Star Adventurer - Serious Periodic Error?

[Knight of Clear Skies]

I tested out a newly purchased Star Adventurer for my Dad the other night, using a 135mm lens, and got these results:

(2 minutes, 100% crop)

(5 minutes, 100% crop)

To me, this looks like serious periodic error. Things I checked: polar alignment (not perfect but fair), balance (pretty good, tried to make it a touch East-heavy), made sure everything was tight, put a weight on the tripod tray and stated well clear of the mount when it was taking the 5 minute exposure.

Do people agree with me please that this looks like severe periodic error and that the mount needs to be returned?

[ollypenrice]

If there was no wind and the tripod wasn't sinking into a damp lawn  then, yes, it looks problematic.

Olly

PS It rather looks as if the movement might be jerky. The trails are not smooth but seem to have concentrated points, in the short sub at the left of the trail and in the longer sub at various points along it.

Edited April 26, 2019 by ollypenrice

[Knight of Clear Skies]

Think the tripod was reasonably stable and I stayed well away from it for the longer exposure. I don't remember much wind that night but I could probably find some weather station data to confirm.

I've used 135mm lenses on three other mounts before and while I've had streaks due to balance problems or misalignment I haven't had a flock of seagulls before.

[fwm891]

First do a good PA with the weight of the camera/lens etc on the SA then try your exposures again. My first results were similar because I 'had to use it' but never took the time or care over the initial set-up.

Mine works fine now with a 430mm fl scope mounted and DX format camera. 

These images were taken with mine when reviewing a WO ZS 73 APO: 

 

Edited April 26, 2019 by fwm891
text added

[JamesF]

Given that only the RA axis is driven, if there's periodic error and that's the only problem, shouldn't the trails be straight rather than jagged?  Or perhaps only deflect in one direction?

James

[fwm891]

3 minutes ago, JamesF said:

Given that only the RA axis is driven, if there's periodic error and that's the only problem, shouldn't the trails be straight rather than jagged?  Or perhaps only deflect in one direction?

Hi James - If it is a serious error and the drive wheel is a little eccentric then there may be a speed variation causing the brighter patches...?

[Knight of Clear Skies]

1 minute ago, fwm891 said:

First do a good PA with the weight of the camera/lens etc on the SA then try your exposures again.

I rechecked PA with the camera on, Polaris was still within the ring which shows the tripod hadn't shifted. It was just a rough polar alignment though, straight trails wouldn't have concerned me.

5 minutes ago, JamesF said:

Given that only the RA axis is driven, if there's periodic error and that's the only problem, shouldn't the trails be straight rather than jagged?  Or perhaps only deflect in one direction?

James

I doubt my polar alignment was good enough for 5 minutes at 135mm pointing south, but the longer exposure makes it easier to see any misbehaviour from the mount.

[JamesF]

9 minutes ago, fwm891 said:

Hi James - If it is a serious error and the drive wheel is a little eccentric then there may be a speed variation causing the brighter patches...?

Absolutely.  I'm just trying to get my head around what sort of trails could occur if  there's no tracking in DEC and the only possible errors are polar alignment and RA periodic error.

James

[Knight of Clear Skies]

29 minutes ago, fwm891 said:

My first results were similar because I 'had to use it' but never took the time or care over the initial set-up.

What kind of star trails did you get though please? Did you get any seagull shapes?

[fwm891]

39 minutes ago, Knight of Clear Skies said:

What kind of star trails did you get though please? Did you get any seagull shapes?

They were curved but no bright spots along the length - sorry dumped them.

[Davey-T]

It looks suspiciously like the first example that I received where it was running slower than sidereal time, got it replaced and the new one works perfectly.

Also check that the screws securing the saddle to the mount are tight.

Dave

Gif of the effect here.

Edited April 26, 2019 by Davey-T

[fwm891]

58 minutes ago, Knight of Clear Skies said:

I doubt my polar alignment was good enough for 5 minutes at 135mm pointing south, but the longer exposure makes it easier to see any misbehaviour from the mount.

If your testing something the set-up must be as tight as possible otherwise the test is pointless. Even if you are intentionally off-setting your PA, off-set it a good amount so you can clearly see errors.

Edited April 26, 2019 by fwm891
text added

[Davey-T]

You can test it indoors by fitting a laser pointer and setting it up in the middle of a room, marking the starting spot and then running it for 24Hrs, it should end up pretty much in the same spot.

I used a laser spirit level.

Dave

Here's my result, there are other examples somewhere on here with closer results.

 

Edited April 26, 2019 by Davey-T

[michael8554]

Guessing that the spot is supposed to have returned to the purple mark, then RA and Dec have moved ?

