do you need an apo for narrowband imaging?

[Ags]

I'm just being curious... if you are making narrowband images, do you need to use an expensive apo? Wouldn't a cheap achromat work just as well, as the narrowband filter cancels out the chromatic aberation?

I understand there are other factors at play, such as field flatness, but can't you address that in photoshop with lens effects?

[brianb]

if you are making narrowband images, do you need to use an expensive apo? Wouldn't a cheap achromat work just as well

Yes, for individual images, but you would have to refocus when changing filters & the image scale might be slightly different as a consequence.

field flatness, but can't you address that in photoshop with lens effect

No - you can't add detail which has been lost over part of the image because it's out of focus (which is what field flatness means).

If you're using a small sensor, you maybe get away without a field flattener. With a large sensor (certainly for the APS-C size commonly used in DSLRs) you will certainly need a field flattener with either a doublet or triplet refractor of normal focal length.

[E621Keith]

I'm just being curious... if you are making narrowband images, do you need to use an expensive apo? Wouldn't a cheap achromat work just as well, as the narrowband filter cancels out the chromatic aberation?

I understand there are other factors at play, such as field flatness, but can't you address that in photoshop with lens effects?

Apart from superior color correction, apos are often built to a higher standard, using better mechanical parts, better quality lens, made of better glass grid to higher precision. This may improve contrast and reduce other type of aberrations. Astigmatism, internal reflection, scattering etc...

As for field flattness, I thing you are confusing it with vignetting. I can understand why, as the images you take for compensating for uneven illumination (vignetting) are called 'flats', but as brianb said, field flatness is to do with the variation in focal point along the image plane.

[Merlin66]

This was recently discussed on another thread.

You don't need an APO for narrowband imaging.