Author Topic: Strange Artifacts in Lum only  (Read 1504 times)

Offline Jkulin

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Strange Artifacts in Lum only
« on: 2018 July 16 15:44:26 »
Hi All,

First light with my new Moravian G2-8300 MkII and I decided to do IC 5146 in NB.

I have run the batch pre-processing and as you can see, what appear to be the vanes of my GSO RC8" are showing.

I'm still coming to terms with PI, but wondered if anyone had any thoughts as to what may be causing it and how to get rid of it?

Thanks for anyones thoughts

Offline ngc1535

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Re: Strange Artifacts in Lum only
« Reply #1 on: 2018 July 17 13:25:06 »
Wow...that is interesting. I have not seen that before.
Quick question... when you took the flat- were you focused at your instrument position (focused for stars in other words)?
I am just wondering if you are far enough out of focus, the pupil image of a bright flat would produce shadows of the secondary... I know
you can see the secondary vanes in out-of-focus images of stars.

I look forward to the answer. Usually distinct shadows like this would be caused by something close to the chip. I assume there aren't any fingers holding your filters. :)

-adam


Offline chris.bailey

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Re: Strange Artifacts in Lum only
« Reply #2 on: 2018 July 17 14:51:37 »
What was the light source for your flats?

Offline Jkulin

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Re: Strange Artifacts in Lum only
« Reply #3 on: 2018 July 17 15:59:10 »
Wow...that is interesting. I have not seen that before.
Quick question... when you took the flat- were you focused at your instrument position (focused for stars in other words)?
I am just wondering if you are far enough out of focus, the pupil image of a bright flat would produce shadows of the secondary... I know
you can see the secondary vanes in out-of-focus images of stars.

I look forward to the answer. Usually distinct shadows like this would be caused by something close to the chip. I assume there aren't any fingers holding your filters. :)

-adam

Thanks Adam, yes my focus was exactly as it was after I took the images of IC5146, never had this before when I had my Atik 8300 (Same CCD)

Chris, same light source I always use for my flats, an LCD flat panel, but that woudln't explain the same pattern on my images of IC 5146

Offline ngc1535

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Re: Strange Artifacts in Lum only
« Reply #4 on: 2018 July 17 21:46:21 »


Wow...that is interesting. I have not seen that before.
Quick question... when you took the flat- were you focused at your instrument position (focused for stars in other words)?
I am just wondering if you are far enough out of focus, the pupil image of a bright flat would produce shadows of the secondary... I know
you can see the secondary vanes in out-of-focus images of stars.

I look forward to the answer. Usually distinct shadows like this would be caused by something close to the chip. I assume there aren't any fingers holding your filters. :)

-adam

Thanks Adam, yes my focus was exactly as it was after I took the images of IC5146, never had this before when I had my Atik 8300 (Same CCD)

Chris, same light source I always use for my flats, an LCD flat panel, but that woudln't explain the same pattern on my images of IC 5146

In that case I am less convinced it has to do with your vanes. There are some really quick and easy things to test if you are near the instrument.

1. Does it occur exactly the same in all filters?
2. How about no filter (blank spot in your wheel if you have an open)
3. If you put another rod (temporarily) in front of your telescope... does it show up in the flat? (I am going to predict... no)

How far are you from your flat panel?
A shadow that sharp really needs to be something close to your camera. You should definitely look up your baffle tube (or optical axis) and see if flocking is all attached... no screws sticking out.
Hmm... it is odd.

-adam

Offline sharkmelley

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Re: Strange Artifacts in Lum only
« Reply #5 on: 2018 July 17 23:40:59 »
Are you using a focal reducer/flattener?  It is possible that your particular combination of reducer/flattener, filter and sensor together with their various spacings is managing to create an almost focused image of the central obstruction + vanes, probably via internal reflections off the optical surfaces.

It is easy to test this by adding a fifth artificial vane e.g. a pen.  Does this fifth "vane" then appear in your flats and/or lights?  Adam has already suggested this but unlike Adam, I'm going to predict "yes".

If you then remove the filter, does the problem go away?  If so then you will probably find that moving the filter wheel closer to the sensor or further from the sensor will throw this artefact out of focus, when using filters.

