Hoya nx400 for solar imaging?
I would not recommend any sort of ND filter unless it is capable of blocking infra red radiation, which is the real danger with solar imaging.
I would suggest getting hold of Baader film and making your own. See
I used to use Baader Film until I obtained more specialised equipment and found it to be very good, and a lot better than previous types of filter material.
|Blue and yellow in water by fireplace33|
from Ink and water
|Kylmä joki kopio by Kaappo|
from Shutter speed 1/25 or slower
|WR_2.8_13 copy copy by photoprof|