What started as some simple questions around taking flats led to a series of experiments looking at all sorts of things. I'm trying to summarise the findings in three blogs so hopefully they can inspire/help others to find their own best solutions. Today I finished the second blog, in which I am comparing four different flat panels, two EL panels and two LED panels. To be honest, they are more different than I had expected.... You can find the results here.
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Willem,
Thank you for taking the time to share these very useful results.
I personally use a light pad. The brightness can be modulated by placing sheets of A3 or A4 paper on top. This will also improve uniformity.
Another way to improve uniformity is to rotate the flat panel between sets of exposures so the master flat ends up being more uniform. So far, I have been unable to justify the cost of flat panels costing several hundred dollars when simple solutions can achieve the same effect.
It is also recommended to take flat exposures that are at least several seconds long because there is reduced error in the actual exposure time (as a fraction of the targeted exposure) for longer exposures.
Arun
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Interesting article. I have the Alnitak flip panel but haven't used it due to concerns with wind effects during imaging (no observatory).
I used an EL panel with acceptable results. I'm currently using an Artsky flat panel (made in Italy) which works very well IMO. Artsky makes two version (as well as different sizes for various OTA diameters). One is strictly manual control with a rheostat and numeric readout on the power supply to allow somewhat repeatable outputs (if that was your only gauge of flat panel output). The other version has a USB port to allow direct software control (via ASCOM drivers) of the flat panel by programs like N.I.N.A. to automate much of the flats imaging process.
I use N.I.N.A.'s Flat Wizard to create matched sets of Flats/Dark Flats with a camera mid range ADU target (with a relatively generous tolerance of +/-20%). I have the manual version of the Artsky flat but set criteria within the N.I.N.A. wizard to help me adjust the output so that my exposure times and ADU level criteria are met. If I had the USB version, N.I.N.A. would do that automatically based on the priority/tolerance I set for exposure time and ADU level.
Using the N.I.N.A. wizard with the ArtSky panel has eliminated most of the issues with Flats/Dark Flats as long as I do my part and ensure everything with my rig is setup correctly. My biggest issue now is ensuring that I do this in a dark space or at night to ensure that there is no influence from light leaks. When your optical train encompasses OTA dew shield, field flattener, an OAG, filter wheel, and camera there are always going to be points of light leakage if a person is not diligent/vigilant (obsessive!!) about trying to prevent them.
CS
Clayton
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Interesting test. I use a light panel as well, which I can dim. If needed a white cloth. I never had any issues with my flats using this cost effective setup.
Rob
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Interesting article. I have the Alnitak flip panel but haven't used it due to concerns with wind effects during imaging (no observatory).
I used an EL panel with acceptable results. I'm currently using an Artsky flat panel (made in Italy) which works very well IMO. Artsky makes two version (as well as different sizes for various OTA diameters). One is strictly manual control with a rheostat and numeric readout on the power supply to allow somewhat repeatable outputs (if that was your only gauge of flat panel output). The other version has a USB port to allow direct software control (via ASCOM drivers) of the flat panel by programs like N.I.N.A. to automate much of the flats imaging process.
I use N.I.N.A.'s Flat Wizard to create matched sets of Flats/Dark Flats with a camera mid range ADU target (with a relatively generous tolerance of +/-20%). I have the manual version of the Artsky flat but set criteria within the N.I.N.A. wizard to help me adjust the output so that my exposure times and ADU level criteria are met. If I had the USB version, N.I.N.A. would do that automatically based on the priority/tolerance I set for exposure time and ADU level.
Using the N.I.N.A. wizard with the ArtSky panel has eliminated most of the issues with Flats/Dark Flats as long as I do my part and ensure everything with my rig is setup correctly. My biggest issue now is ensuring that I do this in a dark space or at night to ensure that there is no influence from light leaks. When your optical train encompasses OTA dew shield, field flattener, an OAG, filter wheel, and camera there are always going to be points of light leakage if a person is not diligent/vigilant (obsessive!!) about trying to prevent them.
CS
Clayton What is the shortest exposure time that you can take without any visible banding? I was disappointed by the Flatmaster, who needs at least several seconds to be 'more or less' banding-free. Any chance you could post a 0.1s image? The Artesky looks like an interesting panel and has a 550mm version as well, a size that not many manufacturers make.
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Willem Jan Drijfhout:
What is the shortest exposure time that you can take without any visible banding? I was disappointed by the Flatmaster, who needs at least several seconds to be 'more or less' banding-free. Any chance you could post a 0.1s image?
