Why are my stars eggy in one direction?

[Zuts]

Hi,

I am trying to setup my Celestron edge 9.25 for photography.

In the attached image my stars are all eggy in one direction. The image is a stack of 5 by 120 second exposures. The exposures were all guided with an average rms error of 0.3.

I don't think it is sensor tilt as on my TV85 the stars appear nice and round.

1. However I have removed/replaced the corrector for cleaning so maybe it is replaced in the wrong position?

2. Until I get a counterweight extension bar the scope is severely unbalanced, could this be the issue?

Any thoughts appreciated.

Re-processed with synthetic colour

Cheers
Paul

[glend]

Your guide error seems high to me. I suggest you try shorter subs say, 30s, 60, etc this will tell you if it is tracking or something else. Just my opinion.

[Nikolas]

You said it, the scope is unbalanced and it is affecting your guiding, you will notice it more with longer focal lengths than shorter ones.

[xelasnave]

Drift in RA East / West will be balance for sure..try running unguided and the error should be more pronounced ..see if the stars "move" to the heavy side.

While waiting for your counter weight extension bar you could hang a weight, a little sand bag so you can vary weight, and satisfy yourself that balance is the problem...in any event trying to deal with other problems should wait until you have the correct balance...and remember your mount may like it a little heavier on one side so what may seem like perfect balance actually won't suit the mount...respond by seeing which direction the stars "move".
Good luck.
Alex

[Startrek]

Looks like a tracking issue to me which leads towards an out of balance mount
Guide error rms at 0.30 arc sec seems acceptable for your focal length of 2350mm
Try a 5 sec , a 10 sec , a 30 sec and 60 sec exposure and see if the issue is not noticeable at 5 but progressively gets worse up to 60 sec

[ChrisV]

Geez that's tough. I've never had a tracking error as low as 0.3arcsec rms. But I suppose the +2000mm focal length is the killer here. For me, I see no eggs with 0.5-0.7arcsec rms at 1000mm focal length. So its a bit curious.

If it is tracking, then the eggs suggests that tracking is worse in one axis - probably DEC, its always the culprit. Check the error for each axis and maybe adjust the offender.

Other things?
Check the individual subs, maybe there's just some bad ones with big deflections
I know nothing of SCT correctors so can't comment on that.
I doubt it is tilt as that would end up with uneven distortion over the field.

[Zuts]

Thanks Glen, I did try shorter subs but still a bit eggy.



Thanks Nikolas, I am suspecting unbalanced. If i loosen the clutch the scope will attempt to crash into the mount, so as I said it is severely unbalanced.



Thanks Alex, so you are saying that if I don't guide rather than happing a random error and some trailing, most of the error should still be to one side? Anyway I will try this.



Thanks Martin,

I will try this as well. I forgot to mention that I am using a 0.7 reducer so my FL is actually around 1700mm.



Thanks Chris, I guess I can check the corrector next time if I take some eye pieces with me and verify the collimation. I don't think its tilt either as the camera is fine in the TV85.

Anyway I am going to wait until I get the counterweight extension and then try again. It was just a shakedown so I will go back to the TV85 and Inner City skies :( until then.

[Sunfish]

I would check the collimation. May be focus but may be collimation as well as the drift. Did you put the corrector exactly on the marks and with exactly the same spacers in the same position just finger tight.

[Zuts]

Hi Ray,

I believe so. It is easier to remove/replace newer model Celestron's than older models. The newer models no longer have paper spacers. They have 4 nylon pins on the inside rim and the corrector basically plugs into these and fits snuggly. I did mark the rotational position before i removed. Of course i need to star test when I get a chance just to make sure.

[Sunfish]

That is good. Sounds like a little collimation star testing will fine tune performance.

[Zuts]

Re-processed using composite in StarTools to generate a synthetic colour channel from the duoband image.