Sensor Tilt or Coma Corrector spacing?

[Pitch Black Skies]

Could I get an experienced pair of eyes to have a quick look at my sub please?

My stars not round and I'm wondering what could be causing it.

[pipnina]

If you could provide the raw sub that would make it even easier! The FITS file would be best for us to inspect.

From what I can see, based on how the shape seems consistant through the image. It might be collimation? Sensor tilt might show one corner inter-focual with coma, and the other corner extra-focal with astigmatism. But this seems like the same shape is running through the whole image.

[michael8554]

What camera, OTA, CC ?

Top right corner has the best star shape, but up/down elongation and focus gets worst the further down you look.

Suggesting Tilt.

Michael

 

[Pitch Black Skies]

FITS file coming soon...

294MM

130PDS 

Skywatcher Aplanatic 

[AstroNebulee]

Will wait for the fit file to have a closer look but as @pipninasays they all run in one direction, so could be collimation though there maybe tilt running left to right.

Is your scope collimated? 

Lee

[Pitch Black Skies]

I agree with Michael. Top left looks the best to me too.

Collimated before imaging.Light_IC1396_300.0s_Bin2_294MM_H_gain390_20220813-000724_-10.0C_0001.fit

Edited August 13, 2022 by Pitch Black Skies

[pipnina]

I put it into the abberation inspector of pixinsight so we could see the corners a little clearer.

Something about it makes me think primary mirror misalignment. I think with tilt the center usually stays relatively sharp? I am not sure.

[pipnina]

One option might be to use a high power eyepiece and do a star test. Plate solve the scope to a bright star like vega or capella and see what the airy disk and diffraction rings look like. If it all looks good through an eyepiece, focuser slop or sensor tilt will seem more likely!

[Pitch Black Skies]

@pipnina thanks, that's a useful tool.

I managed to pull this from CCD inspector.

CCD 1.fitCCD 2.fit

It seems like it might be tilt and collimation.

[Pitch Black Skies]

A second attempt last night.

In live view mode I zoomed in on a bright star. I then put some pressure on the imaging train but the star shape didn't change. This has me thinking it isn't sensor tilt. I then adjusted a collimation screw on the primary mirror while watching the star and it went from elongated to more pinpoint. It seems like a collimation issue. I had it exact before adding the imaging train so I'm not sure what is causing it. I collimate using concentre and collimation cap. I analysed a sub from last night and now it looks like a combination of pinched optics in the top right corner and radial stretching in the others indicating the cc is too far from the sensor. 

Light_IC1396_300.0s_Bin2_294MM_H_gain120_20220814-033125_-10.0C_0023.fit

Any further advice is greatly appreciated, apart from getting an Apo 😌

[pipnina]

I've found collimation very hard to achieve. My 130P-DS was hard enough but my F4 TS-PHOTON is pretty much impossible.

If you don't have one already, I can't recommend buying a compression ring fitting for the 130P-DS focuser. The default thumbscrew clamp doesn't hold things anywhere near true and damages equipment that it holds. But it also unscrews (allowing for threaded connections). If the coma corrector can thread into it directly that would make things easier. And if you can use a compression ring holder in place of the thumbscrew clamp your collimation tools will hold true too.

I haven't used a cap or a concenter for primary alignment. Only secondary. I have used a cheshire eyepiece for primary alignment that that served my 130P-DS very nicely!

For my photon I can't collimate with anything less than a laser. And even then I have to be super meticulous about it and rotate the laser multiple times etc. And it still won't be perfect once I'm done!

I found the 130P-DS to hold collimation remarkably well. So maybe you can try the bright star in a high powered eyepiece idea? If you have any visual astronomy kit too, something like a 5 or 7mm eyepiece and a bright star. It will let you see the pattern of rings around the star in the center of the fov and how round it is. Supposedly this is the most reliable method of collimation you can perform.

[Pitch Black Skies]

I'm after stacking all of the frames and while the master isn't perfect I think the finished result looks more than acceptable.

I might try to pick up a laser again for the primary. I'm not a huge fan of them but if used carefully they have advantages over other methods. It's possible the centre of my doughnut isn't the centre of the mirror. The concentre works great for the secondary.

 

[Pitch Black Skies]

Just found a problem with the collimation cap I have been using. 😬

If I rotate it in the eyepiece holder, the peephole doesn't stay exactly in the centre of the doughnut. It isn't very obvious, but under close inspection it isn't perfect. 

I think I have underestimated just how exact collimation needs to be for astrophotography. 

Edited August 15, 2022 by Pitch Black Skies

[pipnina]

2 minutes ago, Pitch Black Skies said:

Just found a problem with the collimation cap I have been using. 😬

If I rotate it in the eyepiece holder, the peephole doesn't stay exactly in the centre of the doughnut. It isn't very obvious, but under close inspection it isn't perfect. 

I think I have underestimated just exact collimation needs to be for astrophotography. 

A good way to think about it, is that if your collimation is off by 2mm, that is more than 10% of your camera's sensor height on APS-C. Naturally this error will notice in your images! And it only gets worse the faster your mirror.

No wonder so many of us spend sleepless nights trying to get it right...