puzzling artefact in Ha

[sonnymoon]

Just wondering if anyone knows what's going on here with Alnitak? I'm not sure if its the scope or the sensor, or an alien megastructure....? Taken in Ha in a white zone, 24x180secs, gain 150, stacked in dss with darks and flats. I've pushed the brightness so you can more clearly see what has happened to the star...

ED80, Zwo 1600mm pro, Avalon Uno

Thanks very much, any thoughts gratefully received.

[carastro]

Yes seen this recently in a topic on my local forum and Shark Melly (Mark Shelley) has done a study on it, it's something to do with Microlensing.  I have no idea of the technical stuff but I'll find his link his report on it on another forum.  

Hope this is in order to post on SGL

https://www.cloudynights.com/topic/635937-microlens-diffraction-effect-explained/?p=8967693 

Carole

Edited December 16, 2018 by carastro

[Thalestris24]

I think it's microlens diffraction - a known problem with the 1600

https://www.cloudynights.com/topic/556795-reflection-around-big-stars/

Louise

[carastro]

Snap Louise 

[vlaiv]

As above it is interference between filter or focal reducer / field flattener and micro lenses on ASI1600.

Not everyone gets those, and it usually happens on bright stars. If you can, try changing distance of your Ha filter with respect to sensor - either a bit closer or a bit further away. You might also try removing field flattener or changing a bit it's distance (I know it will cause trouble with flat field, but just for testing purposes). This effect is probably dependent on spacing as well as angle of light cone coming in on sensor. Maybe some combination of spacing and light cone angle will lessen or almost remove effect.

Edited December 16, 2018 by vlaiv
typo ...

[fireballxl5]

1 hour ago, sonnymoon said:

an alien megastructure

this

[michael8554]

Stargate

Michael 

[sonnymoon]

thanks for the replies; hadn’t heard of that before. I’ll do a bit of fiddling and see if it goes away. Haven’t read the cloudy nights post yet, but wonder if it’s more prone in relation to certain filters than others. 

All the best

[sharkmelley]

Don't bother changing spacing of filter etc. because it won't make any difference.  It's a diffraction pattern generated by the microlenses and a reflection off the sensor coverslip so the position of your filter and field flattener won't make any difference.

The pattern is most obvious with narrowband filters, especially H-alpha.

Mark

[vlaiv]

24 minutes ago, sharkmelley said:

Don't bother changing spacing of filter etc. because it won't make any difference.  It's a diffraction pattern generated by the microlenses and a reflection off the sensor coverslip so the position of your filter and field flattener won't make any difference.

The pattern is most obvious with narrowband filters, especially H-alpha.

Mark

If filters play no part in it, then similar artifact would be visible in red and lum channels as well (those pass Ha wavelength as well), but would not appear with different wavelengths of light - sensor to cover distance is fixed and interference happens when there is matching between wavelength and distance of reflective surfaces.

Here is H-alpha shot of mine (ASI1600 + Baader H-alpha 7nm, pure mirror system):

Effect is barely noticeable on mag 7 star

Same star with OIII filter:

Much larger effect

And I have not seen this effect in any of my broad band images - even with very strong stars.

[sharkmelley]

The key word here is diffraction.  The exact geometry of the pattern is crucially dependent on the wavelength of the light because it is caused by constructive interference.  The (almost) monochromatic light passing through a narrowband filter produces a very clear pattern.  This pattern becomes smaller as the wavelength shortens.  A broad spectrum filter ( e.g. R,G or B ) will produce the continuous superposition of the different sized patterns produced by each wavelength that passes through the filter - in other words it will be an indistinct smeared artefact.  So the only effect you will see in a broad spectrum image are weird variations in colour in a halo around the star.  There's quite a good example here: https://www.cloudynights.com/topic/565014-new-to-narrowband-imaging-question-about-strange-pattern-in-bright-star-halo/?p=7706969

Mark

[vlaiv]

3 minutes ago, sharkmelley said:

The key word here is diffraction.  The exact geometry of the pattern is crucially dependent on the wavelength of the light because it is caused by constructive interference.  The (almost) monochromatic light passing through a narrowband filter produces a very clear pattern.  This pattern becomes smaller as the wavelength shortens.  A broad spectrum filter ( e.g. R,G or B ) will produce the continuous superposition of the different sized patterns produced by each wavelength that passes through the filter - in other words it will be an indistinct smeared artefact.  So the only effect you will see in a broad spectrum image are weird variations in colour in a halo around the star.  There's quite a good example here: https://www.cloudynights.com/topic/565014-new-to-narrowband-imaging-question-about-strange-pattern-in-bright-star-halo/?p=7706969

Mark

I agree with you, but question is where constructive interference comes from? It needs two parallel surfaces to form - and is usually in form of reflected wave that creates out of focus star image on sensor.

