Bad polar aligment or mechanical issue?

[wojetej]

Hello, I image unguided with canon 600d and 650 mm focal length my mount is EXOS 2 GOTO (it's a EQ5). I am wondering if I can extend my exposures so I get less trailing by tuning my mount , not so long ago I did service it and eliminated backlash and tightened everything so there is no play on both axis. Could I do something more to my mount i case of mechanics to make tracking more accurate or now it only depends on polar alignment? Also I did a "test" today I don't know if it brings something but I aligned my dslr so the RA axis was horizontal movement and DEC was vertical movement when i pressed controls on the remote . I took a 10 min exposure and the star trailed to the right in the RA axis. I found a DARV polar alignment on the internet - should I try that before I do anything else? I'd be happy of any advice given. The photo below is the "test" I was talking about the star in the photo is Mintaka it was due south at the time.

Edited February 24, 2022 by wojetej

[sterec]

I think that if there is movement only in the RA axis the problem is with periodic error which is typically a mechanical problem due to manufacturing inaccuracies in the worm.

[vlaiv]

Yes - polar alignment will produce DEC drift and mount tracking issues will produce drift in RA.

You have RA drift - so it is periodic error. To confirm - do shorter exposures and trailing should be shorter and shorter. At some point - most of your subs will have round stars.

Best way to eliminate this issue is to guide.

[rickwayne]

DARV is a great way to double-check or fine-tune your polar alignment, since it measures the end result: drift in DEC. Which, as Vlaiv points out, you don't appear to have.

As an ab-initio method for alignment, it's a little abstruse for most people -- you  have to work out which way a given knob turn narrows or widens the V, both near the meridian (for azimuth) and near the horizon (for altitude). Helps to take good notes! But once you have those, it's easy enough to look up that, for example, when doing the meridian image, when the "dotted" leg of the V is on top, adjust the azimuth knob on the right of the mount, etc..

Personally I really like the direct-feedback methods like Ekos Polar Alignment Assistant, Polemaster, iPolar, etc. But you can't beat drift alignment for reporting what is actually happening -- all those others are one sort or another of derived measurement.

It is possible to introduce errors by getting things too tight, for example gear meshing should be tight enough but no tighter.

[wojetej]

@rickwayne so you're saying I had a good polar aligment? I also don't know if I should align my sensor with axis like I did during the photo shown or can I align it randomly? I am quite confused now.

[rickwayne]

Polar misalignment shows up as a steady drift along the DEC axis, as if you slewed the mount in DEC slowly during the exposure. Since the elongation in your image appears to be only along the RA axis, your polar alignment appears to be good. DARV or some other direct measurement of drift will tell you just how good.

As for rotating your camera, that's merely a convenience for identifying which axis runs which way in the image. You can also plate solve an image to determine its orientation. Some people like to align their camera along the axes as a matter of course; I like to rotate to frame the image the way I want it to look unless I'm debugging something. (Admittedly, I'm nearly always debugging something! )

Edited February 28, 2022 by rickwayne