Acro Scopes and NB Imaging

[MiguelMJr]

Hey All,

I'm wondering if an Apo scope is really needed when doing narrow band imaging or if an acro scope will be able to do the job just as well. Will the color fringing of an acro degrade the image quality compared to an apo?

Thanks,

Miguel

[ronin]

If the band is narrow then an achro should be adaquate, however the focus will shift depending on the filter used. That is the nature of CA, different wavelengths are brought to different focal planes. From the curves filters like the Baader LRGB filters for CCD are not narrow band. So using a set of them would not give good results. Each filter in that set has a spread of around 100nm and that is not narrow so again the focal plane will be spread.

With those the green should be fairly good as on an observational lens green is where the image plane is flattest with regards wavelength, while the red and blue are the wavelengths where the change in focal plane is the most.

Filters like these are bandpass filters not narrow band filters.

Better is an ED refractor, I know a couple of people that use an ED for NB imaging.

[Alien 13]

The difference between an Achro and APO or ED is often more than better colour correction the lenses are better figured producing less coma etc and build quality in general far superior but If you have one then its worth trying.

Alan

[ronin]

Yes, must agree. An achro lens will demonstrate more Spherical Aberration, light through the edges of the lens will not come to the same focal plane even if they are the same wavelength. So NB would not help that specific aberration greatly. SA is more dependant of the objective diameter, bigger diameter = more SA.

That however depends on the achro used and whatever effort went into it. I would think that a Skywatcher achro to be worse then a Vixen achro - picked them as Vixen achros are usually known for having achro's that are good in performance. They likely pick 2 glasses that are better suited, or the negative element is more towards ED type on theirs then on others. There are a lot of glasses out there.

[MiguelMJr]

Thanks for the replies.