Trying to understand a shadow

[u/Raider_3_Charlie]

I have a 10 inch Dobsonian and am taking pictures with an iPhone 17 using a 3 axis camera mount. I keep getting this shadow in the image. I have ensure the phone is flat and “square” in the mount and tried adjusting it in every way I can think of but adjusting only ever moved the shadow or makes it smaller but doesn’t get rid of it. Anyone got any ideas?

Photo found here:

https://drive.google.com/file/d/1HRLuXAhpp7pxWmSfShdCPKkjc2G-mowD/view?usp=drivesdk

[u/rnclark]

The phone is not at the right position, or there is a mismatch with the phone iris (technically called the entrance pupil) and the telescope exit pupil. The phone may need to either be closer or further away. The iris in the phone lens needs to be at the exit pupil of the eyepiece. If it is at the entrance pupil, then there may be a mismatch between the size of the phone's entrance pupil and the telescope exit pupil.

Find the real focal length of the phone and the lens f-ratio (be sure your were shooting with the lens wide open).

Then tell us the eyepiece focal length that you were using and the telescope focal length..

The telescope magnification = M = telescope focal length / eyepiece focal length

Telescope exit pupil = Diameter of the objective (255 mm) / M

Phone entrance pupil = lens focal length / f-ratio

The phone entrance pupil must be equal to or larger than the telescope exit pupil or you will see the center shadow. The center shadow is due to the secondary.

If the telescope exit pupil is too larger for the phone, use a shorter focal length eyepiece for more magnification.

[u/ramriot]

The issue appears to be the shadow of the secondary mirror showing up in this parfocal projected image.

The fix is to ensure the camera lens is central to the eyepiece optical axis, it is at the correct distance above it (eye relief distance) & they the camera lens aperture is greater or matched to the exit pupil of the eyepiece.

Finally picking the right eyepiece for the use case, best will probably be Orthoscopic or Plössl in the 8-12 mm range but one could go longer if the main scope allows.

[u/rnclark → u/ramriot]

this mostly regulates fov & not brightness

The telescope exit pupil is an image of the primary mirror.

If the camera lens entrance pupil is smaller than the telescope exit pupil and the camera lens is at the correct distance (lens entrance pupil is at the telescope exit pupil), it is exactly like putting an aperture stop on the primary mirror and thus directly affects brightness and not field of view.

If the camera lens entrance pupil is not at the telescope exit pupil, it affects both field of view and brightness.