It is definitively due to micro lens on sensor because out of focus stars create square / cross shaped pattern.

So it can be between micro lens and sensor cover window, or it can be between micro lens and filter.

Sensor cover window is AR coated, while filters are interference blocking filters - one is coated to pass as much light as possible - other is created to pass narrow band of light and reflect everything else - key emphasis on word reflect

Simple experiment could show if this is related to cover window or filter - just move filter further away - if pattern grows in size and gets dimmer and possibly change intensity - it is related to filter. Another clue that it might be related to filter is to observe OP image and example that I posted.  My filter is mounted very close to sensor, and my guess is that OP has regular filter wheel and filters a bit further away from sensor - based on size of reflections and distance between first and second order reflections - mine smaller and denser while in OP image larger and further apart (indicating greater distance between reflective surfaces).

 

[sharkmelley]

Unfortunately the sensor coverslip on the ASI1600 is not AR coated.  This is why the ASI1600 is notorious for creating these patterns.

I'll briefly explain the mechanism of the interference pattern. Wavefronts hitting the sensor are being scattered in all directions by the 2D array of microlenses.  At certain angles (dependent on pixel spacing and wavelength) these scattered waves will interfere constructively.  This is reflected back onto the sensor forming a regular grid of points where constructive interference is happening.  This is the same as Laue Diffraction from a  2D lattice that occurs in X-ray crystallography (but scaled up enormously from an atomic lattice).  At each point on this regular grid we see the image of a defocused star.  Typically this forms a pattern of overlapping defocused stars. The diameter of each defocused star indicates the extra distance that the light has travelled and shows beyond doubt that the reflection is off the coverslip and not off any filter.

If you want to understand more then take a read of the Cloudy Nights thread that Carole linked earlier (ignoring the post about the Talbot effect which is another very fascinating diffraction effect but is not what we are seeing here).  There you can also find  some PixInsight code I wrote that allows you to reproduce the geometry.

Mark

Edited December 17, 2018 by sharkmelley

[MartinB]

That's very interesting and helpful, thanks Mark.

Not worth any angst though, Alnitak creates an extreme situation and a quick Photoshop 100% radial blur on Alnitak will have it fixed in a jiffy!

[vlaiv]

7 minutes ago, sharkmelley said:

Unfortunately the sensor coverslip on the ASI1600 is not AR coated.  This is why the ASI1600 is notorious for creating these patterns.

I'll briefly explain the mechanism of the interference pattern. Wavefronts hitting the sensor are being scattered in all directions by the 2D array of microlenses.  At certain angles (dependent on pixel spacing and wavelength) these scattered waves will interfere constructively.  This is reflected back onto the sensor forming a regular grid of points where constructive interference is happening.  This is the same as Laue Diffraction from a  2D lattice that occurs in X-ray crystallography.  At each point on this regular grid we see the image of a defocused star.  Typically this forms a pattern of overlapping defocused stars. The diameter of each defocused star indicates the extra distance that the light has travelled and shows beyond doubt that the reflection is off the coverslip and not off any filter.

If you want to understand more then take a read of the Cloudy Nights thread that Carole linked earlier (ignoring the post about the Talbot effect which is another very fascinating diffraction effect but is not what we are seeing here).  There you can also find  some PixInsight code I wrote that allows you to reproduce the geometry.

Mark

Thanks for explanation, and couple more questions if I may - we are talking about sensor protective cover and not chamber front window that is not being AR coated, right?

By saying that it has been shown beyond doubt that reflection is off this surface, I presume distance from micro lens to this cover has been measured and also its thickness and compared to defocused star at certain F/ratio beam (defocus position being twice distance to first or last surface of this cover)?

Also, by saying it is not filter induced, I can expect in pure mirror system and no filters when shooting bright star to have large halo around it with ASI1600 - even more so than in narrow band image (if we look intensity in small part of spectrum being roughly 1/50 - 1/100 of 400-700 continuum depending if its 7nm or 3nm filter, I can expect it to be about 100 times brighter than with narrow band)?

