Hello, Can you choose your own stars for autofocus? Dont get me wrong but AF does a great job, but it would be nice if I could pick which stars to autofocus on so as to get sharper focus when you zoom in on them. When I take a pic and zoom in, the nebula is focused well but some stars (the further distant ones) are out of focus. It would be nice to choose the stars..
Choose your own star/s for autofocus
SFn8CieR When I take a pic and zoom in, the nebula is focused well but some stars (the further distant ones) are out of focus.
All stars are practically at infinity -- when one star is at focus, another star is also automatically at focus. Even the Sun and the Moon are far enough to obey this.
If some of your stars are in focus while others are not, you have a field flatness issue, and need to improve your optics (or possibly camera tilt).
CHen
w7ay 
Hi CHen, thanks for that. I have a Sky Rover 80ED Triplet refractor with dedicated 0.8 Field Flattener so somehow I dont think field flatness is the issue. Please have a look at my image I have uploaded so you can see. The nebula itself looks well focused but when you zoom in to the stars at say 200% they look out of focus and not pinpoint sharp.I am not sure if is camera tilt, everything is square on the tube including the ASI533MC Pro camera, so how can I tell if it is a field flatness or camera tilt issue as you mention?
Many thanks again..
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SFn8CieR Please have a look at my image
I usually don't do this since I am seeing the same image (with JPEG artifacts) that you also already see. But...
Notice that the stars on the top left corner are slightly "non-round." Except for some chomatic aberration (typical of non-premium optics even when they claim being "APO"), the top right looks mostly OK. Bottom right looks better. And bottom left is similar to top right. My guess is that you have s slight left-top-bottom-right tilt -- i.e., a small tilt along the bottom-left-top-right diagonal axis.
If some corners look good and some corners don't, it is usually a tilt problem. If all corners look bad while the center looks good, it is a field flatness problem.
Remember that stars are never a "pin-points." Even perfect (finite sized) optics will obey the Dawes limit (i.e., diffraction limited). Their intensity profile obeys a Bessel function in 3 dimensions -- similar but not precisely the same as the familiar sin(x)/x profile in 2 dimension processes. The "tails" of the profile is very long and brighter stars will therefore look larger.
On top of that, atmospheric turbulence ("seeing") will also bloat your stars.
This is what I can do with my FSQ-85 (4-element Petzval + 1.01x flattener, ASI2600MM, 45mins of integration)
http://www.w7ay.net/site/Images/Lagoon.png
You can see what I'd described above -- brighter stars appear larger. Seeing was OK but not great that night (July 24 2021). My notes say HFD of 2.12 pixels (3.59").
(Filter was a little weird, while I was waiting for my Chroma narrowband filters. It is a stack of a "color" Radian Quad filter with a red Antlia filter to pull only the H-II side from the Radian Filter passbands.)
Chen
w7ay
Thanks Chen. I see what you mean about top and bottom left stars being slightly out of round in my pic. I was not sure if I should send you a jpeg or fit file but the jpeg was smaller although it has some artifacts..
So all that I have on my scope is a field flattener screwed directly to the focuser tube with a ZWO filter holder and ZWO camera so its the best I can get as far as keeping it all square and in line. Is there something I can get to take the tilt out? Also the stars in your image seem pretty much perfect albeit the slightly bloated bright stars, but what is that on the right middle area, a reflection, looks like someone has written an S on it? Thanks again for your great advice!
SFn8CieR Is there something I can get to take the tilt out?
I am surprised there is so much tilt since you are using a small sensor.
The tilt could come from anywhere -- the OTA itself, poor spacer tubes, filter drawers, etc. And of course the camera.
Rotate the camera 90 degrees. If the tilt stays in the same corner, the tilt is coming from the camera. Otherwise, it is coming from somewhere between the camera and the OTA, or the OTA itself.
Best way to treat tilt is to learn not to be obsessed by it :-). I have an early WhiteCat51 that has enough tilt, I threw the scope into the junk pile.
Chen
By the way, are you familiar with Siril? (If not, it is definitly worth downloading; lots of really useful tools in it.)
This is what Siril shows as the tilt in your image. Sure enough, top left corner has the largest error -- the eyes don't lie :-).
Take another image one night, with a different camera angle (any star field will do). It will show if the tilt is in the camera or somewhere else.
Chen
w7ay
Hi Chen, Wow that is amazing! Yes I use Siril but never knew it could detect tilt. Where can you find this feature?
There seems to be a lot of tilt using a small ASI533 MC Pro Sensor as you mentioned. I am surprised by this. Mind you this picture was taken over 5 nights of imaging. Maybe that has something to do with it too? I have taken many different images with this scope and camera combo so maybe you can look at one of it as well to see if the same result for me. The camera and scope are exactly the same as before and untouched in configuration. Again thank you Chen.
SFn8CieR Yes I use Siril but never knew it could detect tilt. Where can you find this feature?
I don't know about other OS, but on macos, there is a hamburger icon (three parall horizontal lines) near the top left of the Siril main window.
Click on that to get a contextual menu, one item is "Image Information." That is a submenu with an item named "Dynamic PSF." Select that.
It opens an aux window called (what else) Dynamic PSF. Towards the bottom right are some icons, one of which looks like a trepezoid with lines connecting diagonals. Click on that ans you will get a tilt report in the main window.
Instead of Dynamic PSF, if you click on aberration instpector, you'll get tat 3x3 array of corner and central part of the full image -- saves a lot of time doing it yourself.
Chen
SFn8CieR The camera and scope are exactly the same as before and untouched in configuration.
Aha! The camera angle changed.
First image:
Second image:
Camera angle changed from 119º to 208º. Camera was rotated about 90º and the tilt followed. So, the tilt is not in the camera sensor, but in the rest of your OTA train.
Chen
Hi Chen,
Again thank you for your time and trouble here!!
Thank you also for information on Dynamic PSF and Aberrator Inspector in Siril, I am sure I will use them to check things from now on.
Yes I understand that the 2nd target is about 90 deg away from the first one but that should not make a difference because everything is screwed together, no clamps or screws, so maybe there is movement in the focus tube? Maybe one of the spacers is not true? I will investigate this for sure.
You say the focus had changed so I take it you are referring to the Radius figures being 1.171 and 1.191 being a measure of seeing conditions, as I use ASIAir to autofocus and the seeing can obviously affect that.
cheers..
Nick
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SFn8CieR Yes I understand that the 2nd target is about 90 deg away from the first one but that should not make a difference because everything is screwed together, no clamps or screws, so maybe there is movement in the focus tube?
If there is a tilt in your optics (OTA, reducers, flatteners, etc), or bad spacers, then you would get what we are seeing. That is why I had earlier suggested changing the camera angle, to binary chop the problem (viz. camera or elsewhere).
I use ASIAir to autofocus and the seeing can obviously affect that.
Yeah, ASIAIR "autofocus" has problems. I mentioned a few of them on the forum in the past. Just use a Bahtinov mask if you are using ASIAIR.
Chen