More Issues with FLATs
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- Опубликовано: 1 фев 2023
- I've moved my imaging train to the Explore Scientific ED102 and am using the ZWO ASI1600MM as the imaging camera with this scope. I performed a Dew Experiment by accident: Do we need dew straps for refractors with long dew shields? YES! So, I've been having to take and re-take a lot of flats ... and ran into another problem. My off-axis guider shadow is not calibrating out and I have those pesky circular reflections. I've solved the circular reflection artifact problem by changing a setting in the ZWO Filter Wheel driver, but the OAG shadow is still there.
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James, I was getting the same artifacts in my flats, and was cleaning filters and saw no change. I did change the driver setting like you did and it seemed to solve the problem. I stumbled upon the solution but didn't know why. But thanks to your due diligence, I now have a much better understanding as to why, so thanks!
Glad to help, Dave. If you’re using unmounted filters, you might consider the 3D printed filter masks from Buckeye Stargazer (Agena AstroProducts) - they provide additional positional accuracy. Thanks for watching!
You are the best. Clear explanation supported with data...Science is Magic That Works
Thanks John! But "the Best" would've plugged in his dew strap!
Fantastic as usual, thanks James! Truly valuable, no BS assessments.
Haha. I disseminate plenty of BS
thanks for another excellent analysis + video and all the work you put in! I suppose the "no-short-flats-recommendation" is also concerning the use of cameras with mechanical shutters, especially leave shutters: even if the shutter is fast there is a moment when the image is vignetted by the moving shutter. In long exposures the relation between this moment and the exposure is almost zero. In very short exposures this relations gets relevant and you get additional vignetting by the shutter itself.
This is just brilliant James! I shoot with an ASI294mm Pro and a ZWO filter wheel and am struggling with the very same “crater” issue. It never occurred to me to check on the filter rotation as the possible source of the problem. I think you just allowed me to keep a few more of my hairs from being pulled out! I’ll be testing this out at the Winter Star Party down in the Keys a week from now. Can’t wait to see the results. Thanks again!
Good luck, Ken!
That was brilliant problem solving demonstration James! This is really helpful like most of the videos on your RUclips channel.
Great! Thanks for watching!
Great! You answered and solved two issues - the circles and "why the universal direction check box?" Thanks! Again, always good to have a back pocket engineer close to hand for we liberal arts types who don't even know the questions to ask!
Haha. Remember, these problems were created by engineers in the first place. Kind of like my lawn service that cuts through my sprinkler lines and charges me to have them fix it. The circle of life as we know it.
@@Aero19612 Ha! I always keep that in mind! My training as a historian!
Great video. Thank you.
Thanks for watching, Matthew!
You have been way more patience with that OAG than I have been. LOL. Love the vids. Thanks for sharing your experiences.
My patience won't last long if I keep getting the Bermuda Triangle in my subs. Funny thing is, this a very rare occurrence. Can't help but think there's something about my line of sight to this DSO. Can't blame the Moon (not up and near new Moon).
Always enlightening and entertaining, James. I suffered the crater syndrome and also decided unidirectional was the way to go. The one thing I will do the next time I must open my filter wheel (and not before) - is to "focal offset order" my filters to minimize my "spinning" time - I do take images in a high to low offset order but haven't reordered the filters within the wheel. If I remember right we both use Antlia filters but my measurements have them a little off par focal. My offsets range +22 for red to -3 OIII, at least with my refractor/focuser combination. On the dew straps and my PPAdv - I use the auto-dew selection to avoid tracking which to port to plug in, this may or may not help on all Powerbox models.
Interesting that you get different offsets. I do see some difference, but not worth shifting focus for. I may re-visit that conclusion. Thanks for watching, David!
The reason for recommending long flat exposures is non-linear sensor performance. For ASI1600 the limit is 0.2s (IIRC), for ASI294 1.0s. The 294 at least seemed to use a different mode for short exposures - I remember shooting flats at 0.999s and 1s, and the image statistics were so far apart that you'd think there was a whole lot more than 0.001s of difference.
I don't know how that would affect calibration, though. There are threads on Cloudy Nights about it.
