Adding Extra Data - How is it done ?

[johnb]

Ok hope this is not a silly question !!!

I don’t have the luxury of an observatory so I setup for each session, now I have finally managed to image and on my first session I got myself 20 x 60 secs on M31 so next time I go out I would like to get yet more data and build on what I have so as always a few questions

1 – I hopefully focus correctly every time but reality says it may not be an exact match every time – is this going to be an issue ?

2 – Whilst I setup the same every time its possible that I may not be pointing 100% at exactly the same place I assume there is an allowable variance that DSS can cope with ?

3 – Do I stack from the beginning or do I stack week 1 and then save some how and then stack week 2 so I end up with 2 images to stack ? – does that make sense ?

4 – I assume each weeks data need its own Darks and Flats ?

Thanks in advance whilst I feel I have learnt a lot there is so much more to learn

Regards

John B

[teh_orph]

I too would be interested to find out how to do this - so far I'd figured you treat each session as it's own job. Darks and flats for each, then stack the results for each night.

Then let's say you have ten nights worth of data you adjust your ten images to have roughly the same white balance and black/white points and do a quick alignment of the feature within each image (eg ensure that the same star is in the centre of the shot). Then stack those ten images, and process!

Unless each night has very consistent seeing and sky brightness etc I don't think you'd be able to stick everything into one jumbo stack...right?

[johnb]

So if you treat each session as unique you then Stack in DSS and save the resutling tif file, after severl session ( and of course you stack using the same parameters) you end up with say 10 tif files which you simply stack - if i understand correctly ?

John B

[Euan]

Just pile it all in there

The closer you are to the original position the less you will have to crop out, but DSS in mosaic mode should be able to cope with just 10-20% overlap between frames

Conditions change as an object moves across the sky during one session anyway so I wouldn't worry about that

The only thing I can see happening is that if you disconnect the camera each time your flats might change slightly. Take separate flats for each session and keep them just incase

It's then possible to calibrate the individual files separately, I know who to do this in Maxim but never tried DSS, maybe just try stacking with one light frame in but all the calibration files?

[teh_orph]

Do you use the same set of darks for all shots then? Also, what if you were to photograph at different exposures (or even with some filters)?

[Euan]

Do you use the same set of darks for all shots then? Also, what if you were to photograph at different exposures (or even with some filters)?

Your darks have to be the same length as your exposures, so you need a set for each different exposure length. There are different views on how many you actually need but personally I go for at least 20. It's important that you try and get them at the same temperature as well, this can be done on a cloudy night, but you just need the camera to be outside with a cover over the front of it

Different filters don't make any different to darks or bias, but make a huge different to flat frames

So in summary, say you have these light frames (lets say this is M31 with short exposures for the core):

4 x 15 mins RED

4 x 15 mins GREEN

4 x 15 mins BLUE

4 x 2 mins RED

4 x 2 mins GREEN

4 x 2 mins BLUE

You would need the following calibration frames:

20+ Bias

20+ 15 mins Dark (as close to the same temperature as possible)

20+ 2 mins Dark (as close to the same temperature as possible)

20+ Red Flat frames

20+ Green Flat frames

20+ Blue Flat frames

[johnb]

Im glad i asked the question but im getting more confused as to how to address this with regards to unique sessions which darks and flats to use etc ?

Regards

John B

[Euan]

What I would do then is to stack the 15 min and 2 min subs separately in DSS, using their own mix of calibration frames

Then use either Photoshop or PixInsight to blend the two together, and that is a long thread in itself

[Euan]

Im glad i asked the question but im getting more confused as to how to address this with regards to unique sessions which darks and flats to use etc ?

Regards

John B

Assuming the ambient temperature is close, the only thing that could change for the calibration frames between sessions is the flat frames. As long as you have flats for each session then it should be ok

[martin_h]

Imagine this -

Day 1. 20 light frames at 5 mins

Day 2. 20 light frames at 5 mins

Day 3. 20 light frames at 3 mins

You will need:-

1 master set of 5 min darks and 1 master set of 3 min darks.

