• Cameras

  • Problem with halos? Not anymore.

I have this same problem, your NGC2024 looks exactly like mine, my camera is the ZWO 2600 MC-Pro. Is there no way around this, there is no way I could attempt your engineering feats!

Three days ago I had finally clear skies and run some single frames with different exposures at Alnitak. The masking didn't work so good, hardly notably change if any. I cleaned the eloxated casing with isopropyl alcohol but kept the masking on the AR window.

The truth is that the original window will never work on some filters. A few weeks ago I noticed a document that I had missed. It is on the Baader site by Michael Risch, co-founder of www.astronomie.de:

"Small cause - big effect. The anti-reflective coatings of the camera windows of different manufacturers differ slightly. Shown here are two cameras from different manufacturers, the left one reflects more in the green spectral range, the right one more in the blue spectral range. These small differences are responsible for the fact that the photos taken with the left camera in combination with OIII filters show strong halos, while with the right camera and the same filter halos will appear hardly noticeable."

The document is here at the Baader site: https://www.baader-planetarium.com/en/blog/halos-viewed-without-prejudice/

I have ordered a new window with different coating and ditching the green coated original. Should arrive in next week.

19 days later

My new window finally came. It is the Baader Clearglass 31x2mm filter. 1mm less diameter, but the same BK7 glass with different coating. It leaves a 0.5mm gap all around compared with ZWO AR-AR glass. That's all right when Baader filter holders usually allow some sideways movement to not get pinched. Thickness is 2mm, the same as the original.

Clearglass is the C-filter (clear filter) in the Baader L-RGB-C set and have the same specs as the original window. It has also been used as dust protection.

ZWO original AR-AR window. The reflection is my LED lamp in the ceiling.

Baader Clearglass.

The new window has a more natural color and didn't confuse the auto focus of my DSLR when taking these pictures like the original did. Next will be some tests. Unfortunately the Alnitak is gone for this season up here in the north.

Two days of tests and this is what I found out: Neither window didn't make a difference, not even removing the window didn't change the result. Nor any different focal location of filters, 1.25" or 2", did any changes. So the window itself do not introduce any anomalies.

My conclusion is that the properties of the scope are the cause of halos. Filter halo is not the reason, it is the result of the scope. The narrower band, the stronger is the halo. And the CMOS microlens effect comes with too much stretching and has nothing to do with halos.

10 days later

Flocked the scope with 50mm Mil-Tec canvas tape, no difference. Printed some bafflers and there it was, bingo! Halo is gone from my overkill artificial star.

Now waiting for some clear sky for a real test. Any constellation star should do the job.

I'm glad you figured it out Leivotech. I also have quattro 8" F4 newton with baffles. When I replaced tube with carbon one, I also put baffles into new tube to strenghten structural integrity. Didn't tried imaging without them, so can't say about halos. But I think it helps. Your discovery is good for all who have problem with halos and maybe can solve it with this approach.
Good luck and clear skies.

Andrej

Thanks Andrej.

I did some tests at Arcturus that is one of the brightest stars. Filter wheel had a set of different Baader neodymium filters. Arcturus is a real monster and can fill sensor wells instantly. The only way was to roll the filters one at a time and compare them with different exposures. All filters did now bloat and halo about the same time.

I have a southside balcony and there is no sensible target to do any test on for a smaller wide field scope right now. I bought this scope originally to go piggyback as guiderscope on my larger refractor, but I started to use it also for imaging later. 102/600 f/5.9 is nice for larger nebulas.

All filters has different properties and be more sensitive to produce halos, especially Baader neodymium that was originally made for visual use. They added UV-IR cut later and made a few more combinations of them. What I was looking at is halos that came with very short exposure. I guess everything is now well because I can't ask the UHC-S to perform better than the rest of the filters. If you use the same stuff in the optics then the results will always be the same. You can only suppress it, not remove it. Filter and camera are the scapegoats for a bad telescope.

One weird thing happened after installing bafflers. Off axis light is suppressed clearly, but instead of the larger halo there is multiple smaller halos inside it, like bafflers in the scope. I switched camera window to original ZWO and it is still the same. That is of course when you use too long exposures. I am happy to scrap the window theory for good and ZWO cameras are good as they are from the factory.

I can't do any more before end of august when stars comes back and next season starts. Dark season is over here up in the north. I will be in touch when I know the results.

Clear skies to everyone.

