Mirrors are culminated but not aligned?

[Savage234]

Okay, so I recently got back into the hobby...and I have not been short of problems since. My reflector had some pretty bad pinching (which I fixed), but there still seems to be some distortion of the stars. 

I took an image over the past couple nights of ~10 hours and you can see the distortion pretty clearly. 

I think the problem is coming from my coma corrector. According to my flat frames the center of the mirror is off center. I have collimated the mirrors with a laser and they are perfect. But according to the flat frame it might be pointed in the right direction, but it doesn't seem centered. This off center mirror reflecting light through a centered coma corrector I think is the source of the problem. How do I fix this? Do I need to go in and maneuver the secondary mirror differently?

Thanks. 

Edit: Converting the Flat frame to a Jpg really ruined it, here is a better link. The vertical lines are not present in the actual flats, just when uploaded here:

 

[Merlin66]

Could be the alignment of the focuser to the optical axis?

 

[almcl]

Making some assumptions about your equipment and using MaxPilote's Maxselector to analyse the image above gives a collimation offset of almost 16% (image below).  While this isn't large, it might be worth trying to reduce and see if it helps? If you have a sight tube or a Cheshire eyepiece, it might be worth following Merlin's suggestion and checking focusser alignment. Maxselector suggests that there is ~14% tilt.

[Savage234]

I will check the focuser, though I think it is fine (the problem is new, I have taken many photos where there wasn't any offset). Maybe simply taking it off and putting it back on will adjust it slightly, it has been through a few moves since last use. Taking it off and putting it back on seems to fix plenty of problems.  

Without the coma corrector the distortion is much less noticeable. The coma just centers towards a different point.  Adding the corrector just seems to "correct" improperly. 

I don't have a Cheshire eyepiece, but they are fairly cheap so i'll pick one up.  I looked into properly using one and it seems that is probably my problem. 

I was kind of under the assumption that a laser and Cheshire did more or less the same thing, I guess I was wrong.

The scope is a 254mm 1000mm F/3.9. I don't know if that would make it better or worse, but either way the problem seems to be the same.

Thanks for the help!

[jacksdad]

Is your laser collimator itself correctly adjusted?

[Savage234]

On 20/06/2018 at 14:09, jacksdad said:

Is your laser collimator itself correctly adjusted?

Pretty sure the laser is correct. I have another reflector that is working fine using the laser.  I did a pretty extensive cleaning before I put up the scopes for storage and probably didn't put the mirrors in exactly the right orientation. Just looking through the focuses it looks like the secondary mirror is a little twisted. I messed around with it a bit, but my old laptop finally kicked it, so It'll be a little before I can really test it with photos.

 

[spillage]

sorry to hijack this thread but @almcl could you let me know how you got maxselector to show the collimation. I have loaded up a .fits and in the file list it shows a reading for collimation but no visual display. Is this not possible with the free version?

 

Mark.

[almcl]

4 hours ago, spillage said:

sorry to hijack this thread but @almcl could you let me know how you got maxselector to show the collimation.

PM sent, Mark.

[Savage234]

No problem, I was interested in the same thing. I downloaded the software, but my laptop died so couldn't really go much further with that. 

Update: I think the problem was that when adjusting the secondary mirror it had spun (slight clockwise) when I loosened the main screw. I used a Cheshire eyepiece to adjust that better and a laser to center the secondary. It appears centered in both laser and Cheshire now. I do not have the ability to take flat frames so I won't be able to test that yet, but I think my problem is solved. 

 

Thanks for the help