Optimal distance from barlow lens to sensor?

Hi,

I've been imaging planets with my C8 and ASI120MM camera recently. I normally use a 2x barlow which gives an effective focal length of about 4000mm when used directly with the camera.
Now when I use a filter wheel in between the camera and barlow, the distance between the sensor and the barlow is about 15mm more. This results in an effective FL of about 5000mm.

I understand how the barlow works and the relationship this distance has with the effective FL, but I was wondering would there be some aberrations (spherical) introduced by too much distance from the lens?

Thanks for any advice.
Derek

Hey Derek,
I've never noticed a deterioration in image quality when say using a 2x barlow at a a higher power cos of different spacing tbh.
but if you're really concerned with this, then you should have a look at the televue powermates, they're designed to be power stable at different distances, unlike a barlow which is entirely dependent on spacing.

Thanks Dave.
The powermate looks interesting. I thought it was just an expensive barlow but seems to be a different system altogether.
Derek