Is this dew?

It looks like a tracking error causing those degraded stars.

Thank you for your replies!

Hmm, star trailing/tracking error? It is true that I use to take these short exposures without the guiding active, but I only get this issue from time to time. And the first few subs were fine (between the second sub that I attached here and the first one there were another 7 subs of which were OK).

I will activate the guiding next time to see what I get.

Like others have mentioned not dew, but some kind of tracking error.

The stars are double, not elongated, which suggests a discontinuous error. Something flops, bumps or settles or so. Could be many things. Bump against the rig, cable snag, wind gust, backlash in the mount, unstable tripod, unstable ground, play in one of the mirrors, play in the imaging train, etc.

Try to walk back exactly what happened or what you’ve done during the times where you see this behaviour and hopefully it will give you a clue where to look.

What software do you use to control your setup? Most programs offer a method to plate-solve and re-align to certain coordinates. It will probably not take more than 10s instead of 10 mins.

You’ve clearly looked at many factors already and ruled out several potential causes.

When you say that the problem went away after opening the imaging train, you did a lot more than open the filter drawer and disconnecting the camera. Something must have been unscrewed, the imaging train was touched, so was wiggled a bit, the whole rig was wiggled a bit as a consequence. The issue you see is very small and not much needs to happen on your rig to move the stars a few pixels. Perhaps good to also check the focuser. Focusers can sometimes have a little play in them, allowing the whole imaging train to shift a bit. And keep in mind, a few pixels is hardly anything you see very easily. Also, when after 6-7 images things got worse, what did your guiding graphs look like? Any suspicious corrections one way or the other?

One thing you can be quite certain of, this is not a dew issue. Dew would soften stars, not duplicate them.

I don’t know APT, so can’t comment on that. A quick google search came up with this, perhaps it is helpful.

In general the methodology is as follows:

• You tell the mount where to go to, for example the target you’re going for. Most software will allow you to search on target and subsequently feed the RA/DEC coordinates to the mount.

• Then you take a picture and have it plate solved by the software. This plate solve returns the actual RA/DEC coordinates of where the mount is now pointing to. These are often not exactly the same as what you’re aiming for.

• The mount is now ordered to slew to the coordinates you’re looking for, from the coordinates where it is.

• Then you take another picture and have it plate solved again. You now get a revised set of RA/DEC coordinates, hopefully closer to where you need to be. The software will have a tolerance set, so will keep repeating this process until the actual coordinates are within the tolerance of the desired coordinates.

You say you want to keep things simple using the hand controller. I understand that, but there are many people out there that would argue that using a hand controller to control your rig is more challenging than to make use of standard software routines in imaging software such as NINA, KStars or others.

Good luck with trouble shooting!

CS, Willem Jan.

Willem Jan Drijfhout · Jun 19, 2026, 08:25 PM

You say you want to keep things simple using the hand controller. I understand that, but there are many people out there that would argue that using a hand controller to control your rig is more challenging than to make use of standard software routines in imaging software such as NINA, KStars or others.

I would totally agree. I might seem simple because it’s understood but in the end, a more modern approach would actually end up being the simplest solution.

Big shrug….

Anyway, back to your problem. There’s no doubt that something moved during your exposure. As you’ve checked everything else, the last possibility is your mount. If you can check if the displacement lines up with DEC or RA, that will at least tell you where to look for a problem. I guiding graph will tell you a lot too. If it is the mount when you are running un-guided and it happens only occasionally then just throw out the bad frames and stack the rest.