H-alpha DS filters - guidance?

I am considering buying a hydrogen-alpha filter for DS work. Astronomik
do one for £98 and locally 'Kenko' at SCS-Astro do them for £23. There
must be a difference.

Any guidance from anyone?

TIA

Lawrence

In message m,
Lawrence writes
I am considering buying a hydrogen-alpha filter for DS work. Astronomik
do one for £98 and locally 'Kenko' at SCS-Astro do them for £23. There
must be a difference.

The first will be an interference filter with a relatively narrow
bandpass centred on H-alpha - but still far too wide to be safe for
solar work. It is optimum for deep sky. It tends to give odd effects on
the continuum of stars but enhances contrast on hydrogen alpha emission
nebulae.

The second Kenko R-64 is almost certainly a deep red colloidal coloured
glass long pass filter with the 50% cutoff at 640nm. It may need to be
combined with an IR blocking filter to give the best results.

A Wratten 25 red filter combined with Nonad or Baader filters to block
out the orange yellow will also give something roughly approximating an
H-alpha filter (50% cutoff at 600nm). Again an IR blocking filter will
help.

The more continuum you let through the lower the contrast.

Regards,
--
Martin Brown

In message m,
Lawrence writes
I am considering buying a hydrogen-alpha filter for DS work. Astronomik
do one for £98 and locally 'Kenko' at SCS-Astro do them for £23. There
must be a difference.

The first will be an interference filter with a relatively narrow
bandpass centred on H-alpha - but still far too wide to be safe for
solar work. It is optimum for deep sky. It tends to give odd effects on
the continuum of stars but enhances contrast on hydrogen alpha emission
nebulae.

The second Kenko R-64 is almost certainly a deep red colloidal coloured
glass long pass filter with the 50% cutoff at 640nm. It may need to be
combined with an IR blocking filter to give the best results.

A Wratten 25 red filter combined with Nonad or Baader filters to block
out the orange yellow will also give something roughly approximating an
H-alpha filter (50% cutoff at 600nm). Again an IR blocking filter will
help.

The more continuum you let through the lower the contrast.

Regards,
--
Martin Brown