General Question: Narrowband imgaing with an achromatic refractor ?

[Christoph Puetz]

Dear PI Users,

I own a small ED refractor and I can access a large one in an obseratory (a Starfire 6" Full-APO).
If I want to shoot narrowband images during travel with a CCD Cam only: Is an achromatic refractor sufficient ?
I came to this question, because solar photographers (who only use narrowband Ha filters) get cool results with "achromats".
So why not take Narrowband images with them, if I care about some principles (take care of focus, use flattener ....)

Whats your opinion ?

[marekc]

Hi Christoph,

I don't know the answer to your question, but I've wondered the same thing. If a person were shooting solely narrowband, would a cheap achromatic refractor suffice? At first it seems like it would, but I'm not really sure.

I guess it would depend on some of the things you've mentioned, like the flatness of the field, and so on. I guess there would be the problem of shooting RGB stars with such a scope, too. But if a person was happy with, say, the Hubble palette and its pink stars, then maybe an achromat would be the way to go.

Hopefully someone will know if there's a hidden optical issue that we're not thinking of!

- Marek

[Geoff]

As far as the chromatic aberration is concerned you won't have a problem with narrowband, as long as you refocus for each filter.  You could probably get away with RGB as well, again refocussing for each filter.  The images through different filters will have a slightly different scale because the focal points are different, but PI can handle that.
Other aberrations (coma, curvature, etc) will neither be helped nor hindered by this process.
Geoff

[Christoph Puetz]

Dear Geoff and Marekc,

thanks for your replies. Your feedback made be courageous :-)
I will check this with an 125/900mm FH Refractor, as soon as we will get clear skies in Germany and I will post some results.