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Old 26-09-2020, 10:41 AM
sunslayr (David)
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Example of 1600mm microlensing

Thought I might post this for people deciding between a 1600mm and 294mm so they could see how microlensing issue resent in the 1600mm affects a final lrgb image. I haven't really noticed it in narrowband however. This is from an Esprit 100 F5.5 refractor so there should be no diffraction spikes but the bright stars to the lower right are showing diffraction effects.
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Old 26-09-2020, 02:23 PM
glend (Glen)
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David, of course Orion (M42) is notoriously difficult to process correctly, easily fixed in Photoshop using a layer mask. I believe the 294 has significantly more issues than the 1600, such as the AMP glow and poor sensor cooling scene as, leading to dependance on gymnastics with Flats. The 294 backlit architecture does not lend itself to even substrate cooling, producing gradients when used in traditional long sub imaging. The 294 is fine for high frame rate video, and EAA use. By comparison the 1600 does not require Flats at all, I never found them necessary.
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Old 28-09-2020, 09:37 AM
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That’s not microlensing on the 1600. It’s much more obvious than that...it’s a square grid pattern. If you point it at Alnitak or another similarly bright star it’ll become very obvious after you’ve integrated a few subs.

I also have an Esprit 100 and 1600 and the only image I’ve felt the microlensing effect has ruined an image is around the Horsehead nebula, with above mentioned Star featuring heavily :LOL:

You’re most likely seeing something actually causing diffraction, like the leaf of a tree or something else funky.
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Old 28-09-2020, 05:22 PM
sunslayr (David)
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I've grabbed a cropped sub from the blue filter it shows the grid a bit better, it's definitely not an obstruction, the star probably isn't bright enough to really show it. I can also agree that the 294mc is not without its issues but with the 1600 sensor being discontinued it will soon be the only new option. Is there a way of using something like a layer mask to fix microlensing? I've had some success with the clone tool but I admit my photoshop/gimp experience is fairly limited.
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Old 28-09-2020, 05:34 PM
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David,
Why do you say the ASI 1600 is being discontinued??
I use it regularly for solar imaging, with no issues.
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Old 28-09-2020, 06:34 PM
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multiweb (Marc)
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+1 That's not microlensing. These are internal reflections and/or diffraction somewhere in your imaging train.

This is microlensing on a QHY9 mono sensor (KAF8300) taken with an FSQ106N that has no diffraction spikes whatsoever.
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Old 28-09-2020, 06:50 PM
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Quote:
Originally Posted by sunslayr View Post
I've grabbed a cropped sub from the blue filter it shows the grid a bit better
If I had to guess I'd say this is an image of the sensor made by the light bouncing from it to the back of a filter or piece of glass close to it.
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Old 28-09-2020, 10:25 PM
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Ok now I'm confused, Is in not the reflections of the microlens array being reflected back due to the lack of AR coating on the sensor cover glass? Or are you saying the sensor itself is being reflected back from my filters and/or AR Cover glass on the front of the camera. As far as I know my Astrodon filters are AR coated both sides? Also If it was the filter, corrector or something else would the reflections not be further from the star? Each circle is no more than 65px apart.

I guess I just assumed because it looks exactly like what I'd expect with all those circles.

http://hamamatsu.magnet.fsu.edu/arti...photodiode.jpg

Ken the Panasonic sensor AMN34230 used in the 1600mm is listed as EOL. I'm not sure how much stock ZWO and QHY have left however.

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Old 29-09-2020, 12:33 AM
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David,
Thanks for the update on the ASI 1600 sensor......what do I say? It worked very well for me. RIP
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Old 29-09-2020, 10:40 AM
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Quote:
Originally Posted by sunslayr View Post
Ok now I'm confused, Is in not the reflections of the microlens array being reflected back due to the lack of AR coating on the sensor cover glass? Or are you saying the sensor itself is being reflected back from my filters and/or AR Cover glass on the front of the camera. As far as I know my Astrodon filters are AR coated both sides? Also If it was the filter, corrector or something else would the reflections not be further from the star? Each circle is no more than 65px apart.

