Please share your thoughts about when the flats should be taken during a session. - Forums

Topic starter

Nov 10, 2023

I am looking at all the information and different opinions available on the internet on whether flats should be taken after the subs, or that it is ok to take them at the beginning. There seems to be a consensus that it is absolutely imperative to take them for every session, meaning if one shoots the same target many nights in a row, flats should be taken each night.

Frankly, I can't make sense of it. I am not saying that flats are not required, obviously they are but I don't see how taking them before or after the subs, or taking them only once when shooting multiple nights (as long as focus and temperature is the same) vs taking some every night makes a difference.

Flats are used to substract anything that is on your lens/scope (usually dust) during stacking. Every youtuber astrophotograher emphasize about how important it is that nothing moves, that one has to be extremely careful because you don't want that dust to move.

That's what does not make sense to me. If I shoot for 7 hours, dust will come and go all the time, if I take flats at the beginning then I only pick the dust that is already there but none of what will accumulate during shooting. Then on the opposite, if I take flats at the end, I only pick the dust that's there which only matches the last few subs that I took.

Another thing that bothers me is that people say not to move anything when taking flats but then when they show how to do it, they rotate they gear to have the lens/scope facing up so they can put a sketch pad on top.

And while they say not to move anything because we don't want the dust to move, they don't mention the fact that things moved constatly during shooting with tracking and they moved a lot when a meridian flip takes place.

My point here is no matter when we do it, we will take flats for just a few minutes, there is no way they can cover hours of shooting where things may change constantly.

So with all that in mind I ask myself……..does it really make a difference whether I take them at beginning, the end, once for multi-night sessions on the same target or every night ?

I'd like to have some opinions on this. I am new to astrophotography and I don't think I know better than everybody who has been doing it for years so………what am I missing here ? There's got to be something I am not getting.

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Kristof Vandebeek

Nov 10, 2023

Flats don't only correct for dust, but also for vignetting, that's a good reason why you shouldn't rotate single parts of your optical train independently.

The dust that lands on top of your telescope (eg: if you have a refractor) or on your mirror (eg: if you have a newtonian) will not be as visible as dust closer to your sensor (eg: on your filters, on your sensor, ...). You should try the following link to do a calculation of dust size on sub-exposures and then you'll understand this: https://astronomy.tools/calculators/dust_reflection_calculator

I take my flats for all my filters (eg: L, RGB, Ha) at the end of my session, this also makes sure that my focusing is more or less the same as during my imaging session.

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Tom Boyd

Nov 10, 2023

As I travel to my observing location, I set up my rig before every observing session and usually do it while it is still light out. I take my flats and flat darks at the beginning of each session while I am waiting for it to get dark so as not to waste my limited dark observing time.

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Christian Großmann

Nov 10, 2023 · Edited: Nov 10, 2023

Hi Eric,

reading your message makes me feel bad, because I do not care too much about it.

I do flats usually before a session when it begins to get dark. The reason is, that I have to go to work in the morning and I have no time or motivation to do it after sleep. On weekends, It might happen that I do them in the morning. But I don't really care about the right time.

If I am not too busy, I take flats for every session, indeed. But because I have a lot of responsibility in my life, that isn't always possible. So I sometimes reuse flats for 2 to 3 days. But I have a kind of permanent setup and do not move the equipment in and out all the time. If I would, that would be a very different story. Of course, if I rotate the camera or change some filters and I have to remove the cam, I have to take a new set of flats.

I do astro photography just for fun and for my own pleasure. So the somewhat reduced quality is usually not a big deal. Most of us process the images to make them look nice. However, if you do scientific work, that might be a different story. If you try to measure star brightnesses of variable stars for example, you want to get the most accuracy out of your subs. Reusing flats might not be a good idea in this case.

I might not be the best astro photographer here, but I never realized any problems with dust spots in my images. Flats do a good job, here, but as you said, there must be some new dust particles during a session that are not covered in my flats. I never saw one of those in my images, although I never actively looked for them.