[sharkmelley]

We assume it's the protective cover on the sensor itself because the reflection is from a surface less than 1mm above the microlenses.

Yes we would expect the "halo" caused by this diffraction effect to be brighter with no narrowband filters.  In very rough terms the star would be 50-100x brighter and its halo would also be 50-100x brighter.

Mark

[vlaiv]

9 minutes ago, sharkmelley said:

We assume it's the protective cover on the sensor itself because the reflection is from a surface less than 1mm above the microlenses.

Yes we would expect the "halo" caused by this diffraction effect to be brighter with no narrowband filters.  In very rough terms the star would be 50-100x brighter and its halo would also be 50-100x brighter.

Mark

I appreciate your explanation, and certainly think it is plausible. I'm still not 100% convinced however (can't be helped )

What do you make of this image then?

Ha and OIII clearly show patterns and filter presence (large defocused aperture image superimposed over star). Luminance also shows this, and what I believe is mild reflection from chamber window (enough light to make it detectable since it is AR coated).

No filter has no filter signature, only halo that I would say is from chamber window (and some diffraction spikes that are probably from mirror clips and focuser or what ever - since this is newtonian scope). Not sure that there is halo from this effect though - unless I've mistaken halo around star to be from chamber window and it is in fact from this source - but I would expect it to be progressively brighter towards the star?

[sharkmelley]

You have multiple things going on there!  I think we're heading a bit off-topic but I'll play along.

First of all, all the filters show a very large doughnut with spider vanes.  The doughnut is the same size in each case and disappears when there is no filter.  That indicates reflections off the filters.  The extra length of the light path can be confirmed by using the Dust Donut Calculator (http://www.ccdware.com/resources/dust.cfm). and this will help confirm which other surface is implicated - either the chamber window or the sensor stack.

The centres of those big halos have different offsets to the star causing them.  Either the star is in a different position in each image or the filters are mounted with slightly different tilts to the optical axis.

The H-alpha filter also has a series of increasing rings around the star.  This is usually caused by internal reflections within the filter glass because the AR coatings (if any) aren't too effective at the wavelength of H-alpha.  I'm guessing it's not an Astrodon filter otherwise you need to send it back and complain.  Again the dust donut calculator will give you the additional optical path travelled within the filter glass (at least its equivalent in air).  The refractive index of the glass is then required to calculate the actual filter thickness.

As for the Laue Diffraction off the microlenses, I can't be 100% certain I'm seeing it in the H-alpha image but it is certainly there in the OIII image.  The distance of the sensor protective cover can be implied from the geometry of this diffraction pattern.

Mark

Edited December 18, 2018 by sharkmelley

[vlaiv]

3 minutes ago, sharkmelley said:

You have multiple things going on there!  I think we're heading a bit off-topic but I'll play along.

First of all, all the filters show a very large doughnut with spider vanes.  The doughnut is the same size in each case and disappears when there is no filter.  That indicates reflections off the filters.  The extra length of the light path can be confirmed by using the Dust Donut Calculator (http://www.ccdware.com/resources/dust.cfm). and this will help confirm which other surface is implicated - either the chamber window or the sensor stack.

The centres of those big halos have different offsets to the star causing them.  Either the star is in a different position in each image or the filters are mounted with slightly different tilts to the optical axis.

The H-alpha filter also has a series of increasing rings around the star.  This is usually caused by internal reflections within the filter glass because the AR coatings (if any) aren't too effective at the wavelength of H-alpha.  I'm guessing it's not an Astrodon filter otherwise you need to send it back and complain.

As for the Laue Diffraction off the microlenses, I can't be 100% certain I'm seeing it in the H-alpha image but it is certainly there in the OIII image.

Mark

Hopefully we are not going off-topic, I just found that image online as example of people complaining at mentioned artifact (I believe it was posted on zwo forum, but can't be sure - forgot where I copied it from).

We agree on most points - how certain halos were created, but what I wanted to point out is that "no filter" does not generate much larger (like we agreed roughly x50-100) effect - if anything it seems that there are no artifacts at all related to micro lens reflection.

This leads me to conclude that your description does not account 100% for what is going on - clearly presence of filter and it's reflective surfaces in some way contributes to interference pattern. It is also very indicative that different people get different results based on type of filters they are using. Some even have this effect on R, G and B interference filters.

