Author Topic: Dealing with reflections (Oh Alnitak!)  (Read 350 times)

Offline schwim

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Dealing with reflections (Oh Alnitak!)
« on: 2018 January 06 00:13:39 »
I have 13 hours on the Horsehead Nebula in Ha and I need some ideas on how to deal with the multiple reflections present in the data. I'm assuming the reflection is from Alnitak (pretty sure actually). Here's what I'm seeing (very stretched):



This is a copy of the master frame:

https://www.dropbox.com/s/18616hmyyvq8df0/hh_master.xisf?dl=0

I haven't put much effort into fixing this yet, but one idea I have is to use range selection to create a mask, then use curves to match the reflections to the surrounding areas. Another is in using wavelet layer processing to target them. For example, if I use the ExtractWaveletLayers script I can see the reflections quite well. Maybe there's a way to target them that way?

Does anyone have any other ideas on how to handle a problem like this?
- Greg
Scottsdale, Arizona, USA

Offline RickS

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Re: Dealing with reflections (Oh Alnitak!)
« Reply #1 on: 2018 January 06 02:35:55 »
Yes, RangeSelection is worth a try.  Another technique you might find useful is to draw a preview around an area of interest and then copy it into a black image using SubstituteWithPreview.  That can save a lot of work with CloneStamp.  You might also find it useful to subtract a proportion of the original image from the mask (PixelMath something like "$T-Orig*0.65".)  This allows you to remove bright things, like stars, from the mask so that you don't dim them along with the reflection.  Convolution is your friend for smoothing the mask when needed.

Cheers,
Rick.

Offline sharkmelley

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    • Mark Shelley Astrophotography
Re: Dealing with reflections (Oh Alnitak!)
« Reply #2 on: 2018 January 06 09:11:05 »
I don't have any ideas for dealing with the image but that's one of the best examples I've seen of a microlens diffraction effect, reflected back onto the sensor, probably by the filter.

Which scope, camera and filter did you use?

Mark
Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/

Offline schwim

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Re: Dealing with reflections (Oh Alnitak!)
« Reply #3 on: 2018 January 06 16:39:22 »
Thanks Rick. I like the additional pixelmath step you suggested. I'll give that a try.

Mark - the camera is an FLI ML16200. The filter is an Astrodon luminance filter. OTA is an AGOptical iDK10.  I've never had a reflection like this before. Would you see the primary and secondary shadow out of focus like that with microlense diffraction?
- Greg
Scottsdale, Arizona, USA