On Mon, Nov 11, 2019 at 07:59 AM, Khalid Baheyeldin wrote:
The longer the focal length, the harder things become. Even with a 1300 mm 8 inch Celestron SCT, things are hard.
So, 2600mm is out of the question, and of course the higher focal length.
Stay below 1000mm or 800mm, and you can get good results.
Most amateurs use 500mm or 600mm, and get great shots.
Another thing is that a tele converter increases the F-stop.
So, if your zoom is originally F/5.6, it will be F/11, and requires double the exposure time (assuming everything else remains the same, aperture, ISO, ...etc.)
Remember planetary imaging too where some longer FL's are necessary.