Don't get me the wrong way - not trying to say that your analysis is flawed and that you are wrong - I'm just questioning if it's the complete picture of the issue. From above, it seems to me that it might be the case that type of filter, it's position in optical train and F/ratio of system all have "a say" in how pronounced effect is, or if it's there at all.

[sharkmelley]

5 hours ago, vlaiv said:

This leads me to conclude that your description does not account 100% for what is going on - clearly presence of filter and it's reflective surfaces in some way contributes to interference pattern.

That is incorrect.  The reflective surfaces of the filter contribute nothing to the microlens diffraction pattern.  You can convince yourself of this fact by going away and performing a few experiments. Unfortunately I don't think there is anything more I can add that will help you.

Mark

 

[sharkmelley]

Yesterday I was looking for another example that I've come across before.  I have now found it.  It's interesting example because this one is another microlens diffraction but it reflects back off the narrowband filter instead of the sensor protective coverslip. 

http://geoastro.co.uk/equipment/ghosts.htm

The pattern is huge and the overlapping defocused star images are also huge.

Mark

[ollypenrice]

On 17/12/2018 at 23:40, MartinB said:

That's very interesting and helpful, thanks Mark.

Not worth any angst though, Alnitak creates an extreme situation and a quick Photoshop 100% radial blur on Alnitak will have it fixed in a jiffy!

This is the processing solution. I have the process saved as an action - indeed I have three versions of it for different sized stars. Use the magic wand to select the star and then

Start recording.

Select, modify expand (by x.)

Feather (by y)

Filter, blur, radial blur, spin, best quality. (Put this command in twice.)

Deselect

Stop recording.

The values for x and Y you have to find by experiment and the larger the star the larger the value, which is why I have three saved. If you save the action to a function key all you have to do is click on the star with the magic wand and then hit the key to run the action. It is one star at once but not many will be affected so the whole process is quick. You could experiment with the best stage in the stretch to run the action.

Olly

[Whirlwind]

On ‎18‎/‎12‎/‎2018 at 00:54, vlaiv said:

This leads me to conclude that your description does not account 100% for what is going on - clearly presence of filter and it's reflective surfaces in some way contributes to interference pattern. It is also very indicative that different people get different results based on type of filters they are using. Some even have this effect on R, G and B interference filters.

Don't get me the wrong way - not trying to say that your analysis is flawed and that you are wrong - I'm just questioning if it's the complete picture of the issue. From above, it seems to me that it might be the case that type of filter, it's position in optical train and F/ratio of system all have "a say" in how pronounced effect is, or if it's there at all.

I think you may be correct in that the filter has an effect but perhaps not in the way you are thinking.

I would suggest the following reason for an enhanced effect in the narrowband filters.  It appears to be generally accepted that the diffraction pattern is off the microlens.  However it is a lens that likely diffract different wavelengths by differing amounts.  In a broadband image you have many different wavelengths that are all being diffracted by a slightly different amount.  As each wavelength is diffracted slightly differently than they all overlay at the sensor in a slightly different way.  As the pixels are (at a broad assumption) broadly equivalently detected then these patterns merge and form one continuous 'blob' (highly technical term).  Hence you likely get a broadened star but with no pattern.  

As you place narrowband filters in the imaging train you now have one specific wavelength which has one specific diffraction pattern.  As there is no other wavelengths affecting the CCD you get to directly see that diffraction pattern.  

In effect the broadband image is like taking thousands of individual narrowband images and then slightly altering the pattern so that when they overlay they have merged.  

As such you can clearly see the effect in narrowband but not in the broadband images, but it is still there.

This should be testable by using consecutively broader filters.  If you started with a 3nm Ha filter and then tested the same exposure using a 5nm, 6nm, 9nm, 12nm Ha and so forth then as you broadened the wavelength range you should see that the effect 'fades away'.  It might also explain why some people don't see the effect.  They are using wider narrowband filters compared to someone that easily sees it in a Chroma/Astrodon 3nm. 

[vlaiv]

41 minutes ago, Whirlwind said:

I think you may be correct in that the filter has an effect but perhaps not in the way you are thinking.

I would suggest the following reason for an enhanced effect in the narrowband filters.  It appears to be generally accepted that the diffraction pattern is off the microlens.  However it is a lens that likely diffract different wavelengths by differing amounts.  In a broadband image you have many different wavelengths that are all being diffracted by a slightly different amount.  As each wavelength is diffracted slightly differently than they all overlay at the sensor in a slightly different way.  As the pixels are (at a broad assumption) broadly equivalently detected then these patterns merge and form one continuous 'blob' (highly technical term).  Hence you likely get a broadened star but with no pattern.  

