Ellinor William

 You won't see a clear image through the viewfinder of your Nikon D3000 with the lens removed. Because, the lens focuses light onto the mirror which then reflects it to the pentaprism (or pentamirror) which creates the image. Also, you're on the right track about using a T-ring for telescope photography. As this helps light from the telescope to enter the camera body and reach the sensor to capture the image.

My personal favorite is Outdoor Photography and Capture. BTW there are some good recommendations in the comments. I would suggest, how about visit the nearby library and check for yourself.

We usually think blur in the photos is due to shaking of the camera or movement of the subject at the time of capture, or incorrect camera settings. However, there are many other reasons that can make your photos blurry. Some popular applications that remove blur from the photos are Adobe Lightroom, Adobe Photoshop, PaintShop Pro, PhotoLemur, and more you can search online. I have got this article from web on how to repair them.

I understand your dilemma. Shooting in RAW has its advantages and disadvantages, especially for sports photography. Ultimately, the choice between RAW and JPEG depends on your preference and purpose. If you want to have more control and flexibility over the editing process, you may want to keep shooting in RAW, but be prepared to invest in more storage space and processing time. If you are satisfied with the JPEG files and do not plan to do much editing, you may want to switch to JPEG.

Strong suggestion: Try JPEG, which are ready to use out of the camera, saving you time and storage space.

Can't say for sure, but it seems that you are seeing two frames of each photo because your camera is saving both a JPEG and a RAW version of the image. To avoid having two frames of each photo in the future, you can change the settings on your camera to save only one format, either JPEG or RAW, depending on your preference.