Problem with Flat Frames - Forums

Topic starter

Jul 21, 2023

Hi all,

I haven't really been happy with an image I took of LBN 1093 back in May. I did do an initial edit which turned out ok, but I would like to re-edit this data with new techniques I have learnt over the past couple of months.

Whenever I try and stack the data, it comes up like this. I have no idea why it's doing this for. What I have attached is a starless version with DBE applied using similar settings as what Adam Block used in his Starless DBE YouTube video.

I think this issue might be because I accidentally shot at 400 ISO when I imaged and the reason I did that was so the exposure would be correct, but upon inspection of taking the images, as LBN 1093 rose higher in the sky, my images got substantially darker with each image I took. I don't know if this would be the culprit or not.

I did shoot my flat frames at the same ISO as my light frames, but I suspect that my flat frames might be over-correcting as in the image it appears that the image hasn't been corrected by my flat frames. As it has been quite some time between imaging sessions, I am definitely not going to be able to re-shoot my flat frames again as the dust has been cleaned and the imaging train was disassembled after the imaging session.

I hope that there is a fix somewhere that someone knows or if I just need to accept my loss, throw the data out and re-shoot LBN 1093 again in the future.

Zak

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andrea tasselli

Frequent contributor

Jul 21, 2023

You don't say with what and what settings (if this applies) but I suppose it's a DSLR (you're still sitting on the fence as far as getting an astro-cam I gather). My guess is you got the flats wrong for one reason or another. Given how poor the data looks I'd go and ditch it and start over again once you get your flattening technique sorted (easier said than done, I know…). So I guess you'll need to tell us how you go about taking your flats (and with what exactly) and we'll take it from there.

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Zak Jones

Topic starter

Jul 21, 2023

andrea tasselli:
You don't say with what and what settings (if this applies) but I suppose it's a DSLR (you're still sitting on the fence as far as getting an astro-cam I gather). My guess is you got the flats wrong for one reason or another. Given how poor the data looks I'd go and ditch it and start over again once you get your flattening technique sorted (easier said than done, I know...). So I guess you'll need to tell us how you go about taking your flats (and with what exactly) and we'll take it from there.

Whoops! Should have specified how I took my flat frames. Sorry about that! Need to get better at actually including the relevant information in the initial posts instead of having to reply and amending what I should have specified in the beginning.

I used a white t-shirt which was rubber banded at the end of the lens and then pointed my rig towards an indoor light like I normally do and shot 39 flat frames with the following settings:

1/25th
f/5.6
400 ISO
Canon 6Da
Canon EF 70-200mm f/2.8L IS II USM

Yes, I am still using my 6Da until I upgrade to a dedicated astro camera in the near future lol.

I agree, it does look like that the data is unsalvagable unfortunately, which is a shame as it's a great target to shoot at the moment. I might re-shoot it again the next clear night.

I might actually invest in a good LED tracing panel next so I can take flat frames properly. I have seen really good ones that are cheap on Amazon, might pick one up from there.

Zak

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andrea tasselli

Frequent contributor

Jul 21, 2023

Right-o…

Now, right off the bat your technique is at minimum risky and at worse downright wrong. As a diffusing screen a t-shirt is rather marginal and it is certainly so in the context of a zoom lens (which I would be loath to use in any AP settings but you gotta do with what you got, I get it). If you want a diffuser screen then get a proper translucent (milky) piece of plexiglass or such. If you don't have a flat panel yet (but get one ASAP) you can get by using said diffuser screen and the sky, taking dawn or dusk flats (but make sure the sun is actually below the horizon and point the camera 90 degrees away from where it was/it is going to be). At any rate, take always flat-darks and remve the master flat-dark (taken with the same settings of your flats) from the flat frames. This alone should improve you flats significantly. Laso, make sure there is no zoom creep while you using your zoom, especially not when taking flats (use sticky tape to prevent any changes to the zoom settings).

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Arun H

Frequent contributor

Perfect post

Jul 21, 2023

Zoom lenses are notoriously tricky, certainly the ones with IS.

