same locations. If so then DSS should subtract them. Per Paul, ensure you take your darks in the same duration and pref after you've taken your light subs (that way, your sensor is "properly" warmed up when you get take your dark calibration frames).
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Old 13-09-2019, 08:36 PM
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astronobob (Bob)
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And if you notice any of these = Dead Pixels
Nice work, you have accomplished a lot
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