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Imaging the same object over multiple nights..


feilimb

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I have only ever imaged an object before in a single night, but am having a go at getting more data on M33 tonight after getting just an hour's worth of 4 minute exposures last night.

Just a couple of newbie questions:

* If one is taking flats, and there is eg. slightly different camera orientation on night #2, how would you handle the stacking / use of flats when it comes to putting all the data together ?

* Does it matter if exposure times differ on night #2, eg. 4 minute exposures on night #1, and 3 minute exposures on night #2 ?

* Does it matter if ISO speed is changed between sessions (when it comes to stacking all the data) ?

Many thanks for any info / advice in advance!

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I have found when using DSS that ISO must match across the sessions if using the groups function. Each sub group in the stack must be the same exposure length for their lights. I load my flats, dark flats and bias first with one set of lights. Then add sub groups one at a time for each set of other but matched exposures. Every calibration file in the first group is applied to all sub groups in that stack. Whilst it is technically possible to add calibration files to each sub group (the trick is here you add one file only then expose the groups tab and load into the groups as needed then tick off the file in the first group this forces DSS to get calibration files from the sub groups but you would need unique calibration files for each sub group.. I do not take that many or want to littler my hard drive up with multiple copies of the same file and have the hassle of making the name different otherwise DSS ignores it.

If the ISO changed then I would do a different stack.

If you add more nights of data into one stack then you may have a smaller overlapping data area.

Edited by happy-kat
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When you say 

Quote

If one is taking flats, and there is eg. slightly different camera orientation on night

Do you mean the orientation is different because you removed the camera between imaging sessions?  If so then you'll need new flats, if the orientation is different because of slightly different GOTO then you can use the same flats.  

I always find it easiest to stack the different nights individually and then stack the stacks.  there is a group stacking function on DSS but I am not sure if it works well/at all if the exposures etc are different.  Even when I used to use the group stacking method I still found I got better results from stacking the stacks.

HTH

Carole   

 

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Hi

Regarding orientation, it's a good idea to align the imaging camera with RA and DEC. It's also much better if you can leave your camera connected and undisturbed across nights. That way you should be able to use the same set of flats. It's a good idea to stick to the same exposures and iso settings for lights if you can though it's possible to stack a mixture.

hth

Louise

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I found that using different time exposures is necessary to get more depth in your final stack.

I use Nebulosity 3.3 to stack my captures and regularly capture ISO 400, 800 and 1600 for the same image, and when stacking it has absolutely no visible detrimental effects, so I don't think it matters if your setting are different, including gain, and/or it might also depend on the program used for stacking.

 

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38 minutes ago, carastro said:

When you say 

Do you mean the orientation is different because you removed the camera between imaging sessions?  If so then you'll need new flats, if the orientation is different because of slightly different GOTO then you can use the same flats.  

I always find it easiest to stack the different nights individually and then stack the stacks.  there is a group stacking function on DSS but I am not sure if it works well/at all if the exposures etc are different.  Even when I used to use the group stacking method I still found I got better results from stacking the stacks.

HTH

Carole   

 

Hi Carole, what do you mean by 'stacking the stacks'?

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11 hours ago, carastro said:

When you say 

Do you mean the orientation is different because you removed the camera between imaging sessions?  If so then you'll need new flats, if the orientation is different because of slightly different GOTO then you can use the same flats.  

I always find it easiest to stack the different nights individually and then stack the stacks.  there is a group stacking function on DSS but I am not sure if it works well/at all if the exposures etc are different.  Even when I used to use the group stacking method I still found I got better results from stacking the stacks.

HTH

Carole   

 

Thanks Carole, I meant removal of camera after night #1, and then setting everything up from scratch on night #2.  I guess I'll try to leave the camera attached sometimes now, especially if it looks like there will be a couple of clear nights in a row.  I ended up shooting 80 minutes of images last night, but failed to notice some dew forming on the corrector after the first 30 minutes or so (I have a dew shield but have not progressed to dew heater yet).  In any case I did get some useful extra images to add to the previous night.

To be honest I haven't really gotten into the habit of shooting flats yet - I have been trying to get a handle on the process and software for shooting lights, I know any resulting images will suffer from the lack of calibration frames but am getting to the stage now where I am ready to start learning to shoot flats also after a *lot* of frustrating sessions trying to figure everything out..

