Fat blue stars

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oopfan
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Fat blue stars

Post by oopfan » Sun Jun 24, 2018 9:22 pm

Hi Everyone,

I have a color image of NGC 6791 having blue stars that seem to indicate that they are brighter than they actually are. Please compare my image with this one:

http://darkhorseobservatory.org/images/ ... 1_full.jpg

(Sorry, east is up in their photo but north is up in mine.)

What might account for this?
1. My scope is a 71mm f/5.9 ED doublet from William Optics.
2. Their scope is a 250mm f/5 Takahashi Baker-Ritchey-Chretien.
3. My camera is an Altair 290M uncooled monochrome CMOS camera.
4. Their camera is a SBIG ST-10XME cooled monochrome CCD camera.
5. My filters are Optolong LRGB.
6. Their filters are Astrodon Series E.

In conversations with 'turfpit' he recalls experts saying that blue filters are noisy. If this is true, which I don't doubt, why should a blue star appear fatter in my image?

Any thoughts will be GREATLY appreciated.

EDIT: I wonder if this could be mitigated by increasing the number of darks from 30 to 50 or more?

Brian
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turfpit
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Re: Fat blue stars

Post by turfpit » Mon Jun 25, 2018 8:04 am

Brian
In conversations with 'turfpit' he recalls experts saying that blue filters are noisy
To clarify, I checked through my notes from a Damian Peach High Speed Planetary Imaging workshop back in 2017 and held at Banbury. Damian suggested taking more blue images (than red and green) when doing LRGB planetary imaging, the reason given was that blue was noisier (i.e. stack more frames to help reduce noise). I think this is to do with greater atmospheric distortion of the blue wavelength - cue ADC to compensate. Also suggested was to use the BLUE filter when the object is at or around highest elevation in order to reduce distortion or refraction. Damian also emphasised a re-focus at each filter change.

Damian also emphasised allowing time to re-focus at filter changes on his 2017 Remote Astronomy workshop, again at Banbury.

This link https://pixinsight.com/forum/index.php?topic=11774.0 mentions blue star bloat can be caused by UV sometimes and a difference in focus for blue as opposed to red and green. There is a link in this post to http://pixinsight.com.ar/en/info/proces ... ction.html where the writer explains about Multi Scale Linear Transforms and star masks as a way to resolve this. This is well outside my comfort zone now.

Dave

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oopfan
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Re: Fat blue stars

Post by oopfan » Mon Jun 25, 2018 11:08 am

Dave,

Thank you, I'll give those two links a thorough read. Already I am capturing 20% more frames of blue in order to get the correct white balance. If I collect even more then I will have to either (a) collect more red and green, or (b) boost the red and green channels in StarTools. That second option is not perfect since what I really want to do is boost the signal-to-noise not the signal and the noise!

I have a sneaking suspicion that switching to a larger aperture is the answer. A few months ago I posted a question on a forum for photometry that asked why my star images were not seeing a boost in signal-to-noise when I stacked. Someone wrote back bringing up the topic of scintillation and how smaller apertures suffer from it more.

Brian

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turfpit
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Re: Fat blue stars

Post by turfpit » Mon Jun 25, 2018 12:10 pm

Brian

I will have a go at NGC 6791 using an Altair 183C on an Altair 66ED refractor (could try the Celestron C8 also) and see what happens with say 30x60s for as one shot colour image. I need the practise ……

Dave

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