Apo vs Achromat for Narrow Band Imaging

[beka]

I was thinking, does an apo produce better images than an achromatic refractor for narrow band imaging? Chromatic aberration should not be an issue for NB but I read somewhere that for apo's spherical aberration is corrected for two wavelengths vs one for achromats.

Cheers!

[Peter Drew]

Whilst achromats can give excellent visual results, the long focal ratio needed to do this makes them unsuitable for imaging DSO's compared to apochromats even in narrow band.  Triplet apochromats, by their nature, are expensive items and tend to be made to a higher specification.     🙂

[vlaiv]

Fast achromats usually suffer spherical aberration in interesting parts of the spectrum and hence are not best suited for narrowband imaging as results that they produce are not quite as sharp.

It also depends on actual sample as there is a lot of sample to sample variation.

Sometimes you can optimize the telescope for Ha wavelength (or rather skilled optician can do that by varying distance between lens elements) but that would throw off correction in OIII (other part of spectrum).

Have a look here for example:

http://interferometrie.blogspot.com/2017/06/3-short-achromats-bresser-ar102xs.html

Interesting excerpts from that article:

Quote

- monochromatized they suffer still from considerable spherochromatism, if they are good in blue they are the more bad in red and vice versa

 

Quote

- some use them for narrowband imaging (even if I critizice them sometimes for their not-so-sharp details)

However, you can use longer focal length achromat that has good reputation (TAL100R for example) if you are willing to give away some of the FOV and know adjust your sampling rate to suit your need ("slow" scope is not necessarily slower than fast scope if you match aperture and sampling rate - only thing you loose is FOV).

[Rusted]

iStar make f/10 achromats specifically for H-alpha solar work.
These seem to work well on lunar imaging with a green filter too.

[beka]

3 hours ago, Rusted said:

iStar make f/10 achromats specifically for H-alpha solar work.
These seem to work well on lunar imaging with a green filter too.

That is strange isn't it? H-alpha is red so they should work best at that wavelength. 

[Rusted]

20 hours ago, beka said:

That is strange isn't it? H-alpha is red so they should work best at that wavelength. 

I haven't seen any data on the colour correction of their H-a achromats.

I also have one of iStar's R35 180" f/12 achromats.
This defocuses to magenta rather than the usual violet. 
It is claimed that the R35 offers colour correction equivalent to f/16.
Rather than the colour correction to be expected from a nominal f/12.