untracked astrophotography - Forums

Topic starter

Jan 10, 2020

well, i am doing untracked astrophotography for a long time, but i have some questions about it…
first thing, the results of the carina nebula: when untracked (and not modified) the core gets a bluish natural coloration, and when tracked, it gets white. don't you have an idea why this happened? even in others untracked pictures the core gets blue!

Frédéric Auchère

Jan 10, 2020

Dear Olga,

Do you have an example of a tracked picture to compare with? I suppose that you checked that but I'm trying anyway

Could it be that your detector saturates when tracked? With 2" exposures, you get a drift of 30 arcseconds, which corresponds to 6.5 pixels given your camera's plate scale. When untracked you spread the signal over this many pixels. In other words, when tracked you get in a single pixel the equivalent of a 13 seconds exposure.

Frédéric

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Arun H

Jan 10, 2020

The other thing that may be going on is that the untracked sub exposures are shorter and hence don’t saturate the core.

Olga

Topic starter

Jan 10, 2020

Frédéric Auchère:
Dear Olga,Do you have an example of a tracked picture to compare with? I suppose that you checked that but I'm trying anyway
Could it be that your detector saturates when tracked? With 2" exposures, you get a drift of 30 arcseconds, which corresponds to 6.5 pixels given your camera's plate scale. When untracked you spread the signal over this many pixels. In other words, when tracked you get in a single pixel the equivalent of a 13 seconds exposure.

Frédéric

https://apod.nasa.gov/apod/ap160323.html in this apod, for example, the core is white.
https://imgur.com/r/astrophotography/BGMEqdx and here a untracked version (with the same gear as mine), where the core is blue.

Frédéric Auchère

Jan 10, 2020

Indeed. I was assuming identical exposure times for the tracked and untracked subs.
And BTW, what I wrote is strictly true only for punctual sources. If the source is uniform, there should be no difference in signal. I don't realize how sharp the core of Eta Carinea is. Not my latitude, unfortunately :C

Olga

Topic starter

Jan 10, 2020

Frédéric Auchère:
Indeed. I was assuming identical exposure times for the tracked and untracked subs.And BTW, what I wrote is strictly true only for punctual sources. If the source is uniform, there should be no difference in signal. I don't realize how sharp the core of Eta Carinea is. Not my latitude, unfortunately :C

maybe different exposures lengths can lead into a different kind of image- and i don't even have saturated it too much, the original tiff file already have a bluish color!
the carina is even visible with my 70mm little scope, one of my favorite nebulae! ;)

Olga

Topic starter

Jan 10, 2020

The other thing that may be going on is that the untracked sub exposures are shorter and hence don’t saturate the core.

what do you mean by hence?
yeah, my exposures were from 2.5 seconds, with 454 subs. normally a tracked exposure shot would go for several minutes, using 30-50 subs.
maybe different exposures times (and different numbers of subs) get into different images... but i don't know why...

Frédéric Auchère

Jan 10, 2020 · Edited: Jan 10, 2020

maybe different exposures lengths can lead into a different kind of image- and i don't even have saturated it too much, the original tiff file already have a bluish color!the carina is even visible with my 70mm little scope, one of my favorite nebulae!

Dear Olga,

When I wrote saturated I meant saturated at detector level, i.e. red, blue and green neighboring pixels have the same value. In that case the colour information is lost and the region appears white. If your tracked subs are saturated in the core of Eta Carinae, there won't be any colour information in that region. In that sense different exposure times can lead to different results. That hypothesis could be tested with 6.5 times shorter tracked exposures, with the same camera + lens (if you have a means to track) and everything else being otherwise equal.

Frédéric

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Frédéric Auchère

Jan 10, 2020 · Edited: Jan 10, 2020

https://apod.nasa.gov/apod/ap160323.html in this apod, for example, the core is white.https://imgur.com/r/astrophotography/BGMEqdx and here a untracked version (with the same gear as mine), where the core is blue.

Sorry I missed your links before replying ...
I thought you were comparing tracked and untracked exposures taken by you with the same camera & lens.
It is difficult, if not impossible, to compare the colours in pictures taken in very different conditions. You'd need to know all the acquisiion details in both cases, plus all the processing steps.

Anyway, I really like short exposure untracked images. That's a lot of fun!

Frédéric

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Olga

Topic starter

Jan 10, 2020

Frédéric Auchère:
maybe different exposures lengths can lead into a different kind of image- and i don't even have saturated it too much, the original tiff file already have a bluish color!the carina is even visible with my 70mm little scope, one of my favorite nebulae!
Dear Olga,

When I wrote saturated I meant saturated at detector level, i.e. red, blue and green neighboring pixels have the same value. In that case the colour information is lost and the region appears white. If your tracked subs are saturated in the core of Eta Carinae, there won't be any colour information in that region. In that sense different exposure times can lead to different results. That hypothesis could be tested with 6.5 times shorter tracked exposures, with the same camera + lens (if you have a means to track) and everything else being otherwise equal.

Frédéric

great idea!! nowadays i don't have any tracking device, but when i get one i will probably do this test.
when i use 30 seconds exposures (the stars get completely elongated), but only for comparative reasons the core gets white- as you said, the saturation in the detector levels depends on the exposure times.
thank you for the help