Flats and darks

[chubster1302]

Did a couple of hours on M81 last night and took my darks straight after and my flats in the morning. I'm going to do another couple of hours this eve. Do I need to do darks and flats again and if so do I just throw them all in  with the other flats and darks. Same with my subs, all in one folder?  Will be using dss to stack

[Elp]

If you're using an uncooled camera you'll have to retake darks and stack each set of images with their respective darks. If your image train remains completely untouched including focus position and filter used then one set of flats will work. Depending on the quality of your focuser it can sometimes shift when there's a camera loaded onto it and the scope points higher up, if so new flats required. Then stack the stacks as long as they remain linear unstretched.

Edited June 24, 2023 by Elp

[chubster1302]

OK, so I'll take more darks tonight, thanks. So are you saying I need to stack lastnights subs and calibration files in dss, then do the same for tonight's? 

[Elp]

You can, as stated if nothings changed to affect the calibration frames then you can stack it all together. For good practice I always stack each nights, at least that way you can see if there were any issues during the session.

[ollypenrice]

The key thing is to understand what you're doing and why you're doing it.

Darks record the noise that is there in your camera during an exposure even when no light at all gets in. It shouldn't be there so it will simply be subtracted when darks are incorporated into a stacking software program. However, this noise must be nearly identical to the noise in your exposures. With a cooled camera, it will be. With an uncooled camera, it probably won't. With an uncooled camera, not running at the same temperature during successive exposures and not necessarily at the same temperature as the darks, darks are likely to do more harm than good. So... what's your camera?

Flats photograph the light path of your system. It should be perfectly even (flat) but won't be. All optics are brighter in the middle than in the corners and bits of dust create shadows. In a sealed system, left unchanged between nights, there is unlikely to be any change so flats can be re-used. If there has been a change then, clearly, flats taken before the change must be applied to subs taken before the change and vice versa. For amateur purposes, very small changes in focus between filters and during a run of exposures can be ignored.

Olly

Edited June 24, 2023 by ollypenrice

[chubster1302]

13 hours ago, ollypenrice said:

The key thing is to understand what you're doing and why you're doing it.

Darks record the noise that is there in your camera during an exposure even when no light at all gets in. It shouldn't be there so it will simply be subtracted when darks are incorporated into a stacking software program. However, this noise must be nearly identical to the noise in your exposures. With a cooled camera, it will be. With an uncooled camera, it probably won't. With an uncooled camera, not running at the same temperature during successive exposures and not necessarily at the same temperature as the darks, darks are likely to do more harm than good. So... what's your camera?

Flats photograph the light path of your system. It should be perfectly even (flat) but won't be. All optics are brighter in the middle than in the corners and bits of dust create shadows. In a sealed system, left unchanged between nights, there is unlikely to be any change so flats can be re-used. If there has been a change then, clearly, flats taken before the change must be applied to subs taken before the change and vice versa. For amateur purposes, very small changes in focus between filters and during a run of exposures can be ignored.

Olly

Thanks Olly. Camera is D5300. I took darks after my second set of subs. Have to admit, I'm struggling with the flats. Trying to use NINA and I either get too bright or too dim, so my goal today is to understand exactly how to set up the flat wizard. Scope and camera are permanently set up so, once I suss out the flat wizard, plan will be, darks after each session and then flats every now and then. 

Edited June 25, 2023 by chubster1302

[chubster1302]

Im trying to get the histagram to peak roughly in the middle but im getting three spikes, what would be the reason for this and what would I need to do to get all spikes in the middle. Do i even need too ?

[chubster1302]

Do these "look right" ? Dark flats produced by NINA directly after taking the flats, scoped covered. 

[alacant]

1 hour ago, chubster1302 said:

Dark flats

Hi

Keep it simple...

D5300, so best to lose any type of dark frame. Noise will be better controlled simply by subtracting the offset; Nikon NEF usually set 600 . Don't bother with in-camera bias. 

We recommend Siril for all calibration and stacking.

Cheers and HTH

[chubster1302]

5 minutes ago, alacant said:

Hi

Keep it simple...

D5300, so best to lose any type of dark frame. Noise will be better controlled simply by subtracting the offset; Nikon NEF usually set 600 . Don't bother with in-camera bias. 

We recommend Siril for all calibration and stacking.

