LRGB imaging ! Exposure length advice please!

[Tycho65]

Hi,
I recently bought an Atik 314L+ mono camera (second hand) and am learning APT and autoguiding and plate solving as well. I have got a set of Baader narrowband filters and some less expensive ZWO LRGB filters (all mounted 1.25 inch filters).

I read in SkyAtNight Magazine that, R,G,B frames should have 3 times the exposure of the L frame (assuming the light pollution does not lead to overexposure !).

For example, I take a 60 second L exposure.
Then R frame (or B or G) needs a 180 second exposure.
If you bin 2x2 the R frame, then it is 180/4=45 seconds.

Summary: L (bin 1x1)= 60 seconds
R (bin 2x2)=45 seconds
You will then need to resize the R image (double the size) in order to combine the L+R,G,B frames.

Is this correct ?
Is another ratio correct, why is it 1:3:3:3 for LRGB (as explained by Sky At Night magazine) ?

Many thanks.
Magnus

[Gordon]

If you look at the filters, the luminance "L" filter is almost clear, and the color filters are much darker (hence less light can pass through them). To gather enough light for the exposure it requires more time for the color filters. I usually don't bin my images, it seems to work better having them all the same size to stack them.

[AstroBee]

There really is no hard set rule for exposures but you are correct that your RGB filters will require longer exposures than your Lum filter because of the reason @Gordon has already stated.
Also, whether to bin or not, binning should only be done if you are severely over-sampled and without knowing which scope you are using, it is impossible to answer that part.
Here's a great online calculator to check. Just put in your gear and look at the graph at the bottom.
https://astronomy.tools/calculators/ccd_suitability

[KathyNS]

Don't get carried away with exact ratios. Yes RGB should have more exposure than L because the filter is blocking roughly 2/3-ish of the light. And yes, you only need 1/4 of that if you bin 2x2. But the exact numbers aren't worth stressing over. I shoot 300s for L and 120s (2x2) for RGB.

For sizing, I just rely on the star alignment phase of stacking to take care of it for me. As long as I pick one of the L frames as the reference, the alignment phase will adjust the R, G, and B frames to match. I use PixInsight, and the stacking script (WBPP) is smart enough to always choose a 1x1 frame rather than a 2x2 frame for the reference frame.

[sdbodin]

From my experience, salt grains apply, I shoot 50% L and 50% color. Like this, 30 minutes total L, usually in 2 min subs, and 10 minutes of each color also 2 min subs. So that is 15 luminance frames and 5 frames each of RGB. When stacking the frames I stack ALL in luminance AND color, presently black and white too, that is ALL 30 frames are stacked as a 'super' luminance image then each color separately stacked and converted to its respective color then the three colors stacked for a color frame.

All then combined in Photoshop layers as luminance and color. Basically color just overlayers the detailed image and applies color to it. Just like old-fashioned tv used to work, basic black and white image with blurry color over the top.

Anyway, IMHO, color should reflect the actual contribution of each color, e.g. if red is weak then tripling the exposure will distort the red contribution to the true color balance.

Of course, one can say, that all this is academic anyway as all we humans can see at the eyepiece is maybe a slight green color. All is artwork in the end.

Clear skies,
Steve

[Graeme1858]

Looks like everybody does it differently Magnus!

I do 2 x Luminance and 1 x each RGB:

But I'm a relatively new LRGB user. I'm probably going to do a few more targets like this then try 3 x L + 1 x RGB and see how it goes, if the clouds ever go away!

I'd say ponder all the advice given and try out a few combinations of your own and see what works best for you.

Graeme

[carastro]

Hmmm, well I don't know where the original poster got their info from, but the Atik314L is a CCD camera and requires much longer exposures than a CMOS camera. Having said that the total imaging time is roughly the same.

I was always told (from a number of sources) to do 600secs Lum and half that for RGB, but I tend to bin the RGB x 2 so I do either 150Secs binned or 300 secs binned depending on target. (150n secs binned is normally from home Bortle 8 as 300secs is too long and whites out the sensor).

For narrowband I do the same, treating Ha to 600 secs and O & S to 300 binned x 2.

The Atik314L is a very capable camera - I used one for a fair number of year.

Carole

[Stuart]

I'm going to take a different answer and say it depends on the target, your imaging train, and your mount. First determine what the maximum amount of time you can go without getting egg shaped stars, and that's your top limit. For example, someone may say "10 minutes" but if your mount can't handle 10 minutes, then you need to figure that out first.

Let's say you can, and I'll take my own situation. If I'm imaging a star cluster, I won't image with lum at all, and I'll just do RGB subs, and I'll go whatever length I can that will result stars not being saturated. If I am saturating a star, I will decrease the time. Nebulas I will do longer times, again making sure to saturate as few pixels as possible.