Great info, Riku. Thanks! I'm not certain this issue did affect my calibration. It has not in the past (0.3 sec). Newer flats at 2 sec. Same problem.
+1 for this. Some people expose longer just precisely for this reason
Another thing to ensure (you may already be doing this, but it wasn't mentioned) is that it is *essential* that the camera gain (and offset) used when taking flats match that of the lights. So if you ever modify the camera's gain for a new target/new filter, you'll need to take new flats with that identical gain setting to match. As you've demonstrated, exposure time differences aren't that big of a deal (I often take flats at less than 1 sec with my sketchpad panel and they work fine). But gain settings are more critical. -- Not sure if that explains the OAG shadow, but I thought I'd mention it. You might have better luck mechanically backing the OAG's pick-off prism out a little bit.
Hey Mark. Mostly agree. Here's where I differ a bit. The gain used for imaging does not have to match the gain used for FLATs. HOWEVER, it is critical to match exposure time, temperature, gain, and offset for FLAT subframes and the corresponding DARFLFLAT subframes so that you can properly calibrate the FLATs. Once the Master FLAT is calibrated properly, you're good to go. In my case, I do use the same gain and offset for imaging and calibration. I use -5C for imaging and +10C for (indoor) calibration. My OAG isn't the problem - it always gets calibrated out properly - except for this target. I tried this target again the other night with a different telescope and camera. Similar type issues. I think reflected light I get in the telescope for this target (over neighbor's lighted/not lighted tree and their house) changes during imaging so that no FLAT can properly calibrate all of the subframes. If it's not one thing, it's another...
@@Aero19612 I think you mean matching temperature and exposure times are necessary for *dark* frames (i.e., matching your light frames). But not so much for flat frames (temperature and exposure times are not particularly critical for flats). Camera gain (and offset for ZWO cameras), on the other hand, is (are) important for both darks *and* flats matching the lights. But if you already use the same gains for your lights and flats, then that's not the issue you're seeing, and you're all good to go on that front.
Regarding the OAG pick-off prism shadow: Even if you calibrate that out with flats, you're only calibrating out the "mean" value caused by the shadow. The standard deviation of pixels within the shadow region will be larger compared to what there would be with no shadow region.
In other words, while flat frames can compensate for the mean values in the shadow regions, they can't compensate for the noise. In your calibrated frames, those regions around the pick-off prism will be noisier, comparatively. Hence my bringing up the idea of mechanically backing off the pick-off prism a little bit.
Neat info.
I have a KAF16803 sensor with a mechanical leaf shutter with significant open and close time. I had to take flat exposures that were long enough to swamp any effect of shutter vignetting.
Yep. A slow shutter requires longer exposure times for Flats. So many other ways Flats can go wrong too. Thanks for watching!
Thanks James! I've been having the same issue with my qhy268m and cfw3m, and I guess this is the cause. I'll test the unidirectional fix once I'll get clear skies again :(
Great Paolo! Note that if you continue to see the "embossed" circles after selecting the uni-directional option, it probably means that the filter wheel does not precisely go back to the same position for each filter. Some people clean the sensor in the filter wheel that detects position. There's also a "calibration" option in my ZWO filter wheel driver. Good luck!
@@Aero19612 ah cool, I wasn't aware of such a thing on the filter wheel. Thanks for the pointer!
Afaik, qhy cfw drivers do not have such option so far. However there is a firmware update just to prevent the fw rotates in both directions. I applied it on mine and works like a charm. Qhy support provides such firmware
Thanks for the vid and tip, i have both zwo and qhy setup and I have noticed the donuts on some of my zwo images. Will be making a change to the driver. I will also look at the QHY driver to see if they have a simular setting. As for your OAG shadow, have you done any tweeking on it's position. You may have to slide the prism up just a scosh. I've had to do that when I switch from the 2600 to the 6200.
Hey Mark. I haven't played with the position of the OAG stem, except in the beginning when I was setting focus. I've never had that shadow not calibrate out before. Makes me think there's something unusual about the light coming from that DSO direction. Playing with the stem position is tricky. As you move the stem out, you also have to move your imaging camera back (to maintain focus). And that may not be possible given that you also have to have a certain back focus to the imaging sensor. So, in reality, you start by getting the back focus correct, then adjust the stem height to get simultaneous focus with the OAG and the imaging camera.