Thinking about it, I do not feel that bad anymore. In theory, all the things you mentioned above has to be taken into account. In reality, the world isn't as critical as it seems. At least in my experience. If I would've read your lines as a newbie, I might have never pointed a telescope to the sky. But with a good set of "standard" calibration frames, you can reach a really high level of quality. It doesn't matter too much, when you take them. All the things you do to get even better quality takes a lot of effort and the outcome only slightly increases. So it's up to you where you draw your border line.

That's my personal opinion. I guess, other users do not agree with me. But for me it works quite well. Everything more would drive me crazy.

CS

Christian

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Brent Newton

Nov 10, 2023

Before vs after is fine so long as you have achieved focus. Practically, however, your focus is much more likely to change a few times at the start of the night vs the end/ Temperature, at least in colder months, may drop much more rapidly during the first few hours of darkness compared to early morning, where any continued drop in temperature is usually more gradual. If using some kind of automatic refocusing method then that could mean many more of your exposures taken throughout the night will be better suited to Flats at the end of a session versus at the beginning.

Does it make an actual difference? That is harder to say, and I've taken Flats at either end of the night for years without a single instance of them not correctly removing dust and light fall-off. And if you have an automatic Flats panel set up along with a program which allows sequencing like SGP or NINA, it basically doesn't matter. I just have Flats automatically capture once Astronomical darkness ends with no extra effort on my part.

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Scott Badger

Nov 10, 2023

If nothing is rotated or swapped in the optical train, every couple months works fine for me.

Cheers,
Scott

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Eric Gagne

Topic starter

Nov 10, 2023

Christian,

You are right on the spot with my reason for reflecting on this. Taking flats in the morning is not an option for me because I will never leave my equipment unattended outside while I am sleeping. I take flats and dark flats right after I set things up. Then when I start taking subs I will let it go by itself (asiair plus here) but I keep an eye on thing with the app. This actually turned out to be a good just 2 days ago because auto-guiding went completely nuts, I don't know why, and while trying to reposition it had no clue where it was and was doing a 360 on the dec axis, rolling some cables pretty tight around the mount head.

Anyway, back to topic, since I won't leave it out until morning and I want to have as much time as possible to take subs, I do it first. I just shot NGC 7822 for 3 nights over 5 or 6 days and I only took flats and dark flats the fist night. Reading the replies above I guess that was a mistake since I move my equipment in and out for each session. That's probably something I should take into account and do it for every session from now on……..which means I will need to learn how to do multi-sessions stacking in PI

That said, I agree that this is only a hobby, it's not for science and it does not have (and cannot be) perfect so I guess the perfectionnist in me will just have to live with the fact that some dust may have moved during a meridian flip and no longer match my flats…………………which by the way I don't even know if it would be visible on the final images.

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apennine104

Nov 10, 2023 · Edited: Nov 10, 2023

I used to take flats every session. Now, I use the following rules:

- Take flats per multi-day imaging run at the same temperature, gain and relatively same EAF focus position as my lights. Meaning as long as I don’t physically rotate or disassemble the optical train, I re-use the flats for that target run, which could be up to 1-2 weeks.

- I use NINA to keep a constant flat exposure time, adjusting flat panel brightness for each filter. This allows me to re-use flat darks like regular darks.

I have not noticed any issues with this technique. My caveat is that I just image at my house, so no extended car drives to shake around dust.

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Christian Großmann

Nov 10, 2023

That said, I agree that this is only a hobby, it's not for science and it does not have (and cannot be) perfect so I guess the perfectionnist in me will just have to live with the fact that some dust may have moved during a meridian flip and no longer match my flats.....................which by the way I don't even know if it would be visible on the final images.

I guess, there are lot of technical issues to solve in the beginning of an astro photographers career, which will increase the image quality much better than reducing some dust spots (if you already do flats, of course). In your case, taking flats every night might be a good solution.

Eric Gagne

Topic starter

Nov 10, 2023

- Take flats per multi-day imaging run at the same temperature, gain and relatively same EAF focus position as my lights. Meaning as long as I don’t physically rotate or disassemble the optical train, I re-use the flats for that target run, which could be up to 1-2 weeks.

