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pixueto

Veil Nebula Canon 350D, 200P on EQ5 mod

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This is my last processing for the Veil nebula. I took the lights in a warm August night. I can see loads of noise; the night was warm indeed but I took 10 minute subs (3h 40mins!!!).

I sorted out my kit to be able to take 10 min subs using a LP clip filter (I live in a badly light polluted area). I thought that such a long subs would bring up much more detail but I'm wondering if by increasing the time I'm actually making things worse as the chip in the canon 350 heats up a great deal under those exposition times. Maybe I would be better off with 5min subs or less and maybe that would remove the need of the LP filter?

Thanks

post-18331-0-86301200-1349386023_thumb.j

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That looks a nice piece of work. Very well processed with one of the very few good looking star spikes !

If I remember correctly the chips warm up, as you say, then stabilizes. A few experimental shots may prove or disprove this for your camera. IE. If it gets to it's warmest at say 3 minutes then anything after that may not make it any worse. I think Gina has done temperature measurements like this.

Dave.

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I've read that super-long subs are not always beneficial. I believe what this guy has to say:

http://www.samirkharusi.net/sub-exposures.html

I think DSLRs are not really designed to take 10 minute exposures - the noise gets out of control and the stars turn white. And with long subs you can't take as many, so you stacking is less effective at reducing noise.

However, you have certainly captured a lot of detail. It would be nice if you could kill the color gradient in the lower left corner.

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Interesting link.

I've been reading elsewhere that one way of approaching the problem is to use short subs at a reasonably high ISO or long with a low ISO. Either way stop before saturation !

Dave.

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Thank you very much for your comments. I really appreciate it. It would be nice if Gina can say something about it.

Dave: how can I measure the temperature in the chip? Can APT do that?

Agnes: after what you said, I gather that 30min total exposition time made up of six, 5 mins subs is better than three, 10 min subs? I've got a feeling after experimenting a bit that this is the case but I don't understand the theory behind it. Why would people go through the trouble of taking long exposure lights and having to deal with the complex issues that involves such as accurate tracking then?

Edited by pixueto

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APT does it in *F or *C

Gina may not have done it. I've read the cooling threads but can't now remember who did say it.

Dave.

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In light polluted skies you should be reducing your exposure, I think. Also, as well as taking darks, try to dither between exposures so that a given star visits a few pixels nearby and doesn't stay on one bad one. That helps in those cases where the darks are not consistent (when the temperature varies, for instance).

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I've read that super-long subs are not always beneficial. I believe what this guy has to say:

http://www.samirkhar...-exposures.html

I think DSLRs are not really designed to take 10 minute exposures - the noise gets out of control and the stars turn white. And with long subs you can't take as many, so you stacking is less effective at reducing noise.

However, you have certainly captured a lot of detail. It would be nice if you could kill the color gradient in the lower left corner.

Thank you. Is that a real gradient or is it part of the Hydrogen in the other parts of the loop? I wonder.

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Nice capture of the witches broom, if you don't mind me saying, you made a classic DSLR mistake! I think you already realised that unless the camera is cooled the 10min subs are just causing you to record more and more thermal noise... I made this mistake as well, trying to do 20min subs, it just doesn't work, you would be better of sticking to 3mins or 5mins max and allowing some cool down period in between each shot as well.

3min + 1min cool down etc... The more shots you get the more control you will get over the noise which sit mainly in the RED channel.

Still its a nice capture and your definitely well on your way to getting better results.

Edited by Darth Takahashi

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Very nice image, Pixueto!

APT can read and use the EXIF temperature for all models that support it. Unfortunately the Digic II cameras like your 350D doesn't have temperature sensor :(

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I regularly use 10min subs (for narrowband) and don't find thermal noise to be that much of a problem. Sky noise, however is a different matter altogether and I never use more than 5 min exposures when capturing broadband. Generally, I think you're ok to continue increasing your exposure time while your sky background level isn't too high (e.g. below about 8% according to DSS). So long as you capture enough subs, that is!

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I'm not familiar with the 350D so don't know if it's possible to prevent image sensor heating using the Pause in APT. With my 1100D it doesn't help because it uses LiveView to simulate mirror lockup to wait for any vibrations to die away. Some EOS models allow true mirror lockup with the Pause in APT - I imaging this would allow the imaging system to cool a little between exposures. What I do know is that the noise is very dependent on temperature, increasing exponentially with temperature - doubling for every 7C rise. As can be seen in my cooling threads (particularly the cooling to ambient one) in the DIY Astronomer forum, cooling the chip is very good at reducing thermal noise in the camera.

Now regarding your image... Firstly, well done on the capturing and processing :)

Whatever exposure or ISO you use you want to use an LP filter - there are ways of reducing the effect in processing but nothing like stopping or reducing it in the frist place. In urban areas the main sources of LP are sodium and mercury vapour street lights and these emit light in narrow bands in parts of the spectrum we are not interested in for DSO AP. This means filters that block soduim and mercury emissions will not block our beloved faint fuzzy emissions. The best I have come across is the CLS filter - available as a clip-in filter for EOS models from Astronomik and available to buy from Bern at Modern Astronomy (whom I wholeheartedly recommend). I have one of these even though my LP isn't too bad except very low in the sky. When I took two photos across the valley at the nearby villages (and main A30), with and without the CLS filter I was amazed at seeing the street lights completely eliminated. I'll see if I can find them.

With an object like the Witch's Broom with a very bright star you need to keep exposures low to avoid the star not only saturating the image but spreading out sideways and spoiling the dim nebulosity. This is particularly the case with earlier EOS models with Digic 3 or earlier image processors, as these only give 12 data bits meaning just 4096 luminance levels. (The later Digic 4 models give a 4x improvement - 14bits and 16384 levels.) So it's particularly importand with earlier models not to over-expose each sub. You can make up for shorter subs by having more of them. What matters is the total time within certain limits.