Thank you! I likely have the same dew strap on my Esprit100. When I've touched the strap after having it on for several hours I can't tell if it's warm. I guess that's normal.
Yep. The dew strap works, but I sure don’t feel that much heat.
Your videos are always so thoroughly done, James, very well done, and thank you!
It would be interesting to see how your new unidirectional approach will hold for the future. I've had the same disks and like you discovered that it is related it to variations in filter positioning. But I always use it unidirectional...😞. The latest I have done is to run the calibration process. Not exactly sure what that does, but hope it will improve things. Also I have read that an infrared sensor inside the wheel that checks wheel position can get dirty, and may need a cleanup. I havent tried that yet.
Also, I wondered if temperature might have any influence on the friction control of the wheel. Ultimately it is a metal pin pushing a rubber band. Normally I take flats around the imaging session, but sometimes I take the scope inside and take flats then. Will this friction control be identical between a cold imaging night vs comfortable room temperature?
Excellent points. The friction pin could very well be sensitive to temperature as you say. I'll have to see if I can find that sensor. And maybe I'll run the calibration process, but, like you, I'm not sure what it does. Thanks for watching!
I had the same flats problem about four years ago and drove me crazy! I called them craters on my flats but luckily I managed to figure out the unidirectional motion of the filter wheel was the solution.
The off axis mirror I would make sure it’s on the long side of your CMOS chip and bring it up a hair. I’m quite sure I was watching your video on off axis guiders to help me set mine up so I don’t think I’m really qualified to help you out!!🤔😁
Craters is good description. Totally agree with putting the OAG on the long side of the sensor, but this ZWO OAG combined with interference with the filter wheel only gives me one position where things fit. And this is it - the worst position possible along the diagonal. Thanks for watching, Gregg!
@@Aero19612 my bad! I now remember that you were using your Celestron OAG on your SCT and the ZWO OAG on your refractor?
Yeah. The Celestron OAG provides total 360 deg freedom, but it's heavy and takes up a lot of back focus distance. The ZWO is good for back focus and is lightweight, but with the filter wheel, you have no flexibility in orientation relative to the sensor.
I have also the same issue - last time I took flats it was partley sunny (I`m using an Artesky flat panel) and I`m pretty sure there was a leak of light somewhere through the filterwheel. So thanx for tips about taking flats in a dark room and make FW only go one way. I`ll try it 👍
Who knew taking a picture of light could be so complicated! Thanks for watching, Arnstien!
just remember that you're putting a light source at one end of the scope....If it's larger than your dew shield then you'll also be sending light across the outside of your imaging train, potentially letting light in, somewhat negating the affect of the dark room.
@@MGee1 That's true. I've tried taking flats with and without a towel draped over the flat panel. I don't see an effect. Also, it's probably darker in my dark room than it is outside. The perfect is the enemy of the good...
James, excellent video. Not too long ago I came across the stacked flats issue you reference. I can’t say how many hours across a weekend I spent on this but it was way too many. Like your video, one frame flat calibrated appeared to handle vignetting but after stacking and calibrating not! I finally gave up and sucked it up and cropped my pictures. Honestly I hated doing it but just couldn’t really find a way to fix it. I am now wondering if taking exactly the number of flats as I have images may get to nirvana. Maybe an experiment for another day. As always thank you for the thought provoking and thorough treatise on the pesky flats!
Hi Jerry. You may be onto something. My FLATs are correcting 99.9% of the OAG shadow as is. So I'm experimenting with a much larger number of subs in my Master FLAT. We shall see... Thanks for watching!
Welp. A master FLAT composed of 150 subs didn’t do the trick… Another weekend bites the dust
@@Aero19612 well that's a bummer! Back to the drawing board on this one. Thanks for the update...weather not so great here but you saved me some time for sure!
Longer flat exposures are needed with mechanical shutters, professionals used to suggest 5s to avoid seeing shadow from the moving shutter - I usually aim for 4-5s with my QHY9. Shutters aren’t so common these days but there was a reason for the rule of thumb
Ahhh. Very good point, Pamela! I assumed that the "2-sec" rule had a rationale behind it. As with most things in our hobby (and life in general) our game of telephone where one person says something, and it gets repeated for several iterations tends to filter out the rationale and to leave us only with a "rule". Always good to question and verify when we can.
