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ISO Settings for unguided images


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Hi I have not been able to get my auto guiding equipment to work yet so am doing short exposures at the moment and stacking them.

Can you get just as much faint detail by doing 4x 30secs at 800 iso as 1min at 1600?

Thanks Dean

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Ok Thanks I have been doing 1min at 1600 and like you said have been getting alot of noise. I will try more at 800 and below tonight. I was thinking that I would not be able to capture the faint parts of a object even if i stacked them after.

Thanks Dean

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I have been doing 1min at 1600 and like you said have been getting alot of noise. I will try more at 800 and below tonight. I was thinking that I would not be able to capture the faint parts of a object even if i stacked them after.

If you're using DSS (or any of the other dedicated astro image processing packages) you'll find it doesn't make any difference what the ISO setting is. The more light you get in, in total, the better the result will be. Having said that you want the subs to be reasonably long as you add read noise every time a frame is taken ... but 4x30 sec will certainly show fainter objects than 1x60 sec & you also have the chance to "rescue" a session where e.g. an aircraft spoils a frame, in fact if you take loads of frames (more than 10) and use kappa sigma stacking transient objects simply disappear automatically, very useful.

If you can get away with 60 secs then try stacking 30+ of them.

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Just for interest : I've made a stack of 25 x 5 sec exposures I took of the Scutum region last week - these were made with the intention of getting accurate measures of some of the brighter variables in this region, the individual frames were 5 secs at f/2.8, ISO 100, 50mm lens, Canon 40D. On a STATIC tripod (no drive). Stopping the lens (it's the f/1.4 version) down a bit sharpens the star images.

R%20Sct.jpg

2009 Sep 13, 2022 - 2030 UT.

Note how the star clouds are clearly visible despite the short subs. Nothing has been done to this image other than adjusting the levels in DSS! Incidentally the review screen on the camera shows an essentially black screen when used to examine these images, with great care you can find half a dozen or so stars ....

No filter was used, there is some reddening of the background especially towards the bottom of the frame. (No filter is required to get a known response for photometry - also when measuring the individual frames I seperate out the red, green & blue layers and use only the green, this gives a reasonable approximation to the photometric V response curve).

And yes, it would have been better to take even more subexposures - if I'd been after a pretty picture I would have opened the lens up to f/2, used a CLS type light pollution filter and either taken very many more subexposures (at least 100 x 5 sec) or increased the subexposures to 30 secs and used a driven mount to eliminate trailing.

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