Hi! I'm very new to astrophotography and wanted to calculate the effective focal length for my setup and have come up with a problem with my calculations. I sheepishly ask for some guidance or possibly a link to old fora that discuss this.
I have an old Celestron Ultima 11 SCT equipped with a Celestron f/6.3 focal reducer. I am imaging with a Nikon Z7 mirrorless camera with a full frame sensor and 4.34 um pixels. My camera sensor sits 106 mm behind the FR (Celestron told me the F/6.3 ratio is for an optimum distance of 105 mm behind the FR). The FR is on the rear cell of the telescope, the Celestron T-adapter is 50 mm in length followed by a Fotasy T-ring (40 mm) and he flange distance of the Nikon Z7 is 16 mm.
My Celestron Ultima 11 has a published focal length of 2790 mm and a focal ratio of f/10 so I'd calculate the focal length with the FR would be about 0.63*2790=1758 mm. To double check, I put the same values in the calculator on http://www.wilmslowa...re/formulae.htm and got a slightly different number of 1727 mm which is close.
So I took an image of M44 the other night and plate solved the raw image in Pixinsight and obtained an effective focal length of 1442 mm with a pixel resolution of 0.61 arcsec/pixel. I next plate solved the same image on Astrometry.net and also obtained a resolution of 0.621 arcsec/pixel. Being the obsessive type, I opened the image and compared it to the photo view in Stellarium and when I visually adjusted the Stellarium image to match the photo. I used a multiplier on the sensor definitions page of 0.51 to match my photo which would give an approximate focal length of 0.51*2790= 1422 mm as well which is close to the plate solved values.
Why the discrepancy? I know that FL changes as my primary mirror changes during focusing, but can't imagine it's that much. Could someone please explain so I can head-slap myself and continue on with my life? Thanks for the help!