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Post by Stephen McGovern on Oct 12, 2004 13:20:19 GMT
I recently bought a Nikkon F55 SLR for my 4" refractor. I have done a fair bit of reading into astrophotography and understand most points. However I am wondering why do some people do multiple exposures when talking a photo of an object what is the purpose of this?
My scope is motorised would this suffice if I where to take a long exposure say of 15mins or so. Finally I was wondering as to how long of an exposure you should use for planets, nebulae, and galaxies?
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Post by markdj on Oct 12, 2004 20:02:17 GMT
Hey Stephen The reason for multiple exposures is for the purpose of stacking the images on top of each other. This can cancel out the grain or noise in CCD and film cameras increasing the signal to noise ratio and depth in an image. Also, if you have a bright core of a nebula but dim arms, then you can stack so as to use the correctly exposed core and a longer exposure to bring out the fainter parts of a nebula. This is all done in post processing. Stacking can be done in Registax. Many film users scan in the image into a PC and stack images as this works for increasing signal and reducing noise. See the astrophotography resources page... www.eaas.co.uk/club_members/astrophotography_resources.htmlIf you have your mount polar aligned accurately and are using a wide angle lens, piggybacking on mount, then you could have long exposures but around 5-10mins would probably suffice. The longer the focal length, the more accurate your mount needs to track the stars. You will start to see a sine wave pattern to the stars at higher magnifications. This is due to the gears in the mount not being perfectly made and is called periodic error. For each spoke in the gear, it usually takes about 8 minutes to move one spoke. At this point, the scope can jolt very slightly if the gears are not smooth. Accurate tracking comes with high magnification and using a reticle eyepiece to manually keep a star in the same place in an eyepiece while the film is being exposed. You can also get software to control computer enabled mounts to automatically compensate for you. As you can imagine, this is rather expensive to do but is worth it if you are going to do serious astrophotography. For widefield images though, the movement can be so small that it is not noticable. So far, the long exposures I have taken have been unguided widefield. The M57,M27 and M31 images using the Starlight Xpress MX7C were much shorter exposures so that the stars did not streak. What we are planning to do is to use our Meade LX200 as a guidescope and piggyback the Orion ED80mm APO and attach the camera to the Orion. For bright planets, eg. Venus, Jupiter, Saturn, an exposure of 1/2sec would probably be ok. The best thing to do is to take a note of you scope setup, if you are using any barlows etc. and bracket your exposure, taking a note of your exposures. When you get the pictures back from the lab. you can then see which exposure time turned out best. Bracket between 1/25 and 2 seconds (John McC and Paul might know better though...). For manual SLRs the key is bracketing. For faint subjects, anything up to 8minutes. Above this, you could get film fogging with light pollution. Hope this helps. Clear Skies MDJ
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Post by stephenmcgovern on Oct 13, 2004 17:25:27 GMT
Thats a great help thank you very much. Hopefully it will be clear skies tonight so I can get a few shots.
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