5 replies to this topic

[UP4014Fan]

I've purchased an AVX GEM for my 8" SCT and I really do like it.  I haven't had to toss any subs due to trailing or egg-shaped stars yet.  Well, to be honest, last night, I didn't notice that, because M86 was right at the meridian when I started, NINA/CPWI elected to aim the scope from the wrong side of the tripod, so it hit the RA limit I'd set after three images.  My own fault, as I hadn't set a meridian flip trigger after seeing that my session would start shortly after transit.  Good news is that I only lost three images.

But that's not the point.  In a fit of madness or hubris, I elected to set up my original Alt/Az/Wedge combination as well.  I mounted another Canon 200D with a Sigma 120-400 (I know, primary lenses, but it's what I have right now).  The Sigma carried the load of the DSLR, like most long lenses, and I attached that to a dove tail that went to the mount.  I did a three-point manual alignment in CPWI and then a TPPA in NINA, getting within less than arc minute.  

 

I selected my target, in the vicinity of the Markarian's Chain, and everything plate-solved within three or four attempts.  As the night went on, it seemed to track very well, even without a guider.  When I ran the images through Bling in PixInsight, everything looked rock-solid, although I noticed that the stars were twinkling a bit in the animation.  When I zoomed in to the point of looking at one or two stars, I could see that, every five or six frames, there'd be some slight trailing back and forth for about four frames, and then things settled down again.  The trailing was all in RA (azimuth to the mount), and only about double the apparent width of a star, and not noticeable when zoomed out.  Obviously, I tossed those images into the reject directory (OK, "folder" - I'm old).

 

So, what could have caused this?  The camera/lens combination are significantly lighter than the SCT and, although I had these issues when using the SCT on this mount, it wasn't nearly as severe.  Is there not enough load on the mount, so it's bouncing off the azimuth gear when everything is tilted 38^o?  Is there some binding occurring?  Should I just give up on the wedge and live with field rotation at shorter focal lengths?  Or is there a guider that's compatible with the hardware/firmware that I should consider?

 

Thanks in advance.

[Son of Norway]

Is this like an SE mount? possibly make sure the altitude nut is adjusted properly and also try a backlash adjustment.

[UP4014Fan]

Is this like an SE mount? possibly make sure the altitude nut is adjusted properly and also try a backlash adjustment.

Thanks!  Hadn't considered that.  I had done a backlash adjustment with the SCT, but this is virtually no load in comparison.  It might be overcompensating.

 

BTW, it is my original SE mount.

Edited by UP4014Fan, 23 May 2025 - 04:56 PM.

[PhilHoyle]

If the mount is driven by a worm gear, as most are, what you are seeing is periodic error.  Periodic error is caused by the gears.  Even perfect gears have it.  It causes the mount to be driven a little too fast then a little too slow in a sinewave pattern.  When your stars look good is when the period is reversing.  When they look bad is when the speed error is the greatest.

[smiller]

How long were your exposures with the camera?

 

 

If all else fails in solving the periodic error, and the error is small as you say, perhaps cut exposure time to the point the error is within your desired tolerance as a stop gap…

[UP4014Fan]

How long were your exposures with the camera?


If all else fails in solving the periodic error, and the error is small as you say, perhaps cut exposure time to the point the error is within your desired tolerance as a stop gap…

30 seconds, which is about as short as I want. I need to solve the mount issues.