Go Back   IceInSpace > Beginners Start Here > Beginners Astrophotography

Reply
 
Thread Tools Rate Thread
  #1  
Old 19-05-2018, 11:03 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,495
Mono Camera exposure balancing per channel?

Hi all,

Quick question about mono imaging.

If you're imaging an object that is mostly, say, red, do you expose the blue channel for the same amount of time as the red, knowing that the data will be thin, or do you expose the blue for longer to get cleaner blue data and then balance in post?

At this point, I'm suspecting the answer is 'same exposure on all channels' since slightly noisy blue data will be masked by the much brighter levels of the red channel.

Is that correct? Does it make a difference if planetary or DSO?

Cheers

Markus
Reply With Quote
  #2  
Old 20-05-2018, 01:13 PM
Camelopardalis's Avatar
Camelopardalis (Dunk)
Drifting from the pole

Camelopardalis is offline
 
Join Date: Feb 2013
Location: Brisbane
Posts: 5,425
Markus, my strategy is (often) to expose to get roughly the same sky background values, as it's the background where the noise is most troublesome (least signal). When I'm feeling lazy, just the same exposure time

The response of the camera sensor to the different wavelengths of light isn't linear across the visual spectrum - have a look for the QE (quantum efficiency) plot for your sensor. You may be able to estimate what sort of exposure times to aim for from that.

The noise in each channel is independent of each other - but will be dependent on the QE and exposure time as above.

There will be a sweet spot, regardless of planetary/DSO...but the number of frames you stack for each will be vastly different (typically thousands for planetary!)
Reply With Quote
  #3  
Old 20-05-2018, 03:11 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,495
Thanks Dunk!

As with so much of this stuff, it's a case of try it and see what results you prefer!

My access to dark skies is limited, so I'm trying to find a starting point that will yield results that are likely to work. But yes, loking at the graphs, the blue channel is always going to require about 20% more as it's only about 80% relative to the other channels. Thanks for pointing that out.

I was coming at this from the point of view of planetary, but exposing for the same skyglow levels makes a lot of sense for DSO.

Best,

Markus
Reply With Quote
  #4  
Old 20-05-2018, 05:07 PM
Camelopardalis's Avatar
Camelopardalis (Dunk)
Drifting from the pole

Camelopardalis is offline
 
Join Date: Feb 2013
Location: Brisbane
Posts: 5,425
Yeah totally!

Essentially you're using the same principle whether it be DSO or planetary, to get the best signal in each sub/frame and then fine tune in processing.

With DSO, so long as you're comfortably over the read noise, and take enough subs, you can balance well enough in processing so long as you have good SNR.

Obviously, with an object that shows up strongly in the red channel (most likely Ha), you want to avoid using the object itself as a sampling point.
Reply With Quote
  #5  
Old 20-05-2018, 05:35 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,495
Is SNR such an issue for planetary though? As long as you're well exposed, ie; not crushing blacks or clipping whites the skyglow would be negligible, surely?

For example, are darks, bias, flats and flat darks so important with planetary given how bright the signal is?

Cheers

Markus
Reply With Quote
  #6  
Old 20-05-2018, 05:39 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
The idea is that you want sufficient exposure to get the boring featureless background above the noise floor. If you can get that above the noise floor then any signal is going to be more than sufficiently exposed.

I personally image with the same exposure as it means a smaller Master Dark library and it just makes it easy to run the calculations for total integration. As long as you know that for each filter the exposure is long enough that a dark featureless background is sufficiently above read noise, then it doesn't matter whether it is a strong Ha emitter (red emission) or a strong reflection nebula (blue emission), it'll always be above the read noise.

Although I've never done planetary, I'd imagine that most of the time you'll be about half filling the wells where the planet is in frame so I'd suggest that unless you've got a VERY dark planet (frame rate too fast), you're going to be well above read noise.
I also don't often hear of anyone using calibration frames when doing planetary or luna imaging.
Reply With Quote
  #7  
Old 20-05-2018, 07:47 PM
Camelopardalis's Avatar
Camelopardalis (Dunk)
Drifting from the pole

Camelopardalis is offline
 
Join Date: Feb 2013
Location: Brisbane
Posts: 5,425
Yeah you got it, ideally in planetary you want to set your gain/frame rate to maximise the bit range you’ve got, typically only 8-bit, although this precision is promoted by stacking a large number of frames. About 3/4 full on the histogram usually gives enough headroom for highlights.

I’ve thought about trying my 178 on the bright planets because of its higher potential dynamic range, but it’s compromised by lower frame rates...the 290/224 are just faster.
Reply With Quote
  #8  
Old 22-05-2018, 08:23 PM
rustigsmed's Avatar
rustigsmed (Russell)
Registered User

rustigsmed is offline
 
Join Date: Mar 2012
Location: Mornington Peninsula, Australia
Posts: 3,950
- also remember you can cheat a bit if you have to when LRGB'ing, maximise the Luminance ... the colour channels can handle stronger noise reduction, as the resolution is predominantly in the L.

cheers

russ
Reply With Quote
  #9  
Old 23-05-2018, 09:12 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 1,495
Yes, I've heard of ppl binning the RGB at 2x2 and reducing exposure by 1/3. Is that the kind of thing you do?

-Markus
Reply With Quote
  #10  
Old 24-05-2018, 06:02 AM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Binning isn’t overly effective with the ASI1600, you don’t get the same improvement that you do with CCDs.
Reply With Quote
  #11  
Old 24-05-2018, 02:41 PM
Camelopardalis's Avatar
Camelopardalis (Dunk)
Drifting from the pole

Camelopardalis is offline
 
Join Date: Feb 2013
Location: Brisbane
Posts: 5,425
I typically kick the gain up for the colour channels with the 1600 as the noise still fizzles nicely with a good number of subs.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 10:27 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement