Quote:
Originally Posted by gregbradley
Luminance is usually shot 1x1 although sometimes in a mosaic you could get away with 2x2 as you are getting very wide fields of view.
2.5 is way too short. At AAIC it became clear the top guys are doing really long exposures like 30 minutes or an hour. I switched to 15 minutes from 10 minutes recently (I used to use 15 minutes but my mount was not up to it so I went to 10mins). I see John Gleason the Ha imaging expert routinely uses 40 minute subexposures for his Ha and that is with a fast Tak FSQ106ED refractor.
I use 20 minutes for narrowband but if there was clear weather then I think I'll be extending that to 30 minutes to 40 minutes.
This is a common question that often leads to different points of view about stacking to reduce noise versus long exposure.
My take on it is you will never get certain detail from short exposures no matter how many you take as they don't get above the noise floor of your camera. This then does not get stacked out and is lost in the noise.
A very long exposure though gives the opportunity for faint data to be registered and accumulate above the noise of the camera.
I would put a caveat on this strategy and that would be often the longest exposure is limited by the accuracy of your mount, whether you are using self guiding, OAG and no differential flexure and also the stability of the weather.
There is no point in doing 30 minute subs if the weather is unstable. You will lose a lot of images.
Also some objects have a very bright core and a high dynamic range image is the right approach - ie a series of very short images for the core and longer ones for the dim outer nebulas etc.
Galaxies really need long exposures and long total exposure time.
A similar argument exists for binning colour exposures or using 1x1.
To me it would depend again on other factors. If you want maximum detail and have the time then 1x1 is the go for everything. If time is limited then 2x2 is the go. If seeing is poor then 2x2 may also be a good strategy for colour. Binning 2x2 also rounds out guiding errors so if your tracking is not so hot then 2x2 makes sense also. 2x2 also rounds camera lens aberrations so some less than perfect lenses respond to using 2x2 as well as stopping down the lens.
Greg.
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Hi Greg,
I appreciate your voluminous reply! You make some great points. My experience so far has been mostly in photographing galaxies so it has been fairly straight forward having an approach to capture as much light as I can before the light pollution gets in the way. With M16 the problem seems somewhat different with stars very bright and a lot of nebulosity showing even in shortish exposures. I might try the CCDWare exposure calculator based on the backgrounds of some test images.
As I mentioned earlier I intend to shoot 1x1. The "test" shot was just a quick grab at 2x2 to set up the camera framing, etc. I intend to do LRGB first then add Ha. For sure my Ha exposures would be quite long without worries about light pollution! For LRGB though I think LP will be the limiting factor, but your point about getting faint details out of the noise through long exposure is well taken. I think my guiding problem is finally resolved so I hope it won't become a limiting factor.
Peter