A set of flats for each session.

Load the day 1 lights in to DSS + the 5 min darks + the flats for that session - This is the "main group"

If you look at the bottom of the frame in DSS there will be 2 tabs - "Main Group" and "Group 1"

Now click on "Group 1"

Load the Day 2 lights and the day 2 flats....no need for darks, it will use the darks in the Main Group.

Now click on the "Group 2" tab

Load the Day 3 lights and day 3 flats and the 3 min darks

Then process as normal..

The Main Group holds calibration framse that are "common" to all the light frames (day1 and day 2 darks)....but if the light frames change you need to add new calibration frames for that set

[martin_h]

Taken from DSS help file:-

File Groups

File Groups may be used to simplify multiple nights on the same object file management by logically grouping files for each imaging session.

If you use only the Main Group DeepSkyStacker is working exactly like before the introduction of File Groups.

There are two kinds of File Groups: the Main Group and all the others groups.

• Light Frames from the Main Group may only be associated with Dark, Flat and Offset/Bias Frames from the Main Group.
  This is the behavior of DeepSkyStacker before the introduction of the File Groups.
  
• Dark, Flat and Offset/Bias Frames from the Main Group may be associated to Light Frames of any group.
  
• Dark, Flat and Offset/Bias Frames from others groups may be associated only with Light Frames of the same group.
  

You can create as many File Groups as you want knowing that a file may belong to only one File Group.

When you start DeepSkyStacker only the Main Group is available. As soon as you add a file to the last available group a new empty group tab is created.

Example:

You shot the same subject two nights in a row.

For each night you have a set of Light, Dark and Flat Frames but the temperature was not the same each night and the Dark Frames are not compatible and the orientation was slightly different so your Flat Frames are also different between the two nights.

To associate each Light Frame with the good Dark and Flat Frames you just need to put all the Light+Dark+Flat Frames from the first night in one File Group and all the Light+Dark+Flat Frames of the second night in another File Group.

Since Offset/Bias Frames are common to all the nights they must be put in the Main Group.

DeepSkyStacker will automatically associate the Light Frames of the first night with the Dark and Flat Frames of the first night and the Light Frames of the second night with the Dark and Flat Frames of the second night.

The Offset/Bias Frames from the Main Group will be associated with the Light Frames of the first and second night.

[celestron8g8]

You picked a hard one as i have found out myself . DSS and PS are going to be your best friends in software . If you have considered or know about some plug-ins you might try Noel Carboni plug-ins . Affordable and extremely helpful . I'm just trying to make up my mind which ones i want .

Astronomy Tools Actions by ProDigital Software

[Bizibilder]

Thank you Martin H !! I've wondered how to do that myself!! This should be posted where everyone can find it as there have been several rerquests for exaclty this information and this is the first time I have seen DSS groups mentioned.

(Sorry for jumping the thread!)

[teh_orph]

<snip>

What happens if you want to share some dark frames between groups? If I re-add them to a seconds group they get ignored. The only way I've found so far is the copy the files then add them in. Is this the right way to do it?

[martin_h]

If all your light frames are the same time exposure then you put the darks in the first tab "Main Group"

The "Main Group" holds all the common calibration files - say Bias frames - which would be 'common' to all the frames

You only need to add extra calibration frames in other groups if something has changed in your light frames eg exposure time or flats for that session.

So going back to my previous post -

Main Group would hold the: 5 min lights from Day 1 + 5 min darks+ Flats for that day+Bias

Group 1 would hold: 5 min lights form Day 2 + Flats from that day

(no darks or bias - they are shared from the Main Group)

Group 2 would hold: 3 min lights from that day + 3 min darks + flats from that day.

(no bias - they are shared from Main Group)

Thats if you break down the kit between sessions - if you dont, then you can just use 1 set of flats in the main group because they will be common to all groups like the bias