4 months later

Thank you for your effort in trying to resolve this issue. I am also having a problem with off-centered halos in brighter stars. I have two WO Redcat 51s and ZWO 071MC Pro and 2600MM Pro. Both cameras get the same size halos around the same stars. Every filter I own gives the same results along with using no filter as well. I was suspecting the AR protect window to be the cause of the halos, but your test results prove this is not the case. I was going to replace the AR window with an IR Cut filter glass, but now I will not proceed with that plan. I am not sure of my next step of troubleshooting this issue, so I look forward to your next post to see your latest findings. Thank you again for all your info in this thread.

Are these halos not just caused by internal reflections within the filters on bright stars? Apologies if none of this is new as I'm sure others have come up with this formula before, but my back of the envelope calculation says that the diameter of the first reflection halo should be approximately 2 * filter thickness / (focal ratio * filter glass refractive index) / pixel size. So for my Sharpstar 15028HNT with an Optolong L-eXtreme filter and an ASI294MC-Pro, I calculate the diameter of the halo to be about 183 pixels - a bright star on a recent image of IC1318 had a halo of 183 pixels. I did the same for my 190MN scope, using a Baader OIII filter (2mm thick glass AFAIK) at 2x2 binning with an ATIK 383L and the calculation gave a size of 46 pixels - a bright star in my M27 field gave exactly that size halo. My Baader broadband filters don't give halos (at least not that I can see particularly), so I assume it is really a problem with narrowband filters and it is one made worse at faster focal ratios. The other thing about these internal reflections is that you might also see second and third reflection halos at 2 and 3 times the diameter which is what I believe might be occurring in the Horsehead Nebula photo above.

10 days later

Thank you dtc1999. Are the halos only in one direction or are they leaning out of the center? If in one direction then your back of your light train is offset or you have a collimation problem. If they are leaning out of the center then you have a problem with your field flattener distance to camera (spherical aberration) . My guess is you have both since you have both halos and offset. Another thing is what I suspected in my scope: at your f/4.9 normal filter is too slow and gives you halos. There are filters for fast scopes. Try to check collimation and the specs distance of camera and focal reducer when using your RedCat. The shorter f-rate, the worse is the tolerance to get it right.

terry56. Yes, you are right. This is internal reflection in the filter. It was clear from the beginning because UHC-S was the only one of similar filters that made big halos. The math was correct but I wanted to test this claim https://www.baader-planetarium.com/en/blog/halos-viewed-without-prejudice/ and because filter wheel is very close to the window. There is a lot of people that blames the window, so I did some tests. We have to understand that all Baader Neodymium series is originally made for achromats and visual use, often with bigger f-rates. They added later UV-IR cut to them for cameras and made some new iterations. They are not new dedicated astrophotography filters. UHC-S doesn't work with long exposures with fast scopes, its as simple as that.

I will soon do some tests on the nebulas in Orion when they show up. Dark season is only starting here at 60 latitude.

    Leivotech fast scopes are brilliant for collecting photons, but they do bring lots of issues like collimation and filter sensitivity! But just one thing on the RedCat51 - the Petzval design means that the spacing between the rear flange of the scope and the image plane is not that important - you need to get it around 55 mm but that is because if you get it much more than 55 mm, you may not be able to achieve focus. The Petzval design, AFAIK, has the lens spacings preset and so focusing moves the entire assembly backwards and forwards. If there is a problem with off-centred halos, it may be that is more to do with tilt in the image plane? I certainly have a small issue with that on my RedCat, but I'm struggling with clear nights to try to adjust the tilt. At 55 North, at least our dark nights started a few weeks ago, it's just the clouds don't want to play ball!

    • w7ay replied to this.

        I thought RedCat was a air spaced 2-lens design ED. Can it really be a 2-lens achromat with Petzval lens in front of the focusing tube? Are you sure that it is not a ED with ready installed field flattener inside? My guess it has a preinstalled FF to make the whole package smaller and because it is a dedicated astrophotography scope. ED/APO need to have a FF because of the spherical aberration.

        If all halos are offset despite good FF, there is a problem with the collimation. Usually the back end of the light train, image plane. There no way to confirm the optimal setting without collimation tools. Luckily they are not so expensive. I use a cheap laser- and Cheshire collimator to all calibrations, About 50€ for both together. The funny thing with these cheap laser collimators are that you have to collimate them before use.

        My workflow is this:

        • Cut a paper that fits on the lens cell and make a small hole or mark in the spot exactly in the center.
        • Put the laser in and check if it hits the center mark. If not, you have to straighten up your focuser back end.
        • Change to Cheshire.
        • Look thought the Cheshire and find the two diffraction rings. If they are perfectly centered inside each other then everything is fine,
        • If they are offset or starts to separate you have to adjust the lens cell.
        • It is important to line up your light train first, then the lens cell.

