Am I seeing periodic error?

[oishiimsg]

Hey everyone!

I have have been shooting DSOs for a few months now and have always run into the same issue with my image sets. Given a set of 30 second exposures over a 1 hour duration, roughly 2/3 of my images will be unusable due to star trailing. For the longest time i couldn't figure out why until recently. With one of my recent sets i was flipping through the images and noticed the image will shift back and forth over a few min period. The usable frames came from the 30 sec that the image seemed to be still. I went back and checked other sets and notice the same behavior. The image will shift back and forth throughout the total duration of the session. After a bunch of googling I am thinking this is due to periodic errors in the mount. Does this sound like the issue to you guys as well? If it helps my mount is a CGEM DX.

Thanks for helping!

[Leveye]

You really should give PHD2 and autoguiding a shot and see how your graphs look.It would help diagnose the problem. If your 30 seconds exposures have trailing in them it could be an alignment/balance issue. Does your mount have PEC? Try using that as well to correct for PE

[oishiimsg]

Thanks for your reply Leveye. The mount does have PEC. I just haven't been able to record the corrections as I am waiting on my illuminated reticle eyepiece. I have been looking into autoguiding but am not sure which solution to go with yet (OAG or guide scope). If it helps i can create video or gif when i get home of the described behavior. Not sure if it makes sense for it to be a alignment issue as the image never moves out of frame, and actually never moves out of the zone in with it shifts. It just shifts back and forth over a few min duration.

With reguards to autoguiding, do you have any recommendations on a guiding solution? I have an Edge HD11 on a CGEM DX. For imaging I am using a DSLR.

[kalasinman]

Have you attempted to find a correlation between the period of error and the time for your mount's worm to make one rotation? Is the error in RA or DEC?

[oishiimsg]

I have created a short video to demonstrate the motion i am talking about. Keep in mind each frame of this video is 20 seconds, the video is played at 30fps so the speed at which the image wobbles is greatly exaggerated. Doing a rough calculation based on the movement throughout the frames it would seem 1 cycle is ~7 min. I just looked up the worm gear rotation speed and it looks like its 8 min, this is what is making me think its periodic error causing this motion. Anyhow, here's a link to the video https://www.youtube.com/watch?v=z-ARgmcPOIY

Thanks once again!

[kalasinman]

Most align the x axis parallel with the DEC axis so that horizontal error is DEC and vertical is RA.

It is also significant to note whether the error is affected by pointing near the zenith, the CP or CE. Knowing this would help eliminate various mechanical conditions other than PE.

[kens]

Take a series of shots over about 20 minutes and overlay them on one another, much as you did with your video. Make sure your PA is not too good so you have some dec drift. Your PE will show up as a "worm" for each star. If it really is PE the pattern will repeat during the 20 minutes. Try it near declination zero to maximize the RA movement.

[kalasinman]

My experience with PE shows 25% of subs are most affected from a bad spot in the worm with 120 sec subs and a 300 second period of rotation.. If the entire worm is bent, then 50% would be most affected. As the error in RA for example can either put the star out of position high or low, but not left or right (DEC error). As the worm rotates through 360 degrees, just like a broken clock, it will be right twice a rotation. How many subs are affected is partly determined by the length of the subs as a percent of the period of rotation.

[Astrosurf]

Going back to guiding, a guide scope is the easiest option. What's your scope's focal length?

Alexxx

[dph1nm]

That is periodic error plus a bit of polar alignment offset.  The 'shake' is entirely in the RA direction, and there is a general drift in Dec. Looks very roughly like 30 arcsecs peak to peak, which seems about par for the course for these types of mount.

NigelM

[oishiimsg]

Thanks for all the replies guys. A lot of good information here for me to play around with. I will definitely be looking into a guiding solution as well as doing some PEC with the mount. Until then i will take everything you guys mentioned and test it out when i get a chance. Thanks for all the help once again!

[kalasinman]

Having beat my head against unguided imaging with a less than premium mount, I gave in to guiding. DSO unguided imaging is the realm where ASAs, 10 microns, Avalons, etc. rule.

[ollypenrice]

Unless you have an incredibly serious mount you cannot do deep sky imaging at C11 focal lengths without autoguiding. Unless you have at least a very serious mount you will struggle to do it with autoguiding!

A typical DSLR in a C11, even with focal reducer, is imaging at about 0.6 arcseconds per pixel. This is a phenomenal resolution which the sky itself may be unwilling to allow and which only a very good mount-autoguider combination can hope to deliver.

In a nutshell, 0.6 arcseconds per pixel takes some guiding.

I had a spell imaging at 0.66 arcseconds per pixel but that was on an autoguided Mesu 200 which is, it must be said, a serious mount. http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-Sc3kgzc/0/X3/M51%20DEC%20VERSION%20clip-X3.jpg

Olly

[Scorpius]

Seems like I experience periodic error every time I venture out. However, it's of the human variety and has nothing to do with the mount!

[kalasinman]

If it were only as predictable.