The Artesky looks like an interesting panel and has a 550mm version as well, a size that not many manufacturers make. I keep my flats to no shorter than 1s; there was an article on this a couple of years ago concerning CMOS sensors and Flats. I don't remember the author(s), but there is an issue with CMOS and extremely short exposures (moire patterns being one of the potential problems as I remember). Anyway, the recommendation was that the sweet spot for Flats (on the shortest exposure side) is around 2-3 seconds. Keep in mind this was before the latest generation of sensors, back when amp glow was a notorious issue for every sensor on the market. Since my Artsky is not the USB version, I set the output high enough that combined with my ADU target, NINA Flat Wizard can start generating my flats with a Lum exposure of 1-2s (I have a 2"x7 EFW). That gets me through the LRGB filters with a max exposure time of 8s on the B filter. Then I have to step up the panel output for the 3nm NB approximately +2.0 relative to the output used for the LRGB filters. After the Flats are done, NINA waits for me to indicate I have covered/capped the scope and it runs through and creates the matching set of DarkFlats. I could set the tolerances in the Flat Wizard so that it would require me to change the output to keep the exposure time on the high side shorter, but <10s that I have implemented seems to work. Light leaks messing with the DarkFlats and Darks have been my primary bane since starting the workflow/process above. You would think once fixed it wouldn't be an issue, but I've found it requires constant vigilance. ;-)
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Willem Jan Drijfhout:
What is the shortest exposure time that you can take without any visible banding? I was disappointed by the Flatmaster, who needs at least several seconds to be 'more or less' banding-free. Any chance you could post a 0.1s image?
The Artesky looks like an interesting panel and has a 550mm version as well, a size that not many manufacturers make.
I keep my flats to no shorter than 1s; there was an article on this a couple of years ago concerning CMOS sensors and Flats. I don't remember the author(s), but there is an issue with CMOS and extremely short exposures (moire patterns being one of the potential problems as I remember). Anyway, the recommendation was that the sweet spot for Flats (on the shortest exposure side) is around 2-3 seconds. Keep in mind this was before the latest generation of sensors, back when amp glow was a notorious issue for every sensor on the market.
Since my Artsky is not the USB version, I set the output high enough that combined with my ADU target, NINA Flat Wizard can start generating my flats with a Lum exposure of 1-2s (I have a 2"x7 EFW). That gets me through the LRGB filters with a max exposure time of 8s on the B filter. Then I have to step up the panel output for the 3nm NB approximately +2.0 relative to the output used for the LRGB filters. After the Flats are done, NINA waits for me to indicate I have covered/capped the scope and it runs through and creates the matching set of DarkFlats. I could set the tolerances in the Flat Wizard so that it would require me to change the output to keep the exposure time on the high side shorter, but <10s that I have implemented seems to work.
Light leaks messing with the DarkFlats and Darks have been my primary bane since starting the workflow/process above. You would think once fixed it wouldn't be an issue, but I've found it requires constant vigilance. ;-)
Thanks George, good to see that you can go down to 1s without banding. Indeed, recommendation has long been not to go shorter than 1s, but not sure if that still holds today. It does require a good flat panel though, but with a good EL panel, I seem fine to go down to 0.1s. Your approach makes a lot of sense, one brightness setting for LRGB and then a higher setting for SHO, and the Artesky can apparently go bright enough to keep all narrowbands under 10s. You are fully right that in daylight, Darks of any kind will reveal the smallest of light leaks. So best to build a dark library taken with the cap on the camera. But that would require even more control over the light to keep exposures the same. So if I go with an ArteSky, perhaps the USB version might be best.
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The recommendation to keep flats greater than 3 seconds long is based on work by John Upton. You can read the linked thread, but it comes down to: - That particular CMOS sensor is non linear at exposures lower than 3 seconds. One reason is because the timing for short exposures is dealt with on the sensor itself, and the increased heat load causes inconsistencies because the cooling cannot respond fast enough.
- For slightly longer exposures, the timing is controlled from the driver. But there can be errors in this timing, and the impact of the error is greater for short exposures than long ones. For example, an inaccuracy of 0.1s is 10% of a 1 second exposure, but only 2.5% of a 4 second exposure.
While his work is specific to the 294MC, these are excellent guidelines for all CMOS sensors, since the linearity is not well known, nor the error in the implementation of exposure time.
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For my qhy268m I asked directly to QHY, and they told me that >=1s should be used for flats.
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Willem - one other point I want to make about your writeup.
You mention increasing gain to achieve reasonable flat exposure times.
But remember that gain does not impact the number of photons collected. And you need a large number of photons to get enough SNR to accurately distinguish something like a dust mote. So very high gains (like 300) which are much above the gains where you normally take lights would not be recommended. Or if you use them, take a larger number of flats to compensate so you get the same total exposure time.
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I use an artist's light pad on to which I have attached a 5mm thick sheet of white translucent acrylic to act as a diffuser. I set it to the lowest brightness setting then use NINA's Flats Wizard in auto-exposure mode. Setting the histo peak to be 30-35% typically sets an exposure time of 2 to 3 sec, then takes 20 of them followed by 20 matching flat darks (once the lens cover is in place). I find it works really well, so I have never felt the need to invest in a flats panel (which seem to be about 20 times the price of my arrangement).
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Arun H:
The recommendation to keep flats greater than 3 seconds long is based on work by John Upton. You can read the linked thread, but it comes down to:
- That particular CMOS sensor is non linear at exposures lower than 3 seconds. One reason is because the timing for short exposures is dealt with on the sensor itself, and the increased heat load causes inconsistencies because the cooling cannot respond fast enough.
- For slightly longer exposures, the timing is controlled from the driver. But there can be errors in this timing, and the impact of the error is greater for short exposures than long ones. For example, an inaccuracy of 0.1s is 10% of a 1 second exposure, but only 2.5% of a 4 second exposure.
While his work is specific to the 294MC, these are excellent guidelines for all CMOS sensors, since the linearity is not well known, nor the error in the implementation of exposure time. I worked with John and did the exact same testing for the 294MM. Results were the same for the IMX492 sensor. Non-linear response below 1". Avoid 1" like the plague because that's where it switches. By 2-3" you're well into the typical linear response. Recently another issue with the 294/492 sensor has been shown: non-saturation at certain gain levels. This was evident in the range of gains 120-200 on the ZWO branded cameras (not sure equivalent numbers on the other brands). Basically, you can expose for as long as you want and you'll not completely saturate the sensor. Here's a post from the thread on the SharpCap forums. Sorry about going a bit off topic...
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Jonny Bravo:
Recently another issue with the 294/492 sensor has been shown: non-saturation at certain gain levels. Yes, I read about that here as well. In the context of taking flats with that sensor, you'd probably want to avoid any part of your pixels getting close to saturation because you want to avoid hitting that nonlinearity as well. I'd think that targeting 20-30,000 ADU keeps you safe. I have not seen that particular issue affect my lights - possibly because only a relatively small number of pixels are affected. I image at Gain 120 all the time with my 294MM.
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While we are talking of flat panels, I would like to put a word in for DeepSkyDad flat panels. I am not an expert in this domain but I find the panel for my AT115EDT to be very easy to work with. I can control brightness to ensure my exposures are at least 2s with NINA Flat Wizard and I generally haven’t noticed any banding or other artifacts when taking flats for my ASI533MM and 7 OptoLong filters. And as mentioned above, setting an ADU target of 32K with a 10-15% tolerance works very well in this combination.
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Chiradeep Chhaya:
While we are talking of flat panels, I would like to put a word in for DeepSkyDad flat panels. I am not an expert in this domain but I find the panel for my AT115EDT to be very easy to work with. I can control brightness to ensure my exposures are at least 2s with NINA Flat Wizard and I generally haven’t noticed any banding or other artifacts when taking flats for my ASI533MM and 7 OptoLong filters. And as mentioned above, setting an ADU target of 32K with a 10-15% tolerance works very well in this combination. What is the exposure you get for your SII filter (6.5nm?) at gain 100 and 32K ADU? And for that setting, is the panel at maximum brightness?
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Willem Jan Drijfhout:
Chiradeep Chhaya:
While we are talking of flat panels, I would like to put a word in for DeepSkyDad flat panels. I am not an expert in this domain but I find the panel for my AT115EDT to be very easy to work with. I can control brightness to ensure my exposures are at least 2s with NINA Flat Wizard and I generally haven’t noticed any banding or other artifacts when taking flats for my ASI533MM and 7 OptoLong filters. And as mentioned above, setting an ADU target of 32K with a 10-15% tolerance works very well in this combination.
What is the exposure you get for your SII filter (6.5nm?) at gain 100 and 32K ADU? And for that setting, is the panel at maximum brightness? I’ll have to go back and check. I do recall that I have to either have longer exposure time for a couple of those filters (Sii and Ha IIRC) or bump up the brightness although I don’t think I’ve ever hit max brightness. I’ll revert later in the day after checking.
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In case people are interested - here is a procedure by John Upton on measuring uniformity of a flat panel. I believe it is adapted from a publication by a group that does photometry. And here is Rouzbeh's measurement of the Aurora panels using this method.  |
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With my ZWO585MCPro I just take sky flats at whatever exposure is necessary to get the histogram peak between 40 and 50%. The resulting master flats seem to work fine even though my exposures are far shorter than suggested. In fact, I've always done sky or wall flats and have never had an issue. I just take them in the morning, before breaking down my rig from the night before. I can certainly see the utility of using a panel in certain situations but for some people at least, sky flats will work fine and won't cost a dime.
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Willem Jan Drijfhout:
What started as some simple questions around taking flats led to a series of experiments looking at all sorts of things. I'm trying to summarise the findings in three blogs so hopefully they can inspire/help others to find their own best solutions. Today I finished the second blog, in which I am comparing four different flat panels, two EL panels and two LED panels. To be honest, they are more different than I had expected....
You can find the results here. Willem, thank you for sharing the comparison. I was also using a Lightpad with satisfying results but I switched recenty to the RBFocus eXcalibur EL Flatpanel with a servo motor in order to automate flat captures with calibrated exposures. The functionality is similar to the Alnitak FlipFlat Panel but it is available for diameters up to 415mm (>415mm upon request) but much cheaper than the Alnitak. For my 12" scope the costs are 511€. The results are pretty good, no banding and no more issues with taking the flats, as this can be done after every bunch of shots automatically using NINA. CS, Xeno
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