Cheers and HTH

Oh...hmm, ok. that does make it simplier, well "quicker anyways. So, no darks needed at all. Still do flats though, yes?

"Noise will be better controlled simply by subtracting the offset" So is that done in the stacking programme ? And when you say "dont bother with in-camera bias", sorry I dont know what that means. 

I will have a look at Siril, thanks for the recommendation.

[alacant]

1 hour ago, chubster1302 said:

Still do flats though,

Yes.

1 hour ago, chubster1302 said:

I dont know what that means.

https://siril.org/tutorials/synthetic-biases/

HTH

[chubster1302]

Thanks very much. Just watching a tutorial now 👍

[ollypenrice]

As Alacant says, don't use darks with a DSLR,  just subtract a bias value.

Don't over-complicate your flats. There is no need for any fancy software,  just set up your flat light source (what are you using?) and experiment with exposure time till you get a histogram peak between 1/3 to half way over to the right. If you can, it's good to put in a delay of a couple of seconds between flats to let the sensor cool. Rapid shooting of subs increases temperature and, therefore, noise.

Oly

[chubster1302]

On 25/06/2023 at 13:44, alacant said:

Yes.

https://siril.org/tutorials/synthetic-biases/

HTH

Eek, just finished reading and that seems even more complicated than just taking darks 😳

[chubster1302]

On 26/06/2023 at 07:58, ollypenrice said:

As Alacant says, don't use darks with a DSLR,  just subtract a bias value.

Don't over-complicate your flats. There is no need for any fancy software,  just set up your flat light source (what are you using?) and experiment with exposure time till you get a histogram peak between 1/3 to half way over to the right. If you can, it's good to put in a delay of a couple of seconds between flats to let the sensor cool. Rapid shooting of subs increases temperature and, therefore, noise.

Oly

Im having so many issues with my "flats"....this should be the easy part surely. Ive managed to sort the scope, download a myriad of drivers and get all the software working correctly, learnt how to polar align and also do a three point polar align in totally alien (to me) software. Have learnt how to plate solve and sort issues when it didnt work correctly, managed to get guiding working correctly, when i didnt even know what guiding was....and I have managed to capture light from galaxies 12 million light years away YET..I cant take a fecking picture of a flat white background sitting on top of me scope...arghhhh

So...deep breath. I got Nina to the stage where it wasnt complaining and actually taking the flats, I managed to get the histogram just to the right of centre and on the screen it "looked" ok, (see dust bunny pic above). But, when I look at it in camera raw it looks like this. What am i doing wrong ?

 

Edited June 27, 2023 by chubster1302

[woldsman]

The D5300 is a DSLR so you can take flats by switching from manual to aperture priority setting. The camera will work out the exposure time. You just need a uniform light source and a white t shirt & elastic band. In the Summer months, temperature varies a lot so probably best not to take darks. You should take bias frames though. I usually take 31 flats & 101 bias. I like using backyardeos with DSLR astrophotography - there is a version for Nikon cameras. IMO this software is well worth purchasing & is relatively inexpensive. 

[Elp]

42 minutes ago, chubster1302 said:

What am i doing wrong ?

Do the flats calibrate your light frames to flatten the field (remove vignette, dust spot and relative even illumination minus any light pollution gradient)?

It's not what they necessarily look like, it's what they do which counts (well the actual numerical pixel values count). The software might not be displaying it how you want, when I use Siril I use the histogram stretch viewing mode and it shows the flats as you expect with extreme vignette and dust motes, opening the image normally just usually looks like a dim grey/colour image.

Looking at your histogram I'd say the peak isn't wide enough, needs to be 2-4 times as wide, maybe increase the duration? What is their duration? When I started I used to take quick flats, now I dim the light source as much as possible so flats are at least 0.5s to 1s long (luminence/RGB), 5s odd with an osc narrowband filter and maybe up to 10s long mono narrowband. Does NINA have an auto flat routine so it can work out the duration itself?

Edited June 27, 2023 by Elp

[chubster1302]

27 minutes ago, woldsman said:

The D5300 is a DSLR so you can take flats by switching from manual to aperture priority setting. The camera will work out the exposure time. You just need a uniform light source and a white t shirt & elastic band. In the Summer months, temperature varies a lot so probably best not to take darks. You should take bias frames though. I usually take 31 flats & 101 bias. I like using backyardeos with DSLR astrophotography - there is a version for Nikon cameras. IMO this software is well worth purchasing & is relatively inexpensive. 