I'll add that I image with CCDs, and not a CMOS.

[Tycho65]

Thanks all for your great advice. I see there is no ""right answer"" ! I will have to experiment a bit more, very hard to do in these changing climate - cloud, humidity etc.
Can I ask Stuart, how do you assess ""star saturation"? Do you have to look at the PSF graph of the star: if the peak is cut off, then it is saturated ? Or is there a wizard app or subroutine you use, I have problems assessing if a star is SATURATED !

Magnus

[Tycho65]

From my experience, salt grains apply, I shoot 50% L and 50% color. Like this, 30 minutes total L, usually in 2 min subs, and 10 minutes of each color also 2 min subs. So that is 15 luminance frames and 5 frames each of RGB. When stacking the frames I stack ALL in luminance AND color, presently black and white too, that is ALL 30 frames are stacked as a 'super' luminance image then each color separately stacked and converted to its respective color then the three colors stacked for a color frame.

All then combined in Photoshop layers as luminance and color. Basically color just overlayers the detailed image and applies color to it. Just like old-fashioned tv used to work, basic black and white image with blurry color over the top.

Clear skies,
Steve

Hi Steve,

Do you need Maxim DL or PixInsight to stack a Super Luminance frame ?? I am not sure if this is possible in DSS or Siril (free stacking software).

Thanks,
Magnus

[sdbodin]

Hi Steve,

Do you need Maxim DL or PixInsight to stack a Super Luminance frame ?? I am not sure if this is possible in DSS or Siril (free stacking software).

Thanks,
Magnus

Anything should work as all frames are the same, just monochrome images. I actually just use the freeware Nebulosity.

Steve

[Stuart]

Hi Magnus,
In PI there are a couple of quantative ways. The easiest way is to hover your cursor over the offending star and look at the readout mode. 0 is black and 1 saturated. You can also run the Statistics process to see what percentage of your pixels are saturated. But frankly what I just do is look at an unstretched sub and make sure that none of the stars are bright.

[Tycho65]

Don't get carried away with exact ratios. Yes RGB should have more exposure than L because the filter is blocking roughly 2/3-ish of the light. And yes, you only need 1/4 of that if you bin 2x2. But the exact numbers aren't worth stressing over. I shoot 300s for L and 120s (2x2) for RGB.

For sizing, I just rely on the star alignment phase of stacking to take care of it for me. As long as I pick one of the L frames as the reference, the alignment phase will adjust the R, G, and B frames to match. I use PixInsight, and the stacking script (WBPP) is smart enough to always choose a 1x1 frame rather than a 2x2 frame for the reference frame.

Hi Kathy,
Thanks for your really sensible reply. Can I just get some help with the alignment issue. I do not have PixInsight yet. Can this bin1 for luminance frames be aligned with RGB frames binned (2x2) in something like DeepSkystacker or Siril. Apologies if you have never used DSS or Siril...

Magnus

[KathyNS]

Don't get carried away with exact ratios. Yes RGB should have more exposure than L because the filter is blocking roughly 2/3-ish of the light. And yes, you only need 1/4 of that if you bin 2x2. But the exact numbers aren't worth stressing over. I shoot 300s for L and 120s (2x2) for RGB.

For sizing, I just rely on the star alignment phase of stacking to take care of it for me. As long as I pick one of the L frames as the reference, the alignment phase will adjust the R, G, and B frames to match. I use PixInsight, and the stacking script (WBPP) is smart enough to always choose a 1x1 frame rather than a 2x2 frame for the reference frame.

Hi Kathy,
Thanks for your really sensible reply. Can I just get some help with the alignment issue. I do not have PixInsight yet. Can this bin1 for luminance frames be aligned with RGB frames binned (2x2) in something like DeepSkystacker or Siril. Apologies if you have never used DSS or Siril...

Magnus

I don't know Siril at all, and it has been years since I used DSS. My understanding, though, is that both of them (and other commonly-used stacking software) will handle it. The important thing is that the reference frame, to which all the others are aligned, must be one of the 1x1 frames. The 2x2 frames will be stretched (in physical dimensions, not talking about dynamic range) to the same 1x1 scale. If you accidentally specified a 2x2 frame as the reference, the 1x1 frames would be reduced to match the 2x2 frame. PixInsight picks a 1x1 frame automatically. I presume the other software does, too, since it is not a rocket-science decision.

Just to emphasize, it is never a 1x1 frame being aligned to a 2x2 frame. It is always the 2x2 frame being aligned with a 1x1 frame.