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DSLR Imaging - Curious Issue


beka

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Hi All,

I noticed a few times while imaging with my Canon 700D (KStars/Ekos) that one of a series of subs will have much less background noise than those immediately before and after. At first I just though it was the way the FITS viewer displayed the images but when I check later there was a huge difference. I attached a screenshot (to reduce size) with successive images side by side opened in AIJ. I captured the sequence automatically and did nothing between subs. The subs are 60s and maybe max 30sec between the subs while the image downloads over WiFi. Any ideas?

Best

dslr_noise_mystery.png

Edited by beka
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Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

Edited by almcl
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1 hour ago, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

I will check if I have not disabled the noise reduction in the camera, I don't have the raw files - just the FITS.

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  • beka changed the title to DSLR Imaging - Curious Issue
5 hours ago, beka said:

I don't have the raw files - just the FITS.

The latest version of EXIFLOG (referenced above) can read the temperature data from FITS files if your imaging ap has recorded it.

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On 18/01/2023 at 20:10, beka said:

I will check if I have not disabled the noise reduction in the camera, I don't have the raw files - just the FITS.

On 18/01/2023 at 18:14, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

Hi almcl,

So both High ISO and Long Exposure noise reduction features were off. I found the "CCD Temp" value in the FITS header, as can be seen by the KStars image viewer and all subs were at 16 degrees.

Best

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On 18/01/2023 at 18:14, almcl said:

Not sure that this will answer the question, but it may be worth examining the EXIF data for the subs.  Although I've never seen quite such a dramatic a difference as your screen shot shows, I have seen  differences of up to 25°C in recorded sensor temperature between subs in a session and this might account for some of the difference.

The 700d also has a noise reduction mode which (I *think* from memory) subtracts an automatically taken dark from a sub.  If you have this mode active it might make a difference.  Mine seemed to have a mind of its own when deciding whether it was active or not.

If you don't have an EXIF data reader, there's quite a nice one available from https://www.covingtoninnovations.com/astro/astrosoft.html

 

EDIT:

Sorry, just noticed your image acquisition software is storing your subs as FITS rather than .CR2 files so unless you also have the raw files you may not be able to read the EXIF data.

So the problem seems to be fixed. It may have been that the camera was not on manual mode. I remember seeing a warning in Ekos but had disregarded it since everything seemed to be working. Anyway I did not think it would lead to errratic exposure time  - maybe the warning should be more strongly worded :)

 

Best 

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