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david_taurus83

Oiii flats uneven?

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Whats going on with my Oiii flats? Has anyone else seen a similar effect? My Ha flats look like you'd expect. Darker in the corners. But the Oiii appear to show some evidence of the ring around the brighter part and where you'd expect it to get darker into the corners it gets brighter?!

 

Oiii and Ha below for comparison. I use an Artesky flats panel and both were exposures of 3-4 seconds, calibrated with flat darks.

Oiii master flat.jpg

ha master flat.jpg

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That's odd.  I presume the flats were taken in the linear range of the camera?

As an aside it might be worth cleaning your sensor window - you obviously have some dust bunnies which the flats should clean up but that spot top left is pretty dark...

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Did you by any chance mix something up here? Like calibrated OIII flats with Ha flats by mistake?

I'm asking because filter dust shadows look suspiciously same in distribution. Note small dark one with smallest diameter in top left part - that would be dust on sensor cover window. That one should stay the same between filters as nothing about sensor changed. Then there are medium size shadows, that I suspect are dust on filter. These should have different distribution between filters. To me it looks like they match almost perfectly - you can check this by flipping between images to see if they move or stay in the same place.

One more thing is pointing to mixing up between flats - Upper left larger bright circle in OIII matches in shape and position to dark one on Ha.

 

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Nope. Not a mix up. Below is an Oiii master flat from a few weeks ago. Then I was using the 0.8 reducer. Above, from Thursday night I was trying out the Hotech SCA 1.0 flattener.

 

It was this one from a few weeks ago that left my head scratching.

Oiii master flat 2.jpg

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And back to the recent one, here is a double stretch of a single Oiii flat. You can clearly see how uneven it is to the corners.

Oiii_2019-02-15_00-43-22_Bin1x1_3.40625s__-25C_2019-02-15_00-43-22_Flat_c.jpg

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The important thing is whether or not the flats correct your subs properly. If they do, then they are correct :)

I have strange looking OIII flats too, but they seem to work OK :)

 

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On 16/02/2019 at 20:46, vlaiv said:

Did you by any chance mix something up here? Like calibrated OIII flats with Ha flats by mistake?

I'm asking because filter dust shadows look suspiciously same in distribution. Note small dark one with smallest diameter in top left part - that would be dust on sensor cover window. That one should stay the same between filters as nothing about sensor changed. Then there are medium size shadows, that I suspect are dust on filter. These should have different distribution between filters. To me it looks like they match almost perfectly - you can check this by flipping between images to see if they move or stay in the same place.

One more thing is pointing to mixing up between flats - Upper left larger bright circle in OIII matches in shape and position to dark one on Ha.

 

My flats show little or no difference between filters which is why I often use only luminance flats for all filters. Very occasionally I see a filter-specific dust shadow but it is not more than once per year and often less than that.

As Paul says, try the OIII flats. Then try calibrating the set using no flats and then try the Ha flats on the OIII data. See what this tells you.

Olly

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Both your Ha and OIII flats show a 'weave' pattern, are you using a fabric diffuser over the OTA by any chance? if so there is no need for a fabric diffuser with a LED flats panel such as the Artesky.

A 'weave' pattern may also be caused by the brightness control of the panel when set to low levels if it uses PWM to control the brightness (strobing effect) though this usually results in light/dark bands or lines that are horizontal. vertical or diagonal (at any angle) and the lines/bands are normally straight whilst your images show lines that are curved. 

Looks very like a fabric pattern in these images.

The Artsky, being directed specifically at the Astronomy market would, one hopes, use a linear brightness regulator and not a PWM controller however I can find no specific information about this panel on the web.

If suspect, use the panel on full brightness and attenuate the output with a photographic neutral density gel filter (£7.00 for a 1.2m x 0.53m sheet) :

https://www.sblite.co.uk/neutral-density/265-409-rosco-299-12-neutral-density.html#/23-sheet_roll-sheet

The bright corners 'might' be light leaking around the edge of the filter, if it is an unmounted filter check for complete edge blackening of the filter substrate, correct orientation (which side is supposed to face the OTA) and seating of the filter in the FW.

Lastly, off-axis light distribution inside the OTA varies with wavelength and the quality of the internal baffling and flocking. Plain black anodised surfaces reflect differently according to wavelength leading to a different distribution of the flat field within the OTA, check that all internal OTA metallic surfaces are correctly blackened with non-reflective mineral based black paint or flocking material.

See the linked document for further information:

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

HTH

William.

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6 hours ago, Oddsocks said:

Both your Ha and OIII flats show a 'weave' pattern, are you using a fabric diffuser over the OTA by any chance? if so there is no need for a fabric diffuser with a LED flats panel such as the Artesky.

A 'weave' pattern may also be caused by the brightness control of the panel when set to low levels if it uses PWM to control the brightness (strobing effect) though this usually results in light/dark bands or lines that are horizontal. vertical or diagonal (at any angle) and the lines/bands are normally straight whilst your images show lines that are curved. 

Looks very like a fabric pattern in these images.

The Artsky, being directed specifically at the Astronomy market would, one hopes, use a linear brightness regulator and not a PWM controller however I can find no specific information about this panel on the web.

If suspect, use the panel on full brightness and attenuate the output with a photographic neutral density gel filter (£7.00 for a 1.2m x 0.53m sheet) :

https://www.sblite.co.uk/neutral-density/265-409-rosco-299-12-neutral-density.html#/23-sheet_roll-sheet

The bright corners 'might' be light leaking around the edge of the filter, if it is an unmounted filter check for complete edge blackening of the filter substrate, correct orientation (which side is supposed to face the OTA) and seating of the filter in the FW.

Lastly, off-axis light distribution inside the OTA varies with wavelength and the quality of the internal baffling and flocking. Plain black anodised surfaces reflect differently according to wavelength leading to a different distribution of the flat field within the OTA, check that all internal OTA metallic surfaces are correctly blackened with non-reflective mineral based black paint or flocking material.

See the linked document for further information:

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

HTH

William.

No material used to diffuse the light. Both flats were around the 3 to 4 second mark so any flicker shouldn't show up. Artesky though, advertise a flicker free panel. It has a meter on the back and can select between 24v to 26v. Both flats calibrated with matching flat darks.

The Baader filters have an arrow pencil marked on the edge of the filters to show which way to face them. Mine definitely face the right way but I think I'm going to try and flip the Oiii the other way as I get overwhelming halos with it. Perhaps a duff filter? Baader had issues with these before? I've read about people using permanent markers to blacken the edges of filters but I'm reluctant to do that just yet in case I need to send it back for a replacement.

I have sone LRGB data yet to process so will check out those flats when I get round to it.

 

Thanks

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My flats look exactly the same. OIII is the worst or has the most visible stripes across it. I have an Astrodon 5nm OIII filter. It shows up some on SII as well, and not really visible on HA. It doesn't matter whether I shoot flats with a t-shirt, use a light box, an iPad, or the sky, they all have the same pattern. I suspect it's the coating on the filters. 

But in any case, it fully corrects out using any of the above methods. So I regularly use a flat panel.

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8 hours ago, Lead_weight said:

My flats look exactly the same. OIII is the worst or has the most visible stripes across it. I have an Astrodon 5nm OIII filter. It shows up some on SII as well, and not really visible on HA. It doesn't matter whether I shoot flats with a t-shirt, use a light box, an iPad, or the sky, they all have the same pattern. I suspect it's the coating on the filters. 

 

Thank you for this reply Andrew, it is quite timely as I had been discussing these stripes yesterday with a retired work colleague who was an optical engineer.

His opinion was that they are polishing marks, either in the surface of the filters or the sensor itself, tiny variations in surface smoothness of the substrate, like small hills and valleys, an artefact created by the polishing machines used in the silicone wafer construction from which the the sensor was cut or final polishing of the filter substrate or camera window, and are made visible by illuminating the sensor with virtually monochromatic light. They should become more pronounced as the bandwidth of the illuminating light is reduced, in other words more visible in a 3nm OIII than a 12nm OIII for example.

His test to determine if the polish marks are filter artefacts or sensor artefacts are simply to rotate the filter (round or square) by 90 deg in the filter wheel, if the direction of the stripes changes then the stripes are filter polishing artefacts, if they remain the same they are either sensor or camera window polishing artefacts. You would have to remove the camera window to determine if that was the source of polishing marks but hardly worth the effort if they are corrected by flats, more of an academic pursuit really.

Apparently they are quite common in sensors as the silicone wafer substrate is thinned by polishing but only become visible when illuminating the sensor with monochromatic light, either from a monochromatic source or via a narrow band filter, and will calibrate out with a flat frame taken using the same filter. They should be absent from broadband filtered images such as LRGB.

The mistake I made was ignoring that in @david_taurus83's Ha and OIII images there are two sets of curving stripes at roughly 90 deg to each other and true fixed frequency beat pattern interference stripes caused by PWM controllers in flat panels or electrical noise caused by ground loops in cameras would always be straight lines, in one direction and never in the same place in two random images (unless the camera shutter was in some way linked to the frequency of the intruding interference source).

Always something new for me to learn in this discipline ?‍?

William.

Edited by Oddsocks
Missing text.
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Hi William. I had a look at some LRGB flats the other day and I couldn't see the same pattern. This weekend I think I might open up the FW, flip the Oiii around to see if halos improve and get the blower and vacuum cleaner on the dust!

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Ahh, good to know that this probably is. It did look like something a buffing motion might create. I don’t see them in the LRGB filters, so assumed it was something related to the coating on the narrowband filters. 

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