Absolutely beautiful analysis! What camera are you using? ASI294MM?
Ooops! I should have mentioned that I'm using the ASI1600MM with this scope. Thanks! I'll edit the description.
Great video James, what sensor do you use? I have been told that flats need to be over 3 secs if you have a 294 sensor due to the inherent issues of that sensor? I have an Altiar Astro 294C Pro cooled camera and don't think my flats are correcting properly? I tend to get green and red fringes to my flats although the dust motes calibrate out nicely. I have been doing flats at 2 secs but am to try going to 3 and see if I get better results?
I note you use PI, I can nver get to see my flats in PI, is there a way to be able to see them in PI? If I use Astro Pixel Processor they show up nicely??
In this video, I'm using the ASI1600MM. I also have an ASI294MM. I have been getting good results with LRGB flats ranging from 0.3 sec to 1.75 sec. The 294MM sensor is different from the MC version. And maybe there are some sensor-specific reasons to use longer exposure times. Not sure about the color fringes. If you've got cloudy skies or a full Moon, that's a great time to conduct a series of FLAT experiments. Yes, the flats I'm showing in PI have the ScreenStretch applied so you can see the light variation across the frame.
Thanks James. Don't know what I do wrong in PI but when I apply the screen stretch it is just white and cannot see any dust motes, vignetting. However when I view in APP I can see the flat nicely with dust motes and alll??
@@julianmatthews3262 what is the median level of your flat. Go to process/statistics and click the check mark with flat selected. What does it say for “Median”?
@@Aero19612 Thanks James, I'll be back home at the weekend and I can check my processing computer for that median value.
First of all, thank you for your well reasoned videos. I noticed your #8 filter is secured to the wheel by threads while the other filters use 3D printed rings. I just wonder if taking filter #8 out will result is not seeing the embossed artifacts even when you use the non-unidirectional option. I'm thinking of potential unusual reflections inside the wheel.
Good question, Quique. That's my ZWO O3 filter in the #8 slot. I was comparing it with the Antlia O3 in the #6. The Antlia filters are unmounted and so do not have that threaded mounting ring that the ZWO set have. When I open the filter wheel again, I'll remove the ZWO filter. Another thing: it's still good to use the unidirectional mode just in case you have a smudge on a filter - those will leave 3-D artifacts as well. Thanks for watching!
Thanks for the EFW tip to Rotate in One Direction only, I'm sure we that would save lots of people's nights of imaging. I've also heard that the dew strap should be placed just behind the primary element, so that's where I've been putting mine and it's always been dew free. Looks like you're having success with it on the dew shield in front of the primary element too, so it's good to know it might be fine either way.
Ahhh, yes. The great "where to put the dew strap" debate. First, if what you're doing works, stop reading NOW! The first, most important step, is to plug it in. Geez. There is more to learn on this topic. There is so much thermal mass with the thick dew shield and 3 pieces of glass and the tube itself that the poor little dew strap can't possibly heat all that material up (my reasoning). So, the next best thing, is to heat up the air in front of the lens. I've got to dig into this more. But first, I need to deal with this FLAT issue... Thanks for watching, Andy!
@@Aero19612 Great video thanks. I also have a 102mm refractor and place the dew heater directly around the lens cell inside the dew shield, should I reconsider and place it around the dew shield on the outside?
Hey Julian. I say, if it works, don't change a thing. How do you get the dew strap between the lens and the dew shield - do you have a wrap-around dew shield (like I have for the SCT)? Sounds like a good place for the strap.
@@Aero19612 hi James i have a refractor abd the dew cover slides up and down from the primary lens. When pushed in, I can get the dew strap fitted around the lens cell nicely and then pull the cover back out to image.
@@julianmatthews3262 ahhh, ok. You’ve got mor of a gap between the lens cell and the dew shield than my scope has. If mine had that gap, I’d do what you’re doing
Any thoughts of returning to a guide scope for the refractors James?
