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Star registration fails when Flat Frames are added
Yesterday I did an imaging session of M81 using DSLR (Canon EOS 5D MKII) and a Celestron 127 SLT MAK (1500mm) on an alt-az GoTo mount. I took around 100 25 seconds exposures of lightframes, 30 darks and 20 bias frames. I left the rig assembled and went out with it this morning to take flats. I covered the lens with a white t-shirt and a rubber-band, set the camera to AV-mode and took about 20 flat frames. (First time attempting flats, so I might be doing something wrong).
Downloaded all of my frames, sorted them, deleted frames that had vibrations and/or satellites in them and happily added everything into APP like I always do. Everything went fine up until registration. All of them failed. I tried the various tips in the pop-up, but to no avail. Think only a couple of the frames had more than 500 stars in them, but the rest of them varied between 300 and 490. Seemed odd to me, because APP have stacked worse subs than this for me earlier.
I then loaded all of my files into Deep Sky Stacker, just to see and all of them loaded and stacked fine. I am not too happy with DSS (which is why I bought APP in the first place, it really was a life-changer for me), so I went back to APP to try something else. I loaded all of my lights and darks, but skipped flats and bias. Just to see if it made a difference. Registration went through just fine, but now star-count was reduced to between 120 and 150. APP is currently integrating those just fine.
The obvious then seem to be: I messed up my flat-frames somehow. But what did I do wrong? Why can DSS apply them and APP not? Any tips appreciated.
Some perhaps useful data:
Light-frames: 100x ISO 3200, 25seconds
Dark Frames: 30x ISO 3200, 25 seconds
Bias Frames: 20x ISO 3200, 1/8000
Flat Frames: Put the camera in AV-mode, so the exposures varied between 1/350, 1/400 and 1/500. Is this perhaps where it went all wrong? Should I try to only use flats that has the same exposure? Took 20 flat frames, same ISO and same focus/camera adjustments as the night before.
APP Version: 1.079
This would be best answered by looking at the data ourselves I think. Would you be able to upload a subset (like 10 of each) to our server? Login and password: appuser and then please create a directory with the name "TommyKnutsen-starregfail". Thanks a lot and hope to be able to see the issue.
Thank you so much for the reply Vincent.
I also did another integration, including my bias-frames as well and that also worked fine. I am pretty confident that the culprit is the flats, but exactly what's wrong I do not know. Especially since the registration fails when I add them.
Uploaded 10 of each to the folder you requested.
Thanks, I'll try to get to it today or tomorrow.
After two other sessions, the same thing happened. Couldn't register stars when I added the flats I'd taken.
I experimented abit, and instead of going through each step from 2-6 like I normally do, I set all the parameters I wanted in each tab and instead went straight to tab 6 and pressed "Integrate". For some weird reason, when it went through all the steps, this time registration went fine and it is now integrating.
I'll go back and try to integrate the session I reported in this thread using the method above and see if that works better.
My apologies for the delay, life got in the way. I'll have a look right away.
Oh, no worries at all.
I can tell you that I also was able to successfully integrate the same frames if I used the method above. (Only setting parameters in each tab and then "integrate"). Is that the preferred way of doing it perhaps?
I'm not sure, I don't know what you exactly do when doing it "manually", can you give a small step by step? I'm downloading the data at the moment anyway. 🙂
What I meant was that I've previously gone through each tab one by one after loading the files. For example like this
1) Tab 2 Calibrate - Set parameters and then click "Create Masters & Assign to Lights"
2) Tab 3 Analyze Stars - Set parameters and click "Analyze Stars"
3) Tab 4 Register - Set parameters and click "Start Registration"
4) Tab 5 Normalize - Set Parameters and click "Normalize Lights"
5) Tab 6 Integrate - Set Parameters and click "Integrate"
What I've done the last couple of times is skip the... eh... "do action button" (in lack of a better term) in each of the tabs and instead just set parameters, adjustments check/uncheck the options I want and go straight to tab 6 and click "integrate"
Working on the data now. So I always check my calibration frames before integration, which you can do very nicely in APP. Load your master Bias, master Dark and Master flat. Open a light frame and then select, on top of the window, l-calibrated. This will show a fully calibrated single light frame to quickly see if the calibration is working well. There you can see that the flat is not working properly.
As you can see below, it's over-correcting the gradient and didn't remove the dust at all. Which are indications that the flats are not correct. When this is the case, it's always better to leave them out as wrong calibration data will affect your processing negatively.
So, without them, registration worked fine, about 100-130 stars per frame and this was the result;
Light pollution correction can still correct quite a bit of it then;
But ofcourse, good flats are always preferred.
Thank you for taking the time to do this.
I already suspected that my flat frames weren't optimal (I am still learning how to take them), so that helped me confirm some of it. I guess what I don't understand is why registration doesn't fail if I go straight to tab 6. But you're right. If APP can't register them properly, there's probably something wrong with them and they should be left out.
These are the complete subsets. On the left is without flats (using the one tab at a time process), and the one on the right contain all the flats I took that session but I loaded them in, set parameters on each tab and clicked "Integrate" on tab 6.
I wholeheartedly agree with you - good data is always key and in this case my flats seemed to be the culprit. I'm just trying to understand the process, what the program looks for and what it deems as acceptable for registration so that I can do better next time.
Yes, that does look better with flats, even if they aren't perfect. So I couldn't get it to really fail, maybe because we do have different data-sets (you have more data ofcourse), might be that some of the calibration frames throw it off. Then why the processing does work with going straight to 6, I also don't have a very good answer for that unfortunately. When you do things manually, do you change any of the parameters in between?
No, I keep the same settings for both methods.
For this particular project, I suspected that it was the flat frames that caused registration to fail and didn't really put more thought into it until I started to see a pattern. APP won't accept any of my flat frames, regardless of session unless I skip to tab 6, press integrate and let the process do what it does.
I even went back and tried to stack a session I did of M101 a few months back. That time it accepted the flats (earlier version of APP), but now it wouldn't. (New version of APP), so I thought perhaps something had changed.
It works for me to do it this way, so it's not a deal-breaker. I realize this was a very specific and (from the looks of it) very individual error that I haven't seen mentioned anywhere else either. When you couldn't get registration to fail using the exact same files, there has to be something on my end.
Thank you for trying at least. I'll keep taking flat frames until I eventually get them right. 🙂
Very interesting, if this happens with new flats again, please send us the data and we'll try to reproduce it. Thanks for noticing! 🙂