Originally posted by mctaveck That is very very interesting! So you think astrotracer or a "professional" mount does not make any difference for that trails in the corner? And it is NOT a matter of precision of position, GPS, hours/time etc.? That's calm me down: I do not have the ambition to shoot professional astrophotographs but to take the maximum from my gears. So, if I have to end up with panoramas (like I did), it's perfectly fine with me. If this is the best result I can have with a single shoot (technically speaking) it's fine , I'm happy.
The boat. I'm very surprised many of you doesn't accept it or consider it distracting. To me seem a star fallen down and, although it's not clear what it is, at least can rise curiosity. Well ....at least this was my intention
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Maybe, like you are suggesting, the intention doesn't reach the goal. I got it, that's why I'm asking for critique!
So at the very end, really thanks to you for this technical explanation and to everybody for the critique (and also for the positive comments)
Danilo
Careful! A professional EQ mount will make a huge difference. If you were to really get into this, i'd suggest the Skywatcher Star Adventurer.
What I was referring to was a professional Alt-AZ mount. Alt-AZ mounts are the quintessential mount that people think of when you tell them to picture an old telescope in their minds. They track the sky on the X-Y axis, so as the star arcs across the sky, they move up one step, over one step, like a set of stairs. The elongated stars in the corners comes from the field rotation of the stars (they don't just move linearly but as an arc). The Alt-AZ mount is great for visual because it is so simple to use and set up but because it doesn't compensate for the field rotation, the longer you image with one the less of the frame you can use due to the rotation. You get the same result if you were to stack a whole bunch of untracked shots and anyone who tries to say otherwise is selling you a unicorn. You can get away with stacking 20 or so 10-15s shots, and the wider the lens, the longer it takes to notice, but if you were to try stacking say 30x 30s shots, you'd end up with stars in the corners elongating.
An EQ mount tracks the sky properly, following that arc in a smooth motion on one axis so you don't get the field rotation and you can stack as many images as you want with no issues.
I went out the other day to make a test, to see how much of Orion I could fit in 50-55mm for a shot I want to try but not sure if it's possible. This was taken with my camera attached to my Skywatcher HEQ5 Pro equatorial mount. It's a stack of 9x 5minute exposures, no star elongation.
This next one was taken Sept 2018 with my ZS 71ED Telescope with Field Flattener/Focal reducer so my focal length was 330mm (ish) also taken on my Pro EQ tracking mount.
As you can see, with a proper EQ mount, you don't get those issues in the corner.
However don't confuse comatic aberration, which is a lens defect that elongates the stars in the corners, with field rotation. They are different! But, the former (Comatic Aberration) will make the latter (field rotation) far worse. This is why in the shot with my telescope I used a field flattener: it corrects my telescopes comatic aberration and turns the stars in the corners round again, but even with that, if I were to use my K3II's built in astrotracing, I'd end up with elongated stars.
Hope this isn't too confusing, just want to arm you with more knowledge.
---------- Post added 10-16-2019 at 06:48 AM ----------
Here is an example I found of one of my full milkyway images. It was taken with Astrotracer and that night I took a bunch with the astrotracer. The top is a single astrotraced shot, with some elongation. The bottom is a bunch of those stacked. As you can tell, stacking astro shots without the use of a proper EQ mount is what causes the worst of the elongation. I always shoot more of the sky than I will use int he final photograph. In this case I shot the sky in landscape but knew my final image would be portrait, so I cut off a huge amount from the sides and top.