Still struggling with eggy stars

[rubecula]

Since I last posted on this subject things have not improved.  After the last post I thought it might be sag due to threads not being sufficiently tightened. They are all tight now.  Also it was suggested that the polar alignment might be off.  I have spent a couple of nights getting that better.  When I did a PHD2 drift align last night the azimuth Dec line had zero slope and the altitude had an error of 0.03" so I presume that the PA is pretty good.

I slewed to Capella, re-calibrated and then took a 600 sec exposure via BYEos. The result was dreadful   I nearly threw it away, but then thought it might help in resolving the issue.

The centre stars are reasonably round but are egg-shaped in the corners, particularly the top corners.  Also the reflection (???) around Capella is not centred on the star.  Does that give a clue?  Apologies in advance for the image, as I said it's dreadful:

Equipment used all as in signature below.  Any help gratefully received.

[Davey-T]

Round stars in the centre and eggy ones in the corners are usually indicative of wrong reducer to sensor spacing or droop in the image train.

Hopefully someone will run you image through CCD Inspector.

Can you post an enlarged image of the four corners ?

Dave

[gnomus]

I am no expert so take what follows with a large pinch of salt....

The coma in the top corners is quite marked, whereas, as you say, it is significantly less in the bottom (indeed, in the bottom left it almost looks like it is heading in the 'wrong' direction).  I'm not entirely sure what the doughnut is around Capella, but it is non-concentric.  Are you sure that the scope is properly collimated?  Also, as Davey-T says you need to be very particular about the spacing of the sensor from the back of the focal reducer.  If you run the Guiding Assistant in PHD2 for a few minutes it will tell you what your PA error is (although I am not sure how reliable the report is), but this doesn't look like a polar alignment error.

I tried for a time to get results from my Edge HD 8 with focal reducer on a CGEM mount, albeit without an OAG.  I think that imaging at these long focal lengths is very demanding of equipment and user, and eventually I admitted defeat and bought an ED80!!   I started to enjoy things a bit more after that.

[jambouk]

What scope is it?

Jamee

[Owmuchonomy]

...and what do short exposures of a few seconds look like?

[Astrosurf]

Could it be that the sensor is tilted to the optical train?

 

Alexxx 

[gnomus]

27 minutes ago, Owmuchonomy said:

...and what do short exposures of a few seconds look like?

This is a very good point.  A brief exposure with the scope pointing straight up would be helpful. 

[Physopto]

 

Looking as closely as I can at your image it would appear that the stars at the top left are egg shaped outwards at about 45 degrees roughly from the centre of the image. The exact same thing appears to happen to the stars towards the top right of the image but pointing out wards again from the centre towards the right diagonal. All the stars around the bottom of your image look reasonably normal or roundish !

To me that suggests that the optical train is not set up correctly and is skewed at an angle to the image plane. The off centre halo around Capella also suggest something wrong with the optics as there is also some non concentric flaring. It could be a problem with the reducer. I would first remove the reducer and try the image again to see if the problem still exists. If it does then it is most probably not the fault of the reducer and it can be excluded from the equation. It may then be down to the camera not  being exactly centrally located  possibly at some tilted angle. The camera could just be cross threaded onto the receptacle thread on the scope.

It may not be the camera angle to the scope but  the CCD chip is not correctly aligned with the imaging plane, (more likely than cross threading).

Finally in my mind the scope may need the front elements collimating.

There are many possibilities. You need to eliminate one at a time to find out what the cause is.

I hope this helps. Do not loose hope usually everything can be fixed.

Derek

[jambouk]

"Finally in my mind the scope may need the front elements collimating."

Which is why I wondered what the scope was. 

[Physopto]

Yes, without knowing what scope it could be a multitude of things! Mind running over all sorts.

I suspect a doublet as colour fringing is present. Could easily be wrong though.

Derek

[gnomus]

Just now, Physopto said:

Yes, without knowing what scope it could be a multitude of things! Mind running over all sorts.

Derek

I think he told us the equipment was in his signature: Celestron AVX 8" EdgeHD, 0.7 Focal Reducer, 13mm & 21mm Baader Hyperion | Canon 7D, Canon 550D (modified) OVL Slim OAG with Altair GPCAM (mono)

We may not be sure which of the two cameras he is using though. 

[Physopto]

I'm going blind!

Could be the mirror flop then?

 

[rubecula]

Thanks for the ideas so far. The image was taken with the modified Canon 550D. 

Alexxx, I was also wondering if it could it be a problem with the camera modification?  I'm not sure whether it's possible to introduce a sensor tilt when doing the modification.

Is there an easy way to tell whether it's the camera sensor rather than the FR or colimation?

 

[gnomus]

1 minute ago, rubecula said:

Thanks for the ideas so far. The image was taken with the modified Canon 550D. 

Alexxx, I was also wondering if it could it be a problem with the camera modification?  I'm not sure whether it's possible to introduce a sensor tilt when doing the modification.

Is there an easy way to tell whether it's the camera sensor rather than the FR or colimation?

 

You could try your other Canon camera.

[jambouk]

Have you checked the scopes collimation? Have you ever adjusted the scopes collimation?

What happens if you image without the focal reducer in. Is the reducer a flattener too... doesn't an EDGE already have a flat field?

James

 

[Physopto]

Read FLO page on the Celestron Edge, I just did. Interesting! Separate flattener and reducer.