        Looks difficult but is really a easy and fast process. Refractors needs to be collimated and checked like any type of scopes. Especially important with those that are very fast scopes designed for astrophotography and if you transport it often.

        terry56 you need to get it around 55 mm but that is because if you get it much more than 55 mm, you may not be able to achieve focus.

        Yes, the spacing between the Petzval's flange and the camera flange is flexible because the focuser's draw tube is in between the rear most element of the telescope and the sensor.

        It is not that the spacing is not important. The focuser is the one that determines correct spacing. When you are in focus, you have the correct "backfocus spacing".

        With the RedCat51, you can use up to 59.7mm of spacing between its flange and the sensor ("draw tube" in the shortest position). I forget what the shortest backfocus is (I have not touched my RedCat in a year now), but it is that 60mm minus the maximum draw tube travel.

        As such, you also need to focus a Petzval very carefully since any defocus will also bring up the coma at the edges of the frame. Just 1ºC temperature change could make an FSQ-85 (another Petzval) require refocusing.

        However, if you ever place a reducer (not that William Optics makes one for the Cat51), you will then need to maintain accurate distance between that piece of glass and the sensor. For example, the FSQ-85 has a strict backfocus when you attach a flattener, a reducer, or tele-extender (Takahashi chose 56.2mm as the design distance; a number etched in my memory).

        If there is a problem with off-centred halos, it may be that is more to do with tilt in the image plane?

        That is also correct.

        The RedCat51 is well known to have poor QA regarding tilt (mine had a tilt that can be seen even on a APS-C sized sensor). So much so that they quietly introduced a second version, about a year after the initial release, that included a tilt adjuster. You could (I don't know if you still can) buy the tilt adjuster independently.

        Chen

          That is one weird scope. Why do it have both ED fluorite glass and Petzval lens? Is it to line up the colors and make it to act like a real APO?

          Yes on both of my Redcats I will get halos that offset the bright star towards the nearest edge. One of my Redcats I noticed elongated stars in one corner. I have not tested yet if this this tilt in the scope or in the camera.

          I am currently testing the location of the built-in FF of the Redcat. Terry is correct that when you adjust focus, the built-in FF lens will move inside the OTA. The focal plane is 59.7mm and I was using 55mm of spacers (including the backfocus of camera). This was putting the FF lens near the end of the OTA. The focus adjustment of the Redcat will move 33mm of space, so I decided to remove the 16.5mm spacer. Once I was back in focus, the FF lens was now that 16.5mm further up inside the OTA and the camera did show an improvement with halos. Granted the area I was imaging after that change had mag4 and 5 as the brightest stars and those did not produce any halos, as my other redcat was still using the 55mm of spacing and was still producing halos on those stars (I have a dual scope setup to allow me to compare). I have not done enough testing to say for sure that is the fix, but it does seem the further up the FF lens is in OTA, there are signs of improvement. The type of filter used can also play a part in this as was stated in previous posts. I am not sure how to determine the "speed" of a filter.

          The idea of using less spacers came after speaking to another Redcat user with a 2600MC who only used the 16.5mm spacer and no filter as the 2600MC's AR window also is IR-Cut glass. His image of M42 and a portion of the Horsehead had zero halos and looked perfect. Unfortunately, Alnitak was not in the FOV for the best test.

          My 2600MM and 071MC's window is only AR protect and not IR Cut, but ZWO does sell the windows with IR Cut. They are fairly inexpensive, so I ordered them so I could mimic his exact setup with just the 16.5mm spacer and no additional filter other than the window with IR Cut. This will not solve the problem if it is the filters reflecting to the camera window and if you would need to use additional filters, but will bring us through a another step in troubleshooting the issue. The IR window has not arrived yet, so I will report back once I have received and tested.

          The idea of using less spacers came after speaking to another Redcat user with a 2600MC who only used the 16.5mm spacer and no filter as the 2600MC's AR window also is IR-Cut glass. His image of M42 and a portion of the Horsehead had zero halos and looked perfect. Unfortunately, Alnitak was not in the FOV for the best test.

          I'm not sure how using less spacers with the RedCat would help with halos since it just means the focuser would have to be racked out by that extra distance, but maybe I'll give it a go just to see - there are so many unknowns in this pastime that you just never know! Maybe the reason he got no halos was just that he wasn't using a filter (did I understand your post correctly there?) - remember the halos are not caused by light bouncing between the camera window and the filter, but between the front and back surfaces of the filter.