Unless Im doing something wrong, or there is a setting I do not know about, I cant do this. If i switch to AP, all I get is "lens not attached" message and unable to take a shot.....hmm, hang on a mo. I was trying to press the shutter release on the camera itself when getting that message. Do I need to switch to AP then go back to NINA?

[Elp]

2 minutes ago, chubster1302 said:

lens not attached

This usually means you're using a manual lens, a vintage lens, a non native lens or the electrical contacts between your camera and lens body has an issue which can sometimes be cleaned but usually it's a different issue. Either way the camera is saying it cannot control the aperture of the lens automatically so won't take an image. Try it in Nina, it might override this function.

[chubster1302]

17 minutes ago, Elp said:

Do the flats calibrate your light frames to flatten the field (remove vignette, dust spot and relative even illumination minus any light pollution gradient)?

It's not what they necessarily look like, it's what they do which counts (well the actual numerical pixel values count). The software might not be displaying it how you want, when I use Siril I use the histogram stretch viewing mode and it shows the flats as you expect with extreme vignette and dust motes, opening the image normally just usually looks like a dim grey/colour image.

Looking at your histogram I'd say the peak isn't wide enough, needs to be 2-4 times as wide, maybe increase the duration? What is their duration? When I started I used to take quick flats, now I dim the light source as much as possible so flats are at least 0.5s to 1s long (luminence/RGB), 5s odd with an osc narrowband filter and maybe up to 10s long mono narrowband. Does NINA have an auto flat routine so it can work out the duration itself?

Don't know haven't got that far yet, loaded the flats up on main pc and immediately started stressing lol. 

These were 1/500s , ISO was 1600, as I believe they need to be taken at the same ISO as the Lights?

[chubster1302]

Just now, Elp said:

This usually means you're using a manual lens, a vintage lens, a non native lens or the electrical contacts between your camera and lens body has an issue which can sometimes be cleaned but usually it's a different issue. Either way the camera is saying it cannot control the aperture of the lens automatically so won't take an image. Try it in Nina, it might override this function.

Camera is attached to the William Optics scope. 

[windjammer]

>>My idea of OK being it was grey, with a slight darker border and with several large dust spots, easily visible.

Yep, looks about right

 

[Elp]

13 minutes ago, chubster1302 said:

same ISO as the Lights

That's how I've always done it.

AP mode will throw up that error on a scope as far as the cameras electronics are concerned, it doesn't know that there's a lens attached.

Your flat duration is 0.002s, that's way too short, it's more like the duration of a bias. Try exposing for longer and see the histogram expand whilst you do them.

Only once you do the stack will you be able to tell, and experience will guide you in future what makes a good flat or not. Been doing it for years and still I get issues with them, but I can look at a histogram and see what should be right.

Edited June 27, 2023 by Elp

[Budgie1]

22 minutes ago, chubster1302 said:

Unless Im doing something wrong, or there is a setting I do not know about, I cant do this. If i switch to AP, all I get is "lens not attached" message and unable to take a shot.....hmm, hang on a mo. I was trying to press the shutter release on the camera itself when getting that message. Do I need to switch to AP then go back to NINA?

I found some info in THIS THREAD on the APT forum and it seems there's limitation with the Nikon D5300 in that it won't take an image in "A" mode (AV mode in Canon cameras) with no lens attached. So you have to enter the shutter speed manually.

Edited June 27, 2023 by Budgie1

[chubster1302]

1 minute ago, Elp said:

That's how I've always done it.

AP mode will throw up that error on a scope as far as the cameras electronics are concerned, it doesn't know that there's a lens attached.

Your flat duration is 0.002s, that's way too short, it's more like the duration of a bias. Try exposing for longer and see the histogram expand whilst you do them.

 

I had three layers of thick white tshirt over the scope and I was inside a conservatory with an opague roof, ie nice and flat light, on a grey day. That was the setting I needed to get the histogram just right of centre, which I believe is the way most say it needs to be. So what can I do....more layers of tshirt?? Most vids Ive watched they just seem to throw a tshirt over and point to a morning blue sky....Im fairly sure my "light" was dimmer than in those situations, but maybe not ?