I am going to repost what I said in another thread...
This line of questioning is very similar to threads you have posted previously.
What I am about to say is my opinion only. Also, because I am becoming interested in using my observatory for science, I am currently evaluating my equipments performance precisely.
The first thing to say is that in order to talk about unguided imaging, you need some objective standard to measure mount performance. You can't simply say "if the stars look good to you then why does it matter". That level of subjectivity just doesn't cut it if your goal is make fair comparisons. Don't get me wrong - there is the whole debate about how "perfect" do you need. That is a valid debate. But it is not valid if you want to make fine judgement claims about one mount over another mount.
This leads me to my second point. There are many people here (and on Stargazers Lounge) that claim unguided imaging performances of 20 minutes or more at moderate focal lengths. If you ask them what their threshold for acceptability is, they often say 'my stars look round from corner to corner". I am a Scientist and I work in Computational Imaging Analysis and my job is to analyse images. I can assure you that many of these unguided images have elongated stars. No question. But the imager says they are perfect. This leads people to think that unguided imaging is a reality for long exposures and then get frustrated when they can't replicate this. One person on SGL recently claimed 30 minute unguided imaging with an NEQ6 with an Esprit 120 - this preternatural performance from a mass produced mount is not helpful to anyone who is having difficulty with their mounts tracking.
I now have two 10 Micron Mounts. A 2000 and a 3000. To measure performance I look at star eccentricity. There are other ways to do it and other metrics but this is the one I have found useful. I ran experiment after experiment doing unguided imaging up to 30 minutes on targets all over the sky. First at 1200mm FL (LZOS 152) and then at 530mm (FSQ106). The results weren't that good. Subs were inconsistent - sometimes ecc was 0.47-0.50 but often there would be 0.6's and the occasional 0.7. To get consistent results at either focal length, I had to drop to somewhere between 7 and 10 mins. Even then, the occasional sub was thrown out.
This year I have used a MX+ and AP 1100 and an ASA DDM85. Only the ASA outperformed my 10 Micron by a fraction and it wasn't on the same night and was at a different location so the comparison isn't even valid.
So I tried guiding. I quickly read the manual, set exposures to 10 seconds. I kept dual tracking (encoder guiding) on. Now I can easily image up to 1 hour and keep my star eccentricity below 0.45 ON ALL SUBS, even under mediocre conditions.
So the point I am making is
1. Treat claims of unguided imaging cautiously. I have yet to see consistent round stars (as measured objectively - not with eyeballs) on long exposure (>10 min) unguided imaging even at reasonable focal lengths (i.e 500mm or more). You will say...but wait! Here is an example. Yes - you may have a couple of examples (I have many), but can you reproduce this night after night? That matters, a lot. You don't want the lottery of checking your images in the morning. You want to know that on all but the most infrequent night, mount performance has not negatively impacted your subs. There may be other reasons - clouds etc, but it won't be the mount.
2. The benefit of a premium mount is not that you can do unguided imaging. It's that the mount will guide so well and with ease. Guiding essentially locks everything down so its consistent. My system is automated now so guiding just gets activated and off it goes. I don't intervene at all. When I started guiding I was fiddling with everything and changing stuff. Once I just let it do its thing, it did. Attached is my guiding graph from last night - this is very average behaviour from my mount under nonideal conditions. It does this night after night after night. I have seen the RMS to drop below 0.3 arcseconds for sustained periods of time when the night is still.
So to make the point in a different way -
- If you are the sort of person* who requires that your stars are objectively round by a reasonable measure (in PI, eccentricities of 0.42 or less are considered by some to be visually indistinguishable from perfectly round), then the arguments above may apply. In other words, if you knew that if the sub length and focal length were shorter, your unguided images would improve, then the arguments above may apply.
I have used an example of most of the premium brand mounts this year including a direct drive mount and none could consistently perform, unguided for more than about 15 minutes at focal lengths over about 800mm.
With trivial guiding that requires no supervision and involves no "fear" in the morning that things might have gone wrong, I can image for all practical purposes indefinitely (certainly more than 3600s) and maintain star eccentricities below 0.5. If I want to run 10 min subs unguided, I can with similar results. That is what you are paying for with a 10 Micron mount.
- If you are the sort of person* who is happy to look at your images and understand that actually, the stars aren't round but you don't notice, then the arguments above probably don't apply and I think you may be better served saving your money.
I hope this helps - I have analysed and analysed during lockdown on various mounts and under different conditions and these are the conclusions I have come to. Many people grossly over blow their unguided imaging performance and I think that this makes decisions difficult for people who are hovering over that "guided vs unguided rabbit hole" as they pull the trigger on a new high end mount.
*Both these viewpoints are equally valid. What is not valid is to make claims about mount performance for the purpose of comparing two mounts using a metric that is not fit for purpose. "To my eyes, the stars are round from corner to corner" is perfectly fine if it satisfies you, but in a mount shoot out, it isn't good enough.
Edited by SimonIRE, 25 September 2020 - 06:13 AM.