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PixInsight => General => Topic started by: Jkulin on 2018 July 16 15:44:26

Title: Strange Artifacts in Lum only
Post by: Jkulin 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
Title: Re: Strange Artifacts in Lum only
Post by: ngc1535 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

Title: Re: Strange Artifacts in Lum only
Post by: chris.bailey on 2018 July 17 14:51:37
What was the light source for your flats?
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin 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
Title: Re: Strange Artifacts in Lum only
Post by: ngc1535 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
Title: Re: Strange Artifacts in Lum only
Post by: sharkmelley 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
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin 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.
Title: Re: Strange Artifacts in Lum only
Post by: sharkmelley 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
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin 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.
Title: Re: Strange Artifacts in Lum only
Post by: gvanhau 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
Title: Re: Strange Artifacts in Lum only
Post by: pfile 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
Title: Re: Strange Artefacts in Lum only
Post by: Jkulin 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
Title: Re: Strange Artifacts in Lum only
Post by: Barry-Wilson 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
Title: Re: Strange Artifacts in Lum only
Post by: pfile 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
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin 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
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin on 2018 July 20 13:05:41
Now rotated the camera 45degrees, so it would appear that the camera is picking up the vanes
Title: Re: Strange Artifacts in Lum only
Post by: ngc1535 on 2018 July 20 13:54:47
OK.
So in your first message from today- you indicate that a rod (or something) put in front of your telescope DID NOT produce an artifact?
This is what I predicted as I recall. The shadows are too distinct to be the vanes.  They must be near the camera. However, they are beyond the
camera itself. Note that when you rotated the camera how the dust donut on the filters remained unchanged (of course). The rotation also eliminates the shutter/iris short exposure idea.
So the issue is something between the filters and the front of the telescope..and it must be something closer to the camera. This just leaves anything you have attached to
the front of the camera, adapters, or something in the baffle tube right in front of the camera. Only something fixed in the optics will rotate in the image when
you turn your camera. With no lenses and no filters (open slot), it would seem reflections are eliminated as well.

Now in your second message you say the camera is "picking up the vanes." I would argue based on your first message there are no vanes visible at any time. What I do find disconcerting
is that the artifact happens? to be aligned to your diffraction spikes...but there could be something else on your telescope along these line symmetry. In addition, your longer exposures do actually show another artifact pair... making 6 bits.

-adam
Title: Re: Strange Artifacts in Lum only
Post by: pfile on 2018 July 20 15:01:36
in the last "empty" image 2 posts above there appear to be 2 horizontal spikes in the image for a total of 6 'vanes'. so the horizontal spikes seem to be present even before rotating the camera.

i wonder what happens if you take the camera off the telescope and take an image of the wall or something, just to see if it's got something to do with the sensor or the shutter.

rob
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin on 2018 July 20 16:35:06
I have now tried it with the camera removed and there are no artefacts showing.

Looking inside the tube there is nothing getting in the way.

I never had this with my Atik 383+ which I sold and this is the first time of using it with my new Moravian G2-8300 MkII, it does not appear with any other filter and does appear when there is an empty slot and with Lum, so am totally lost.
Title: Re: Strange Artifacts in Lum only
Post by: WillB on 2018 July 21 06:49:51
so am totally lost.

See this web document, scroll down half way to see the issue you have.
I had the same problem with an Altair Astro RC8 and a QSI 683.

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

In my case the off axis reflections were primarily from the focuser barrel which was anodised black but remained very shiny, particularly to IR wavelengths, the distance spacers and baffles supplied with the RC8 were just as bad. To cure the problem I used high temperature matt-black barbecue paint to paint the inside of the focuser barrel, the telescope's distance spacers and the primary baffle, this cured the problem entirely.

The reason you don't see it with the other filters is because the anodised surfaces don't reflect all wavelengths equally so if the strongest reflections are not in the pass band of the filter they don't make it to the sensor.

They may have been absent from your previous camera because of the way it was coupled to the telescope, the size of the CCD chamber aperture, the baffling in the camera nose piece,  the IR cut-off point of the camera's AR window etc, etc.

