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NBZ filter problem


MartinB

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I am new to the NBZ filter.  I bought it to use with my ZWO 2600MC camera coupled to either a 200mm F2.8 or a 135mm F2 lens.  I have managed one image using the 200mm lens without any significant problems.  This week I have made a start on a widefield Orion project using the 135mm lens (also using an Asiair Plus for the first time).  I have found that the focal point for red differs from blue and green.  I am attaching images through a Bahtinov mask

Mask_red_focus.jpg.55c5a1bcdf3122ee8500525d29553363.jpg

This image has the blue and green in focus.  The red spikes are fuzzy and the central spike is over to the right.  This focus produces ugly red halos.

Mask_compromise.jpg.c01758ba499d575697776ad2ce5eb50a.jpg

The above image is a compromise with the blue and green central spike to the left and the red a little over to the right.  This diminishes the halos but the stars are slightly off focus

1113945463_M45cyanfocus.thumb.jpg.4d489f8afd3e8a9e17c1118312c5919a.jpg

This cropped M45 image shows the effect of focussing blue and green

M45_red_focus.thumb.jpg.c30aec545e6212cedd88804e1a94da71.jpg

The above has the red closer to focus.  The halos have gone but the stars are distorted, out of focus and overly cyan.  Obviously the filter is inappropriate for a broadband target like this but I was investigating prior to Orion appearing.

Below is an Orion image.  For this I got a compromise focus using the mask but then removed this and made tiny adjustments looking at a small dim star.  This seems to be the best approach but the stars still aren't perfectly focused and are slightly distorted even in the centre so I don't think this is a lens issue.

 

M42_hh_M78_small (1 of 1).jpg

I understand that the NBZ is rated down to F2 and would have thought that if the lens was too fast it would affect band pass rather than focus.  Have any NBZ users got any experience of this problem.  Could it be a problem with the filter or am I doing something stupid (which is my default setting).

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I use this filter with the same camera, but with a telescope.  I cant say I've noticed this effect and I have focused with a Bhatinov mask with this filter.  SGPro generates a grey scale image with the colour camera, but i just get three nice lines in the X shape/one intersecting.   Could it be the glass in the lens?

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I suspect it is the speed at f2.  Both the 200mm and 135mm are Canon L series lenses, so same glass I guess.  Very reluctant to stop it down but might need to.  Don't know if Samyang 135mm have a similar problem.

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1 hour ago, MartinB said:

I suspect it is the speed at f2.  Both the 200mm and 135mm are Canon L series lenses, so same glass I guess.  Very reluctant to stop it down but might need to.  Don't know if Samyang 135mm have a similar problem.

I was thinking more about the glass not bringing the wavelengths to focus at the same point, but you’d think the canon lenses should be ok. 

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1 hour ago, MartinB said:

Don't know if Samyang 135mm have a similar problem.

Hi Martin.  Although not with the NBZ, I definitely see the R & GB focus difference in the Samyang 135mm (mine and others).  My Green, Blue (and of course OIII) focus at a similar point but Red (and Ha) is totally different.

While waiting for my gear to warm up the other night, I finally did a test rotating through R, G, B, OIII and HA filters with a bahtinov mask.  I was focused for OIII.  G, B and OIII produced a perfect pattern and Red (and Ha) as expected were way off.

I use my Samyang at ~F2.8.

Your Orion image looks great.

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19 hours ago, geeklee said:

Hi Martin.  Although not with the NBZ, I definitely see the R & GB focus difference in the Samyang 135mm (mine and others).  My Green, Blue (and of course OIII) focus at a similar point but Red (and Ha) is totally different.

While waiting for my gear to warm up the other night, I finally did a test rotating through R, G, B, OIII and HA filters with a bahtinov mask.  I was focused for OIII.  G, B and OIII produced a perfect pattern and Red (and Ha) as expected were way off.

I use my Samyang at ~F2.8.

Your Orion image looks great.

That is good to know thank you!  I might try a bit of trial and error with stopping down.  I think I will use F2 on a big mosaic both for speed and because the iffy stars shouldn't really be noticeable except for the most obsessive of pixel peepers which I definitely am not!

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