 

11 hours ago, Thalestris24 said:

Hi

Regarding orientation, it's a good idea to align the imaging camera with RA and DEC. It's also much better if you can leave your camera connected and undisturbed across nights. That way you should be able to use the same set of flats. It's a good idea to stick to the same exposures and iso settings for lights if you can though it's possible to stack a mixture.

hth

Louise

Thanks, I usually do orient the camera in this way but it would be a very rough visual orientation (eyeballing the back of the camera to RA movement etc).  Is this something that one should try to achieve in a more precise way or is rough RA/DEC camera alignment enough?

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2 hours ago, feilimb said:

Thanks Carole, I meant removal of camera after night #1, and then setting everything up from scratch on night #2.  I guess I'll try to leave the camera attached sometimes now, especially if it looks like there will be a couple of clear nights in a row.  I ended up shooting 80 minutes of images last night, but failed to notice some dew forming on the corrector after the first 30 minutes or so (I have a dew shield but have not progressed to dew heater yet).  In any case I did get some useful extra images to add to the previous night.

To be honest I haven't really gotten into the habit of shooting flats yet - I have been trying to get a handle on the process and software for shooting lights, I know any resulting images will suffer from the lack of calibration frames but am getting to the stage now where I am ready to start learning to shoot flats also after a *lot* of frustrating sessions trying to figure everything out..

 

Thanks, I usually do orient the camera in this way but it would be a very rough visual orientation (eyeballing the back of the camera to RA movement etc).  Is this something that one should try to achieve in a more precise way or is rough RA/DEC camera alignment enough?

It's a helpful thing to do. It doesn't have to be to the nearest arc sec but the closer the better :)

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I think that by 'stacking the stacks' Carole means combining a stack from one night with a stack or stacks from other nights, and you can certainly do this. You'd need to find the best way to weight them in the process. Under no circumstances would I put different sub lengths or settings into one stack. So, assuming consistent skies and the same settings, you'd just weight the stacks based on their total integration time.

The best way to do it, though, is to calibrate each individual sub in the entire shoot without combining them and so generate a new collection of individual subs duly calibrated. You then take these calibrated subs aside and stack them as a separate operation, obviously without any calibration files since that has been done to each sub already. This way you get the maximum value from the Sigma Clip algorithm and the best benefit from dither (even if you weren't dithering between subs.)  I suspect that this method would be of even greater benefit to DSLR imagers since it would attack the background 'colour mottle' problem most effectively.

Now, different sub lengths. Do it only if you know why you are doing it. Of the 100 or so images on my gallery site I've done it precisely twice, once on M42 (which everybody has to image in multiple sub lengths) and once on M31 (where I'm not even sure it did any good at all.) I never worry about white clipping stars. You can pull the colour into the cores in post processing and if you expose so as to avoid clipping stellar cores you won't go deep enough. The key thing is to look at your linear stack. If a galaxy core isn't burned out there then it doesn't have to be burned out in the final image because you already have the data in the stack.

Olly

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Well here is what I achieved complete with dust bunnies and hot pixels - no flats, no darks and no bias frames - I can live with the noise and imperfections for now.. I'll get around to starting the habit of taking those in future hopefully. Combination of approximately 72minutes of 4minute subs.

 

Autosave002_DBE_linear.jpg

Canon 450D (unmodded)

Celestron 8" SCT with F6.3 reducer

Orion mini guidescope and QHY5LII Mono guide camera

Edited by feilimb
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5 hours ago, ollypenrice said:

I think that by 'stacking the stacks' Carole means combining a stack from one night with a stack or stacks from other nights, and you can certainly do this. You'd need to find the best way to weight them in the process. Under no circumstances would I put different sub lengths or settings into one stack. So, assuming consistent skies and the same settings, you'd just weight the stacks based on their total integration time.

The best way to do it, though, is to calibrate each individual sub in the entire shoot without combining them and so generate a new collection of individual subs duly calibrated. You then take these calibrated subs aside and stack them as a separate operation, obviously without any calibration files since that has been done to each sub already. This way you get the maximum value from the Sigma Clip algorithm and the best benefit from dither (even if you weren't dithering between subs.)  I suspect that this method would be of even greater benefit to DSLR imagers since it would attack the background 'colour mottle' problem most effectively.

Now, different sub lengths. Do it only if you know why you are doing it. Of the 100 or so images on my gallery site I've done it precisely twice, once on M42 (which everybody has to image in multiple sub lengths) and once on M31 (where I'm not even sure it did any good at all.) I never worry about white clipping stars. You can pull the colour into the cores in post processing and if you expose so as to avoid clipping stellar cores you won't go deep enough. The key thing is to look at your linear stack. If a galaxy core isn't burned out there then it doesn't have to be burned out in the final image because you already have the data in the stack.