Ha. Not yet! The OAG isn't really necessary for the refractors, but I do like the cleaner look and lower weight with the OAG. This shadow though is a bit irritating. Not sure why it didn't calibrate out like it has in the past. Welcome back, Rob!
Hello James. Why u dont try the dew heater ring of celestron in your 9.25 ?
Indeed! I may try that out. This video is about my smaller refractor (ED102). For my SCT, I also use a dew strap wrapped around the dew shield and I get through the night OK. But I do like the Celestron heating ring. Well worth looking into. Thanks for watching, Guillermo!
Hi James. Great video as usual. I had problems with short duration flats with my ASI533MC camera. I did some testing and found that below about 1 sec exposure the adu was nonlinear and more variable. The result was similar to your oag issue (gradients more noticeable with increased # of subs. I am not sure if my ASI2600MC has the same issue (I went to 4ish sec flat/dark flat exposures). Thanks for the info.
Ahhh. See?! That's a good reason. I don't doubt that there are pieces of equipment or combinations of equipment that can/will lead to restrictions like the 2-sec rule (not to be confused with the 5-sec rule when we drop a Dorito on the floor). But that 2-sec gets engraved in stone and passed around without the supporting "...and here's why". Thanks for bringing a legit "...and here's why" to the discussion, Bob!
Wow 🤩 Thanks for this in-depth analysis. Will change my filter wheel setting now! And take new flats, fingers crossed that my donuts disappear
Give it a shot, Bengt!
Jay, if you run local normalization on your calibrated lights it will help with the oag dark spots, give it a try during your calibration process. Bill
Thanks for the suggestion, Bill! I'll look into it.
@James Lamb Jay, one thing I forgot to add is that you may need to correct a light frame and use it as a reference for the local normalization. This is a work around, however I do find all of this odd as the oag shadow should have been fixed with flats to begin with. Have you tried using different stacking software to see if you get the same results?
@@billblanshan3021 Well, as you say, this is odd. It's always worked fine before. I am experimenting with including many more subs in the Master FLAT stack from 40/50 to 150. There's just a tiny bit of error left after the calibration with 40-sub Master FLAT, so maybe I just need to squeeze that little it more out of the flat correction.
Nice Media Room . It would be nice to see your setup (being an HT enthusiast myself) . Anyway , great catch with the Filter wheel . It's always something ./SRK
I need a media room where my teams win...
Perhaps you are producing too much heat which may cause unnecessary turbalance. I have used a Kendrick ambient temp controller but now use a celestron 2x & 4x controller with the additional sensors that are placed under the dew strap (next to lead in before the actual element). The sensors provide feedback to the controller so that the power to the strap is applied only enough to get to the desired temperature. In the case of the celestron controller, just above the dew point. You can adjust the agressiveness of the control. In this way, large or small dew straps are all controlled evenly (ie. main OTA or guide scope) and you don't have to manually guess to save power or brute force to control dew because you have that feedback loop.
Hey Greg. I doubt that my poor little dew strap can generate enough heat to produce turbulence. I swear it's barely warm to the touch at max heating. A feedback option is a nice idea. I run my setup off of house power, so I'm not worried about conserving energy. Even so, all of the power for cameras, the mount, focuser, etc is coming through the Ultimate Power Box so It's not like there's an infinite source of power. Thanks for watching!
@@Aero19612 Re turbalance, how do you actually know? An interesting call for a person who tends to measure and collect data. Re feedback, it's something that Kendrick do with measuring ambient temperature and controllers like Celestron and PrimaLuci do with Dewpoint. But many people seem fixated on the Pegasus type controllers or even manual and don't really get the feedback idea. Any manual control is just a guess and you have to over do it just to make sure. (bad for portable unless you have heaps of stored power). BUT, I think it might be interesting to know if a dew strap can cause measurable turbalance and how one might go about measuring that impact.
I do not know that turbulence is not an issue. I surmise that based on what I do know:
1. I've using the dew strap the same way for years and never had this problem, so if turbulence were the issue, it seems like I'd be having the problem consistently
2. Turbulence is a great hypothesis but how do you prove it? Any sensor (pressure sensors?) placed in front of the lens would block the light. How do you differentiate turbulence caused by the dew strap vs wind blowing across the front of the scope? How do you differentiate turbulence from the effects of seeing in a long-duration exposure? How can turbulence affect only one corner of the image?
