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[Note: this topic was previously posted to Cloudy Nights]I recently imaged regions containing exceptionally bright stars (e.g. Alnitak) and noticed some unusual reflections. The reflections appear strongest in the blue channel but are also apparent in the luminance. I imaged with a ZWO ASI6200MM Pro in HaLRGB. My RGB filters are Astronomik and the luminance (LoGlow) and Ha filters are Chroma (LoGlow for lum., 3 nm for Ha). Interestingly, the reflections lie on radials from the image center and appear to be defocused bright stars. The distance from star to reflection increases with increasing distance from the image center. I don't see the reflection in red, green, or Ha. (Okay, maybe a slight reflection in green.) My imaging train consists of: SW Esprit 150 (f7) > Moonlite Nightcrawler focuser > SW flattener > PrecisePart spacer > ZWO OAG > ZWO EFW (7X50mm round) > ASI6200 MM Pro. I attached images of the blue channel exposed for 60s, with zoomed in captures of the stars and their reflections. So, what might be causing these reflections? Given their size and proximity to the stars I think their origin may lie within the camera. FWIW, I flipped the filters and the reflections were unchanged. TIA, David   |
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| My bet is the optical window of the camera and not perfect anti-reflection coatings. |
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| Flattener and Correctors are possible Candidates, too. |
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| IMO, almost certainly from the filters. These look very similar to the reflection haloes I used to get using Optolong L-eXtreme filter with a 2600MC. Changed to Antlia ALP-T and no more haloes. |
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| Andrea, wittinobi, and Roger, thanks for sharing your thoughts! I'll probably start swapping out components to see if I can isolate a cause. My thought that it might be something with the camera was because of the reflections' very small size (versus a typical halo) and their relative proximity to the star. |
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| Try flipping the side of your Blue filter. See if it helps. |
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Hi David, I get very similar reflections with my Esprit 100 and Astronomik Deepsky set. I suspect it's a bit of a combination of certain filters and the coatings on the SW flattener. When testing the same camera and filter on my Quattro with a Sharpstar MPCC, I was not able to reproduce the reflection even when using a super bright star like Sirius, where as my Esprit setup would show these reflections on even moderately bright stars. I haven't got around to testing without the flattener as it's not a regular issue for me and sufficient dithering often rejects the reflection if it has moved around enough, but that would be the first thing I'd try. Finding the right adapter to allow you to leave the flattener out of the train might not be so easy though |
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Matthew Sole: Matthew, that's interesting concerning the Esprit-Astronomik combo. When I get a break in the weather I'll pull the flattener out and see what happens. I may just use a different scope for bright-star targets. Thanks! |
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| I thought blue was due to CA ? i wouldnt absolutely rule out CA and focus in the blue channel. |
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David, I had a similar phenomenon with blue astrodon filter when using an apex reducer on an At92. I sent the files to Scott at starizona and he modeled it and determined that it was a reflection off of the front of the filter to the rear element of the reducer. By changing the position of the filter you can change the location of the reflection at the focal plane. So, by changing the spacing between your sensor and filter you might be able to push the reflection off the sensor and eliminate the issue. |
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Thanks everyone for the replies! The feedback definitely makes take a closer look at the flattener and the filters. I reviewed some older data where I used an Atik 16200M and an Atik FW. The scope, flattener, and filters were the same. I did not find the small, almost planetary nebula-like reflections with that imaging train, but the OAG, FW, and camera were different. |
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David Andra: That seems to point to the inevitable conclusion that it must be the camera as neither FW nor the OAG interfere with light collected at the focal plane. BTW, by the size of the ghost image, the focal length/ratio and camera spec one could arrive at the location where these ghost images are being produced. |
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andrea tasselli:David Andra: My original thought when I posted was that the camera was the culprit. However, that seemed hard to accept since I didn't find similar tiny reflections discussed in the online forums for the popular 6200 camera. Nevertheless, I performed the calculation you suggested with a "ghost" reflection of 28 pixels diameter, camera bin 2, f7 scope, and the ASI6200MM Pro camera. The resulting distance was inside the camera at 1.47 mm from the sensor. If the origin lies within the camera, I'm at a loss as to how I can prevent the reflections(?). Surely others have dealt with this problem. |
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David Andra: If your calcs are right then the culprit is the glass cover of the sensor. Hard to spot with the typical tests these sensors go through. If it so then your particular sensor just to happen to be the odd one off. |
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| If it was the cover slip of the sensor the problem would be localized around the offending star itself. In this case the issue is opposite the bright star, on the other side of the sensor. This indicates a reflection is bouncing off of the curved element of the flattener. |
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| The reflections are very close to the offending stars and displaced in proportion to the field angle. Besides, the OP states that the image train except camera and two other bits not relevant to the case were the same without giving the issues seen here, hence having excluded all other possibilities whatever is left, for however improbable, must be culprit. |
