I'm on a steep learning curve lately, related to modifying my 8" f6 Newtonian OTA for imaging purposes, and could use some advise from those who understand how optics setup effects imaging results.
It all started when I noticed the shape of out-of-focus stars on my camera live view screen, near the edge of the field, were clearly showing that my camera sensor could not "see" the edge of my primary mirror except at the very center of the screen. This caused the out-of-focus star to look like cat's-eye shape with the secondary shadow way off to one side (nearly half the primary mirror was not visible at the edge of the screen!!). What I later discovered, as you'll see below, was that my 100% illumination zone at the sensor was tiny.
First, some details about my setup and goals:
- Goal: astrophotography of DSO's, no visual, minor interest in lunar/planetary imaging (have a Maksutov for that)
- 8"f6 Newtonian on GEM
- 1.82" secondary mirror minor axis (modify to 2", 23% obstruction increases to 24%)
- Camera is Canon M50 mirror-less, sensor size 0.88" x 0.59"
- Based on analysis using Newt for the Web, my current OTA setup yields the following (values in parenthesis is where I think I will be after modifications):
- 100% illuminated diameter = 0.185" (0.87")
- 75% illuminated diameter = 1.13" (1.49")
- Focuser min height = 2.8" (0.675" - custom super low profile camera mount, minimal focus range)
- Primary mirror face distance to focuser hole = 37.5" (39.95")
- Now and with the modifications, there are no issues according to Newt with vignetting or admitting 100% of the rays by the secondary mirror.
So the bottom line is my 100% illuminated zone is tiny (0.185") and I want to expand it to fill my camera sensor as much as possible (0.87" 100% zone vs 0.88" sensor width). I plan to pull my primary mirror back 2" in the tube, and push the camera about 2" closer to the secondary mirror by building an ultra low profile camera mount.
Besides the obvious issue that I'm loosing nearly all of my adjustment in focus position, am I heading in the right direction with this modification? Are there any serious flaws you can see?
One thing I'm unsure about is whether or not the image coma will get better or worse, since the camera sensor will see the entire primary mirror image.
Anyway, sorry about the detailed read. I appreciate any help or comments!