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Old 10-05-2010, 09:15 AM
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Manav (Yugant)
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Question Calculating optimal exposure length for astrophotography

Hi Everyone,

I have a question regarding optimal exposure length for DSLR (Canon 50D) when conducting in LP areas such as Sydney. Just to give you background I live in Sydney and use the Astronomik LPS clip-in filter. I've read several articles online and can't figure out how to find the ideal exposure time for astrophotography. My light frames range from 30sec to 3min and I would like to know if there is an way/program that will help me get optimal data so I can spend more time taking same exposure length images instead of changing ISO/exposure after every shot?

Furthermore, recently, I've also started to look at using a lightbox for creating flats. Once again how do I calculate the exposure time required to get the best flat image? I know there is a target ADU to reach but how do I calculate what that is for my Canon50D?

Thanks for your help!

Regards
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Old 10-05-2010, 09:43 AM
Mighty_oz (Marcus)
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This should get u started Happy reading

http://www.samirkharusi.net/sub-exposures.html
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Old 10-05-2010, 09:52 AM
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Octane (Humayun)
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Manav,

5 minutes at ISO-400 is a good starting point.

Ignore the light pollution -- you're still capturing data through it. When it comes time to set your black and white points in post, select a region of the image void of objects and assign a black value of something like 25,25,30. This will give you a dark background with a very minute blue cast to make the sky appear natural. You shouldn't have to set your white point then, unless you're into G2V calibration.

This should at least give you a starting point to further enhance your image in post.

For flats, take an exposure and observe the histogram on the LCD preview. You're looking for a histogram which peaks somewhere between 1/2 to 2/3rds along the x-axis of the histogram.

H
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Old 10-05-2010, 10:35 AM
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Mighty_Oz: Thanks for the link it was quite a good read. The author mentions keeping the exposure times in line with the histogram and shooting 20-100 shots.

H: For the flats I'll be shooting with the filter in place so my final image will be slightly blue do I need to worry about that?
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Old 10-05-2010, 11:00 AM
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Octane (Humayun)
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Manav,

Nope, not at all. The colour of the flats is not an issue.

Samir is an advocate for short exposures and lots of them. I disagree with that. If you want nice images, take long exposures, and lots of them.

Of course, going to dark sites helps infinitely more so, but, that still shouldn't stop you from capturing photons from a light polluted site.

H
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Old 11-05-2010, 05:59 PM
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hey manav, as i have the same scope as you and pretty much the same camera i can let you in on what my experiences are. That is anything over 3 minutes died in the A55 admittedly i wasn't using a LP filter.

I would seriously consider Narrowband filters as then your not worrying about the LP. but in which case you need to have your camera modded or a CCD cam.

Hows the MPCC adapter going anyhow did it work the way we intended it?
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Old 11-05-2010, 10:15 PM
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Yup it sure did! I'll update the original thread and I apologise for not doing that earlier (soo busy in recently). Brendan did you use the hystersis for the exposure length (flats) as well?
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Old 12-05-2010, 09:57 AM
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with the flats i just eyeballed it, using the histogram, its the easyest way to run the show with the KISS theory.

Im unsure what you mean by the hystersis (i associate that with PHD guiding). Im glad that the mpcc and 2" mod works well! I just fitted my 3" FT focuser last night finally had enough of the skywatcher gig... the 3" focuser is huge and the baseplate only just misses the spidervane thumb screw by about 2-3mm hooray! The only downer part, is i think im going to have to shorten the tube by about 20mm...
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Old 12-05-2010, 10:11 AM
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multiweb (Marc)
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Quote:
Originally Posted by Manav View Post
I have a question regarding optimal exposure length for DSLR (Canon 50D) when conducting in LP areas such as Sydney.
In a light polluted environment I'd recommend shorter subs and lots of them. Flat fielding will also make a big difference in your pictures contrast and will allow you to push your data harder.
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Old 17-05-2010, 09:36 PM
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So here is the data varying from 15sec to 2:30min @ ISO1600 from Paramatta using a 254mm Newtonian @ 1200mm focal length.

Flickr Link

The imaging consists of Canon 50D with a MPCC and Astronomik CLS LP Clip in filter. As you can see the saturation at exposure greater than 1min. I wish there was a program that would let you calculate the point at which the data being collected clips off and the noise overtakes.

The contributions above leads me to believe that best bet is lowering ISO and capturing 2min + shots and lots of them!

Brendan: I'll PM you regarding my lightbox and how to use the camera histogram.
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Old 17-05-2010, 09:57 PM
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I did say 5 minute exposures at ISO-400 in my post!

2:30 at ISO-1600. The equivalence at ISO-400 would be 10 minutes.

H
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Old 17-05-2010, 10:19 PM
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Sorry H! Have taken note of your suggestion and will now experiment once the clouds go away!
BTW Did you like my blue LP
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Old 17-05-2010, 10:35 PM
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CCDWare have just updated their calculator.

http://www.ccdware.com/resources/
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Old 18-05-2010, 11:04 AM
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thats pretty useless unless you have a Sbig, FLI, or apogee. thanks for the link though.
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