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andrea tasselli: The reflections are not the exact same distance from the stars though nor the same orientation from the offending stars. This suggests light bouncing off of a curved element. Most likely the flattener. Its easy to test. If the OP increases or decreases the spacing, the reflections would move. If they are interacting with filters (which since it is only in Blue and Lum, they are) then changing the position of the filter should also change the location of the reflection. Even sherlock would agree that there are still possibilities that have not been excluded. I highly doubt this has anything to do with the sensor cover slip., however until the OP does some testing, all any of us can do is guess. |
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| I did the calcs and I can confirm it's a 1.47mm of defocus. Since it is got to be an internal reflection, that is twice the optical path within the glass causing it. Factoring in the refractive index typical that means that the thickness of the object causing the reflection is around 2.1 mm thick. The distance of the ghost image from the source is a function of both the incidence angle and the distance from source point but at the moment I can't recall the formulae... |
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| My first chance to test under the stars will be Thursday night when I expect a break in the local weather. Perhaps the easiest first step would be to remove the flattener from the image train. I'll collect some data and post the result. |
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The weather improved as expected yesterday so I configured the system to remove the flattener. The imaging train now consists of: SW Esprit 150 (f7) > Moonlite Nightcrawler focuser > SW flattener > various spacers > ZWO OAG > ZWO EFW (7X50mm round) > ASI6200 MM Pro. To test, I shot a series of 60s exposures, bin 2, with the blue filter. A sample image, oriented as before in the Horsehead region, is shown below: When compared to the images in my initial post you can see that the "ghost" reflections disappeared when the flattener was removed from the imaging train. Not the results I expected, but compelling nonetheless. As a further test, I shot one 180s sub (not shown) of the same target and still no reflections were observed. I finished testing by targeting Sirius with another 60s, blue exposure (below). Sirius looks pretty much as expected - blown out but no reflections.  The lack of a flattener is very obvious at the corners of the images so I need to find a way to put it back. If the reflections are from filter to flattener and back to the camera it seems like my only option is to change the spacing from the filters and camera. This will be challenging since the camera-EFW-OAG are bolted together and my back focus is precise using a Precise Parts spacer. Perhaps I'll have to unbolt the camera and replace the custom spacer with a series of not-so-precise generic spacers? Or, maybe I'll just have to not shoot bright stars with this system(?). Finally, thanks to all that helped me work through the problem! David |
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Exactly what I was expecting you to find. Couple things to try. 1) flip your filters and see of the orientation of the AR coating can help mitigate the issue. 2) rearrange spacing so that you move filters either closer to or further from the chip to see if you can move the ghosts off the edge of the chip. |
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Here's an update on additional testing: I reinstalled the field flattener and moved the filters 5 mm farther from the sensor. I could not try moving the filters closer because they were already at the minimum possible distance to the sensor. I also removed the OAG and used spacers to preserve back focus. The results for the blue filter are shown in the first image (below). The result is essentially the same as the original image shown in my first post - a defocused star reflection. Flipping the filters yielded the same result. Finally, I defocused the image inward 1.45 mm and obtained the second image (below). In the defocused image the reflection shrank and appeared to focus. I was surprised that moving the filters didn't really make a difference. Clearly, I have some more thinking to do! If you have a thought, let me know! Thanks - David   |
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| If you have another camera you could try swapping cameras to see whether this still occurs or not. While I don't think the issue is with the filters, the fact is that moving the filters up and down does not change the amount of increase of optical length they create. I suppose using the L filter still create the issue. |
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| If moving the filter doesnt change anything that suggests that the initial reflection is not off of the filter back to the field flattener. Perhaps it is off of the camera chamber window or the sensor cover slip itself. You could remove the protective window and take an exposure to see if that is the cause. You wont be able to cool and you may need to recharge your desiccant, so just be aware. Its a bummer its not the filters, but there are a few things still to rule out. Andrea's suggestion to test with another camera would be a good test to try. |
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Chris White- Overcast Observatory: Yes, great suggestion! Last night I was able to test a different camera. I used a ZWO ASI294MM with the same filter wheel as before. I had to change the image center point a little due to the smaller sensor. The results in LRGB are shown below without cropping. The reflection, slightly up and to the right of Alnitak, is there in L,G,B, but not in R. Recall that the filter manufacturer varies with RGB=Astronomik and luminance=Chroma (LoGlow). Very interesting and frustrating. I now know the reflection occurs with different cameras, with variable filter spacing, and with different filter manufacturers. Only when I removed the flattener did the reflection go away. Perhaps it's time to reach out to Sky-Watcher?     |
