When to use UV/IR cut filter

[LewisM]

When imaging with a refractor, is a UV/IR cut best left on all the time, or only with blue?

If using an OSC CCD, any detriment to leaving an UV/IR filter in place on the front of the camera? Any detriment stacking one on top of an H-a filter? (I extract the red channel of OSC H-a filtered images)

Appreciate thoughts here, as I have read conflicting advice.

[Merlin66 (Ken)]

Generally yes. Just leave it in place.
Check the specs for the Ha filter, it may already have effective UV/IR blocking.

[gregbradley]

No need for Ha as its narrowband which means by definition it only lets the narrow part of the spectrum through around Ha. IR and UV are not in that band so they are filtered out already.

Greg.

[Merlin66 (Ken)]

Greg,
I agree 100%!
(However, I have see some multi coated "narrowband" filters where they actually have a secondary resonance pass band.....this can be up in the IR or down in the UV)

[LewisM]

Thanks guys!

[PRejto (Peter)]

Lewis,

Why do you feel you need a UV filter? And, which refractor is this for? And which camera? I only see an issue using the Sony ICX694 and not my KAF8300 so I wouldn't jump to any conclusion about the need for a filter until you run some tests without one.

I think my discussions re the TEC140/blue issue are pretty specific to the way that lens is figured. My recent tests of UV blocking filters shows that one needs considerably longer integration time with the filters and the end result may not justify the trouble. At least I can say that in relation to some targets.

In any case, what filter are you considering? I've tested the Hutech IDAS, Both the Schott GG420 and GG435, The Baader Fringe Killer and the Baader Semi-Apo. I've listed them in order of UV cutting with the Hutech starting around 410 nm. The Baaders cut a sort of notch pattern starting around 430 nm. In my opinion the semi-apo is not suitable for imaging especially if left in place for G and R or luminance. There is way too much light lost. The Fringe-Killer is pretty good but suffers 40-50% loss in blue but can be left in place for R and G. The GG435 and GG420 both need perhaps 30% more time and really clean up blue stars quite nicely with my TEC140. They can also be left in place for R & G. The Hutech is quite nice if you also need a mild LP filter. Depending on the lens you are dealing with the Hutech cut might be enough. It isn't for my TEC140/Trius combination.

Peter

[White Rabbit]

I hAve an sbig stf8300 and a 102mm refractor and I need an ir/uv block or my stars get very bloated. I have a CLS filter in place as my lp is really bad and that cuts uv and ir, and doesn't interfere with Ha. Kills my star colour but it's either that or I don't image at all.

Cheers
Sandy