Shooting Oiii

[multiweb (Marc)]

Hi Guys would you use an Oiii filter with another kind of filter like UV/IR to minimise stars bloating or is it a bad idea? If yes, before , after the Oiii or doesn't matter?

[rally]

Marc,

The Oiii filter is normally a narrow pass band filter meaning everything else is filtered out.
Just the band covering the 2 main Oxygen emissions around 500nm are allowed.

That should include IR and UV at either ends of the spectrum
So there should be no need to use other filters.
In fact more filters is potentially likely to result in worse problems with reflections and a reduction in light.

Can you look at the transmission curve for your filter ?
It should tell the story

[multiweb (Marc)]

You're right. I had a look at the transmission curves and there is very little point. I thought I read somewhere (optcorp I think) that you could use them together. I always get bigger stars when doing Oiii than Ha or Sii and was wondering why and if there's a 'cure' for it.

[JohnH]

I suspect the bigger stars are due to the effect of atmospheric dispersion. Ha and Sii are both in the red end of the scale, Oiii is green/blue. I see this effect most strongly when using RGB - the B stars are larger - even when optimising focus for each colour (refractor). The only cure I know of is avoidance - that is shoot Oiii (or B) when your target is at maximum elevation. Would love to hear if anyone has alternate solutions.

[multiweb (Marc)]

Originally Posted by JohnH

The only cure I know of is avoidance - that is shoot Oiii (or B) when your target is at maximum elevation. Would love to hear if anyone has alternate solutions.

Makes sense. I also recall Jase saying to shoot the blues at the meridian. I've been shooting a lot of Ha scaffold frames right now and I'm going to start shooting Oiii soon so I'll do some testing, try a few things and see what happens.

[gregbradley]

Yes its interesting isn't it that when you use programs like CCDSoft blue is last. So that often means it is shot when it is lowest. Considering blue is usually the most bloated and hazy then shooting blue early on would be good CCDSoft is not good for that. It labels the colour wrongly even though you can tell it which filter to use in what order. Makes for confusion when processing the image.

I had a German guy send me an email saying you can add more filters in the colour box in CCDsoft. Its in the script ability of CCDSoft and there is an example in the software of how to do this. I haven't gotten in that yet but it would be handy. CCDSoft as-is only allows 4 filters. So if you want to shoot HaLRGB you have to do Ha separately which means more trips to the observatory.

Greg.