HEQ5 Pro + Rowan - periodic error ?

[matthasboldlygone]

I've always noticed some star movement, not drift as in a bad polar alignment. It's a back and forth movement over 10-15 minutes.

The Rowan belt was installed by FLO, level tripod, good polar align, light and balanced payload.

This is two exposures to show to extreme movement. The most I could get was 30 second exposures before star trails got really bad.

It seems to be worse with the scope in certain positions, pointing low East isn't too bad, but this pointing South and higher in the sky.

I've looked at programming periodic error on the handset but that looks like it could take maybe 1 hour every single time I use the mount. That would probably be the end of it for me, there's enough setting up without adding that level of hassle.

I'm sure a HEQ5 Pro with Rowan should do much better than this, I hear of other people doing 5 minute exposures without guiding.

Any suggestions?

 

[michael8554]

The legend says "Up is -85.5 degrees E of N".

I think this means that RA is running from top to bottom of the image.

If so the drift is in RA.

That means it's not due to poor PA, that movement would be in Dec.

Simplest explanation would be slop in the image train, since you say it varies the sky position.

The Belt Mod removes Periodic Error caused by the gearbox, but the Periodic Error due to the worm and wormwheel  remains.

However I seem to remember the worm period is typically 480 seconds on these mounts ?

So 10 to 15 minutes would be 2 or 3 cycles, are you sure it's not showing the full drift after 8 minutes ?

If you don't want to use the mount PEC you will need to guide if you want longer exposures.

But all this assumes I'm right about the image orientation  🙂

Michael

[matthasboldlygone]

Thanks @michael8554, I will do some playing and time the movement more accurately. I'll also watch the belt / gears to see if anything looks odd. Sounds like guiding is going to be the way forward.