Does having a clip in filter change the focus?

[swag72]

Imaging with Ha is a nightmare to set up - well for me it is anyway! I use a 12nm clip in filter in my DSLR and struggle to focus and find stars with it in.

So, if I remove it and focus without a filter, align and frame without the filter, can I then pop it back in and the focus will be maintained? Or, would I be better to use for example my clip in LP filter, then swap them over?

Not sure whether the clip in filter will change the focus plane and if it does whether using another 'less strong' filter will maintain focus. Is the word parfocal?

[Merlin66]

If you check out the Astronomiser site - you'll find an explanation of the issue.

The focus is different.

[swag72]

I think I explained myself badly - Is there a difference in focus with a Astronomik EOS clip in filter and no clip in filter? Alternatively, are the differences in the focusing between different clip in filters?

So if I focus on a star with no clip in filter, nail the focus, then put the clip in filter into the camera, will the focus be maintained?

[jgs001]

I would be tempted to refocus anyway, as the removal and refit of the camera may change something. I've never actually tried it though.

[Psychobilly]

It will change...

[Carbon Brush]

Glass (filter) in the line will change the focal point. Think about apparent depth vs real depth in water. I think the change is by around 1/3 of the filter thickness. I haven't done the maths to verfiy this so don't shout at me!

If you replace the Ha filter by any filter that allows enough light in for focus, you should be near enough. Aiming for a filter with the same glass thickness as your Ha is obviously best.

[Merlin66]

Yes, in the converging beam the focus moves backwards by 1/3 of the filter thickness.

If all the clip in filter are the same thickness, there shouldn't be a problem.

[Psychobilly]

I used to use a bahtinov and take test shots when I was imaging with a Ha filter.. you wont get the normal lines but a series of dashes. at short FL these become very hard to see so your better of going for min star size...

You will get enough of a Ha signal from the brighter stars.. it's a slow process... use max ISO and widest aperture then when your focused stop the lens down if it needs to be to sort ot the corner stars...

[swag72]

Cheers folks - I guess I'll be looking at aligning with out a filter, then carefully adding the filter into the mix for focusing. I have managed with the Ha filter before, but it is tricky that's for sure!