Olly

Hi Olly. So if stack a set of subs from one night, get an image and then stack images from a different night and get an image; if I stack the two produced images is that stacking the stacks?

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Quote

Hi Olly. So if stack a set of subs from one night, get an image and then stack images from a different night and get an image; if I stack the two produced images is that stacking the stacks?

That's correct. 

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7 hours ago, cuivenion said:

Hi Olly. So if stack a set of subs from one night, get an image and then stack images from a different night and get an image; if I stack the two produced images is that stacking the stacks?

As Carole says, yes, that's right. In truth I often do this myself but I'm using cooled CCDs which produce low background noise. The advantage of calibrating the subs individually and then combining them using a sigma clip routine is that the total number of images used to define what's normal (as opposed to what are rogue outlying values) is higher so the rogues are better identified and normalized. If you have only two stacks to stack the only option is to combine using 'average' which will not be as effective against noise.

Olly

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On 9/1/2018 at 08:07, ollypenrice said:

As Carole says, yes, that's right. In truth I often do this myself but I'm using cooled CCDs which produce low background noise. The advantage of calibrating the subs individually and then combining them using a sigma clip routine is that the total number of images used to define what's normal (as opposed to what are rogue outlying values) is higher so the rogues are better identified and normalized. If you have only two stacks to stack the only option is to combine using 'average' which will not be as effective against noise.

Olly

I'm no expert, but I imagine one should strive to do this every time, for exactly the reasons you mention? It seems like less than ideal to stack the stacks.

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I calibrate my lights (bias, dark, flat) after every session. I store these as my "input" files (suffixed with a "_c" to show they are calibrated and with a "_cc" if I have also cosmetically corrected them for hot/cold pixels. If I image the same target, I just add more files into that target's folder. I can then, at any stage, realign and stack all the images. It also means I don't have to recalibrate and use different flats for different batches.

It also avoids the need for stacking the stacks. It is possible, but you're losing some of the benefits of stacking larger numbers of lights. 

It does take longer because as your library of lights grows, so does the time it takes to align and stack them. It also requires more storage as you're saving every light and not just the stacks (but I can never bring myself to delete original data).

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  • 4 weeks later...

Here is my first attempt at M42 - the image is made up from lights and calibration frames achieved over 2 nights.  All lights were 3.5 minutes @ ISO800, with a total of ~87 minutes integration. I used a SW ED80 DS Pro (recently got this 2nd hand through the forum) and a Canon 450d unmodified.

Next thing is to try to blend in some shorter duration exposures for the core area which is of course over saturated in this version :)

 

Integration_Main_DBE_HarryMethod_2ndPass.jpg

Edited by feilimb
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  • 3 years later...

Hi Feilimb, I read your post with interest. I've been asking the same question on other sites and never got a satisfactory answer. I'm torn beween OSC and mono. I would prefer mono if I was confident that I could stack them well. I know it is more work but I can process on cloudy nights. Surely if you tackle mono as though you were  I get told if I din't gmhave pernanent set up I should go OSC. Then there is the weather issue.. then lack of knowledge of processing software, and yet to learn how to guide... got to buy a guide camera. On the other side I want to get the best out of the eauipment I buy. I'd be happy stacking session months apart.

 Can I adk how you got on in your endevour? 

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I just came up with an idea of how to accurately align camera on mount on seperate sessions. I'm going to useful binary star systems to align the planetary camera against the cross hairs on the target in the viewining software. Just align the two stars along the North south or East West axis. Of course you need to find a binary with suitable seperation. But there are plenty around

Mizar and Alcor for example have a seperation of 12 arc mins if that helps.  Thats just less than half the full moons apparent size.

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6 minutes ago, Robbie4clearskies said:

I just came up with an idea of how to accurately align camera on mount on seperate sessions. I'm going to useful binary star systems to align the planetary camera against the cross hairs on the target in the viewining software. Just align the two stars along the North south or East West axis. Of course you need to find a binary with suitable seperation. But there are plenty around

Mizar and Alcor for example have a seperation of 12 arc mins if that helps.  Thats just less than half the full moons apparent size.

Hi and welcome to SGL.

There is much simpler method for doing that - that does not require binary stars.

You start exposure and slew bright start in RA or DEC while exposing. You don't need to expose for long - like 5 seconds is enough. Star will make a trail in your image. Make trail horizontal or vertical (for landscape or portrait orientation of FOV) by rotating camera and checking line again (you might need 2-3 rounds).

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