Mounted vs unmounted CCD filters?

[choochoo_baloo]

I'm planning to upgrade to mono CCD imaging soon, and wondered why unmounted filters exist at all?

1. Surely the faming piece is required to support the glass?
2. How are 'unmounted' filters actually secured in filter wheels?
3. Why would one chose unmounted over mounted? Is vignetting a concern on mounted?

This has bothered me for a while!

[Adam J]

29 minutes ago, choochoo_baloo said:

I'm planning to upgrade to mono CCD imaging soon, and wondered why unmounted filters exist at all?

1. Surely the faming piece is required to support the glass?
2. How are 'unmounted' filters actually secured in filter wheels?
3. Why would one chose unmounted over mounted? Is vignetting a concern on mounted?

This has bothered me for a while!

Taking 1.25 inch vs 31mm filters as an example.

The 1.25 inch is actually the size of the filter thread not the clear aperture so the clear aperture on the filter is more like 24mm to 26mm dependent on the brand. That is because you need a retention ring to hold the filter within its mount. Also the filter is smaller to begin with.

So essentially you get a bigger clear aperture for your cash with unmounted.

They both mount into the same hole on a filter wheel, just via different methods.

However my view is that they are a pain to handle, you have to work out which direction to mount them in and they are more expensive too.

Generally unmounted filters are thicker glass than mounted but not universally so.

Personally I would say, get the cheapest filters that cover your sensor at the fastest f-ratio you intend to use. As such you need to chose your camera and scope before you decide on what filters to get.

Adam.

 

 

 

 

[choochoo_baloo]

Thanks Adam, very useful.

I acknowledge your advice of waiting till I'm sure of camera + scope, but can I take it that mounted are superior in terms of user-friendliness?

[Lead_weight]

They are, however, there are brands that have taken steps to aid in putting unmounted filters into the filter wheel correctly. Some use a tape label around the rim of the filter indicating the direction, or some put anti-reflective coating on both sides of the filter so it doesn't matter which direction they face. But just do a little research so that your decision is sound. Definitely do a forum search for the type of filters you're looking into, you might find they have other issues that would warrant choosing a different product anyway.

You can also use the filter size calculator at Astronomy.tools to check your scope and camera configuration to ensure you get filters large enough to accommodate the FOV available to you. You may find you need unmounted filters.