Does a Triplet Matter?

[Sunshine]

Regarding deep space AP, does a triplet make a difference? i am aware of the whole purpose of a triplet in aligning red green blue to reduce/eliminate CA. In the case of deep space objects where CA is not evident is a triplet needed? if so, please explain.

[Peter Drew]

Seeing the large number and variety of triplets currently offered to imagers I presume that it does matter.  Imaging reveals uncorrected artefacts that are invisible to the visual observer.     🙂

[andrew s]

10 hours ago, Sunshine said:

 In the case of deep space objects where CA is not evident is a triplet needed? if so, please explain.

...but it is evident in deep space objects. Field stars can show halos due to CA.

Regards Andrew 

[Sunshine]

10 minutes ago, andrew s said:

...but it is evident in deep space objects. Field stars can show halos due to CA.

Regards Andrew 

Uhhhm, disregard my dense question, I seem to have forgotten that DSO images include multitudes of stars.

 

Edited December 5, 2020 by Sunshine

[vlaiv]

It matters.

It is not just what we understand as "color correction" that is important - purple halo around bright stars.

It is the fact that doublet scopes have only two wavelengths of light in focus at the same time, while triplets have three. That fact alone means that there will be wavelengths of light that are more out of focus in doublet that there will be in triplet scope (provided that they use appropriate glass types - good ED doublet will outperform triplet scope made out of ordinary glass - although I have not seen one yet - tripled made out of ordinary glass, but I wonder why there aren't any).

Any unfocused light causes blur / lowers contrast. More of it you have - less sharp / contrasty image is.

Only case where it does not really matter and you can use even singlet lens / and regular achromat works well (if it is well corrected) - is narrow band imaging. Since every type of scope can bring at least one wavelength of light to focus - in narrowband you are effectively shooting only one wavelength (or wavelengths at very small vicinity of it) of light.

[ollypenrice]

Yes, Vlaiv is right to point out the big difference between NB and broadband with regard to the optics. You can refocus a less-corrected doublet between NB filters. A purist might argue that, to combine them into a single image, you would really want not just to co-register them to the selected reference image but also to resize them to it because the focal change has a small effect on the image scale. Registar would do this and I think it's also possible in PI and APP if you know how.

There are some very well corrected doublets out there, though. One of our guests gets great broadband results with a TeleVue 85.

Olly

Edited December 5, 2020 by ollypenrice