First light moving from DSLR to ZWO ASI1600MM-Pro - Elephant's Trunk Nebula LRGB - comments and advice appreciated

[SiD the Turtle]

Hi folks, I traded up from a DSLR to a ZWO ASI1600MM-Pro, with ZWO filter wheel, LRBG and SHO filters. After waiting a month finally the sky gave me two nights to image, which despite all my reading up, research and prep was a fun and frustrating first time, making sure the filters were set, focusing correctly and so on. Here's the rig I used and the data I processed:

Kit:

• HEQ5-Pro with Rowan belt mod (PHD2 guiding was thankfully spot-on)
• Polar aligned using a PoleMaster
• William Optics ZS73 II APO with field flattener
• ZAO ASI1600MM-PRO, EFW with 36mm filters, EAF for focusing
• Running off of Astroberry/Kstars/INDI.

Data:

• Bortle 5 UK edge of suburbia, I think it's closer to a 4
• 30x180s lum
• 10x180s RGB (i.e. 30x total)
• Unity gain, offset 10
• 20x darks, 20x flats and 60x dark flats (20x per RBG channel), no bias
• Each channel stacked in DSS, then to PixInsight (I'm very new to it so not an expert at all): EZ stretch, LRGB combine, curves, mask the background and colour saturation, dynamic crop.

Here's the result. I'm both chuffed that I can see the Elephant's Trunk, and disappointed that it's nowhere near looking like other images I've seen, even without proper time spent on post-processing:

Some thoughts:

• I don't have enough data yet.
• I would have though the whole picture would have been a whole lot redder, seeing as I'm using RGB and the luminance and red channels had far more data.
• I think I have astigmatism somewhere in the setup, look at the stars in the corner. I'm using the field flattener and didn't see this with the DSLR. Maybe my backfocus is wrong.
• There's a line coming out of either size of the central star.
• There's still a gradient top left to bottom right, that I thought the flats would have killed.

Would appreciate all input and advice on next steps for me, so I'm ready for the next clear night. First I think I need to experiment with the back focus and fix that astigmatism.

Cheers!

[Whistlin Bob]

@SiD the Turtle - I'd say a very well done for a successful first light- that's not bad at all. My next steps on this would be to gather Ha data on your next clear night and then follow the tutorials at Light Vortex astronomy to learn how to blend it with your RGB data (I have no connection to them- it's just how I learned) - it'll make a huge difference on what is a popular, but quite faint target. Oiii and Sii are a bit trickier and need more time, but they will add a fair bit too and give you the opportunity to play with different palettes.

To your specific points-

- I think you have enough RGB data, its narrowband you need now.

- The Ha will blend in as Red and transform the image.

- I think possibly your backfocus is slightly short, but not by much.

- Aren't those two stars either side?

- I think that gradient might be Hydrogen nebulosity, but I'm not sure. In any case, flats won't remove a gradient unless it's a feature of the image train, most gradients are caused by light pollution. You could remove it using Dynamic Background Extraction, and Ha data will likely swamp it anyway.

[vlaiv]

Yes, I would check flattener distance as corners don't look very good.

17 minutes ago, SiD the Turtle said:

I would have though the whole picture would have been a whole lot redder, seeing as I'm using RGB and the luminance and red channels had far more data.

That really depends on the way you do LRGB combine. Since you did PI method for that - well, that is what it is producing.

18 minutes ago, SiD the Turtle said:

There's a line coming out of either size of the central star.

Yes, indeed - it looks like there is something causing (very faint) diffraction.

It can be either outside telescope or inside. Outside - any power lines or anything similar in direction of your target? Internal - could be as simple as very small hair on sensor or flattener.

Difference would be visible on other stars in the image - if they all have it - it is probably external to telescope or at least somewhere near the front lens. If only few stars suffer from it - it is close to sensor and in that general area.

23 minutes ago, SiD the Turtle said:

There's still a gradient top left to bottom right, that I thought the flats would have killed.

That is light pollution gradient. Flats won't deal with it and you need to remove it.

 

What is your luminance like in terms of signal in general? If you say that red and luminance have enough signal - then RGB combine method is not very good and image should have more signal and be more red in nebulosity in the background.

Did you refocus on each filter? There are a lot of red halos around stars and that is ok with ED doublet if you keep single focus position. Benefit of doing LRGB is that you can refocus on each filter and reduce this effect. L will still have it unless you invest in L3 from Astronomik.

27 minutes ago, SiD the Turtle said:

EZ stretch, LRGB combine

You should combine RGB data first and then perform stretch. In fact - you should stretch L and then apply RGB linear values to that stretched L if you want to maintain RGB ratios.

[Richard_]

I'd say you did a pretty good job for your first go, well done!

You've got a good outline of the trunk so just get more integration time on there. Your stars have nice colour so maybe you can relax on collecting RGB for now. If you want to see more detail around the trunk, I'd recommend capturing some H-alpha data. Once you've captured your H-alpha data, take a look at the stack of just this data and you should see a lot more detail in the dust and less signal from stars.

I've just recently processed my image of the same target (7 hours with a OSC camera using the Optolong L-eXtreme dual band pass filter). In my final image, the stars are smaller with less emphasis, so the detail in the dust really shines through. I started seeing some nice detail at around the 2 hour mark.

With regards to strange star shapes in the corners, you're spot on that this looks like a back focus issue. Both the ZWO and DSLR require a back focus of 55mm but for the ZWO you will need to use spacers etc. to achieve this instead of a simple T-ring for the DSLR. ZWO have some nice flow charts on their website (link below) to show how you can achieve the 55mm back focus depending on what equipment (e.g. filter wheels, off-axis guiding) you use (technically, you should aim for 56mm since you should add about 1/3rd of the thickness of your filter to the back focus). Perhaps there's a spacer you've missed out?

https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html

[SiD the Turtle]

Thanks @Whistlin Bob, @vlaiv and @Richard_ for the input, all good advice!

Firstly here's a raw luminance frame, just resized and stretched, so you can see what I'm capturing:

A few responses:

• Backfocus: I'm already using the guide that Richard mentioned to make sure I have the right backfocus (but I will double check). However the William Optics field flattener is adjustable (https://www.firstlightoptics.com/reducersflatteners/william-optics-adjustable-flattener-for-zs73.html) so wondering if the problem is while I've got it set to 11.4mm on the flattener as the guide suggests, it's not quite there. I'm also conscious that Ekos' plate solver is complaining that while my focal length on paper is 430mm, when solving it thinks it's around 450mm. Is that normal? Obviously the draw tube is out some way. I don't remember it complaining when I had the DSLR hooked up.
• Focus: I did refocus with the filter changes using Ekos' auto focus routines. I was struggling with this at time, so perhaps my focus was slightly off. I'm used to slightly less blobby stars with my DSLR.
• Colour: good advice on processing but am I missing something in terms of red balance? I'll take two random examples searching on Astrobin, for pictures taken with OSCs: https://www.astrobin.com/eqy3hx/ https://www.astrobin.com/fub42e/. The overall picture colour is red, which is, from my limited understanding of the science, what you'd expect in 'true' colour, even without Ha data. I'd like to get some Ha data anyway as I have the filters installed and ready to go.
• Dirt somewhere perhaps causing that line in the centre of the image. I can't find it on the raw subs. I think I'll dismantle the whole image path and just recheck the whole thing to be sure!
• Gradient: thanks. Didn't think that the flat wouldn't solve for light pollution. Will look at cleaning it up!

Thanks again all, will give it another attempt at processing later, with the LVA tutorials in hand, while I wait for a break in the clouds and see what I come up with!