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Old 26-02-2015, 07:51 AM
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ZeroID (Brent)
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A good night but ..

Decided to move on from Orion, think I've done it to death. Last night was ok and I had a go at Thors Helmet (NGC 2359) as it was well positioned. Ran 104 exposures @ ISO 1600 for 30 secs ( 16 at 45secs) ) giving me nearly an hour of exposure on the 450D\Lunt102 f7 combo.

Couldn't see anything on the frames but pushing the processing hard revealed it with faint green swirls centred in the stacked image. Nothing like the detailed images I've seen posted.

So the question is just how much exposure is required for this one to get any real results ?

Descriptions of this object always call it 'bright and colourful' so I'm a little perplexed. If it requires a heap more exposure I'm going to have to radically modify my imaging regime in my LP'd zone.
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Old 26-02-2015, 09:10 AM
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jenchris (Jennifer)
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I think it may be a case of 10x6 minutes is a lot more info than 60 x1 minute.
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Old 26-02-2015, 12:00 PM
clive milne
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Quote:
Originally Posted by ZeroID View Post
Decided to move on from Orion, think I've done it to death. Last night was ok and I had a go at Thors Helmet (NGC 2359) as it was well positioned. Ran 104 exposures @ ISO 1600 for 30 secs ( 16 at 45secs) ) giving me nearly an hour of exposure on the 450D\Lunt102 f7 combo.

Couldn't see anything on the frames but pushing the processing hard revealed it with faint green swirls centred in the stacked image. Nothing like the detailed images I've seen posted.

So the question is just how much exposure is required for this one to get any real results ?

Descriptions of this object always call it 'bright and colourful' so I'm a little perplexed. If it requires a heap more exposure I'm going to have to radically modify my imaging regime in my LP'd zone.
Brent, as Jen intimated, it is not so much the signal level that you need to address as it is the case that you need to reduce the noise.
There are a few ways to do this;
Reduce the intensity of the sky background - A 9nm narrowband filter for example will reduce it by a factor greater than 10x (over typical RGB colour separation) .... Bearing in mind that the relationship between signal/noise and exposure is a square (^2) function, a system that has 10x the noise will require 100x the exposure to achieve the sasme s/nr.
Reduce the readout noise of your camera- take fewer exposures of much longer duration.
Reduce the noise associated with accumulated thermal signal - ie) use a cooled camera.

This is a fairly simplified account. It is probably also worth understanding the way the total s/nr is influenced by discrete factors when you have several noise sources contributing to the whole.
You square the value of each source, add them all together and then take the square root of the whole. The significance of this is that if you have one noise source that is numerically much greater than (the) others, reducing the lower level noise contributers doesn't help much at all.

To elaborate further, take for example M42... the nebula is of such high intensity that the dominant source of noise in the image will be the statistical variation in the arrival time of the photons. Thor's helmet on the other hand is of much lower intensity, in which case, technique, equipment and sky conditions play a much larger role.

best
c
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Old 26-02-2015, 06:24 PM
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ZeroID (Brent)
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Interesting. The 450D is somewhat limited in it's scope for modification of performance. Cool box experiments prove the sensor is not cooled easily. My LP limits exposure times considerably. Possibly a UHC filter might help but I'd need 2" filters.
I can also drop the ISO to 800 or 400, it's an older camera so that might help sensor noise.
I may switch to the ZWO 120 which I can get down to sub zero numbers and is also Ha sensitive. And I have a 1.25" Astronomik CLS that could be tried.

Thanks < goes away to ponder and check the piggy bank for filter money >
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Old 27-02-2015, 01:30 PM
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rustigsmed (Russell)
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hi brent,

going through similar issues myself. didn't realise how restricting the light pollution was for my pics at my location. light pollution filter (or NB) will help, winter or a cooled solution is the only way to push it further. summer hasn't been kind, looking at cooling solutions... but by the time I get around to doing/ getting something it will be June, so I guess i'm planning ahead for next summer!

cheers

rusty
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Old 02-03-2015, 06:38 AM
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ZeroID (Brent)
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Hi Russell,
Yeah, not good for sure. I'm lucky sometimes with cold southerlies in Autumn that clear the air but I really need to buy a couple of 2" filters, UHC and O III I think.
I have them in 1.25" for the ZWO which I can cool but I do like the Canon and BYE so might have to bite the bullet.
I'll have a play with the ZWO in both setups and see how it does and think about it ( and build up the astro fund a bit ).
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Old 02-03-2015, 10:03 AM
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If you are guiding, why not try to increase the exposure time?
I can easily get 10 min exposures on my neq6peo.
Though I've found the crossover for heat noise is about 3 minutes.
Once the weather cools sufficiently, the noise will abate somewhat and allow me 5 min subs.
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Old 03-03-2015, 01:18 PM
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ZeroID (Brent)
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Guiding isn't the problem, LP is the problem. SNR goes to hell after about 45 seconds. So much skyglow from CBD. Even a C80A pale blue filter helps to kill some red sky. I may give that another try. Longer exposures with reduced sensitivity but might cut through the red.

The 450D has thermal issues at about 2 mins, the SONY doesn't have heat issues ( lovely camera ) but it's a mission to get critical focus without having a BYE equivalent for it and I'd still need a LP filter or go narrow band to manage longer exposures.

Yoddha (APT) is working on a SONY interface so I've got my fingers crossed it's soon as I'd love to be using it. It also has a customisable White Balance system inside so I can set sensitivity away from the red spectrum end.
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Old 03-03-2015, 04:47 PM
glend (Glen)
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Brent I don't have any thermal issues with my 450D and I run out to five minutes at the dark site we go to at the new moon. I qm pretty sure the 450D produces less heat than many more modern DSLRs. There are things you can turn off that might help - don't need sensor cleaning, dark auto production, etc. There is a good list I ran across of all the functions that should be turned off, I'll see if I can find it.
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Old 04-03-2015, 09:06 AM
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ZeroID (Brent)
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Cheers Glen, I haven't investigated much on that side. I've been limited by the LP situation so longer exposures have not been an option. I'll investigate further but any info welcome.

Also been having a look at Yoddha's APT software. Very clever and more features than BYE but also more complex to use. I'm more than happy with BYE but Yoddha is hopefully adding SONY compatibilty and I have a big SONY just waiting to go.

Always something new to play with hey ?

I just love this astronomy lark !!
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