- I use NINA to keep a constant flat exposure time, adjusting flat panel brightness for each filter. This allows me to re-use flat darks like regular darks.

I have not noticed any issues with this technique. My caveat is that I just image at my house, so no extended car drives to shake around dust.

Do you have a permanent installation ? If not do you leave the equipment outside all the time ?

I got everything mounted on my SA GTI. I never disassemble any of it. I take the whole thing outside when I am ready to shoot and back inside after each session.

GergoB

Nov 10, 2023

When I purchased my electronic camera rotator, I found that my filters and everything in the imaging train needed to be spotless clean to be able to shoot multiple rotations an evening. I went from shooting flats every evening to once every few weeks. I have on multiple occasions tried using flats from 6 months ago as well and they work just fine. This isn't for everyone and requires a really clean imaging train.

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andrea tasselli

Frequent contributor

Nov 10, 2023

I use only OSCs so things are a bit different than when I was using monochrome camera and FW, but I take flats only every now and then (maybe months apart) and I remove and replace both filters and cameras and attendant paraphernalia and does not affect the end results at all. I am, however, religious in taking flats every single time at the end of a session when using a DSLR, reason being that the movable mechanics within (mirror and shutter) with the attendant electrostatic fields do tend to move the dust around in unpredictable way (and sometimes even across a session) so I'm not taking any chances there. If, however, the said DSLR has a clip-on filter (i.e. got to be a Canon) then you can be more relaxed about that as well. In tests I've found the variation being negligible if at all.

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Stuart Taylor

Perfect post

Nov 10, 2023

I take flats at the end of every session using a flats panel with the scope pointed to the azimuth. With NINA's flats wizard it's a very straightforward process and only takes me a few minutes

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apennine104

Nov 10, 2023

Eric Gagne:
- Take flats per multi-day imaging run at the same temperature, gain and relatively same EAF focus position as my lights. Meaning as long as I don’t physically rotate or disassemble the optical train, I re-use the flats for that target run, which could be up to 1-2 weeks.

- I use NINA to keep a constant flat exposure time, adjusting flat panel brightness for each filter. This allows me to re-use flat darks like regular darks.

I have not noticed any issues with this technique. My caveat is that I just image at my house, so no extended car drives to shake around dust.

Do you have a permanent installation ? If not do you leave the equipment outside all the time ?

I got everything mounted on my SA GTI. I never disassemble any of it. I take the whole thing outside when I am ready to shoot and back inside after each session.

My situation is similar to you. I keep the scope fully assembled in a large bin inside, and bring it outside each time. I don’t take apart anything on the scope, only remove it from the mount, which I do store separately or leave by itself outside.

Rajat Kumar

Nov 10, 2023

Hi Eric,

I shoot from a DSLR and a 70 mm F/6 refractor. I do not have any set formula for shooting flats but I can share my process - if one can call that:

1. I am in Bortle 8 area and in a dusty environment. I do not clean my optics in any meaningful way. At best, once in a while I use air blower to clean the objective and sensor of my camera. Someone can explain this but since I have got my camera modified I can see LOT more dust motes than what was visible from my old unmodified camera. So shooting flats is imperative. It has become part of the work flow even at the cost of loosing some imaging time.

2. Normally, I shoot flats at the end of my session and it has never given me any trouble. I use NINA FLat Wizard and after first few failed attempts I have never faced any problem.

3. In case of multiple nights, more often than not, flats taken on the first night work well for subsequent sessions, provided there is not much gap between two sessions. Having said that, I do randomly test my subs by calibrating single and/or 5-10 stacked subs (chosen randomly from the ongoing session) with flats acquired from the previous night. If I see even one dust mote which is not calibrated or any other problem I take fresh set of flats for the subsequent session. This may sound time consuming but its not. I use ASI FITSViewer for this task which can calibrate single subs without taking time. [I don't know about ASIAir - I use a miniPC - I just remotely login and test my subs while sessions is running]

3. If there is a long interval between two sessions then I take fresh set of flats and dark flats. [A week would a long interval for me]

4. In case I have to take the scope off the mount without taking flats then I am careful in handling the scope. I will put dew shield carefully and keep the optical path unchanged till new set of flat frames are taken. Flats taken next day have generally worked for me. But I follow one rule- when in doubt take new flats.

