Dog Biscuit aberration makes the sky look like a grainy photograph.  When the eyepiece is slightly defocused, the Airy disk you should see is nothing but a mottled blob.
This type of aberration is caused by a lens or mirror with a rough surface, for which there is no excuse.  I can only speculate that the mirror maker reached his or her target curve before reaching the finer grades of polishing.
The cure is to polish the mirror properly in the first place.  The optician did not leave enough glass at the end of the grinding stages to arrive at the desired curve when all the polishing stages were completed. This could have been avoided by careful monitoring of the mirror's curve at all times and stages of the grinding and polishing. This applies to the figuring process as well, which is usually considered part of the polishing stages.