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Blurring of Green and Blue Channels


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Here is a grab from an avi, and a jpeg from 90 seconds of Saturn, with the three RGB channels in that order. As you can see red is fine but green is blurred and blue appears double registered:

Video

raw video.jpg

Stack (yes this is RGB aligned!)

02_02_59_pipp_g4_ap33_Resample20.jpg

red

Red-0.jpg

green

Green-0.jpg

blue

Blue-0.jpg

I apologise, as I have asked about this before, but having exchanged a few emails with Emil of AS!2 fame, I now know that these issues are with my setup or captured data, not with how AS!2 is doing the stacking. I don't normally get this problem, at least not bad enough to leap out at you like this. My setup was using my normal kit:

  • 150PL reflector
  • x2 medium-range barlow (celestron type not cheap skywatcher one)
  • Microsoft HD Lifecam

I did not use my IR filter, but I would have thought that would only affect the red channel anyway?

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As you didn't use an ir filter, red was the strongest signal combined with ir. You focused the scope on the red channel leaving the green and blue out of focus.

I can't say why the double image, may be atmospheric dispertion had something to do with it.

It's best to always use an ir filter when planetary imaging, the rest is down to good seeing on the night.

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I have to suspect that the Barlow isn't sufficiently apochromatic. The reflector must be, but for the channels to be out of focus in an OSC image the optics must be non-apochromatic and that means the Barlow is the suspect. Just a thought, but tilt in the system might play a role as well in putting one side of one colour in or out of focus in a non-apo system.

Olly

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Hi again Neil - we are a bit cross-threaded here! I responded to this on my thread, but hadnt really looked at it properly.

I now realise the last image has IR filter and is heaps better, no? So just wonder why you didn't use it this time around if thats not a silly question?

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2 hours ago, Tommohawk said:

Hi again Neil - we are a bit cross-threaded here! I responded to this on my thread, but hadnt really looked at it properly.

I now realise the last image has IR filter and is heaps better, no? So just wonder why you didn't use it this time around if thats not a silly question?

I forgot to fit it :-(

Also Saturn was a bit higher last year.

I've asked for a quote on a pair of 32mm diameter 3.5-degree prisms to make a ADC. No doubt the answer will be deeply scary.

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To close this topic down, the answer was simple in one way - I forgot to fit my IR filter.

The full explanation is that all channels have sensitivity to IR, even green and blue. Mars is so red that the IR was about as strong as the green and blue, but atmospheric dispersion (the atmosphere acting as a prism - google it, I had to!) was causing the IR image to be shifted relative to the blue and green ones. Green just enough to create a 'smeared' appearance and blue rather more so it was a double image. Red was fine as IR and visible red were only shifted by a small amount relative to each other.

Last night I tried again with the IR filter and got better images of Saturn and Mars without the wierd effect.

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