For Internet/Youtube advice: Mostly, its well intentioned, informative and very good. I have learnt everything from there. But it may not always be optimal for your unique circumstance. Calibration use to be my biggest nightmare. I just did not understand why/what/how of calibration. But now, after 2 years or so things are different. I somewhat understand better - at least, from a practical point of view. Now, I dont worry about calibration that much - it happens without any problem. All I mean to say, I have found my own shortcuts which works well within my deviation tolerance level. You will too.

Please note that I have been doing AP for just two years. That too with some sub-optimal equipment under horrible sky conditions. By stating all this, I don't mean to claim I have cracked the AP master code. Far from it. Very Very far.

CS!

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Alex Ranous

Nov 11, 2023

Now this is heresy by conventional wisdom, but I take new flats every few months or when I change between my focal reducer and my flattener. Now my rig is permanently setup in my backyard observatory. I think taking flats nightly is more of an issue if you setup/teardown nightly. Even if you leave your camera attached to the scope, hauling around the OTA has the potential to dislodge dust specs on your sensor cover or filters. I once went nearly 4 months before some dust spot moved and I noticed my flats weren't correcting for it. Basically, I'm lazy and will try and get away without new flats as long as I can pull it off. I carefully look and new subs and if I see a hint of a new dust mote, I'll fire off a set of new flats.

Even crazier, I have a rotator, and I don't bother with new flats when changing position angles. I may shoot 3 or more targets a night, each with their own orientation angle, so having to shoot a unique set of flats every time I rotate isn't really practical. I suppose if you had an automated flat setup like a flip-flat you could do it, but I've found no issues with using the same set of flats regardless of the orientation. I've shot a set of flats at 0 and 90 degrees and compared one to the other and they're virtually the same. Now I have an Astro-Physics refractor, which is legendary for its build quality, so I don't know if you could pull this off with another OTA which isn't as precise and solid mechanically. I also image between 590 and 873mm focal length. I suspect that the longer the FL, the more susceptible that slight changes in the optical train would cause problems.

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dkamen

Nov 11, 2023

If you have a closed system which you usually do, then no you don't need to take flats at every session, as long as the system is not opened or bumped strongly enough for dust motes to change places. Only whenever the camera has been rotated relative to the telescope. I have used the same flats for months in a row.

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Miguel T.

Nov 11, 2023

If you have a closed system which you usually do, then no you don't need to take flats at every session, as long as the system is not opened or bumped strongly enough for dust motes to change places. Only whenever the camera has been rotated relative to the telescope. I have used the same flats for months in a row.

Do you trust that the filterwheel will always bring you back to your filter position at <1px accuracy?

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Alex G

Nov 11, 2023

Hello Eric

What I typically do is both before/after:
1. I focus on stars
2. Do ~10 flat frames + a few of dark flats
3. Do the imaging
4. Do another flats and dark flats in the end

Didn’t have much issues with this approach. What I like about it is a) I have the full set of subs per each session, b) I can “average” my flats before/after imaging in case something happens during the session

I use white T-shirt + phone’s flashlight and NINA flat wizard does everything for me.

Hope that’s helpful

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dkamen

Nov 11, 2023

Miguel T.:
If you have a closed system which you usually do, then no you don't need to take flats at every session, as long as the system is not opened or bumped strongly enough for dust motes to change places. Only whenever the camera has been rotated relative to the telescope. I have used the same flats for months in a row.

Do you trust that the filterwheel will always bring you back to your filter position at <1px accuracy?

I am not using a filter wheel any more and I wouldn't really consider a system that changes filters as closed, but out of curiosity, why would that be an issue? What is the measurable impact that e.g. a 1 mm displacement of the filter is predicted to have on the flat field seen by the sensor 50mm further down the optical path?

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Miguel T.

