This is a pretty good question :) We generally are very welcoming about refunding bad data. Among the classic reasons for a refund are:
- Bad guiding/tracking
- Bad focus (except when out-of-focus images have been requested)
- Wrong position (by more than 5 arc-minutes off-center)
- Artifacts due to clouds or technical issues
And what about data that doesn't get refunded?
Well, let's call that the "risk of the job". These are cases that usually would not get refunded
- When not using moon limitations or started with a fixed time request and having straylight from the moon in the image (imagine luminance frame with bright background)
- With fixed time requests, when focus drifts during the night (can happen if the request is very long. In this case split it up in a few shorter requests so that the system can focus in between)
- Cosmic rays
- Satellite or aircraft trails (they can be removed during processing)
- Hot columns/pixels/rows
- If worse-than-average seeing makes for bloated stars
- Slight elongation of stars in not more than one corner of the image.
- Over- or underexposure. The exposure times shall be set by you in a decent way. Do not expect to have great data with a few 5 second frames :)
- Weather limitations that stop fixed time requests. If a longer series of observations cannot complete due to weather interference.