I do like to measure and collect data...when it seems like a productive use of my time. I don't know how to collect these data or how to answer the questions above. Questions I ask myself before I measure something: (a) do I think it's important? and (b) Can I measure it and evaluate the effects?
What are advantages of OAG for 4” refractor? Thanks
I suppose there are two:
1. Guiding at smaller guide image scale since you’re looking through the main optics rather than, say, a 250mm guide scope.
2. Cleaner (lower mass moment of inertia) setup without a guide scope
But do those advantages mean you get better images? Probably not. At the end of the day, just go with what have/prefer. For my refractors, I just move everything from one scope to the other.
@@Aero19612 I have used OAG on refractors before. Complicated and no advantages. I asked to see what advantages other people get.
Do you use 294 camera? If yes, then I am not surprised that calibration doesn’t work sometimes. 294 has unstable sensor. I take only 5 frames of flats and dark flats to get master flat. Try using only first 5 frames. It may help. It helped me to pull un calibratable cases.
Looks like the filter wheel has backlash and thus the position is not the same when it goes in the opposite direction. If that’s the case, ZWO should fix that in future models of the filter wheel.
The filter wheel uses a friction drive. I believe it also uses a light sensor to ensure wheel filter is properly indexed. Backlash may be in the motor reduction gears, but more likely something is off with the tolerances in how it detects proper index.
Certainly looks that way, Alfredo. Wish I had done this experiment when I first got the filter wheel to see if that "backlash" was present out of the box. I'll get worried if it starts losing location precision in the Unidirectional mode.
@NightOwl This will be something to keep an eye on. There is a "Calibrate" option, but I have not tried that and don't really know what it does. May have to look into that.
@@Aero19612 According to the manual the recalibration function is indeed for the IR position sensor. Maybe give that a try and see if it helps? I used to have the same filter wheel and did not encounter the problem. I don't imagine backlash anywhere to be an issue if the position sensor is functioning properly.
I'm not an imager, just a long-time visual observer so I'm out of my league here, but... 🙂 Am I splitting hairs to point out the objective (no pun intended) of a dew heater is to raise the temperature of the optic above ambient, not the temperature of the (immediately adjacent ambient) air above the objective? Or is that what you meant? For visual I try to place a dew heater as directly as I can over the *edge* of the cell / lens. That's not always entirely possible due to construction details (e.g., dew shield attachment, etc.), but ideally that's where I'd like it to go -- On, not in front of nor behind.
Very interesting video. Although I don't image, I can totally understand: ruclips.net/video/7REI-kBlyQ8/видео.html&ab_channel=rizzlerazzleuno
Hey, Len. I suspect that imagers and eye-ballers are equally affected by dew, so you are definitely not out of your league. Yes, the "heat the optics" VS "heat the air in front of the optics" is an unsettled debate in my mind. I tend think heating the optics is the best approach...if you have enough thermal energy to heat the dew shield/tube + glass. These dew straps don't get all that hot, so (in my mind) heating the air in front of the glass is the path of least resistance. Well worth a "rabbit hole" deep dive!
@@Aero19612 makes sense. My only thought (?) was that to prevent condensation, the glass needs to be warmer than the (wet) air, hence heating the glass not the air. But if it works the other way 'round, I can't argue with results! 😀
In a pinch, it's better to be lucky than take the time to be good. Just be sure to plug your dew strap in!
I never liked ZWO because they have these funny quirks in their products. ZWO have leaking vameras, failing focusers, weird filter wheels and crashing proprietary airpros. I have qhy wheel/camera with senso2 focuser and mini-pc.....never any problems.
I've generally had pretty good luck with ZWO products but have heard of "failures" of this device or that device. I've got a lot of money invested in ZWO products, so I hope they last!
@@Aero19612 To be fair, I recently bought the AM5 because of the weight and I do have other zwo products but some of their stuff is sus.
I imagine there's a cycle there. Early in a business's lifetime, they may cut corners and not have total control over quality, then they get good and things stabilize. But if they go through a fast-growth period, some of those quality control issues pop back up. We just have to hope we're buying their products on the good side of the cycle. But we never know.