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RGBL Live Stack

Posted: Tue Sep 04, 2018 3:50 pm
by mAnKiNd
Live Stacking LRGB can be a simple and new approach to constructing a four channel colour image of an astronomical object by capturing the respective four filters for luminance, red, green and blue over the course of a single night. While it is a more involved and hands on process to produce a color image than acquiring the same integration time with an one-shot color camera, the result nevertheless benefits from all the advantages of shooting channels with a more sensitive mono sensor.

The following image viewtopic.php?f=16&t=1080 of Caroline's Rose was constructed from four Sharpcap live stacks each made up of either of: L - 60x15s (15mins), R,G or B – 30x30s (15min each). Exposures were determined with the sharpcap smart histogram function for unity gain in order to not expose beyond my backyard skyglow. All four stacks were acquired at unity gain (130) with the cooler of the ASI1600MM pro running at -15°C. The stack totals at 1 hour integration, with each of the four channels contributing 15 minutes and was captured through a Canon 300mm USM L (non-IS) f4 lens - a fantastic vintage lens that dates back to 1991 and which contains two fluorite elements out of 7. All four stacks were also correctly calibrated live with their matching flats-bias and darks. Session started at 21:30 and finished at 2330. Two hours from start to finish to integrate a one hour, four channel combined dataset is what I call a quick and efficient imaging session. These four final live stacks were then combined and mastered in Pixinsight/Photoshop.

This simple live acquisition technique demonstrates that at least when it comes to star clusters, a rapid 'quickfire' live stacking approach to four channel acquisition as opposed to traditional long exposure can render a perfectly good four channel LRGB image. As always, integration can benefit from further acquisition time, but with the added consideration of whether or not all four channels can be acquired in a single night. What I enjoyed about it the most was that because it was live, it was more engaging and therefore fun to play with.

One could further consider repeating the live stacking approach over multiple nights and then “double-stacking” the respective stacks, either with DSS or with the new Sharpcap virtual monitor function. This new “double-stack” would benefit from further SNR improvements and allow the image to be stretched and revealed even more in post-process. This could hold especially true for nebulae and galaxies, were deeper integration is indeed needed to reveal more faint details.

Furthermore, a sequence of RGBL might prove to be even better for this approach than LRGB, which is the sequence for how this dataset was acquired. This is because the Luminance data will be closer to the zenith for your target than RGB channels and therefore able to capture the best details. Whilst it is recommended that blue is left for last, I recommend to leave it as penultimate in the acquisition sequence, so that it can still benefit from closer to zenith pointing, but ultimately it is the L channel that I feel should contain the clearest data, even if blue is the hardest channel of the four to capture.

RGBL also benefits from increasing the probability of acquiring at least the colour channels during clear skies, as clouds, rain, sprinklers or some other hindrance might call it a night. That means that if you manage to at least acquire RGB, you get to go cak inside with a colour rendering dataset to play with, and L can be either rendered synthetically later, acquired on a different night or ignored completely. Finally, when capturing nebula where narrowband is beneficial, the L channel can be substituted with that particular filter and acquired last in the sequence as described above. This will be the subject of my next test.

Clear skies :)

P.S. when sequencing finally becomes available in sharpcap, this could all be presetup and then simply set to run acquisition with a single click..or two, as opposed to many clicks, well you get the idea, less clicks. But then again, where would be the fun in that..?

Re: RGBL Live Stack

Posted: Tue Sep 04, 2018 3:52 pm
by mAnKiNd
4th attachment

Re: RGBL Live Stack

Posted: Tue Sep 04, 2018 6:01 pm
by turfpit
Very informative - thanks Minos.

A very time efficient acquisition session. I suppose with set-point cooling you have the benefit of a darks library.


Re: RGBL Live Stack

Posted: Tue Sep 04, 2018 8:20 pm
by mAnKiNd
Thanks Dave, the cost of a cooled camera is definitely small to the benefits of owning one.