View Full Version : 8" SCT and Canon 350 focus issue
5th of November 2008 (Wed), 11:30
I hooked up my Canon 350 to my Celestron Celstar 8. When i take a pic that appears to be infocus on the telescope its isnt in focus on the camera. Its been a while since I tried it so thats all I can tell you . Is this a normal issue ? all the pics of the moon were a little fuzzy.
5th of November 2008 (Wed), 12:06
How did you hook it up? At prime focus (i.e., via a t-ring and t-adaptor)? Very rarely will a camera and eyepiece be parfocal (i.e., both in focus without adjusting the focus knob on the scope). Most astrophotographers use some combination of trial and error, software, focusing masks, and other devices to achieve focus. Here's an excellent resource (http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM) for information on focusing (also a good starting point for all things AP).
Even if they were parfocal, you may also be plagued with problems with seeing (the steadiness of the air in the atomsphere), which is why many ap'ers practice "lucky" imaging - basically spray and pray - take lots of images, and use the ones where the seeing steadies.
This method can further be enhanced by stacking the images and performing further processing like deconvolution using software such as the free Registax, but I wouldn't worry about that until you're sure your focus is pretty good and you've exhausted the spary and pray method.
5th of November 2008 (Wed), 12:17
5th of November 2008 (Wed), 15:45
After attaching your camera to the back of the scope start by adjusting your camera viewfinder dioptric dial for your eyes(different when shooting the night sky than it is for terrestrial shooting). Bring a star or stars to crisp focus using the scope focus knob and tweak the dioptric dial until the stars appear focused. A test shot or two should reveal whether or not you've reached critical focus. There are many focus aids out there like a Hartmann mask(can make yourself) but a quick one is to tape two pieces of monofilamant fishing line(30-40 lb test) in a cross hair configuration at the front of the scope tube. With these in place you will now see a crisp star spike on bright stars when you have reached focus. If you don't want the spikes in your shot remove the monofilament before shooting.
Also be sure your optics are well collimated. If they're not you will not reach optimal focus.
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