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Exit pupil = eyepiece focal length / telescope focal ratio.
For 15mm eyepiece and f10 telescope, exit pupil =15/10 = 1.5mm
For zoom at 15mm setting it will be the same. May have click stop at 16mm in which case it will be 1.6mm. At 8mm setting exit pupil will be 0.8mm.
Exit pupil determines the brightness of the image. 0.8mm is good for planets giving 187x in your telescope. If the image is too dim for your eyes with 1.5mm exit pupil then it will be even dimmer at 0.8mm. In your telescope 1.5mm exit pupil is 100x magnification.
Note exit pupil also = aperture / magnification.
The only way to get a bigger exit pupil in your telescope is to lower the magnification which is not ideal for planets going below 100x. If you want a bigger exit pupil and higher magnification then you need a larger aperture.
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