Stacking frames and ratios

[darrellx (Darrell)]

Hi all

Early this morning (Saturday) I decided to get out of bed and have a go at imaging a Jovian moon transit. I am fairly happy - I got my first pic of the shadow of Io on Jupiter.

But from the experience I have 3 questions. This is the first, and is relevant to this forum, I think. The others are in Cameras, Photography.

My process for imaging is;
A video using my DMK and IC Capture (8inch newtonian and a 3x Barlow).
Export the bitmaps using VirtualDub.
Align using PCFE.
Stack using RegiStax.

Now, until recently, I was happy with the mono images. My questions relates to the colour. Is there much of a difference to the final image, based on the ratio of frames used in each channel? I don't mean that in relation to the detail. I realise that the more frames the better
(up to a point) for the sharpness. I mean it more in relation to the colour depth, or strength, or vibrance, or whatever the word is about how the colour looks.

So does it matter if I use, for example, 300 red, 900 green, and 900 blue frames. As opposed to, say, 1200 red, 800 green, and 700 blue. At the end of it all, I still only combine the final result from each channel.

Darrell

[iceman (Mike)]

Hi Darrell,

In general, I use the same amount of frames in each colour channel. The amount of frames you stack is purely dependent on noise and sharpness, related to the seeing.

As you say - the more frames the better. However only when they still produce a sharp image.

If you capture 1200 frames (1 minute @ 30fps), stacking ALL of the frames will soften the result of that channel because not all of those frames will be sharp. I stack an amount depending on the seeing. If the seeing is average, I might stack 300-500. If it's slightly above average, maybe 600. If the seeing is good or better, I might stack 700-900.

There's always a trade off between noise and sharpness - you could stack only the best 150 frames and have a sharp image but it will be very noisy and won't tolerate much post-processing. You could stack 800 and have a smoother image but unless the seeing is very good, you'll be blurring the finer details by stacking blurry frames.

The quality and contrast of the final stacked image can have some impact on how the colour looks, but it's due to the sharpness and contrast, not how many frames you stack.

So in general, to answer your question, no, the number of frames doesn't impact the colour. Just use colour balance or levels in post-processing to fix up any mismatches in colour. This is due to how it was captured and the filters you use, not how many frames you stack. Some filters allows more light through than others, so the green channel might be brighter - hence giving you a green cast to your combined image.

Generally I stack the same number of frames from each channel, unless one of the channels was captured in poorer seeing in which case I'd stack less for that channel.