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Old 24-09-2008, 10:20 PM
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g__day (Matthew)
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Wierd doughnut glow on bright objects?

I have noticed this recently when I image bright stars for a moderate duration - it looks like a glow on some internal surface - anyone guess what it is (I suppose something needs cleaning and / or alignment)

Here is an shot of Jupiter for 30 secs that clearly shows the problem.

Imaging train is Celestron SCT -> Lumicon OAG with focal reducer lens -> Hutech LPS filter -> Canon 400D

Many thanks

Matthew

PS

What's it telling me that the doughnut is not dead centre of the image - that the focal reducer is not perfectly seated parallel to the Canon imaging chip? Maybe this is the source of coma in my shots?
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Old 24-09-2008, 11:34 PM
Ian Robinson
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Ghosting in the optics.

Probably too faint to see when focusing, but shows up on long exposures.
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Old 24-09-2008, 11:40 PM
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g__day (Matthew)
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er in English what does that mean and is it fixable?
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Old 25-09-2008, 01:52 AM
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Tandum (Robin)
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What happens if you remove all that stuff from the chain? Then add them back in one at a time?
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Old 25-09-2008, 02:07 AM
Ian Robinson
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Quote:
Originally Posted by g__day View Post
er in English what does that mean and is it fixable?
No - I suspect not fixable , as it is internal to glass and due to the optics.
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Old 25-09-2008, 07:25 AM
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Omaroo (Chris Malikoff)
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Matt - Discount the C8 straight away - IF it is collimation. I have many happy snaps though mine at bright objects and have not encountered your experience. Even if I way-overexpose a super-bright object (Venus, Jupiter, Sirius, etc) I just get an overexposed image in the center where the object lies.

Because your aberration is offset, I'd try running the image again and then have a look at the result straight away on your LCD (or on your laptop screen) to make sure you can replicate the problem.

Could you then try spinning the imaging train (it'll be a tad loose in its visual back threads) 90 degrees in reference to the scope, starting with the focal reducer/SCT interface - keeping the filter and camera in line with the reducer as they originally were, and take another exposure. If the aberration has moved then I guess you know it's probably not the SCT.

Repeat by now spinning the camera and filter around an additional 90 degrees by spinning the train at the reducer/filter interface now. Take another image.

Lastly, to the same with the camera.

It may be a clunky method, and by virtue of the fact that by loosening items along the chain you will be altering your focus it'll be difficult - ut you have to start analysing it somehow!

Last edited by Omaroo; 25-09-2008 at 08:05 AM.
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Old 25-09-2008, 01:56 PM
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RB (Andrew)
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It's an internal reflection off your LPS filter.

The objects that are bright enough will reflect light off the camera sensor when exposing, back up the path until it hits the back of the LPS filter.
This will bounce the reflection back down to the sensor causing a secondary image to appear.
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Old 25-09-2008, 03:39 PM
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g__day (Matthew)
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Andrew and all,

Huge thanks for that - was puzzling me! On the second part of the question then - should or shouldn't I be concerned (is a correction needed) because the doughnut isn't over the bright star.

Asked another way - if I dead centre Jupiter on the CCD - take a 1 minute shot and the doughnut doesn't exactly centre on Jupiter does that mean the something isn't exactly square to the CCD?

Mind you if its the LPS - maybe I don't have to worry!
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Old 25-09-2008, 05:13 PM
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I wouldn't be concerned Matt, I wouldn't necessarily expect the lightpath to bounce directly back over Jupiter ie; I don't think you have a misalignment problem.

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