Last night I swapped out the AT66ED with the Celestron 6″ SCT. I purchased this Celestron 6″ SCT OTA second hand back in 2009 or so. The Celestron C6 SCT has an aperture of 6″ (152 mm) and a focal length of 1500 mm so a focal ratio F/10 (1500 mm / 152 mm= ~F/10). It is a great scope but I have not had it on the mount in a good number of years. Got it all dusted off, mounted the SV106 guide scope on it, attached the SV305 camera to the C6 SCT, and the Orion SSAG camera to the SV106. Put it on the CG5 mount and balanced everything.
I have a Celestron F/6.3 Focal Reducer and a Meade F/3.3 Focal Reducer. The F/6.3 Focal Reducer reduces the focal length to ~958 mm (152 mm x 6.3 = ~958 mm). With the F/3.3 Focal Reducer the focal length is effectively ~501 mm (152 mm x 3.3 = ~501 mm). I am going to try out the Meade F/3.3 focal reducer first: one because it is a faster focal ratio, and two it should give me a FOV closer to what I have in the AT66ED.
One of the handy utilities in KStars is the Field of View (FOV) calculator and overlay. You configure your aperture, focal length, camera resolution, and pixel size to calculate and see what the FOV should look like.
You can enable different overlays to compare them. Here are the calculated FOVs overlayed on M81 for the AT66ED with the SV305 (white box), the 6″ SCT and the F/6.3 focal reducer with the SV305 (red box), and the 6″ SCT and the F/3.3 focal reducer with the SV305 (purple box).
This is how I currently have things set up to try this out: Celestron C6 SCT -> Meade F/3.3 Focal Reducer -> 1 1/4″ Adapter -> SV305 Camera.
I pointed the scope at the Moon just to see if I could get focus with the F/3.3 focal reducer in there. I have the SV305 camera directly connected to the laptop and I am using the Live View in SharpCap as I manually focus the scope.
Yep, I was able to get focus. I’ll need to dial the focus in on a bright star with the Bahtinov mask, but that will be another night. I’ll play with the adapters and spacing some until I come up with something that works well, but just being able to get things in focus is a plus.
I have a Crayford Focuser that I want to use with this configuration but I need an adapter (it’s on the way). Once I have the SCT to 2″ barrel adapter the set up will be Celestron C6 SCT -> Crayford Focuser -> 2″ to SCT -> Meade F/3.3 Focal Reducer -> 1 1/4″ Adapter -> SV305 Camera. Adding the Crayford Focuser will allow for a finer focus adjustment.
I was a little worried about being able to get focus, but it looks like that should not be an issue. I am not sure how bad the vignetting might be, but I’ll have to wait for another night for that. The chip on the SV305 camera is pretty small, ~1/3 inch, so I am hoping most of the vignetting is off the chip. Most of the time for EAA the focus is what is in the center of the frame so even if there is some vignetting it probably won’t be that big of a deal (unless it impacts plate solving).
It looks like I shouldn’t have any issues with focusing with the F/3.3 focal reducer. Looking forward to doing some EAA with the old Celestron 6″ SCT. The extra aperture should reveal some more details in EAA observations.