          I'm not sure how using less spacers with the RedCat would help with halos since it just means the focuser would have to be racked out by that extra distance, but maybe I'll give it a go just to see - there are so many unknowns in this pastime that you just never know!

          Basically, I was just trying a different setup and noticed a difference. Even though the spacing between everything remained the same, it was still putting the FF lens in a different position in the OTA.. Pretty much it was a: Let's see what happens....

          Maybe the reason he got no halos was just that he wasn't using a filter (did I understand your post correctly there?)

          Yes, some color cameras come with an AR window that also is IR Cut as well. So you could image without a filter if you wanted to.

          remember the halos are not caused by light bouncing between the camera window and the filter, but between the front and back surfaces of the filter.

          A majority of the time, Yes. This is why I have ordered the windows with the IR Cut so I can test without a filter and confirm that the halos are cured without the filter. I tried imaging without a filter while using just the AR window and even though the stars were very bloated, it looked like halos were still present.. I should look over that data again.

          I had an issue with halos on a previous scope. I troubleshot everything except the FF and could not find the issue. Finally, I replaced the FF and the problem was solved. Granted, those halos were much worse than the Redcats.. I am hoping the FF is not the issue with the Redcat's halos, as we can not replace that FF..

          I actually forgot that I ordered one of those Hubble Artificial Stars years ago. I found it today and replaced the batteries.. still works. I should be able to do some testing now without having to waste a clear night...

          4 days later

          I've just had a pencil and paper out and sketched up what happens when you stick a piece of glass between the the lens (or mirror) of a telescope and the imaging sensor. The halo is (as I noted earlier) 2 * glass thickness / (focal ratio * glass refractive index * pixel size) in pixels. If the star is in the centre of the chip, then the halo will be centred on the star. Away from the centre, though, the centre of the halo will be offset from the centre of the star by an amount equal to 2 * thickness of the glass * distance from the centre of the chip / (focal length of the scope * refractive index of the glass * pixel size), again in pixels. So as long as all of the halos are offset away from the centre of the frame, that is normal and does not indicate any tilt in the system. Any asymmetry in the appearance of the halos may well indicate that there is a tilt in the imaging chip relative to the focal plane, but then that would also show up in the appearance of the stars and how in / out of focus they were, so you could pick that up more accurately with a Bahtinov mask, for example. An artificial star would be pretty good for doing that kind of work. Interestingly, by my calculations, the distance between the piece of glass (filter, whatever) and the imaging chip has no bearing on the size or offset of the halos, so if distance does have an impact, then there must be something else at play.

          I've been focussing my efforts on the filter as the source of any internal reflections, but of course the AR window (or IR cut window) is also a possible source and these things are 2 mm thick on most of the ZWO cameras, in other words about as thick as a 2" filter. I guess that things are just much worse with narrowband imaging due to the ability of the filter to really darken the background allowing the halo to shine through in all its glory :-(

          If halos are being caused by reflections between surfaces on different elements, for instance between the camera window and the filter, or camera window and the final element of the lens system (I think that is unlikely in the RedCat because that final surface is curved as far as I recall), then the formula for the halo size is a little different at 2 * separation / (focal ratio * pixel size) in pixels. So, say you had 55 mm between the filter and the camera window (I use that because of the usual 55mm back-spacing), any reflections between them on a RedCat with an ASI2600, the halo would be 6000 pixels in diameter, or so bigger than the sensor, so not visible. But a 2 mm glass window / filter would create a halo of 145 pixel diameter.

          I have tried and tried to remove halos in post-processing, but I've never had much success - maybe I'm too impatient. I clearly need to change my perspective and start to try to see them as a feature of a shot rather than an imperfection!

          Terry, I can confirm everything you said in your last post as I have put my filters as far apart and as close as possible to the sensor using the Redcat's built-in filter threads and then a ZWO filter drawer connected directly to the camera and the halos were always the same size.

          One thing to note, on my pervious scope that was giving me halos from the flattener, each bright star would give multiple halos. The bright star would have one around it, and there would be several more in random spots in the frame. I assumed this was from the light reflecting back forth multiple times. The Redcat never gives anything like this. Always one halo for each bright star and offset on the star depending the edge of the frame it favors. This gives hope that the built-in FF not tilted as you stated.

          My IR-Cut windows just arrived yesterday, so I plan to swap out the window on my 071 today. I will image with just that and no other filter to test if having one less piece of glass in the light path has any effect.