I guess I just assumed because it looks exactly like what I'd expect with all those circles.
The grid pattern in your blue channel image is the microlens grid pattern, the larger and more obvious circular pattern is something else.

I might have a go when Orion gets high enough over the neighbours, as I've never noticed the grid pattern on those stars you're seeing it on (and I do go looking for it!)
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Old 29-09-2020, 10:56 AM
sunslayr (David)
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Ahh I see, yeah I was only talking about the grid. Not sure about the circular one. Now that I look at it I see it wasn't very clear. The circular one is a mystery to me to to be honest, this is the only image I've encountered it. Maybe the grid is not quite overlapping and making a circle
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Old 30-09-2020, 09:36 AM
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yeah could be nothing ventured nothing gained!

TBH I didn’t notice that in your image when you raised it, it was the bright circular outer ring that stood out.
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Old 30-09-2020, 12:02 PM
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Been doing a little research and learned some pretty cool things. First of all when I examined the channels separately a found the grid was larger and more spread out in the red channel while it was small and tight in the blue. I thought to myself if that corresponds to the wavelength of light, maybe I can use it to calculate the thickness of the cmos cover glass?

Turns out the person who runs Geoastro has done most of the work for me. Unfortunately they did not provide their spreadsheet so I had make a copy for myself to make the calculations a little easier. I will provide a link for anyone who would like to play around with it themselves. Using it I calculated an average distance of 1.3mm, which sounds about right to me. It also appears I was correct in my assumption that an uneven overlap results in a round artifact in the final image.
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Old 30-09-2020, 02:20 PM
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If you push any system on a bright star you'll get ghosts. Here's my CN-212 with just mirrors in the imaging train but for the camera CCD glass.
That's alpha crux.
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Old 30-09-2020, 02:39 PM
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David, I like the analysis - it appeals to me being a spectroscopist.


I have also had to look at the sensor cover plate as a source of resonance banding in the spectrum. The effect we see is very similar to that produced by an F-P etalon filter.


http://astrosurf.com/aras/fringing/opus/
http://www.astrosurf.com/aras/fringi...ter/ripple.htm


I experienced this effect while testing a high resolution spectroheliograph.
The reason for mentioning all this related testing, is to remind you that the sensor cover plates were all measured to be around 0.3 to 0.5mm think.
None were in the range you seem to calculate.
I need to double check my data files, but from memory the cover plate in the ASI 1600 is 0.5mm thick.
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Old 30-09-2020, 04:09 PM
sunslayr (David)
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Thanks for the input Ken. I really don't claim to be an optical expert it's quite possible I'm wrong I wasn't able to find the data on the spec sheet so I just googled cmos cover glass thickness and got a result of about 1mm. I think I might just blame the data, its not a single wavelength so my results say ~=1.3mm +-1mm
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Old 30-09-2020, 04:52 PM
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The “geoastro” ghosts were generated between the sensor and a remote (20mm) filter element.
I think this could be a different process from the narrow band etalon effects we see in Spectra.
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Old 01-10-2020, 09:21 AM
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Good work

David, are your three images above R G and B channels (in that order) ?
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Old 01-10-2020, 09:46 AM
sunslayr (David)
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Quote:
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Good work

David, are your three images above R G and B channels (in that order) ?
I think they are GBR, I think if you hover your mouse over them it should tell you which is which. I Probably should have paid more attention while uploading.
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Old 01-10-2020, 10:19 AM
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Stonius (Markus)
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Dunk and Marc - it seems there is a difference of opinion over what microlensing actually is.


I always thought it was precisely what we see above - a reflection of the microlens array from the back of the sensor window on particularly bright stars (Alnitak being notorious). Also, Marc - I can't see anything particularly wrong with the image you posted - where is the microlensing you refer to in that image?


Sorry for being a dunce - maybe I've misunderstood this all along?


Cheers


Markus
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