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Subframe and Total exposure Time for rgb stars

Acquisition Narrowband RGB Deep Sky
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Anderl avatar
Anderl avatar
Hello Hello,

i have done a lot of narrowband shooting this summer. 
three questions here:

1. do you think narrowband + rgb stars is better than narrowband stars?
2. how long do you guys expose for rgb stars? I am shooting around f5
3. for what total exposure time should I aim to get the best stars possible. is one hour to short? 

cs
Andi
Pablo Gazmuri avatar
Pablo Gazmuri avatar
1. Yes, but you can get better narrowband stars by blending narrowband signals (e.g. with foraxx palette)
2. I shoot at f2.2, and 10 second subs * 30 is plenty. Starlight is so much brighter than nebulas or galaxies that you don't need much exposure to capture color. You don't want to over expose your RGB data or you will end up with maxed-out (pure white) star centers.
3. 5 minutes is more than enough for me. At F5 that's more like 45 minutes if I'm estimating properly, but even that is likely overkill. Stars are bright.
Nick Grundy avatar
Nick Grundy avatar
I'm about 99% confident RGB stars + narrowband works better than Narrowband alone. Of course you can still make both work out, but it seems the best images I've seen usually have RGB stars separated. 

I've been taking the RGB stars at 105gain and 60s on F7-F8, and i'm likely going to cut it down. But…I've been lazy and didn't want to shoot separate flats for RGB stars. 

I'm not sure if there's some formula, but around 30s of zero gain would likely be ideal on my F7.7 scope. Waiting for the next clear night to test this theory though
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Tom Boyd avatar
Tom Boyd avatar
1. Yes
2. I shoot from F5 to F7 with a 2600mc as my main camera. I always use 30 second subs from my stars
3. I always go for a total exposure of 30 minutes - 60, 30 second subs dithered every other sub.
Jerry Yesavage avatar
Jerry Yesavage avatar
You might look up the Glover (sp?) video on calculating optimal time for your F and Bortle.  For me with a C11 F10 and Bortle 7-8 I run 40-60s… might be long but at times I have much clearer skys.  This is for ASI6200… sensitive CMOS.  Do 1/2h total each RGB.

If I am lazy just run NB and do a Spectro calibration… the advice about too long and white centers is right… but too short and you miss stars…. I stretch with GHS and that can help with star colors…

I usually get the RGB too along with the NB since it is a fraction of total time… you might check out Alex W's Narrowband Assistant to merge the two into calibrated colors… if you are a perfectionist… he has the math zeroed in.

Excuse the rambling answer late here…
Joe Linington avatar
Joe Linington avatar
The shortest I have done is 5sx60 per channel which is only 5 minutes per channel. With my gear, that was to short and I was missing some of the fainter stars that I could see in my NB data. I am currently playing with 15-30s for 15 minutes per channel. My challenge is to find the minimum number of subs at the maximum sub length to not blow anything out.

The problem with the question is all of the variables. I am shooting with a 294m in Bin1 with tiny pixels using a 76mm scope at f/4.5 or a 102mm @ f/5.6. This means I have tiny pixels with a tiny well depth. You say f/5 but is that an 8" newt with a 24mp full frame camera or a 60mm scope with a 183m. That makes a huge difference.

The general consensus is a very vague 15-60s for 10-30 minutes per channel. But it all comes down to your particular gear.
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Paolo avatar
Paolo avatar
Bortle 6.5, with an f/4.9 scope and imx571 I do 40s and a total of 40m per filter
Anderl avatar
Anderl avatar
45 sec
f5.4 120mm aperture
imx571c 
bortle 3-4
total exposure time 45 mins

guess i will try that and see if it works for me.

thank you all for your help

cs
andi