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How to calculate actual Focal Length?



Having great success with my CST 9.25. I was using the .63 field reducer to image Orion and I was curious what the actual focal length was of the pics I was taking so I submitted one of them to Astrometry.net to find out the arcseconds/pixel value to use in the equation [Focal Length in mm] = (206.2648 x [Pixel Size in μm]) / [Arcseconds Per Pixel].

The focal length calculated doesn't make sense since its calculates near the actual focal length without a reducer.

The Light Vortex Astronomy article that has this equation does mention that if 2x2 binning is used it doubles the arcseconds/pixel calculation. I'm not sure what to use as a binning value with respect to how EOS takes the photo off the sensor. I admit I'm a bit over my head here. I don't have a clear understanding of how to accurately calculate actual FL.

I'm using the Celestron guider and Canon M6 Mark ii. The guider thickness seems to put the back focus farther back than it should assuming a 105mm back focus for the reducer but I can still get everything in focus including the ASI 174 mini guide camera ( which is awesome ). Last night was the first time I tried guiding and had no problems finding focus as well as finding guide stars. But I am curious what the actual focal length is since it is a value you are supposed to input into the PH2 guiding software. I used 1480 ( .63 X 2350 ).

Am I calculating actual focal length correctly? Your guidance as usual would be great. Really love Backyard EOS!

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1480mm is the correct focal length to for an f/10 SCT with a 9.25" aperture and a 0.63 focal reducer.

Here is the formula: aperture in inches (9.25 for your SCT) * 25.4 mm/inch * focal ratio (10 for your SCT) * 0.63 (focal reducer factor) = 1480.185 mm.

I don't know how astrometry.net is calculating the resolution, but it must be using the focal length that you provided, somehow.

It is not typical to bin color images, where adjacent pixels have different colored filters and cannot be combined. It is typical to bin 2x2 for monochrome images shot through a color (R, G, B) filter where resolution is not as important. For this reason you would always use a binning factor of 1 when calculating the resolution of images shot with your DSLR.

The article is correct that when binning 2x2, the resolution decreases by a factor of 2. That is the number of arc-seconds in a single pixel doubles.

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So I have to confess I solved my issue in an embarrassingly simple way. I've been so focused on the new guide camera that I got that I used its pixel size in the focal length formula instead of the pixel size of the imaging camera. Duh! When I use the right number, things came out a lot more reasonable! 

Rick, thanks for responding to my question.

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