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Old 09-09-2015, 08:53 PM
glend (Glen)
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What the heck is this - DSLR colour bloom?

So my wonderful Canon 450D modified and cooled has developed the blues, or the magentas. I have attached two test shots below that exhibit the problem (it is always in the same location on the frame). The strength of the colour is dependent on the length of the shot, so the one with more colour is a longer sub than the other one.

Had anything changed? Well yes, I took Merlin's advice and removed the front low pass 1 (aka the anti-alysing) yesterday, disassembly and re-assembly went fine as usual (its been taken apart numerous times in the past for cleaning, mods, etc and I know what I am doing). Following the removal of the Low Pass 1 I did these test shots (in my observatory) and got these results. At first I thought I had a light leak, but I even put the camera in a black bag and it still produced these results, so I am thinking its component related, but it seems to have a grain or pattern to it. Anyone seen anything like this before and if so what's the problem? I would hate to lose this great camera. I can put the Low Pass 1 back in but would like to know if it's related or not.

BTW the camera had a Baader UV/IR Cut filter on the front of the T-adaptor so this is not UV light pollution coming in from the front.
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Last edited by glend; 09-09-2015 at 09:14 PM.
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Old 09-09-2015, 10:25 PM
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Octane (Humayun)
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Amp glow. Dark frames will help.

H
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Old 09-09-2015, 10:42 PM
glend (Glen)
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Ok. But why has it never appeared before I removed that Low Pass 1 filter? And why, when I Google amp glow in 450Ds, do I get posts saying 450Ds don't have any_even at ten minute subs? Can this just be a calibration issue or was the low pass filter shielding the sensor?

I have not had to use darks since the cooling mod some six months ago, but did shoot some last week for the dark library, shot at -10C.
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Old 09-09-2015, 11:52 PM
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rustigsmed (Russell)
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hi glen,
it is definitely light leak of somesort.
i had my 600d professionally modified to clear filter. it results in a similar display to your pics IF it is in live view mode. if it is not in live view mode it does not display the 'purple' 'leakage'. give it a try. will say it wasnt extreme as yours though.
to get around the problem it required focus in live view then to switch it back to non live view.

Russ
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Old 10-09-2015, 04:39 AM
glend (Glen)
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I suppose that's a possibility as I use Liveview and leave it on with a flap over the screen so I can check the second count - but that's always been the case and my non-cooled 450D which also has Liveview does not have the problem. It only started after the AA filter was removed on the cold finger 450D. It can't be heat related as there is none - this camera has been running at -10C every time i use it without the colour problem.
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Old 10-09-2015, 08:48 AM
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Merlin66 (Ken)
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Glen,
I have and use ( spectroscopy) both a standard 450D and the full FULL modded 1000D.
Touch wood, I haven't experienced a similar "light leak" issue.
I have had occasional problems when the viewfinder is left uncovered.
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Old 10-09-2015, 08:54 AM
glend (Glen)
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I think I might have found the problem. All of the recent subs were all taken through my GSO RC08, which has a collimation ring mounted just ahead of the focuser. The collimation ring has push-pull screws arranged at 120 degree intervals used to align the focuser precisely with the centre spot on the secondary (much like the technique used to align refractor focusers). The collimation ring therefore has a very small gap between the faces of the two adjusting sections. When the focuser is fully inward there is no way for light to get into the inside of the tube but when the focuser is wound out to the focal point for the camera the inner moveable tube comes back beyond the collimation ring gap and light can then get into the inside of the focuser tube. With the scopes position on the mount and pier in the observatory, the collimation ring is exposed to several leds on pier mounted equipment (heater controller, camera temperature controller, and battery indicators on plugs), potentially also the screen of the guide laptop and my tablet running Sky Safari Pro. So light from these devices, admittedly low level, may make its way into the tube through this collimation ring gap (which would be no more than maybe 0.25mm at the widest point), when the scope is at at high altitude in the eastern and western sky and the camera is nearest the pier (where the test subs were taken). I have verified light leakage with a red headlight, focuser in camera focal point position, and me looking into end of the focuser tube with the scope capped. I need to do some night testing with the camera on the mount, but it looks like the possible source of the light. This also explains why it never showed up in the past when using that camera, as it was always used on my 10" imaging newt or the MN190 (neither of which have collimation rings). I will report back after the test. The simple fix seems to be to wrap a piece of black PVC tape around the collimation ring flange to stop any light from getting in there.
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Old 10-09-2015, 09:14 AM
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Octane (Humayun)
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In my original post, I wrote "amp glow or severe light leak" but when I re-read your post you mentioned you took the photos while the camera was in a bag but still had the same problem, so I edited it to remove the light leak bit.

H
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Old 10-09-2015, 09:51 AM
glend (Glen)
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Quote:
Originally Posted by Octane View Post
In my original post, I wrote "amp glow or severe light leak" but when I re-read your post you mentioned you took the photos while the camera was in a bag but still had the same problem, so I edited it to remove the light leak bit.

H
Thanks H, yeah the bag was around the camera and did not cover the collimation ring (the light source), so you were right.
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Old 13-09-2015, 06:22 PM
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rcheshire (Rowland)
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The attached image shows typical viewfinder leakage and may appear top or bottom and either side of the image, depending on the camera.

Digital electronic interference will show up as horizontal lines, as a rule. You have bonded the cold finger and camera chassis, and there is no evidence of digital interference, as far as I can tell.

I tend to agree, this is light coming from somewhere. It has that look. Your cold finger is virtually identical to my modification, and performance is similar. My images show no gradient with or without the AA filter and with or without sensor heating.

Is there an LED in the vicinity?
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