Whats wrong with my stars?

[cuivenion]

Hi, I thought I'd use a bright moonlit night for some testing of my new system. This is a single guided 300 second sub taken with a Canon 500d, Baader MPCC-III, Skywatcher 130PDS on a HEQ5. Guided by a colour GPCAM v1 on a 135mm lens:

There is obviously the problem of the focuser tube cutting into the brighter stars, but it's more the aberrations in the corners that bothers me. It's even noticable in the center where the stars have a bulge. I don't think its tracking or guiding, as the defects appear on shorter unguided images of about 25 seconds.

Any help would be much appreciated.

 

 

[michael.h.f.wilkinson]

Is it me, or is there some slight tracking error, or field rotation taking place?

[alacant]

19 minutes ago, cuivenion said:

in the corners

Hi. The camera is not quite square and/or the cc needs to be -estimate- 1mm closer to the camera.

21 minutes ago, cuivenion said:

stars have a bulge

 But notice there's a different shaped aberration. I'd say it's guiding.

Both very slight and as tube and focuser flex takes over, I dare say it would change as the tube tracked. When you stack, it removes a lot of the irregularity.

Just my €0.02 and HTH.

[Zakalwe]

My tuppence worth*

The stars in the top left, top right and lower left are elongated as if they are rotating. That could be coma and the spacings might need a slight tweak.

There is some aberration in the brighter stars diffraction patterns, but it's fairly minimal. Like you said, it's probably the focuser tube.

 

Personally, I think that you're getting into the realms of "pixel-peeping". A tweak of RC-Astro's "Star Shrink" (or the equivalent Annies Actions/Carbonis Actions) would probably solve most of it. Or even a mild cropping of the image.

 

 

 

*I am far from being an expert, so take this with a large helping of salt

[barkis]

The distortions are progressively  more prevalent at the edges. 
Symptomatic of spherical aberration, this may be caused by strain on the mirror from the  cell, and  not inherent
in the mirror itself. Have you been doing anything with the scope at all recently perhaps?
The SW 130pds has a good reputation, and some wonderful images have been produced by the instrument.
The above suggestions are quite valid too of course, so you need to do some checks. 
It isn't a problem that can't be rectified.

[Shibby]

44 minutes ago, barkis said:

The distortions are progressively  more prevalent at the edges. 
Symptomatic of spherical aberration

I dunno, it looks more like field rotation to me.

There's a chance that the guide star was a long way from your target; this can cause some field rotation. Most likely, though, is an error in polar alignment. This causes your field to rotate around the guide star.

Edit: The bright, central stars do show a different issue. The diffraction spikes don't look right, which could suggest a problem with the alignment of your spider vanes.

[barkis]

1 minute ago, Shibby said:

I dunno, it looks more like field rotation to me.

There's a chance that the guide star was a long way from your target; this can cause some field rotation. Most likely, though, is an error in polar alignment. This causes your field to rotate around the guide star.

I can see that, possibly polar alignment could also cause  rotation if it was very poor.

[cuivenion]

Hi, Thanks for the replies. I don't think its guiding. Heres an unguided 20 second image, same night same setup:

The defects are still there. I'm not sure about field rotation, would polar alignment have to be really bad for that to occur in a 20 second exposure? I used the sharpcap polar alignment routine and ran it three times, both RA and DEC were within 10 arcseconds of the pole according to the software. I suppose its possible that the tripod sank a little as the ground was soft. I did my best to bed the tripod in before the full set up though.

 

 

[cuivenion]

1 hour ago, barkis said:

The distortions are progressively  more prevalent at the edges. 
Symptomatic of spherical aberration, this may be caused by strain on the mirror from the  cell, and  not inherent
in the mirror itself. Have you been doing anything with the scope at all recently perhaps?
The SW 130pds has a good reputation, and some wonderful images have been produced by the instrument.

 

Hi, The only thing I've tried to do is fine tune the collimation when I first got it. I wouldn't say the collimation is perfect, I ve had real trouble getting the secondary to present as a circle, but it is close.

[cuivenion]

1 hour ago, Zakalwe said:

My tuppence worth*

The stars in the top left, top right and lower left are elongated as if they are rotating. That could be coma and the spacings might need a slight tweak.

There is some aberration in the brighter stars diffraction patterns, but it's fairly minimal. Like you said, it's probably the focuser tube.

 

Personally, I think that you're getting into the realms of "pixel-peeping". A tweak of RC-Astro's "Star Shrink" (or the equivalent Annies Actions/Carbonis Actions) would probably solve most of it. Or even a mild cropping of the image.

 

 

 

*I am far from being an expert, so take this with a large helping of salt

Hi Zakalwe, I'm a beginner with astrophotography. I've dabbled with my GPCAM and long exposure but this is my first time guiding a scope and a DSLR. I don't use photoshop is there a Pixinsight equivalent? I don't think I'm pixel peeping quite yet lol, the scope/mpcc aren't performing as they should. I have been wondering how this large size images are meant to be viewed, do you resize them to taste?

[cuivenion]

2 hours ago, alacant said:

Hi. The camera is not quite square and/or the cc needs to be -estimate- 1mm closer to the camera.

 But notice there's a different shaped aberration. I'd say it's guiding.

Both very slight and as tube and focuser flex takes over, I dare say it would change as the tube tracked. When you stack, it removes a lot of the irregularity.

Just my €0.02 and HTH.

Hi, The CC collar is still on. Would you recommend taking that off before I attach it to the T-Ring?

Here's the guide logs, there was a break in between them for clouds.

[alacant]

5 minutes ago, cuivenion said:

The CC collar is still on. Would you recommend taking that off

Hi. No. The collar is cosmetic. I was hoping you'd say you were using the m48 fitting. Then it's possible to adjust the spacing closer to the sensor. I think if you're going with t2 then just re-seating the camera will do it. To hold the camera better, you could fit a third m4 thumbscrew. I've never had any luck with sw focusers:(

In any case, the aberrations you are seeing are minor enough to be solved in software:)

13 minutes ago, cuivenion said:

the guide logs

The RA error is larger than the dec -and OK for the image scale- but the difference is not large enough IMHO to cause elongation. In any case, if that were to be the case, all the stars would be affected. My money's still on tilt.

HTH and good luck.

 

[cuivenion]

Hi everyone. I've loosened the primary mirror clips, they were quite tight. I've also rechecked collimation and it was worse than I thought. I was also very careful when putting the camera into the focuser. I think this is an improvement:

The star elongation is mostly in the corners which suggests that the MPCC spacing from the sensor is wrong? Am I expecting to much from the MPCC with a sensor this size or should it correct coma across the whole image?

Thanks for any help.