Another star shapes help thread

[cuivenion]

Hi, I'm imaging with a 130PDS, ZWO ASI224 and a Baader MPC coma corrector. Here is an uncalibrated fits files:

It looks to me like coma but across the whole field. I was guiding at the time here is the log:

The guiding looks good to me but I forgot to recalibrate the profile after increasing the guider focal length by 20mm and taking the guide camera off and back on again. I'm just not sure if this a tracking or an optical issue.

Just to add I was only doing 30 second subs on a HEQ5.

[vlaiv]

I doubt it's coma.

With such a small sensor, even at F/5 you can image without coma corrector and odds are that you won't be able to see coma in stars under regular circumstances.

Coma free field of F/5 scope is about ~2.8mm and it takes about 3 times this for coma to actually show against seeing. ASI224 is 6mm in diameter, and that is x2 more than coma free field. Any coma on such scales will be probably masked of by seeing.

I did some imaging on F/6 scope reduced to about F/3 (x0.5 reducer) with similar sized chip (QHY5IILc) and coma was not apparent.

This leaves pretty much two possibilities. MPCC and mount. MPCC has some spherical aberration, and stars tend not to be that sharp, but a bit swollen in center of the field, even at F/5 (at F/4 its worse). But I don't think it would show like deformed stars - more like "round" stars (round in sense of not being sharply defined - profile wise not in image). So I doubt its that either. This is easy to check - just image without MPCC - I think you'll be surprised by absence of coma from your images.

Which leaves guiding. Your guiding is really not up to resolution of your system. With ASI224 and 130PDS you are working on 1.19"/pixel - which is perfectly fine resolution (one might even say spot on for regular seeing conditions), but such resolution requires guiding to be at about 0.6" RMS for guide errors to be minimally visible in the image. You have 0.9" RMS and that is a bit high so stars will be bigger than they could be with better guiding, but that in itself does not mean distorted stars (RA and DEC error are pretty close and that usually means round stars).

You are dithering, right? At 30s exposures how often do you dither? Do you allow enough time for guiding to settle down, before starting new exposure?

You can check one more thing to see if its guiding related. How is your sensor oriented with respect to RA? Is RA close to horizontal? From star shapes, it looks like there is smear in horizontal direction (a bit larger) and small smear in vertical direction. If RA is horizontal (or vertical) - it is probably due to guiding. Often in star shapes if artifact is the same across the whole image and is related to RA/DEC orientation - it's probably due to mount/guiding.

[cuivenion]

Hi, thanks for the reply. I don't dither with this camera, I do when I image with my DSLR. Thanks for the info on guiding on resolution I must of previously misunderstood that, I was under the impression that if your guiding RMS was around the same as your resolution you were doing OK.

Unfortunately I'm not sure what the orientation was in regards to RA or Dec, but the camera was aligned with N, S, E, W. I just can't remember which was up, down, left or right. It's possibly a guiding problem then. I'll make sure I recalibrate before my next run. It was all a bit of a rushed session between clouds.

Would it be better to guide even if your RMS isn't low enough or run unguided?

[cuivenion]

Is it me or does the star scatter graph look like the star shapes in my image?

[vlaiv]

It indeed looks like scatter graph. I think this is the cause of star shapes. One half of imaging resolution is rule of the thumb for guide RMS, at least on those levels guide error should have minimal impact. In this case because guide RMS is larger than half of imaging resolution - it starts to show in exactly that way - scatter graph resembles star shapes.

I think that it's better to guide than run unguided for 30s exposures. Just make sure you calibrate your guide system well. Balance scope and do all recommended things. If you suspect that you are chasing the seeing - use longer guide exposure (like 4s).

You mentioned that you rotated your guide camera? Guide scatter graph looks like "regular" guide scatter graph, rotated for 45 degrees. It's usually RA that is a bit higher than DEC error due to periodic error of mount, so scatter graph tends to look like horizontal ellipse - but this one is at 45 degrees (this does not translate into same direction in image - that depends on angle of horizontal to RA/DEC for imaging camera).

[cuivenion]

Hi, I removed the guide camera to use on another set up then put it back on this one. I recently discovered that my guider focal length was wrong as well and I corrected that in the profile. But I forgot to calibrate, that probably hasn't helped.

It makes sense that the scatter graph and the image don't match up because I had the imaging camera rotated about 45 degrees to get better framing on the galaxy and the imaging and guiding cameras were previously aligned as close as I could get them.

I'll have to make sure I check my setting more carefully as my guider exposure was at 2 seconds and I usually have it at 3-4 like you said. Thanks for the help, hopefully I'll have it running smoother next time.

[ollypenrice]

I think you're being pretty exacting. The stars are hardly disastrous. Maybe the scattergraph does explain the slight asymmetry but might there also be a little pinching?

Olly

[cuivenion]

 

Hi Olly, do you mean pinching of the primary mirror? The primary is secure but can be rotated in the clips. Don't worry I'm not chucking the data away or anything like that, I just hadn't seen that effect before and wanted to track down the cause.

Just need some more data to get more detail out of the outer spiral arms now.

2 hours of 30 second subs. Flats and darks:

[ollypenrice]

1 minute ago, cuivenion said:

Hi Olly, do you mean pinching of the primary mirror? The primary is secure but can be rotated in the clips. Don't worry I'm not chucking the data away or anything like that, I just hadn't seen that effect before and wanted to track down the cause.

Just need some more data to get more detail out of the outer spiral arms now.

Probably that, yes. The stars look slightly triangular. Was it particularly cold on the night in question? This can tighten the clips of the cell.

Olly

[cuivenion]

It got colder near the end of the night. I set up about 2am, got clouded out till about 3.45am started imaging about 4. It was cold but there was no frost on the scope until near pack up at about 6.30. I'll certainly keep an eye out for it happening again. Hopefully the issue will be resolved by recalibrating PHD2.