DSLR ISO choice for Nabulae

[Robculm]

Hi All,

Interested to get your thoughts on this.

I'm using an un-modified Canon EOS800D (with Skywatcher 200PDS), which from my understanding / interpretation of the online data has very good low ISO performance.

My feeling (and experience to a limited extent) is that for Nebula imaging, going as low as possible in ISO gives a better 'dynamic range' (not that I'm even sure what that means!), i.e. allows more enhancement of the fainter areas without blowing out the brighter ones...?

Note, I'm using an Explore Scientific UHC filter with this set-up.

I've been trying to work at ISO 200, but obviously there's quite a trade off in terms of light gathering capability at such low ISO. Even at my current 5min exposures, the histogram is barely off the left.

What are your opinions on this? Is there really a benefit to going as low as this or better to get more light & work at 400 or even 800?

To date, I've kind of settled on 200 for Nabulae, 400 for Galaxies & 800 for star clusters etc...

Although, being a nebie & currently struggling to get much imaging time what with learning how to use autoguiding, crap weather (until now, just as the moon is coming back in play) & the short summer nights, I'm looking for some advice as I'd really like to try & capture M16 before it's out of range for the year...

Many thanks,

Rob

 

[alacant]

1 hour ago, Robculm said:

better to get more light

Hi

The amount of light collected by your 200pds is independent of the ISO you set.

M16 with an unmodified eos800 is not going to be easy, but give yourself a better chance by losing the UHC and dithering between your 5 minute frames.

The best compromise with the 24mp sensor is iso800. But try 1600 too.

Cheers

 

 

[Robculm]

Hi HTH,

Thanks for your response. 

Understood in terms of "light capture". 

Will have another try at ISO 800 to start. But not sure how removing the UHC filter helps? Surely that's improving the level of 'emission" compared with the stars, with an unmodified camera?

Cheers

[alacant]

M16 with a daytime camera?

I'd rather collect the maximum amount of light. A UHC removes a lot of it. 

Adjust the stars if you need to when you process the image.

With your fov, they aren't going to be too intrusive anyway. 

Just our €0 02 so hey, try with and without.

HTH

Edited July 14, 2021 by alacant

[Chefgage]

2 hours ago, alacant said:

Hi

The amount of light collected by your 200pds is independent of the ISO you set.

M16 with an unmodified eos800 is not going to be easy, but give yourself a better chance by losing the UHC and dithering between your 5 minute frames.

The best compromise with the 24mp sensor is iso800. But try 1600 too.

Cheers

 

 

With regards the iso of 800 that seems to be the sweet spot for my canon 200d. I very rarely change that given my bortle 8 skies.

[vlaiv]

1 hour ago, alacant said:

M16 with a daytime camera?

I'd rather collect the maximum amount of light. A UHC removes a lot of it. 

Isn't M16 emission type target?

UHC passes all major emission lines - Hb, Ha, OIII, SII ... and reduces light pollution. Ultimately this should improve SNR.

[Robculm]

Well, the forecast high cloud hasn't materialised yet, it"s a beautiful clear night. Running at 800 without the UHC, so will let you know how it turns out! 

[vlaiv]

As far as ISO goes, there is really no "magic" involved - one can calculate the best ISO setting for their situation - it just involves a bit of measurement.

ISO setting should be related to exposure length used.

When one changes ISO - they also change read noise and read noise should be ideally swamped by another noise source - usually LP noise, but since DSLRs are not cooled - maybe even thermal noise.

Lower ISO usually means higher read noise and need for longer exposure, higher ISO usually means lower read noise and shorter exposure are sufficient.

Process of measuring read noise and LP/thermal noise in given exposure is a bit involved and one can do it if they wish - other than that, just use ISO in 400-1600 range and there is no need to change it depending on target type - there is no preferred ISO for nebulae or for galaxies and such.

 

[Robculm]

So here's the comparison. 1st without UHC, 2nd with. Both ISO800.

Have to caveat that I used 5min exposures with UHC & 3min without as the histogram was well over at only 3m. Also, there's only 72min total exposure without & 110m with (running the mount & ASIair pro drains the battery rather quickly! Lynx astro power supply is in the post!).

Minimal post processing, stacked in Siril, Photometric Colour Calibration, Green Noise removal, auto stretch & then just auto black point set (Pyastro) in GIMP, crop & export as jpg.

Lot of background noise. I usually remove in Siril, but hard with nebulae as it's hard to avoid the 'emission' regions!

So for sure the UHC is highlighting the Hydrogen alpha, but it does also distort the star colour. I guess maybe the answer is to have the camera modified (IR cut removed), so as to capture the emission but keep a more natural overall colour!

Cheers,

Rob 

 

[vlaiv]

43 minutes ago, Robculm said:

So for sure the UHC is highlighting the Hydrogen alpha, but it does also distort the star colour. I guess maybe the answer is to have the camera modified (IR cut removed), so as to capture the emission but keep a more natural overall colour!

I'm afraid it's not that simple. UHC is going to boost emission parts of the target and improve SNR there but it will irreversibly loose true color information in stars.  In principle, you can recover star color if you have additional information about each star in the image - but that requires special processing.

If you want to keep star color and still have boost in emission part of nebulae - it is best to shoot both - stars without filter and nebula with filter and combine data in special way.

[alacant]

10 hours ago, Robculm said:

have the camera modified

Yes:)