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Calibration frames for mono


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Hi,

I have more than one scope but only one mono cooled camera. I use deepskystacker to stack. I create dark, flats and dark flat calibration frames.

I usually keep my camera connected to my scope so that I don't need to keep creating calibration frames. This way I can keep collecting data over several clear nights. 

I wish to remove my camera from my current setup and then later reconnect my camera to my scope to add some additional data to an old target. This way I can mix some wide frame and narrow frame imaging within the same season.

Do I need to keep all my old calibration frames as well as create new ones?

Do you add data by restacking the old data and the new data with calibration frames for each or do you use layers on photoshop, or some other method?

I'll be grateful for tips, ideally I'm looking for a simple method that does mot mean I must keep all my calibration frames every time I reconnect my camera. File management would get very messy if I need to keep calibration data and my lights for every time I return to a DSO. Storage could start beciming a problem too. Perhaps I just need a tidy folder structure but stacki g would be very complicated eg

M57

Nov19>lights 

Nov19>Calibration 

Dec20>lights

Dec29>calibration 

..and so on

 

Thanks for looking

Ian

 

 

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As far as I understand when removing camera from a setup then you should redo the flats but no need to do the dark frames.

Many even take dark frames with the camera removed and with a cover on the sensor and then wrapped in foil in a dark room to ensure there is no stray light getting in there.
So if you take a library of darks to cover any likely exposures you are going to use, including the short exposures for dark flats, then you do not have to take darks again for sometime. 
It is still a good idea to retake these darks every do often as cameras can change over time but probably now only once a year.

Flats however (ideally) should really be taken after every session and if you have a flat frame panel the normally these do not take long. So for a multi session then you would use the same darks for calibration but each night would have a separate set of flats for each session. 
Then so long as you take the flats at the same exposure length for each filter (if mono) then you ideally would already have the correct exposure darks to use so would not need to retake these.
Obviously then you need to keep all the flats until you calibrate your data and then just keep the master flat for that session, if storage is an issue. Personally I like to keep all the flat frames as well as the master just in case I redo the processing at a later date and find an issue with the master I can then examine all the individual flats to see if maybe just one or two of them had an issue that caused a bad master, but as I said if storage is becoming an issue then just make sure you are happy with the master flat before deleting the individual flats.

If for sure you have not taken the image train apart between sessions, or accidently rotated the camera or filters etc, then strictly speaking you can get away with using the same flats for each session. Personally I would still retake the flats as it takes very little time but I know many on SGL that produce perfectly good images do not so I guess that is your decision.

That is my understanding anyway and what I currently do, others with more knowledge than me may say differently, I know from previous similar threads this is not set in stone what people do and many have different views on what is necessary.

Steve

Edited by teoria_del_big_bang
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Thanks Steve,

I don't currently redo flats every session. I have multiple filters and I thought dark flat frames were supposed to be captured at the same exposure as flat frames so when you add it all up it can still take time to do the flats and dark flats for each filter. May I'm in to much of a rush 😀

I take your point with master calibration frames.

I suppose then in deep sky stacker I would stack for instance stack the old green filter lights with the old green filter master calibration files and also stack the new green filter lights with the new green filter calibration frames, ( doing the same for all other filters used)  Then i suppose layer each filter stack in photoshop?

 

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7 minutes ago, Imd said:

suppose then in deep sky stacker I would stack for instance stack the old green filter lights with the old green filter master calibration files and also stack the new green filter lights with the new green filter calibration frames, ( doing the same for all other filters used)  Then i suppose layer each filter stack in photoshop?

You are correct about calibrating old lights with old flats and new lights with new flats. But as far as stacking is concerned, you should, if possible, stack old and new calibrated lights in the same session, and not combine two stacked images as layers in PS. If you have two stacks with approximately equal numbers of lights, then stacking each seperately and combining afterwards is ok. But if the stacks consist of a different number of subs, then snr of the final image will be close to the snr of the stack with the smaller number of subs.

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17 minutes ago, Imd said:

I don't currently redo flats every session. I have multiple filters and I thought dark flat frames were supposed to be captured at the same exposure as flat frames so when you add it all up it can still take time to do the flats and dark flats for each filter. May I'm in to much of a rush 😀

I guess it depends how you take the flats and what camera was being used.
If we were not talking the mono cooled camera then yes ideally take the dark flats each session just same as the flats/ 
Also if using the T-Shirt method and a morning sky so exposure times differ each time you take them then ideally yes you would retake the dark flats at the same exposure.
 

But if you can use a flat panel and the exposure time of the flats can be the same each time then no reason you cannot use a library of darks that you took at the same exposure times.
If using Pixinsight it can also use dark masters at different exposure lengths (and maybe other software can its just that I use PI) but I am a creature of habit and still try to use same exposure length darks as my flats.

As I say this is what I currently do and think what I am saying is correct, I know many do not do all this including some well known members of SGL that produce far better images than myself so for sure none of this is a hard and fast rule of what you have to do 🙂 

Steve

 

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Posted (edited)

"stack old and new calibrated lights in the same session"

Is it possible to stack lights from 2 different sessions when each have different flat frames, wouldn't that cancel the corrections made by flats eg vignetting and dust bunnies?

 

Would all the green lights be added to the tab in DSS as the old and new flats and dark flats to create one stacked green image?

Sorry for all this clarification but I have been wondering this for ages and it really holds me back

Edited by Imd
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I am not sure how to go about this in DSS as I use PI and in PI all this can be done by running one script.

But the basics should still be the same.
Say you have:

  • 10 frames of each RGB from night 1, take flats after that session.
  • 8 frames of RGB from night 2, take flats again for that session. If the exposure times of flats is the same as night 1 then no need to take darks again.

What I tend to do is see what the ideal exposure for flats is for each filter. I don't like taking too short exposures for flats with CMOS cameras so try to keep the exposures at least 2 secs long by reducing the light intensity on the flat panel. So if the ideal exposure is 2.225 seconds for one filter then I will then probably actually use 2 seconds for that filter, as the ADU will be close enough to the ideal ADU, as it means I can use the 2 second master dark from my library, another filter may be 2.994 for another filter so would then take the flats at 3 seconds for that filter and use a 3 second master dark.

Then for calibration:

  • Make master flats for each filter for night 1 using flat frames from night 1
  • Make master dark for each filter for night 2 using flat frames from night 2.
  • Calibrate all 10 light frames for night 1 with master flat from night 1
  • Calibrate all  8 light frames from night 2 with master flat from night 2
  • Stack all 18 calibrated frames for each filter.
  • Combine each filter for master colour frame.

I guess it makes more sense for somebody who uses DSS to explain the exact steps as I have not used DSS for a few years I am not sure the easiest way to do this in DSS. 

Steve

Edited by teoria_del_big_bang
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2 hours ago, Imd said:

"stack old and new calibrated lights in the same session"

Is it possible to stack lights from 2 different sessions when each have different flat frames, wouldn't that cancel the corrections made by flats eg vignetting and dust bunnies?

 

Would all the green lights be added to the tab in DSS as the old and new flats and dark flats to create one stacked green image?

Sorry for all this clarification but I have been wondering this for ages and it really holds me back

In DSS, calibration files in the group tabs are only applied to lights in the same group, anything in the main group is applied to all other groups as well. So put darks (or pre-stacked master dark) in the main group tab. Put lights, flats and flat darks from session 1 in a new group tab. Put lights, flats and flat darks from session 2 in another new group tab (repeat as needed). Stack.

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