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Old 16-07-2014, 08:12 PM
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Camelopardalis (Dunk)
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Which exposure next?

Just as a little tangential discussion from the threads we've had about polar alignment and exposure length... I pose the question... which settings should I try next?

So far I've been taking shots mostly at ISO1600, and as it turns out it looks pretty noisy on my camera. 10 minute shots look like a snow storm

I've just ran a sequence of shots of 1, 2, 3, 4, 5, 6, 8 and 10 minutes at both ISO800 and ISO1600 and the ISO800 shots seem a lot less noisy. I've attached a few of these (an 800x600 area from the top left corner from each shot).

Assuming I can muster a half decent polar alignment next time out what settings should I try to capture the faint photons, and keep the noise under control? Even at ISO800, the 10 minute exposure is quite noisy... (but the 1100D was a bargain!)

How to make the most of what I've got...?

Drop the ISO further? Rely on stacking to smooth it out?

And what sort of exposure length should I be aiming for? (I'm not guiding, yet..)
Attached Thumbnails
Click for full-size image (ISO1600 180s.jpg)
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Click for full-size image (ISO800 180s.jpg)
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Click for full-size image (ISO1600 360s.jpg)
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Click for full-size image (ISO800 360s.jpg)
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Click for full-size image (ISO1600 600s.jpg)
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Click for full-size image (ISO800 600s.jpg)
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  #2  
Old 16-07-2014, 08:40 PM
glend (Glen)
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I usually shoot DSOs at ISO800, simply because I read a recommendation that it was the best choice for the Canon 450D.

What do your Darks look like? What is the ratio of Lights to Darks? Are you using Bias Frames as well in your stacking?

I sometimes switch to ISO1600 when I am attempting to focus but I drop it back to 800 for the exposure run.
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Old 16-07-2014, 08:46 PM
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Eden (Brett)
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Hi Dunk,

Are you auto-guiding yet? If you're looking to push the exposure time, it'll certainly help.
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Old 16-07-2014, 08:57 PM
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Amaranthus (Barry)
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In DSLRs, the ISO is changing the gain, so as to get the maximum range out of your pixels. If you are taking short subs, you'll need a relatively high ISO to distinguish different levels of pixel signal, since the ungained signal differences will be small. With long exposures, you've got more signal and are using more of the full range of your pixels. So you don't need such a high ISO.

Basically, the longer the exposure you take, the lower the ISO you should be setting. But remember that ISO cannot give you more signal, it can just stretch what exists over your dynamic range.

You should aim for the longest exposure you can get without bloated or trailed stars (without guiding) or otherwise (with guiding), you need to think about background and saturation.
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Old 16-07-2014, 09:19 PM
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rmuhlack (Richard)
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this is the approach that I have taken to determine optimal ISO and sub times (using a Canon 400D):
http://www.iceinspace.com.au/forum/s...d.php?t=106775

and again with a Canon 1000D:
http://www.iceinspace.com.au/forum/s...d.php?t=108354

I have have just been through this entire process again, as I've just completed a regulated cooling mod to the 1000D (above) and also a 450D and have followed the same approach to determine optimal sub/ISO settings under regulated cooling conditions.
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Old 16-07-2014, 11:11 PM
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Thanks for the feedback chaps, much appreciated

These images were taken with the lens cap on sitting on the desk today at around 20C ambient temperature. I took 3 shots alike, and the ones above are the middle out of the 3.

I typically take 10-20 darks, and about the same number of bias frames. The number of lights varies by target, but I think the last bunch I've had about 15 minutes worth of data. I appreciate this isn't much exposure time, but I was curious about the exposure settings before taking extended sets of data. After all, I'm still working on my polar alignment

I will get a guider...eventually...I'm just trying to keep it simple for the moment to see how far I can get without needing to run the laptop and the additional power draw that requires. It's a good challenge!

So it sounds like I'm going to need to experiment with some sequences to determine what my best ISO is for the signal to noise ratio...thanks for the links Richard, very interesting reading.

Another question...is it possible that the "optimal" ISO is different depending on the subject? For example, one setting for fainter objects (galaxies for example) and then a different ISO for brighter objects like Omega Centauri, Eta Carina nebula and a select few other bright objects?
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Old 17-07-2014, 07:30 AM
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Couple of things.

1. don't forget to take darks of the same length and subtract those away.

2. even cooled cameras have lots of noise in a single image. You need to stack lots of frames to remove noise and increase signal.

3. What camera are you using? Some cameras are more capable than others. If you are using a camera 3 years old or under that, then you can use ISO1600 but you still need to do darks and stacking to get a smooth shot.
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Old 17-07-2014, 10:06 AM
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Thanks Paul. I'm using a Canon 1100D. I've read a couple of things claiming the noise is lower at ISO1600 than ISO800 which sounds counterintuitive!

How many is "lots"? I'm guess it depends on the brightness of the target? Is there a general rule of thumb for exposure time vs magnitude, without going too much into the territory of diminishing returns.
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Old 17-07-2014, 11:10 AM
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NR follows a quadrature relationship with sample size, so 'lots' is probably 8-16 (for long exposures) or any many as you're willing to do for short subs.
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Old 17-07-2014, 11:19 AM
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Quote:
Originally Posted by Camelopardalis View Post
Thanks Paul. I'm using a Canon 1100D. I've read a couple of things claiming the noise is lower at ISO1600 than ISO800 which sounds counterintuitive!
The way gain is applied makes some odd ISO's cleaner than others. A lot of Canons are cleaner at ISO 160, 320, 640, 1250, 1600, 2500.

eg: http://vimeo.com/10473734
http://vimeo.com/20239453
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Old 17-07-2014, 11:27 AM
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Quote:
Originally Posted by Amaranthus View Post
..... or any many as you're willing to do for short subs.
What Barry said.

When I was doing deep sky video astronomy with 2.5 second integrations and making stills from the sequence, it wasn't uncommon to stack 1000+ frames. I think even for 1 minute subs 1000 wouldn't be "too many". Although the camera's shutter would likely kark it after 100 images.
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Old 17-07-2014, 02:39 PM
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Thanks chaps!

Define "long" exposures...

Those noise videos are really interesting Ed!

Looks like I need a dark sky and time to experiment.

Would it be a reasonable assumption, for the purpose of comparison of noise, to take say an hour at ISO 1600 and 2 hours at ISO 800 and then decide which one works out better? (Using as long subs as I can muster!)
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Old 17-07-2014, 03:00 PM
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Amaranthus (Barry)
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For me, long is >2 min, i.e. when you get into realm of requiring guiding. Short is <30 sec. Everything in between is murky...
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Old 18-07-2014, 02:35 AM
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Unguided, it depends on the focal length of the scope and PA. At 900mm focal length, my scope can only manage a minute with the HEQ5 pro and I lose 20% of the subs to drive error. At 2 minutes I'd probably lose 80% to drive error. With a scope half the focal length at say 440mm, which is where my F5.5 ED80 is, I get 2 minutes unguided and keep at least 80% of subs. The ED80 is shorter and lighter too which helps.
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Old 18-07-2014, 11:43 AM
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OK interesting, thanks chaps!

I don't expect I'll get beyond a minute then, my wide field scope has focal length of about 340mm, and then there's the C8
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