Star Adventurer GTi, sudden change in tracking, Asiair.

[LaurenceT]

This is my new to me mount. A couple of weeks ago I tested it unguided on the Sadr region and got perfect stars at 2 minute subs. Last night I tried to image Andromeda and found I had short star trails at 2 minutes and could only image at 60 seconds which gave good stars.

I haven't yet processed the session. Nothing had changed in terms of gear and setup and I'm at a loss to find a reason for this, PA was as perfect as I've ever got it. 

 Star Adventurer GTi, Asiair Plus, WO ZS61ii with x1 flattener,, Asi 533MC Pro with Baader Moon and Skyglow filter, ZWO EAF, Skywatcher stainless steel tripod, power from Jackery 240 to mount and Asiair individually.

Be grateful for any ideas.

[michael8554]

Hi Laurence.

Was the elongation in RA or in Dec ?

Both objects are at similar Decs, but would probably involve a Side of Pier flip.

So possibly balance.

What were the PA figures on each occasion ?

Michael

 

[LaurenceT]

I'm afraid I'm having to go full novice here, the star trails were horizontal, the PA figure was 0.88.

I hope that's what you mean.

[michael8554]

If we assume that's RA, then may be just night-to-night Seeing and guiding differences.

Any idea what the RA and Dec error figures were ?

Stars would be noticeably elongated if the RA figure was about double the Dec figure.

"the PA figure was 0.88."

Usually measured in arcmins or arcsec, do you know which ?

Michael

[LaurenceT]

Just to clarify, I wasn't guiding in that session. The PA number is in arc seconds. When I was using the EAF before polar aligning the selected focus star was also elongated. Once again, apologies for the novice replies.

[michael8554]

Hi Laurence

We usually ask to see a very short exposure, to reduce the impact of tracking and guiding errors.

You say you had good stars at 1 minute exposure.

But "the selected focus star was also elongated." 

That would point to Tilt, or poor Collimation, or Backfocus of the Flattener if the star wasn't centre frame.

Perhaps you could post a jpeg of one of the Lights.

Michael

[LaurenceT]

These are JPEGs of the 2 minute subs and the 1 minute subs. The elongated focus star was remedied when I did a polar alignment first. If it was a backfocus issue wouldn't the star streaks have been evident in both exposures?

[900SL]

Polar alignment issue, I think

[LaurenceT]

7 minutes ago, 900SL said:

Polar alignment issue, I think

You'd think so but even after PA my 2 minute subs still had star trails, so I reverted to the 1 minute subs.

One thing does occur to me, this was the first time I imaged with the Baader Moon and Skyglow filter, my previous attempt at M31 had been made with a UV/IR cut filter.

When I image emission nebulae I use an L-Enhance filter and just insert a thin spacer to compensate for the filter thickness, I used the same filter this time, maybe I should have used a different thickness spacer.

[900SL]

backfocus causes radial issues, yours are all trailing in similar direction, therefore it's either some serious periodic error, wind, or PA as you were unguided.

[900SL]

On 21/08/2024 at 06:51, LaurenceT said:

This is my new to me mount. A couple of weeks ago I tested it unguided on the Sadr region and got perfect stars at 2 minute subs. Last night I tried to image Andromeda and found I had short star trails at 2 minutes and could only image at 60 seconds which gave good stars.

I haven't yet processed the session. Nothing had changed in terms of gear and setup and I'm at a loss to find a reason for this, PA was as perfect as I've ever got it. 

 Star Adventurer GTi, Asiair Plus, WO ZS61ii with x1 flattener,, Asi 533MC Pro with Baader Moon and Skyglow filter, ZWO EAF, Skywatcher stainless steel tripod, power from Jackery 240 to mount and Asiair individually.

Be grateful for any ideas.

Just for clarification, were ALL your 2 minute subs similar with similar star trails. Is the image you posted a stacked image, or a single sub?

The 1 min subs look good, no trailing. The two minute subs have 'bell' shapes, could be wind, or vibration.

[LaurenceT]

20 minutes ago, 900SL said:

Just for clarification, were ALL your 2 minute subs similar with similar star trails. Is the image you posted a stacked image, or a single sub?

The 1 min subs look good, no trailing. The two minute subs have 'bell' shapes, could be wind, or vibration.

I only took 2 x 2 minute subs which were identical so I continued just with the 1 minute subs. It was quite windy that night, fingers crossed it was just that.

[markalot]

On 23/08/2024 at 08:03, LaurenceT said:

I only took 2 x 2 minute subs which were identical so I continued just with the 1 minute subs. It was quite windy that night, fingers crossed it was just that.

Howdy.  I just retired my GTI to portable applications only.  I was guiding and here's my experience watching how it tracks when hooked up to PHD2 (a guider that tracks errors).

On some nights it tracks very well, and I suspect it's a balance, polar alignment, or tripod issues.  I polar align using NINA 3 point alignment because I don't have a clear view of the north star.  What I ended up doing was not guiding and reducing exposures to 30 seconds, which worked well for 90% of my subs, with a few still having star trails.  Guiding for me usually increased errors due to declination backlash (play in the gears).  It would over correct, swing wildly too far, then get into a back and forth loop.  Not every night mind you!    

Do you know how to calculate what your error tolerance is?   For example, if you are using a 360mm focal length with a 533 (3.76 pixel size) you have a tolerance of 2.15" per pixel (I'm relatively new but I think you'll notice any movement over half that value).   That darn aluminum tripod has all kinds of weird flex, just push it around and see what I mean.  I was never sure if my issues were all tripod or in the mount head itself, but I suspect the tripod was a lot of it.  This also explains why some nights seemed different from others.   

 

Using PHD2 (looks like you're using ASIAir so you'd have to hook up a laptop) I can use guiding assistant to watch it guide without any corrections.  The results were enlightening as you could see the declination wobble even though it should be fixed.  Some nights had little wobble, others a lot of wobble.  In reading up on it it seemed some solved this issue by purposely unbalancing so that there was a slight constant weight on one axis depending on which side of the pier you were on.   

 

Not sure this helps, but bottom line is reducing to 30 second subs should solve most of your issues, especially if you can turn off guiding (does ASIAir guide automatically)?  

 

Edited August 25 by markalot