Strange Artifacts in Lum only

[Jkulin]

Now rotated the camera 45degrees, so it would appear that the camera is picking up the vanes

[ngc1535]

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

[pfile]

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

[Jkulin]

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.

[WillB]

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.

[ngc1535]

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

[Jkulin]

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.

[sharkmelley]

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

[pfile]

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

[Jkulin]

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.

[pfile]

i'm sure something similar is available somewhere here in the US...

rob

[Jkulin]

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.

[WillB]

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.