First Light with an O III Filter

When I bought my first Telescope in 2011 I also bought a BAADER O III filter(10nm) and Light Polution Filter. I had never tried out the O III filter until a few nights ago. The moon was full so I would typical avoid deep sky objects. I decide to try the O III filter because it is a narrowband filter. I decided to image the Crescent Nebula with my 8″ EdgeHD scope at F10 (2032mm FL). I had previously imaged this nebula a few weeks ago using my OPTOLONG L-eNhance narrow band filter so I thought it might be interesting to compare what these two filters will yield.

Crescent Nebula using an OPTOLong LeNhance Filter
Crescent Nebula using a 10nm BAADER O III Filter

I used 10 minute exposures for both images and roughly 4 hours total integration for each. My QHY294C was using a GAIN of 1600 and it’s cooler was operating at -15 Celsius. There are some interesting differences between the two images. The O III image was very dim. Only the green and blue pixels seemed to collect any photons. The red pixels are useless and I needed to tell Startools to push back the red channel as it was contributing noise. The halos on the brightest stars are very pronounced but that is partly because I needed to stretch this image a lot.

Every one of my imaging sessions of the Crescent Nebula always showed a strange phenomenon at the very center of the nebula. If you first identify the very bright star closest to the center of this nebula and then look slightly to the right and slightly down you will see a pronounced black blob that is devoid of stars. I expect it is a dark cloud of dust…but it’s right at the center of the nebula? I did a quick web search with no mention of what this black blob actually is.

Taking FLATs with the O III filter was also interesting. I normally need 1.5 second exposures with the LeNhance filter. With the O III filter I needed 6.5 second exposures using the same camera GAIN. I always use a camera GAIN of 0 when taking FLATs so that my FLAT frames contain the maximum amount of photons. Doing this ensures the highest signal-to-noise ratio (SNR) for my FLATs.


2 Thoughts to “First Light with an O III Filter”

  1. Scott Kuchma

    Hi Peter ,
    Interesting Images . I looked at several dozen other Crescent Nebula photos and they all seem to have that little dark/black circle that you described . However , the older Images do not have that dot in the same place as your current image . It looks like it has moved .
    I’d post the Image that shows it the best but that doesn’t work , Instead search for Crescent Nebula and pick the link for Astrobackyard . Right at the top of Trevor’s Article is a decent Image showing where he saw the black dot . I can’t find a date for the Image though . Interesting stuff.

    1. Scott,
      The image that Trevor displays at the top of his article is spectacular. Do you think it’s his image? He doesn’t take credit for it so he may be using it with permission. I think the black dot is in the same position. My image is rotated 90 degrees counterclockwise so that it tricky to cater for. But imagine if that black dot were to have moved!!! This Nebula is located 5,000 light years away so any movement that we could detect visibly with a small telescope would be big news. I know it can’t be a astronomical black hole…but because it looks like a hole that is black it makes you stop and wonder what it is? I’m pretty sure it’s a blob of cosmic dust that is between this Nebula and us. Probably it’s the coolest astrophotography “photo-bomb” of cosmic dust I have imaged.


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