TheSkySearchers

Hi everyone - Maybe you all know this but I just discovered a "trick" in aligning. I'm now taking 2 minute exposures (optimal in Bortle 4.5 skies) over a long period of time. I stacked the latest images and noted that I had star trails... that is elongated stars. Not huge but enough that all the stars were elongated and the farther from the center the bigger the arc.

I use Nebulosity to stack the images and have just used rotational alignment. For giggles I just now tried rotate and translate and lo and behold I now have circular stars. My thought is that apparently the mount is just slightly out of level and PHD2 is correcting for it. It's not noticeable in my older images because they were of relatively short duration, but now I'm doing hours on the same object. Take this in the "For what it's worth" category.

Nice tip... Thanks.

I like your signature. It's similar in nature to one of mine... "When you stop getting better, you quit being good."

Clear Skies & Stay Safe

Star Dad wrote: ↑Thu Nov 19, 2020 4:20 pm Hi everyone - Maybe you all know this but I just discovered a "trick" in aligning. I'm now taking 2 minute exposures (optimal in Bortle 4.5 skies) over a long period of time. I stacked the latest images and noted that I had star trails... that is elongated stars. Not huge but enough that all the stars were elongated and the farther from the center the bigger the arc.

I use Nebulosity to stack the images and have just used rotational alignment. For giggles I just now tried rotate and translate and lo and behold I now have circular stars. My thought is that apparently the mount is just slightly out of level and PHD2 is correcting for it. It's not noticeable in my older images because they were of relatively short duration, but now I'm doing hours on the same object. Take this in the "For what it's worth" category.

I have Nebulosity 3 but never used it because DSS does the job pretty well. DSS does not ask about rotation and translation, it always does both apparently because usually images have a bit of each. Is there any advantage to using Nebulosity over DSS for stacking? If there is, I would like to know and use it.

Hi Star Dad - When you have rotation of your images it implies that your Polar Alignment is off. If the rotation isn't bad enough to cause star trails in any single image, then, the stacking should take care of it by rotating the individual images to all align. However, if you get star trails in your individual subs then you will need to improve your Polar Alignment.

Have fun.

Steve

My polar alignment is spot on - I use Polemaster. I am going to have to use both rotation and translation on other images to be sure. Last night I did about 4.5 hours on the Crab Nebula (M1), so we'll see how that works out. My individual images are dots, no sign of trails. I do see a regular "heartbeat" in PHD2 indicating an adjustment signal being sent to the mount every 15 seconds or so. I'll have to keep an eye on that next session.

I use Nebulosity for image taking, preprocessing (darks, flats, etc), stacking. Last night I was having severe trouble trying to get my barlow in focus and discovered that I could stretch the image (in Nebulosity) which helped me get into focus a lot quicker. Almost all of my images in the past months have been 1-2 hours of data. My last two sessions I've upped the time to 3-4 hours, and this is the first I've really noticed the oval stars. When I use StarTools I always have to crop about 10 pixels or so on all sides. That has always been the case. So perhaps my bubble is off just a smidge.

My thought is that apparently the mount is just slightly out of level and PHD2 is correcting for it.

• If the polar alignment is, as you state, "spot on," then it doesn't matter if the mount is level or not. This is not your problem.

• Your problem might be polar alignment, Pole Master is not infallible.

• It could also be flexure in the newt OTA especially if you are using the newt with a separate guider.

Good luck

Joe

If the stars in the individual images are "sharp" than your Polar Alignment is good enough.

Getting Polar Alignment good enough to ensure images over 2,3, or 4 hours aren't slightly rotated is not necessary. Any good stacking program will take care of offsets (offsets occur for anyone using Dithering) as well as rotation. The rotation from image to image is likely unnoticeable. However, over the course of an evening of collecting images rotation is inevitable. That's why rotation is an integral part of stacking.

Just keep using the "rotate and translate" option and you should be fine.

Just FYI I know that DSS uses "rotate and translate" as does PixInsight.

Cheers,
Steve