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    Thread: Astrophotography

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      astrophotography

      Hello im putting a discussion on how to go about doing this.We can divide AP into Planetary and Deep space. Planetary photography is very simple( cost wise). Normally everyone starts with moon pics, and all u need is a telephoto lens and DSLR or any camera with zoom.The next level is webcam + telescope and hi res photography . how do we do it? , a webcam has a small field of view and by taking a series of exposures of the same object ie for a duration of 1 minute- 3 min u get about 850 frames as u do about 5 frames per sec. Next u stack those pics, which improves the signal to noise ratio in a freeware called registax and get a satisfatory pic like this http://picasaweb.google.com/drsuresh...40493125043298 . In this example ive used a webcam called philips toucam, here i remove the lens and got an adapter that screws in where the lens was. This adapter will fit the place where the eye piece is supposed to be. here is saturn with a webcam http://picasaweb.google.com/drsuresh...67647589450818 .
      Level 2: Deep Space ie nebula and galaxies
      Some theory:
      Im going to share with you my experience in astrophotography with a series of discussions and essays which i hope will be helpful for the beginner.
      What is so difficult about it(deep sky astrophotography)?The answer is you need to take a long exposure photo of a moving object with no shake in your camera !All stars ,nebulae, galaxies,planets and Sun rise in the east and set in the west bcos we are rotating from west to east hence we need an instrument to hold the camera with a telephoto lens or telescope to view the moving star (from east to west) fixed on your photographic chip to get a picture without a blur.We call this instrument a motorised mount because it has a stepper motor to counter earth s movement,there is also what is known a barn door mount which can be made at home and will work upto 200mm lenses. It is not enough to counter earth s rotation but also be placed parallel to earths rotational axis ; the process we call polar alignment which is beyond the scope of this discussion.
      Do you need telescope for astro photography? yes and no depending on what level u want go . For a start a telephoto lens would do but a 3 inch apo refractor is ideal.
      To summarise we need a mount ; the most critical component, a good telephoto lens and DSLR with long exposure capability .
      The most important hassle one has to overcome with astrophotos is NOISE.
      Lets quickly for the benefit of newcomers run thro the basics. First rule to know is that light itself is noisy !!?? yes that is the truth .Light reaches your photographic chip in packets in a random manner ie not all together. It is akin to a machine gun firing erratically which means the bullets will reach the target but randomly. This randomness of arrival of light packets causes noise in your picture .Put in another way it is due to the uncertain arrival times of light quanta which causes the noise .Noise in other words is the random variation of brightness levels -graininess in the darker part of your image where it is more obvious.
      Why is the grain seen more in the darker or dimmer part of your picture? Because the brighter areas in a picture have more signal and hence a better signal to noise ratio. Dim areas have low signal relative to the noise hence a lower SN value. What i am trying to convey is that noise there throughout the picture but obvious in dim areas due to poor Signal to noise ratio.
      To sum up we need a photograph which has an excellent S/N ratio to overcome noise.
      In the next discussion id like to talk about the types of noise and how we overcome those.
      An example of a pic with a fairly good SN it took me 7 hrs of exposure ie multiple 2minute exposures ie 210x 2minutes were stacked http://picasaweb.google.com/drsuresh...28190961023554 .pls let me know your thoughts
      Suresh

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      I have one doubt here. the telephoto lens can be normally fitted to the DSLR or is it fit to the Telescope?
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      DSLR directly connects with the Telephoto lens, hence performing the function of a small telescope

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      But again the telephoto lens compatibility depends on the body of the camera right? Not all DSLR's can handle this i guess.
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      That is true except for Nikon lenses ( not all) can be attached to Canon cameras by means of special adapter which i have.

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      OT : So suresh which DSLR do you suggest me to buy? Nikon or Canon.
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      Im canon user because the entire astrophotography world uses it hence easier support. Nikon anyday has a better built body , feature wise i like the 500D as it 15mp, live view, and HD video output and it is very light (around 500 gms can sometimes give cheap feeling to PRO camera guys),cost me 30k .My views will be biased towards canon and ive not used much of Nikon except borrowed lenses.

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      Quote Originally Posted by drsureshmohan
      Im canon user because the entire astrophotography world uses it hence easier support
      Wow - I've another Canon fan

      Quote Originally Posted by drsureshmohan
      All stars ,nebulae, galaxies,planets and Sun rise in the east...and will work upto 200mm lenses.
      Doc, I've a question here, though we counter the Earth's movement, the far objects are still moving isn't it? So we would only get their motion picture & not the stationary pic - is my understanding right here?

      Quote Originally Posted by drsureshmohan
      It is not enough to counter earth s rotation but also be placed parallel to earths rotational axis ; the process we call polar alignment which is beyond the scope of this discussion.
      Haha!!! Let me find out what this is about. Sir, if I'm not asking too much, I also request to start a topic on AP in the offline section. Tks.

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      The camera with telescope is attached to the mount, the mount has a stepper motor which compensates earth s rotation ie 15 arc seconds/ second at the celestial equator to a great degree. Therefore the star s motion(apparent) is compensated .The greater the accuracy of tracking so does the cost increase. Starting from 50 k to 5 lac.

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      drsureshmohan thanks for this thread, should be getting lot of Q's from me regarding astrophotography!!!
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