You have lots of moving elements internal to the lens and small changes in their position will ruin your flats. Take your flats right after your lights if possible using a light panel. Or move to a prime lens which is more forgiving.

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Zak Jones

Topic starter

Jul 24, 2023

andrea tasselli:
Right-o...

Now, right off the bat your technique is at minimum risky and at worse downright wrong. As a diffusing screen a t-shirt is rather marginal and it is certainly so in the context of a zoom lens (which I would be loath to use in any AP settings but you gotta do with what you got, I get it). If you want a diffuser screen then get a proper translucent (milky) piece of plexiglass or such. If you don't have a flat panel yet (but get one ASAP) you can get by using said diffuser screen and the sky, taking dawn or dusk flats (but make sure the sun is actually below the horizon and point the camera 90 degrees away from where it was/it is going to be). At any rate, take always flat-darks and remve the master flat-dark (taken with the same settings of your flats) from the flat frames. This alone should improve you flats significantly. Laso, make sure there is no zoom creep while you using your zoom, especially not when taking flats (use sticky tape to prevent any changes to the zoom settings).

Ok cool, thanks for your insights about taking better flat frames, I really appreciate it!

The only reason I've been using t-shirts is I saw a post from AstroBackyard that he also uses t-shirts to take his flat frames. Mind you, this was when he was a lot earlier on in his astrophotography journey. It might be time that I update my flat frame capturing techniques.

What would be a good type of flat panel that I can get from say Amazon or eBay? I have been looking at one for quite some time now, but I haven't ordered one as I don't know what should be the minimum specifications that would be good enough for flat frames.

I do like using my zoom lenses for convenience as there might be an object in the sky that needs a wider field of view, but yes they can be a pain for sure. My 70-200mm is an internal zoom thankfully, but there have been a couple of occasions where I have accidentally nicked the zoom ring. Most of my lenses that I own are zoom lenses, but I do have a couple of prime lenses, including the Samyang 135mm f/2 and Canon EF 400mm f/5.6L USM, both of which I have used for astro, and they are great performers. I haven't used my Samyang for a while, but I am hoping that I can use it more in the coming months.

Are there any benefits of using flat darks with a DSLR? I did take flat darks for a while back in 2021 up until mid last year as I did read that flat darks shouldn't be taken with a DSLR, as they are designed to be used with dedicated astro cameras. Would it be worth me going back and re-running my old data through WBPP that have flat darks? I did recently re-stack them with WBPP using new techniques I have learnt, but I didn't use the flat darks, as I instead used bias frames to calibrate the flat frames.

Also, apologies for the delay in my response, a lot of things have been happening here at home.

Zak

Zak Jones

Topic starter

Jul 24, 2023

Arun H:
Zoom lenses are notoriously tricky, certainly the ones with IS.

You have lots of moving elements internal to the lens and small changes in their position will ruin your flats. Take your flats right after your lights if possible using a light panel. Or move to a prime lens which is more forgiving.

I agree, they are definitely tricky lenses to tame!

I really want to use a light panel for my flat frames now, seeing that a whole night's worth of imaging was ruined due to my poor flat frame technique...

That's why I am posting on here as much as I can, not to spam but to learn. That's so I can learn from like-minded people that are active on here and make the most out of my astrophotography journey!

Zak

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AwesomeAstro

Jul 24, 2023

I highly recommend that you don't take flats on a DSLR with exposures that short (1/25"), it introduces so many problems (unless the camera has some rare issues with linearity). 1 second is a good minimum, 2-4s is ideal.

The issues are likely not from ISO values in the lights either. Small changes in ISO don't make artifacts like that.

I also use a t-shirt to diffuse my light panel with good success, so long as the flats are sufficiently exposed!

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andrea tasselli

Frequent contributor

Jul 24, 2023

I highly recommend that you don't take flats on a DSLR with exposures that short (1/25"), it introduces so many problems (unless the camera has some rare issues with linearity). 1 second is a good minimum, 2-4s is ideal.

The issues are likely not from ISO values in the lights either. Small changes in ISO don't make artifacts like that.

I also use a t-shirt to diffuse my light panel with good success, so long as the flats are sufficiently exposed!

I really don't think so.

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