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Poll: To vary exposures with your DSLR, do you...
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To vary exposures with your DSLR, do you...

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  #1  
Old 22-09-2009, 06:24 AM
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DSLR different exposures - do you vary shutter speed or ISO?

For a target that requires varying exposures, such as M42's bright inner core and fainter outer areas, without even thinking about it I would keep my ISO setting the same (typically ISO800) and vary shutter speeds from 8m or 4m longer exposures down to the 15s or 30s for the brighter parts.

I guess I see the added benefit that the extra time for the other exposures, and the associated darks, is at least much shorter time to do so.

At a talk at Astrofest, and also some discussions with some other guys here, I have realised that another popular option is to keep shutter speeds the same and drop the ISO down instead.

I'd like to know thoughts on the benefits of this.

Is it lower noise in the lower ISO range? Would have thought that the calibration steps we take in astrophotography would pretty much negate the difference in noise values between ISO800 and ISO100 that you would see for terrestrial photography.

I see the downside being much longer exposure times for lights and darks. Not only time I guess, but increased exposure time may also increase risk of tracking errors etc, and therefore more chances of ruining some shots.

What are your thoughts and what method do you use?
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  #2  
Old 22-09-2009, 07:14 AM
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I'm still feeling my way through this myself, experimenting with different exposures and settings.
I guess a lot depends on the object in question and the type of dslr used.
I'm looking forward to hearing what others have to say on this topic.
Excellent thread.
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  #3  
Old 22-09-2009, 01:53 PM
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after the pain of many nights of producing absolute garbage, i have found that i usually keep to ISO800, as its a nice balance of gain vs. noise, and if anything change the exposure. During colder months, i used 10 min subs, now its warming up, 6 mins ... and now i have the QHY8, and thats a whole different kettle of fish !!

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Old 22-09-2009, 07:12 PM
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Didn't even answer your poll, you ISO is much to high 200-400 max.

Leon
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  #5  
Old 22-09-2009, 08:17 PM
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I haven't used my SLR for astro images for a while but I wouldn't ever change the ISO. If you do you then need to do a entire set of darks again at the new ISO. Much easier to leave it the same.
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  #6  
Old 22-09-2009, 08:39 PM
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I did M42 and just changed the time.
Looked pretty good after processing.
I always shoot at iso 800 but might change that to iso 200 because i got my guiding nipped in the butt pretty well
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  #7  
Old 22-09-2009, 08:50 PM
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Quote:
Originally Posted by jjjnettie View Post
I'm still feeling my way through this myself, experimenting with different exposures and settings.
I guess a lot depends on the object in question and the type of dslr used.
I'm looking forward to hearing what others have to say on this topic.
Excellent thread.
Thanks JJJ - hoping to get some good discussion information here.

Quote:
Originally Posted by toryglen-boy View Post
after the pain of many nights of producing absolute garbage, i have found that i usually keep to ISO800, as its a nice balance of gain vs. noise, and if anything change the exposure. During colder months, i used 10 min subs, now its warming up, 6 mins ... and now i have the QHY8, and thats a whole different kettle of fish !!


Quote:
Originally Posted by leon View Post
Didn't even answer your poll, you ISO is much to high 200-400 max.

Leon
Leon, what the ISO is set to is not really relevant for the question. It's about whether you vary the ISO or keep it constant and vary the shutter speed. Doesn't matter if it's ISO 400 or 1600, does it?

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Originally Posted by Terry B View Post
I haven't used my SLR for astro images for a while but I wouldn't ever change the ISO. If you do you then need to do a entire set of darks again at the new ISO. Much easier to leave it the same.
But if you keep the ISO same and vary the shutter speed, you need entire set of darks for the new shutter speeds anyway. Benefit is that if they're shorted times, it's a quicker process. That was the point I was making in OP #1 above.

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Originally Posted by mill View Post
I did M42 and just changed the time.
Looked pretty good after processing.
I always shoot at iso 800 but might change that to iso 200 because i got my guiding nipped in the butt pretty well
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  #8  
Old 22-09-2009, 08:54 PM
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Change shutter, not ISO. If you take bias frames and the temp doesnt change too much the darks can be scaled for time. ie and 8 min dark could theoretically be used for 10 min lights and possibly even 6 min lights. If the temp were controlled then this is definitely possible.
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  #9  
Old 22-09-2009, 09:04 PM
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Thanks Paul. Still interested to hear what the "change ISO" people's school of thought is...
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  #10  
Old 22-09-2009, 09:29 PM
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I have to use a higher ISO to offset my short exposures. But I pay for it in noise.
Once I'm guiding, I'll be able to drop my iso and lengthen my exposures. Hopefully that means an overall smoother image that will cope with stronger processing.
It's all compromise.
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  #11  
Old 23-09-2009, 11:12 AM
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fixed ISO varied exposure depending on target

varied exposure only requried for objects with a high dynamic range

fixed exposure can also vary depending on VB of target
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  #12  
Old 23-09-2009, 05:53 PM
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Hi
i tend to always shoot at ISO 800 with the Canon EOS 20Da.
I vary the exposure length
i dont even shoot darks anymore since i started using the cosmetix function of deepskystacker (set at 1%, 1pix)

frank
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  #13  
Old 23-09-2009, 06:13 PM
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Frank, where/how do you change the settings to use this function?
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  #14  
Old 23-09-2009, 07:18 PM
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Oops, I should have read before I answered the poll. I thought you were referring to terrestial shots. I keep the same ISO and vary the shutter speed with astro shots, but I'm very inexperienced as you know.

Dave
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  #15  
Old 23-09-2009, 08:54 PM
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Hi JJJ,
If that is the setting for cosmetics in DSS, it is under Stacking Parameters at the bottom left of the Register dialogue box that pops up when you select to Register Checked Pictures. When the Stacking Parameters dialogue pops up, it is under the Cosmetic tab.
Cheers.

Oh, and vary exposure, not ISO. Higher ISO can help to achieve a bit better detail in an object you are capturing, but noise is the price. 800 was always my favourite.




Quote:
Originally Posted by jjjnettie View Post
Frank, where/how do you change the settings to use this function?
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  #16  
Old 23-09-2009, 09:06 PM
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Thanks for that. I did have a bit of a look in that section but must have missed it.
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  #17  
Old 23-09-2009, 09:08 PM
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I must have an older version of DSS. There is no cosmetic tab.
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  #18  
Old 23-09-2009, 09:10 PM
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Mine is 3.3.0. It was in the last one as well.
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  #19  
Old 23-09-2009, 09:17 PM
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I've 2.6.3. That explains it.
Must download the new version.
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  #20  
Old 24-09-2009, 06:09 AM
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Sorry JJJ i was too slow to respond
yes you'll need a newer version
i love the function
i did a lot a tests a while back with without darks/cosmetics and in the end checking zoomed in pixels of the test samples i became convinced to keep using it with cosmetix at 1%, 1pixel setting.
hope it works for you as well
frank
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