Newbie Question on Dobsonians
Posted 07 January 2013 - 12:47 PM
Posted 07 January 2013 - 01:11 PM
There are quite a few dob owners with tracking tables using video cameras for shots in the 5-30s range (and sometimes more). This (big fast dob and a tracking table with a sensitive video camera) is IMHO a very effective combination for near real time viewing.
It's not until you want to get into the >60s exposure range that an accurate EQ mount (and even autoguiding) is required or desired for viewing. Now if you are actually doing any imaging (stuff you want to keep and display) then the more accurate EQ mount would probably be required for even the shorter exposures.
Posted 07 January 2013 - 01:18 PM
Posted 07 January 2013 - 02:57 PM
Posted 07 January 2013 - 06:40 PM
It will do fine as an finder and you'll see some detail you won't get visually, but the big thing to remember is that you are not going to get the resolution you'd get with a CCD camera and you won't want to hang the results on the wall.
Posted 07 January 2013 - 07:13 PM
Posted 08 January 2013 - 09:10 AM
The Samsung's sensor needs as much light as you can give it. Try to be faster than f/4. Your XT8i's f/6 will be a bit slow and the focal length of 1200mm is going to give you small fov 5.2x10.8 arcminutes with a SCB-2000. A .5x focal reducer will get you to a good f/3 light wise, but still the fov would be 10x22 arcminutes. I've used my camera on a C14 (3900mm fl) to do a good splitting double stars down to about 1 arcsecond. For a quick conversion, think of the 1/3" sensor in a Samsung as a 6mm eyepiece. Your 1200mm focal length is going to give you about 200x, more than most large DSOs are going to need.
Consider used camera lenses on ebay between 135 and 250mm that are faster than f/4. The 135mm lens I use frequently sells for less than $20. First go to a big online camera store like Adorama or B&H and search for c-mount & Canon, or c-mount & m42, or c-mount & Nikon and you'll find adapters that can be used to connect the lens to the Samsung in the $20-40 range. Decide on the brand of lens you want to use and get the adapter first. If you already have a DSLR and lenses so much the better, all you'd need is an adapter.
Using a zoom lens can be fun too, but they are a bear to get spaced correctly so they hold focus throughout the zoom range.
Check out CCD Calculator at:
to get an idea of what your fov with various focal lengths will be. A guy here got a SDC-435 a few years ago and stuck it on his ETX-125. He complained that the only thing he could see were craters on the moon. It was f/15 & 1900mm. It didn't give the camera's sensor enough light (remember faster than f/4) and his 1900mm fl gave a tiny fov of 3.3x6.8 arcminutes. I don't think he's used it since.
You'll also need a way of seeing the output from the camera. The two normal choices are either an lcd tv or frame grabber to your computer.
If you are really interested in doing video you might want to take a look at a web page I put together about two years ago:
Video is fun, but it does take a bit of time to work out the kinks.
Posted 08 January 2013 - 02:26 PM
Posted 08 January 2013 - 05:01 PM
BTW if you already have a 12v battery all you need to hook up power to the camera is a hunk of wire. There are two screw terminals on the back of the camera.