Tuesday, April 29, 2008

Some ideas about the Lunt's Singlet

Just watched some reports from NEAF, unluckily, people couldn't test the new scopes.

Those 60mm scopes are having singlet objective, which is quite hard to understand at first. But then I guess I know the reason. Since this scope is offered for both H-alpha and CaK, I believe that a doublet which corrects (like spherical aberration) at both wavelength would need to be custom made, that would mean a special cost.

By using a singlet in the objective, and then to use the collimator lens before the etalon to correct the aberration, it would be cheaper to do. Why?

Firstly, the collimator lens is to make the light ray parallel before passing the etalon, that means it will be slightly different in design for different wave length, since different light focus slightly different and thus the lenses should be different for H-alpha and CaK. Therefore, this lens has to be custom made anyway.

And I guess you know what I mean here, since the collimator lens is to be made specifically for H-alpha and CaK, it's thus more economical to design them also to correct for other aberration!

So, let's break down the situation like this:-

1. If you use a doublet as the objective, you will need a special doublet which corrects for both H-alpha and CaK. And in that case, you will also need special collimator lens for H-alpha and CaK respectively. Therefore, we need to have three different custom made lenses.

2. If you use a singlet as the objective, you can have the same one for H-alpha and CaK, since all the aberrations were left to be corrected by the collimator lens. And in this case, you will need a special collimator lens for H-alpha and CaK respectively, but it's just the same as the above. Therefore, we only need to have two different custom made lenses.

This means cost saving. Of course, it might not be the case, I just guess.

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