Nov 11, 2023 · Edited: Nov 11, 2023

Miguel T.:
If you have a closed system which you usually do, then no you don't need to take flats at every session, as long as the system is not opened or bumped strongly enough for dust motes to change places. Only whenever the camera has been rotated relative to the telescope. I have used the same flats for months in a row.

Do you trust that the filterwheel will always bring you back to your filter position at <1px accuracy?

I am not using a filter wheel any more and I wouldn't really consider a system that changes filters as closed, but out of curiosity, why would that be an issue? What is the measurable impact that e.g. a 1 mm displacement of the filter is predicted to have on the flat field seen by the sensor 50mm further down the optical path?

I'm myself trying to figure where to draw the line between absolute perfect flats and a small margin of error. Aside from dust motes, even high quality 3nm chroma filters aren't perfectly uniform so I count every pattern as something that should be calibrated. See the following image for the masterflat of my Ha taken with a FlatMaster. So far I've been taking flats on every session and between every filter switch and never took the chance to see how a slight displacement would look when super-stretched. Never found a clear answer when a filterwheel is involved. I'm not even sure how precise filterwheels are to begin with.

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Arun H

Frequent contributor

Perfect post

Nov 11, 2023 · Edited: Nov 11, 2023

Miguel T.:
Do you trust that the filterwheel will always bring you back to your filter position at <1px accuracy?

well that would mean you have to take flats every time you make a filter change, lol. So if you take RGB, take your R, take flats, take G take flats etc. you’d have watch that filter wheel like a hawk. If you do a pier flip and use a lum filter for your plate solve, your previous flats in the session would be useless or you’d have no way of calibrating those lights. You’d have to take flats multiple times a night. Seems like a recipe for insomnia. I use a filter wheel. I recalibrate my filter wheel every session. I used to take flats every session, but I now reuse my flats for about a month.

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ScottF

Nov 11, 2023 · Edited: Nov 11, 2023

I'm probably an outlier, but I only do flats periodically, as in once for several sessions. Unless I change something in the optical train like rotate the camera, etc. I'v had no issues doing it that way, and I don't have time in the morning to do it after every session. I guess if I started to have issues with the flats, I'd do it more often. Minor focus changes make no difference as far as I can tell. I think dust is a non-issue, but if you get a blob or oil on the sensor itself, then you need new flats.

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Rick Veregin

Nov 11, 2023

A lot of good advice here, and a lot depends on whether you have a FW or drawer and if you keep your optical train setup from night to night. A couple of points not made yet:

Generally dust donuts to not move during the night, and vignetting won't either unless you change something deliberately. Dust is adhered electrostatically and by Van der Waals forces (I'm a material scientist how worked with micron sized particles) to the glass, which is why it can be difficult to dislodge. So as long as you keep your optical train intact, and don't change the focus, you will generally be okay, dust doesn't move. Note if you use a light pad or a white cloth and the sky for sky flats, the flat is not in focus anyway. The important thing is that the image train is the same as when you took your lights. So I've never seen any effect of differing temperature of flats compared to the temperature of the lights. So for the most part, you can take flats before or after, I have even taken them days after sometimes when life intervenes in doing my flats.

WARNING, However, I have had situations where a donut goes for a jump from one spot to another, very occasionally you get creepers that shift. Quite rare, but this can wreck havoc with flats. So what I do is during imaging is to do quick stacks (I use DSS, which can register and stack hundreds of images in a few minutes), using my previous flats and darks. If all is well through the night I don't do new flats. If something shifts in the middle of the night and stays put, I will do new flats after the session. When you do get creepers, they usually settle down to a new location, so I have never had to do more than one flat before and one after, using a rejection algorithm in the stacking if only a few images have the donut creeping.

I don't understand a reluctance to take flats anytime. Flats with a panel literally take 2 minutes to take, so adds nothing to the end of a session if you need them. On the other hand I hate doing darks, they take so much time to do...

My advice is to monitor your lights as you go and do flats when you need to, you will never do more than you need to, and never get caught out.

Rick

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Elmiko

Nov 11, 2023

I take my flats at the beginning of the session. I use the same flats for several nights as long as I keep the equipment covered and protected from dust, and don't change the orientation of the image train.

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