Problem(s) Encountered:

[Although] the seeing is good, there is always strange tails around bright stars in Halpha band.

Solutions:

For some of the KPNO NB filters the issue is that they are not flat enough to be used in the OSMOS collimated beam.  I say some of the KPNO NB filters because we used two of them in February and there were no issues.  These were a pair of 2x2 inch filters that corresponded to a redshifted Halpha line.  Unfortunately, you don’t know a priori which NB filters from the KPNO inventory can be used successfully in OSMOS.  However, there is a list of approved KOSMOS 4” filters given on the KPNO web site.  These might be the place to start.

http://www.noao.edu/kpno/filters/KOSMOS_KPNO_FILTERS.html

For reference, the flatness specification for new OSMOS filters is described here:

 http://www.astronomy.ohio-state.edu/MDM/OSMOS/filterspecs.html

Any of the existing KPNO filters that do not meet these specifications could produce such distortions.