Clear focusing filter question.

[Andrew INT]

Hi All,

After a number of years as an observer, and a little bit of (mainly) DSLR imaging, I've decided to head down the autoguiding/dedicated camera route. I have a mono ASI290MM camera and a seven-filter wheel. At the moment, this is loaded with:

Clear focusing filter (same glass thickness as the narrow bands)

RGB filters

Ha, Oiii, Sii narrowband filters.

My question is this: the clear filter is not (I believe) IR blocking. I do have a L-IR CCD filter. What are peoples thoughts if I added this permanently onto the filter wheel nosepiece? Obviously this would add more glass for the light to pass through. The RGB's are also IR cut but I'm not too sure I could use the clear focusing filter to capture luminance's with the RGB filters. 

Any guidance always welcomed!

Many thanks,

Andrew

[Stuart1971]

17 minutes ago, Andrew INT said:

Hi All,

After a number of years as an observer, and a little bit of (mainly) DSLR imaging, I've decided to head down the autoguiding/dedicated camera route. I have a mono ASI290MM camera and a seven-filter wheel. At the moment, this is loaded with:

Clear focusing filter (same glass thickness as the narrow bands)

RGB filters

Ha, Oiii, Sii narrowband filters.

My question is this: the clear filter is not (I believe) IR blocking. I do have a L-IR CCD filter. What are peoples thoughts if I added this permanently onto the filter wheel nosepiece? Obviously this would add more glass for the light to pass through. The RGB's are also IR cut but I'm not too sure I could use the clear focusing filter to capture luminance's with the RGB filters. 

Any guidance always welcomed!

Many thanks,

Andrew

If you already have an IR luminance filter, then that’s all you need, you can use that for focusing....no need for the extra clear filter... 👍😀

[Andrew INT]

4 minutes ago, Stuart1971 said:

If you already have an IR luminance filter, then that’s all you need, you can use that for focusing....no need for the extra clear filter... 👍😀

Won't the focusing be off on the narrow band filters though?

[steppenwolf]

I wouldn't rely on a clear filter of the same 'thickness' as your other filters to achieve accurate focus. IF you have an accurate autofocuser with an encoder, you can use one filter to focus with and then add (or subtract) a known offset (the offset for which is determined in advance) for use with another filter but I would caution against assuming that filters are sufficiently parfocal for imaging use. If you don't have such an autofocuser and a set of suitable offsets then there is no substitute for focusing with the filter that you intend to use when capturing the image.

Remember that you can manually focus on a bright star and then move to the object you want to image in the knowledge that it too will be in focus until temperature change necessitates a refocus.

[Andrew INT]

Thanks, I want to go down the autofocus path at some point. The baader clear filter is supposed to match my baader narrow band, i.e parfocal (ish).

I'll have a rethink of what I use- thanks for the advice!