Michael

Edited April 26, 2019 by michael8554

[JamesF]

Just now, michael8554 said:

Guessing that the spot is supposed to have returned to the purple mark, then RA and Dec have moved ?

Michael

If the RA tracking is perfect then it shouldn't actually return to exactly the same spot in 24 hours (unless it is set to track at solar rate?).

James

[Davey-T]

6 minutes ago, JamesF said:

If the RA tracking is perfect then it shouldn't actually return to exactly the same spot in 24 hours (unless it is set to track at solar rate?).

James

No just somewhere near, the first SA that I had was well short 

Dave

[Davey-T]

18 minutes ago, michael8554 said:

Guessing that the spot is supposed to have returned to the purple mark, then RA and Dec have moved ?

Michael

No it was heading up towards the mark and eventually reached it,

Dave

[michael8554]

7 minutes ago, Davey-T said:

No it was heading up towards the mark and eventually reached it,

Okay, but you said the image showed your result !

Michael

[Davey-T]

14 minutes ago, michael8554 said:

Okay, but you said the image showed your result !

Michael

I wasn't expecting scientific accuracy, the sidereal day is around 4 minutes shorter than a solar day, about 1 degree in the sky so if I'd ended up a meter out I'd know something was wrong, somewhere around the starting point was good, there is an old thread somewhere that has other folks results on I think.

Dave

[JamesF]

Just now, Davey-T said:

there is an old thread somewhere that has other folks results on I think.

Yes, mine should be there too.  I can't recall my results off the top of my head.  It wasn't spot-on, but neither was it so far out over 24 hours that I was going to panic about it given my intended use for the mount.

James

[FaDG]

16 hours ago, Knight of Clear Skies said:

Do people agree with me please that this looks like severe periodic error and that the mount needs to be returned?

From the height of my experience with this mount (hardly to be envied...) I'd say that you're looking at two different effects: 

A serious drift, in dec I'd say

A mild Periodic Error (more on this in a moment). 

Let me assume that, in your second image, the RA axis is directed up/right, about 60degs up from horizontal, and your dec axis is down/right, about 30 degs down from horizontal. 

Let's start from drift: the long line should be caused by polar alignment. You say that you checked the star position in the polar scope, but is the PS correctly aligned? 

From my point of view, the Periodic Error is rather the small movement up/down, I see two cycles which, alas, make me think of a 160" period. My Star Adventurer mount has the same issue. 

So, I suggest that you spend some time clearly separating the two phenomena. 

Try to get a short image (15") with tracking off, and look at the direction of the drift. That's the RA. 

If I'm wrong, and the drift is actually in RA and not in Dec, then the tracking rate of your mount is wrong . 

Fabio

Edited April 27, 2019 by FaDG

[JamesF]

6 minutes ago, JamesF said:

Yes, mine should be there too.  I can't recall my results off the top of my head.  It wasn't spot-on, but neither was it so far out over 24 hours that I was going to panic about it given my intended use for the mount.

James

A quick search hasn't rewarded me with a link to the relevant thread, but I think the error I found for mine was that it was slow by about one arcsecond in forty.

James

[Knight of Clear Skies]

Thanks for the suggestions everyone. I've plate solved the second image I posted to confirm the camera orientation, the seagulls are caused by uneven movement in RA. Looks like the mount is faulty as I'd need to take the exposure time below 30 seconds at 135mm to get reasonable star shapes. I also checked the weather that night, as I remembered there was very little wind.

6 hours ago, FaDG said:

Let's start from drift: the long line should be caused by polar alignment. You say that you checked the star position in the polar scope, but is the PS correctly aligned? 

Yes, I sighted the polar scope on a distant tree and rotated the mount to confirm this. However, I just used the PolarisView app to check the where Polaris should be so I'm not concerned by the drift in dec.

I'm going to talk to the supplier. While I'd like to run another test I'm now back in Hertfordshire without access to the mount.

[newbie alert]

How can there be drift in Dec as a SA hasn't got a dec gear..it tracks in RA only..

Pe is from the worm gear, which I don't think there's a adjustment for in a SA..

There's a few things on a SA that can become loose..make sure everything is tight..check all bolts and give it a tug to feel any movement as sometimes you can't always see it 

[FaDG]

4 minutes ago, newbie alert said:

How can there be drift in Dec as a SA hasn't got a dec gear..it tracks in RA only..

Dec drift is caused by Polar misalignment. 

Dec guiding (on mounts which support it, that is) can compensate the drift, resulting in field rotation around the guide star. 

Fabio

Edited April 27, 2019 by FaDG