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Old 22-10-2011, 10:38 PM
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hotspur (Chris)
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Time span for subs.

I have a external power supply being made for my Canon 450D-this what I will use for my astronomy photography,on the scope and wide field lens from now on.

I have started using Deep Sky stacker.What is the most regular time span used for doing such projects? I see people mentioning 2 minutes to 5 minutes for the regular type images I am looking at trying to do.

I do not think I will do anything more complicated in astro images,these flats people talk about,is that only if you have a' field flattener' for your scope? Which I heard is rather voodoo to get the exact right one for ones particlular scope.

I think I will mainly focusing on wider field imaging with the 10-22 mm lens,I did a 14 minute single exp of milky way in July,and it got a lot of favourable comment from different groups of people.So stacking lots of images of a smaller time frame would produce better results.

what is the best time frame for subs doing wide field?

Thanks for any advice-Chris
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Old 22-10-2011, 11:25 PM
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Octane (Humayun)
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I guess it depends on how much you stop the lens down.

At 10-22mm, you're taking in enormous swathes of sky; equivalent to about 17-40mm in a full frame system.

You'll probably be looking to shoot at between f/4 to f/5.6 to reduce chromatic aberrations.

I used to shoot for 10-minutes at f/4 at ISO-800 on the 40D.

Flat frames are an absolute necessity when shooting widefields -- the outer portions of the image will receive somewhere up to 20% less light than in the image centre. Vignetting is wonderful for terrestrial photography (framing people/landscapes, etc.,) but it is detrimental to astrophotography.

Taking flat frames is easy. Set your camera to ISO-100 (its lowest native setting), point your camera at an evenly illuminated flat object (wall) and take exposures that give you a histogram on your LCD that peaks about 2/3rds the way across the X-axis. The exposure length will be dictated by your light source.

One thing to remember is to NOT remove the lens from the camera until after you've done your flat frames. That is, once you've captured your light frames, do not rotate the lens, or change focus or focal length -- once you do that, you will shift the artefacts (dust bunnies, streaks, random particulates) to a different location/focal length, and will induce further aberrations in the finally calibrated image.

When I used to shoot with a camera and lens, I would carefully remove the camera from the mount, plonk it down on my laptop's keyboard, open a blank Photoshop document with a white background, and point the camera at it. Then, just used the metering in the camera to give me a proper exposure in aperture priority mode, and then rattled off 16-25 RAW flat light frames. Once that was done, I'd put the cap on the lens, and take flat dark frames -- same exposure duration and ISO as the flat light frames. Median combine the flat dark frames into a master flat master dark, and, subtract that from each flat light frame. Then, median combine the flat light frames into a master flat field.

H
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Old 22-10-2011, 11:32 PM
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Hi Chris I to loved the widefields and did basically all that H has said with great results, it is the way to go, looking forward to your results.

Leon
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Old 23-10-2011, 09:09 AM
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Thanks Humayan-neatly explained!On closer examination I did notice outer edges not as neat as centre.

I see images of wide fields and they have lovely wide field sky,but then have a fore ground of a beach or country scene.These must be composite images.How do they do this.

Also if you had tracking etc etc and did a lovely wide field and had a tree or similar,the tree comes out blurred due to movement of EQ mount.If you wanted the tree or whatever nice and sharp.I imagine you take an image with the tracking off,How do you blend that into your wide field? and make the blurred tree etc go away in the image with tracking?

Are there any links to a bit of this sort of photography with Tut's?

Thanks Chris
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Old 23-10-2011, 09:24 AM
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Now your getting technical Chris, but i get your drift.

Leon
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Old 23-10-2011, 09:54 AM
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hotspur (Chris)
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Quote:
Originally Posted by leon View Post
Now your getting technical Chris, but i get your drift.

Leon

Yeah,well I see these pics out there,of people that must have a small EQ mount a wide lens,they take it to say a cliff top and do all the usual-lots of subs -and do the night sky bit.But then they must switch the drive off get some images of say the shore line or what ever and blend the stacked sky image with the stationary one. Well that's how I figure it-could be wrong.

But there are these Huge Grass Tress in a paddock,they are approx eight meters tall,I've been checking them out and figuring they would look good in a shot with Southern Cross area of sky.So just looking at the jig saw puzzele pieces and seeing what I could do with them.
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Old 23-10-2011, 11:39 AM
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I understand Chris, it dose look good with something in the foreground, I'm sure you will master it mate.

Leon
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