Both the signal and noise levels depend on ISO x exposure so you can use higher ISOs in conjunction with shorter exposures or vice versa to give the same signal to noise ratio. With exposures shorter than a minute or thereabouts you may not need guiding depending on focal length and accuracy of polar alignment. OTOH I've found that the highest ISO seems to give a lot more noise than expected ie. more than twice that of half the maximum ISO. Below that it seems to be pro-rata as expected. Consequently I generally use 3200 rather than the 1100D max of 6400.

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Nice image there and I wont add to the explanations on 10 min subs and DSLAR cooling!

Velvet

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That looks a nice piece of work. Very well processed with one of the very few good looking star spikes !

If I remember correctly the chips warm up, as you say, then stabilizes. A few experimental shots may prove or disprove this for your camera. IE. If it gets to it's warmest at say 3 minutes then anything after that may not make it any worse. I think Gina has done temperature measurements like this.

Dave.

Yes, the sensor warms up and then stabilises at a constant temperature. The rise is fast at first and levelling out later. It's got most of the way in 3-5 minutes. It cools down in a similar way - fast at first then levelling out.

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Very nice image, Pixueto!

APT can read and use the EXIF temperature for all models that support it. Unfortunately the Digic II cameras like your 350D doesn't have temperature sensor :(

Ahhhhh! That explains everything now. I was going mad trying to find that temperature reading!!!! Thank you.

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I regularly use 10min subs (for narrowband) and don't find thermal noise to be that much of a problem. Sky noise, however is a different matter altogether and I never use more than 5 min exposures when capturing broadband. Generally, I think you're ok to continue increasing your exposure time while your sky background level isn't too high (e.g. below about 8% according to DSS). So long as you capture enough subs, that is!

Thanks Lewis. Can you elaborate a bit on how you do narrowband? Is it with a DSLR?

Can I ask you how can I measure the sky background level?

Thanks

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Forgot to say... the redness on the bottom left is nebulosity as can be seen in this one of mine :-

post-13131-0-29550500-1349455854_thumb.p

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Interesting... So if you can get to 5 mins, you may as well carry on to 10, assuming you can take enough subs to smooth out the result. I've certainly learnt something.

Regarding 6x5min or 3x10min, I would think that both are not enough :-) but on a bright target, 6x5 has to be better, and on faint targets 3x10!

Edited by Ags

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I'm not familiar with the 350D so don't know if it's possible to prevent image sensor heating using the Pause in APT. With my 1100D it doesn't help because it uses LiveView to simulate mirror lockup to wait for any vibrations to die away. Some EOS models allow true mirror lockup with the Pause in APT - I imaging this would allow the imaging system to cool a little between exposures. What I do know is that the noise is very dependent on temperature, increasing exponentially with temperature - doubling for every 7C rise. As can be seen in my cooling threads (particularly the cooling to ambient one) in the DIY Astronomer forum, cooling the chip is very good at reducing thermal noise in the camera.

Now regarding your image... Firstly, well done on the capturing and processing :)

Whatever exposure or ISO you use you want to use an LP filter - there are ways of reducing the effect in processing but nothing like stopping or reducing it in the frist place. In urban areas the main sources of LP are sodium and mercury vapour street lights and these emit light in narrow bands in parts of the spectrum we are not interested in for DSO AP. This means filters that block soduim and mercury emissions will not block our beloved faint fuzzy emissions. The best I have come across is the CLS filter - available as a clip-in filter for EOS models from Astronomik and available to buy from Bern at Modern Astronomy (whom I wholeheartedly recommend). I have one of these even though my LP isn't too bad except very low in the sky. When I took two photos across the valley at the nearby villages (and main A30), with and without the CLS filter I was amazed at seeing the street lights completely eliminated. I'll see if I can find them.

With an object like the Witch's Broom with a very bright star you need to keep exposures low to avoid the star not only saturating the image but spreading out sideways and spoiling the dim nebulosity. This is particularly the case with earlier EOS models with Digic 3 or earlier image processors, as these only give 12 data bits meaning just 4096 luminance levels. (The later Digic 4 models give a 4x improvement - 14bits and 16384 levels.) So it's particularly importand with earlier models not to over-expose each sub. You can make up for shorter subs by having more of them. What matters is the total time within certain limits.

Both the signal and noise levels depend on ISO x exposure so you can use higher ISOs in conjunction with shorter exposures or vice versa to give the same signal to noise ratio. With exposures shorter than a minute or thereabouts you may not need guiding depending on focal length and accuracy of polar alignment. OTOH I've found that the highest ISO seems to give a lot more noise than expected ie. more than twice that of half the maximum ISO. Below that it seems to be pro-rata as expected. Consequently I generally use 3200 rather than the 1100D max of 6400.

Thanks Gina. I normally take subs at 800 ISO as I seem to get the best results with those. I noticded that other people like Quartermass using the same camera uses 800 ISO obtaining impressive results.

As for the filter, I have the astronomik LP filter but if my exposures are going to be limited to 3 mins maybe there is not much point in using it?

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Forgot to say... the redness on the bottom left is nebulosity as can be seen in this one of mine :-

post-13131-0-29550500-1349455854_thumb.p

Thanks Gina. Lovely picture.

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I hope you don't mind but I've done a tweak of your image as posted in PS to pull more colour out, particularly the hydrogen alpha nebulosity.

post-13131-0-35433600-1349457821_thumb.p

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Thanks Gina, I haven't been able to remove the IR filter from my canon 350D yet.

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On the subject of sub length, see this thread for a very clever use of a range of sub lengths:

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