Projection astrophotography help!
Posted 24 September 2020 - 03:25 AM
Im trying to figure out what my focal length would be if i used my eyepiece in front of my camera.
I just need someone smarter than me to figure the maths.
So i have a 102ed with a focal length of 714mm
A celestron 21mm ultima duo with 68deg fov
And a asi183mc
Posted 24 September 2020 - 04:03 AM
Technically you shouldn't have a definable answer. The scope and eyepiece should have collimated light in and collimated light out = no actual image. Your scope and eyepiece do not create an/the image. Your eye does that.
IF the camera has a lens then that will create the image from the collimated light on the sensor. The lens is replacing the lens of your eye. Just doubt there is a lens on the camera. Smartphones have but I don't think ZWO ASI's have.
You could form an image, simply move the focuser and the scope+eyepiece will create an image somewhere, may be a real image or a virtual one. Where and size would be a function of very simply where the focuser is positioned, finding where and putting a camera there would be almost impossible/pointless.
You need to understand more optics.
For what you want to appear to do you need a barlow or powermate and in both instances you get a bigger image, this bigger image usually being formed further out.
Posted 24 September 2020 - 04:12 AM
Here is a handy reference for eyepiece projection (scroll toward the bottom of the page for the maths; you will need to measure the distance from the eyepiece to the sensor):
If course, you could just shoot an image with and without the eyepiece and measure the scale factor between the images. The effective focal length of the setup is the focal length of the scope times the scale factor.
Edited by BQ Octantis, 24 September 2020 - 04:15 AM.
Posted 24 September 2020 - 07:53 AM
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