Good day, when entering focal length in the camera properties, does one enter only the telescope's native focal length, or do you enter the native focal length x crop factor. For example, my APO has a 766mm focal lenght, and my camera is a APS-c sensor giving a crop factor of 1.5...so would the correct entry be 1,149 mm?
Thanks!
Crop factor and focal length
Hi bhill,
Use 766mm.
APT uses the focal length and sensor size to calculate the field of view of the image to send to the platesolver. So as long as it knows the sensor size (usually acquired automatically by APT) it has all it needs
As accurately identifying what your actual focal length is (due to filters, extension tubes, etc.) APT can do it for you. If you use ASTAP as your Blind platesolver then using Shift Click Blind will force a fully Blind solve. That is ASTAP is given no starting position or FOV data at all. Instead it just keeps trying different settings until it gets a result.
However, both ASTAP and PS3 are relatively tolerant of an inaccurate FOV size. So if your 766mm is within about 10% of the real FOV it should work. A successful solve always reports the accurate focal length, which you can then use to update the setting.
Just for info, the calculation for FOV is:
FOV = Radian (57.2958) * Sensor size / focal length
E.g. your sensor size is 22x15mm and your focal length is 500mm. Then your FOV is 22/500 x 15/500. To get the answer in degrees we multiply by 57.3 and get 57.3*22/500 x 57.3*15/500 = 2.5° x 1.7°
Hope that helps?
Cheers, Jim
Use 766mm.
APT uses the focal length and sensor size to calculate the field of view of the image to send to the platesolver. So as long as it knows the sensor size (usually acquired automatically by APT) it has all it needs
As accurately identifying what your actual focal length is (due to filters, extension tubes, etc.) APT can do it for you. If you use ASTAP as your Blind platesolver then using Shift Click Blind will force a fully Blind solve. That is ASTAP is given no starting position or FOV data at all. Instead it just keeps trying different settings until it gets a result.
However, both ASTAP and PS3 are relatively tolerant of an inaccurate FOV size. So if your 766mm is within about 10% of the real FOV it should work. A successful solve always reports the accurate focal length, which you can then use to update the setting.
Just for info, the calculation for FOV is:
FOV = Radian (57.2958) * Sensor size / focal length
E.g. your sensor size is 22x15mm and your focal length is 500mm. Then your FOV is 22/500 x 15/500. To get the answer in degrees we multiply by 57.3 and get 57.3*22/500 x 57.3*15/500 = 2.5° x 1.7°
Hope that helps?
Cheers, Jim
Just an additional thought...
People often forget (me included) that you don't need to wait until nighttime and you are actively imaging. You can use any image you have already taken (load it from the Img tab, by double clicking on a thumbnail) at anytime. Do "Point Craft > Shift Click Blind"on that image and get the focal length from.the solve.
Cheers, Jim
People often forget (me included) that you don't need to wait until nighttime and you are actively imaging. You can use any image you have already taken (load it from the Img tab, by double clicking on a thumbnail) at anytime. Do "Point Craft > Shift Click Blind"on that image and get the focal length from.the solve.
Cheers, Jim
