What are 'refracting elements'?

[baileys2611 (Simon Bailey)]

I could google it and spend some time researching, but thought I would tap the combined thousands of years of knowledge in the forum (wel...perhaps hundreds...) basically I'm looking at some filters for CCD photos.

Some are recommend where 'you have a system without any refracting elements', others don't mention that.

Does this mean that some are better for refractors and others can be used in scopes like SCTs? Or have I got the wrong end of the stick there.

Thanks for reading.
Simon.

[multiweb (Marc)]

That must be referring to chromatic aberrations. Basically when light travels through glass different wave lenghts (colors) have different paths so they deviate and will end up focusing at different points.

So you get purple halos, colour fringing, etc... Some filters will help with that. Like the Baader Fringe Killer for lenses.

SCTs are sometime catadioptrics systems. They're a mix of glass (correctors) and reflective components (mirrors) so you can get some colour halos on your stars.

Every scope that uses a correcting lens such as a coma corrector or field flatener will have a refractive element.

High end refractors have many lenses stuck together to get rid of color fringing. You have triplets, quadruplets, etc... with different type of glass in each layer. Some have air spacing, others even oil spacing.

I think it's safe to say that most LRGB filters and narrowband filters would work on all telescope types.
Specialty filters Fringe Killer, etc... are for refractors.

[baileys2611 (Simon Bailey)]

Thanks Marc. Got it I think:

A Celestron C9.25 has a glass cover, but it's not a lens. It bounces the light around until coming out the end. I don't use a coma corrector or field flattener - therefore I have no refracting elements.

If I added a field flattener to the C9.25, it would then have a refracting element.

Whereas the William Optics GT-81 is a tripplet refractor with two field flatteners inside the scope, so it would most definitely have refracting elements.

Is that correct?

[multiweb (Marc)]

Originally Posted by baileys2611

Thanks Marc. Got it I think:

A Celestron C9.25 has a glass cover, but it's not a lens. It bounces the light around until coming out the end. I don't use a coma corrector or field flattener - therefore I have no refracting elements.

If I added a field flattener to the C9.25, it would then have a refracting element.

Whereas the William Optics GT-81 is a tripplet refractor with two field flatteners inside the scope, so it would most definitely have refracting elements.

Is that correct?

The C9.25 front glass is a corrector. It deviates light before it hits the primary. It's not flat. It's actually matched to your primary and secondary was probably figured also. They all work together. So it is definitely a refractive element, so is your field flattener.

Your WO is a triplet so that's probably optimised for imaging and colour correction. Meaning all your colors will focus in the same plane. So you won't need any filter to achieve the same result as it's built-in.

[baileys2611 (Simon Bailey)]

Ah! Light dawns.

Thank you I'm now wiser.