As you place narrowband filters in the imaging train you now have one specific wavelength which has one specific diffraction pattern.  As there is no other wavelengths affecting the CCD you get to directly see that diffraction pattern.  

In effect the broadband image is like taking thousands of individual narrowband images and then slightly altering the pattern so that when they overlay they have merged.  

As such you can clearly see the effect in narrowband but not in the broadband images, but it is still there.

This should be testable by using consecutively broader filters.  If you started with a 3nm Ha filter and then tested the same exposure using a 5nm, 6nm, 9nm, 12nm Ha and so forth then as you broadened the wavelength range you should see that the effect 'fades away'.  It might also explain why some people don't see the effect.  They are using wider narrowband filters compared to someone that easily sees it in a Chroma/Astrodon 3nm. 

One could try progressively wider band pass filters, but even if we don't see pattern without filter, we would still be able to detect it - by light level around bright stars when no filter is present. One can measure background "scatter" levels further away from the star and right in vicinity of the star - they should be different in case of this effect layering on top of itself. I believe it should also be visible "by naked eye" as gradient that starts of at star then fades away further from it. This is what I pointed out as missing in no filter image above.

I do have a "model" of how interference filters and ASI1600 can produce such artifacts due to micro lenses that is dependent on both wavelength and presence of interference filter (and depend on it).

Mark (sharkmelley) stated that it is definitively due to sensor cover window as reflections produced come from source that is less then 1mm away.

I did some quick (but also maybe inaccurate due to this) measurement of reflection diameter (first order) on one of my OIII images. According to this reflection distance calculation:

http://www.wilmslowastro.com/software/formulae.htm#REFLECT

Source of reflection is about 2.6mm in my case. Only thing that I can think of producing such reflection would be camera chamber window that is AR coated and should not produce pronounced reflection because of this on its own (unless of course AR coating is not adequate).

What can happen, at least I think it can, is following: Interference filter placed close to camera chamber window (not sensor cover one) and parallel to it, can maybe create Fabry Perot filter configuration thus turning AR coated window in very effective blocking / reflection window - and this together with micro lenses then proceed to produce artifact. Light would pass forward thru filter + AR coated window because of angle and then get reflected of micro lens in all directions thus changing angle. This reflected light then encounters AR window + interference filter combo that acts as reflection filter and gets reflected back to sensor producing artifact. This is of course simplified explanation - in reality it would be multiple interference of light with itself with one component of the wave being reflected of interference filter.

[Merlin66]

I'm revisiting this topic.....

I found that most (All?) CMOS sensors are fitted with microlenses and the immediate  chip cover plate can vary in thickness from 1 mm to 5 mm (!!!!)

If this was a real problem then surely it would be showing up on more and more images taken with CMOS cameras?

It seems strange the the prime example here and CN is of one star - Anitak

I've never experienced this type of issue.... we do suffer more from "ripples" in our spectrographic profiles - an intensity waviness - thought to be due to either/ both the internal structure of the silicon chip or interference between the cover plate and the silicon surface.

See: 

https://www.lensrentals.com/blog/2014/06/the-glass-in-the-path-sensor-stacks-and-adapted-lenses/

http://www.astrosurf.com/buil/CMOSvsCCD/index.html

https://britastro.org/sites/default/files/attachments/SpectralResponse_WhitePaper_April09.pdf ( thanks to Robin for the link)

 

Is there a real problem here?????

Interesting comments here about the KAF 3200ME with microlenses: 

http://www.wvi.com/~rberry/astronomy/qsitesting/kaf3200me.htm

Quote:

Micro-Lens Artifacts

Micro lenses are tiny cylinder lenses designed to concentrate light on the indium tin oxide half of each pixel in the KAF 3200ME. Although I can imagine scattering from one micro lens to adjacent micro lenses could account for some artifacts, they may also be the astronomical equivalent of the “urban myth.”
Because I aligned my camera precisely north-south and east-west, I was not able to attribute possible low-intensity extensions along rows and columns in my images with any certainty to spider diffraction or blooming. If the effect is real, it is very small.

 

Edited December 5, 2019 by Merlin66
added info