Mark
« Last Edit: 2018 July 18 00:06:57 by sharkmelley »
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Offline Jkulin

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Re: Strange Artifacts in Lum only
« Reply #6 on: 2018 July 18 01:53:03 »
Thanks Mark, no not using any focal reducer/flattener, as I mentioned previously the focus is still at the position for when I was capturing these images.

I'm going to take some additional images today without moving the focus and some further flats and see whether it still prevails.

Offline sharkmelley

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Re: Strange Artifacts in Lum only
« Reply #7 on: 2018 July 18 02:34:23 »
Quote from: Jkulin
Thanks Mark, no not using any focal reducer/flattener, as I mentioned previously the focus is still at the position for when I was capturing these images.

I'm going to take some additional images today without moving the focus and some further flats and see whether it still prevails.

It's still worth adding a fifth fake vane (put it quite close to one of the other 4 vanes) to determine if the artefact is actually an image of the central obstruction + vanes.  If the 5th vane does appear as an artefact in your flat then try it again without the filter in order to determine if reflections off the filter are involved in the formation of this artefact.

Mark
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Offline Jkulin

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Re: Strange Artifacts in Lum only
« Reply #8 on: 2018 July 18 03:34:42 »
Thanks Mark, will do.

I'm not going to adjust the focus as it is currently adjusted for imaging and will portray approx. distances, so will just take a daylight image for and with and with out a filter and with and without a 5th vane.

Will let you know.

Offline gvanhau

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Re: Strange Artifacts in Lum only
« Reply #9 on: 2018 July 18 19:52:19 »
Hello

If the sensor is a KAF8300, and the exposure time is short, may be it is an artifact produced by the camera shutter.
In my atik383 (same sensor) I had to expose at least 10 secconds in order to avoid shutter artifacts.

Regards
Geert
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Offline pfile

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Re: Strange Artifacts in Lum only
« Reply #10 on: 2018 July 18 22:14:45 »
that is a very good point - since this is the L filter it will require the shortest exposure given a constant panel brightness. the FLI cameras i have worked with have an 'iris' shutter which leaves a flower-shaped pattern on flats if the flats are too short. if the moravian camera has a similar type of shutter than that could be it. on my SBIG they use a disc with 3 trapezoidal holes in it as the shutter. the disc moves 1/3 of a rotation on each shutter actuation. somehow even with very short image this type of shutter exhibits fewer artifacts.

rob

Offline Jkulin

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Re: Strange Artefacts in Lum only
« Reply #11 on: 2018 July 19 01:15:49 »
Thanks Lads for your thoughts.

Flats apart, the Cocoon image was done at 10 mins subs, so no risk of the shutter curtain, yet that exhibits the same marks as the flats which were done at 1.61s.

I did a couple of images with the Lum filter and with no filter last night in my darkened dining room, pointed towards a plain wall and no sign of the artefacts, as no stars outside to try outside.

I'll try with some flats as with the light source I have I can use 1.61s at 24955ADU

It just seems strange that PI didn't remove the artefacts when combining

Offline Barry-Wilson

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Re: Strange Artifacts in Lum only
« Reply #12 on: 2018 July 19 10:32:54 »


I'll try with some flats as with the light source I have I can use 1.61s at 24955ADU

With my QSI683 and with the Atik 383L I had previously both manufacturers suggest the minimum exposure time to avoid any effect from the shutters is 3 seconds.

If you need to dim the light source you can place pieces of white paper in front of it to achieve the desired ADU and exposure length.

HTH
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Offline pfile

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Re: Strange Artifacts in Lum only
« Reply #13 on: 2018 July 19 14:08:19 »
i guess i assumed that of the 2 images you posted, that the first was calibrated with the flat and the second was the flat, meaning that the artifact was in the flat. however, you are right, it would have brightened the calibrated light since the artifact is dark in the flat. if this is some kind of reflection then it's possible for it to be of one brightness in the flat and of a different brightness in the light and it won't calibrate away, at least completely.

rob

Offline Jkulin

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Re: Strange Artifacts in Lum only
« Reply #14 on: 2018 July 20 12:10:39 »
Well I am coming to the conclusion that the camera is faulty as I have taken images at 0.2s for both, then added additional gels to get to 25000ADU which meant the Lum was 110.22s and the empty slot at1.47s, with all all of them I introduced an additional vane, which doesn't show at all, so I am wondering if there is actually something displayed on the CCD chip