OIII filter probs at low focal ratios

[physicus]

This looks like something Matthew McConaughey might see on his way into the wormhole, but it's actually a 600s sub through my OIII filter in my latest 50mm Zuiko camera lens/Atik460 combo.  The Ha results have been fine, and I was looking forward to adding another channel for a bit of colour.  But - no way.  The stars here are in fact in nice sharp focus, it's just the massive coma-like artifacts that ruin the image.  I'm imaging at f2.8 (it's a 1.4 lens), which I'll be the first to admit is asking a lot of any geometrically sensitive interference filter to cope with.  This btw is one of the replacement Baaders, which was a great improvement over the original and has been okay at f5.5 with my Esprit 100.  So, question is: does anyone have a OIII solution for imaging with short fl camera lenses at low f ratios ?

Tim

 

[physicus]

Think I can answer my own question; might be useful to someone else though.  Stumbled across a communication from Baader in another forum, which prompted me to reverse the filter - so very shiny side facing the chip, as opposed to the just shiny side - and repositioned the whole thing nearer to the chip.  I think what I was getting here were reflections from the rear lens surface being within about 10mm or so of the filter surface.  Exposing a few tonight at 600s, and so far they look good.

[RichLD]

Glad you got it sorted . I had exactly the same issue with my Nikkor ED180 and in the end found that placing the filter as near as I could get it to the chip mostly solved the problem, although I have ended up stopping down to f5.6 and using 1200s subs, which I found gave me nice tight stars and also retained finer highlight detail in any nebulosity.

I didn't know about the reversing the filter glass trick for reducing reflections, makes total sense if the filter is near the rear lens element.

I have reversed mine but also have the filter body reversed which in my set up (which uses a Geoptik Canon to CCD adapter) gets me nearer to the chip. This of course means that the filter is still in it's standard orientation with the really shiny side pointing towards the lens, so based on your post and ever the tinkerer, I think I'll try reversing the glass again to see if it improves things even further, many thanks for sharing! .

Really looking forward to seeing your finished images, I love widefield me .

All the best

Rich

[physicus]

2 hours ago, RichLD said:

Glad you got it sorted . I had exactly the same issue with my Nikkor ED180 and in the end found that placing the filter as near as I could get it to the chip mostly solved the problem, although I have ended up stopping down to f5.6 and using 1200s subs, which I found gave me nice tight stars and also retained finer highlight detail in any nebulosity.

I didn't know about the reversing the filter glass trick for reducing reflections, makes total sense if the filter is near the rear lens element.

I have reversed mine but also have the filter body reversed which in my set up (which uses a Geoptik Canon to CCD adapter) gets me nearer to the chip. This of course means that the filter is still in it's standard orientation with the really shiny side pointing towards the lens, so based on your post and ever the tinkerer, I think I'll try reversing the glass again to see if it improves things even further, many thanks for sharing! .

Really looking forward to seeing your finished images, I love widefield me .

All the best

Rich

Thanks for the comment.  I have the TS version of the Geoptik (I use a K&F Olympus to Canon adapter on the Zuiko first).  It may be the same, but on mine the filter fits a step-down ring that can be screwed into the body from either side.  The only hassle with the new set-up where I put the filter in the ring first backwards and screwing the ring into the body from the camera side, is it's unremovable without unscrewing the body from the camera, whereas previously, and for the Ha filter, I could just unbayonet the lens.   BTW, to achieve focus, this set-up also needs a small (2 or 3mm ish) T2 spacer in the light train between camera and lens.  The other unknown is how exposures beyond 600s look.  From results so far though, it looks like this particular effect has qualitatively switched off with the geometry change.  Here is a stacked OIII with the new arrangement.  It was a moonlit, low contrast session - which can make a difference - but not a sign of trouble.