Take the camera off the telescope, point the telescope at a brightly lit white wall or ceiling and look critically for light glinting and reflecting off the inside of the focuser draw tube, telescope distance spacers etc then look inside the light path of the filter wheel and camera nose-piece or T2 ring back to the camera CCD aperture, are there any bright anodised surfaces showing?
In an ideal world there will be no reflections visible just a single bright aperture surrounded by a black donut.

If you do need to paint any surfaces using the high temperature black BBQ paint it comes in confusingly gloss and matt varieties, you need the matt black version, also, don't bother trying to use it sprayed, you can't get even coverage inside small tubes, instead, spray some into an empty container and then apply with a brush.
If you need to paint the inside of the focuser draw tube be careful not to paint below where the camera nose piece reaches otherwise you risk scraping the paint off each time you attach and remove the camera which will end up as dust in the optical path, consider flocking paper as an alternative.

(High temperature BBQ paint is a pigment based paint and absorbs IR strongly unlike paint containing dyes which do not absorb all wavelengths equally)

HTH

William.
Title: Re: Strange Artifacts in Lum only
Post by: ngc1535 on 2018 July 21 21:34:13
Wow! That sure looks pretty conclusive. Look at that image in the middle of the document (Figure "Eight").
I don't understand how that works to have reflections give shadow-like features- but if this is JK's issue...that is
a pretty impressive/helpful document.
-adam
Title: Re: Strange Artefacts in Lum only
Post by: Jkulin on 2018 July 22 15:01:52
Will and others, thanks so much for the pointers.

Indeed my mate came around with his camera and his T adapters and there were no artefacts, but as soon as he used my T adapter he got the artefacts, so as a temp measure I applied some flocking material to the inside and the artefacts disappeared proving conclusively that it was reflections from the T Adapter.

I have now painted the inside with blackboard paint which I use to flock the inside of my guide scopes, as soon as it is dry then will try it again, but I am fairly certain that it has fixed the problem.

What surprises me is that I never got this problem with my Atik 383 and have only experienced it with my Moravian G2-8300 MkII.

The T-Adapters by the way were made by Moonlite, so maybe worth my dropping a note to them to say that they need to reduce the finish inside.

I'm going to try and take some flats and if it will remove the diagonals, if not then will have to redo the Lum again which isn't a big deal.

Thanks once again everyone.
Title: Re: Strange Artifacts in Lum only
Post by: sharkmelley on 2018 July 22 16:21:37
Very interesting!  Thanks for following this through and giving us your results.

The description in the article linked by Will makes complete sense and I can now add this interesting artefact to my list of weird imaging artefacts and their causes.

Mark
Title: Re: Strange Artifacts in Lum only
Post by: pfile on 2018 July 22 17:18:53
i was thinking that i should re-do the flocking on 2 of my OTAs... i am not sure if the material that ive used actually absorbs IR light. theres this insane carbon nanotube black paint that is supposed to be blacker than anything, but it actually sounds a little dangerous if you manage to inhale it. probably the hi-temp BBQ paint is good enough.

rob
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin on 2018 July 22 17:28:48
For Flocking I have always used blackboard paint, if you are in the UK then you can buy from Hobbycraft, it's superb and water soluble until it dries.
Title: Re: Strange Artifacts in Lum only
Post by: pfile on 2018 July 22 20:40:04
i'm sure something similar is available somewhere here in the US...

rob
Title: Re: Strange Artifacts in Lum only
Post by: Jkulin on 2018 July 23 03:30:28
Well here we go, no artefacts.

Two coats of blackboard paint on the inside of the t-adapter.

Now sorted. I may even try to capture some flats when it is dark tonight and try to reprocess IC5146

Thanks for everyones input.
Title: Re: Strange Artifacts in Lum only
Post by: WillB on 2018 July 23 04:34:30
That flat looks much better!  :)

Thanks for the info re' chalkboard paint from Hobbycraft in the UK, I have a branch nearby and will pick up a pot to try as I need to paint inside the prism stalk of my OAG.

I did try water based 'blackboard' paint about twenty years ago but found it would not stick to brass and tended to form globs and run off in the same way that watercolour paint runs off waxed paper, it looks as though the Hobbycraft chalkboard paint must have a different formulation from the stuff I tried. I think the high temperature BBQ paint is solvent based and I know it sticks to anything but it has to be used with care on plastics as it melts the surface.

William.