odd star shapes

[spillage]

Hi All,

 

having issues with poor star shapes and not sure if anyone can point me in the right direction. using my quattro 8" with baader cc, Pa is good and tracking was about 0.4 last night. I am sure collimation is okay but not 100%.

Top Left

 

 

 

 

 

 

 

 

 

 

Top Right

 

 

Bottom Left

Bottom Right

 

single frame.

 

[Ships and Stars]

How are you collimating? Cheshire for secondary and laser for primary? Or something like that? Other thing might be the Baader CC isn't fully correcting coma. I'm not sure on either of these, but they are the primary suspects in my mind. Curious to hear what more experienced people here think. 

[ebdons]

Hi, make sure everything is square to the secondary, I had something very similar with the ES CC, I slight tilt across the whole face of the CC, it might be the primary collimation out, easy to check it. although it looks like a diagonal tilt somewhere? also check any spacers or adapters you have fitted. Ton

[Skipper Billy]

I ran your image through CCD Inspector and the results suggest two issues.

1. Your spacing might be a bit off as the image is not flat.

2. There is a fair amount of tilt / slop

See attached results.

Once you have had a play if you want another image analysing just let me know.

[Skipper Billy]

On the upside - your collimation is pretty much spot on! 😉

[spillage]

Thanks for all the replies and the cheers @Skipper Billy for the info and confidence that my collimation is okay. I will find some time tomorrow to adjust the spacing with some more spacers and check my focuser.

[Starwiz]

14 hours ago, Skipper Billy said:

1. Your spacing might be a bit off as the image is not flat.

I could do with improving my star shapes towards the edges of the field. 

How does spacing affect it?  I thought the image would still focus to the same place if a spacer is added for instance.

Thanks

John

[spillage]

Because I am using a coma corrector I need to precise distance from it and my sensor. Too far away and the stars will streak towards the middle of the mirror and too close and they will appear to rotate around the centre a little like bad tracking. I was just worried about collimation but skipper has confirmed for me that this is okay.

[Starwiz]

8 minutes ago, spillage said:

Because I am using a coma corrector I need to precise distance from it and my sensor. Too far away and the stars will streak towards the middle of the mirror and too close and they will appear to rotate around the centre a little like bad tracking. I was just worried about collimation but skipper has confirmed for me that this is okay.

Ah, now I understand.

[alacant]

Hi

Quite often, randomess between frames allows stacking to make the stars rounder. Maybe that will be good enough? Otherwise...

Unfortunately, the Baader gives astigmatism at f4; the cross star shapes you see. If you have to stick with Baader, the Rowe will cure the astigmatism but is super prone to tilt. If all else fails, the -designed for Quattro- GPU just works.

HTH. 

Edited August 3, 2019 by alacant

[spillage]

This might be useful

Too Close

Too Far

[spillage]

I have added a couple of delrin rings and I think its a bit better.  @Skipper Billy if you do get the chance to run this jpeg that would be appreciated and at least I know I'm going in the right direction.

 

[Skipper Billy]

Will do it tomorrow when I get home.

[Skipper Billy]

Better but still some way to go - at least you know you are headed in the right direction - keep going!!!

Collimation is bob on.

 

[spillage]

Thank you very much @Skipper Billy. I have made further adjustments and will see if I can get any test shots tonight. At least I know which adjust needs to be made now.

cheers,

[spillage]

So hopefully this is the best I can get it with the spacers that I have. Tracking on this single 30s light was really bad. I also didnt realise my primary lock nuts were loose and ended up moving one of the adjusters when handling the scope so had to try and redo collimation again, which still needs redoing as I am not that happy with it.

 

Edited August 8, 2019 by spillage

[retret66]

triangular or pear shape stars are signs of pinch optics, if you use some sort of a flattener, it might be that is causing it. If collimation is good then it is probably pinched. I had this issue and found out my dew heater was not hooked up..once I hook it up and crank it after 5 min the stars becomes round.. 

[Adam J]

On 02/08/2019 at 22:06, Ships and Stars said:

How are you collimating? Cheshire for secondary and laser for primary? Or something like that? Other thing might be the Baader CC isn't fully correcting coma. I'm not sure on either of these, but they are the primary suspects in my mind. Curious to hear what more experienced people here think. 

Should that not be Cheshire for primary and laser for secondary??

[spillage]

I think that using the cheshire or sight adaptor to help orientate the secondary and get it round and central in the focus tube, then I would use the the laser and  three adjustment screws to line up laser with the primary doughnut. Finally I would then align the primary back to the target on the laser . I tend only to use the laser to get it nearly there and recheck using a cheshire. A star test really tells the true picture but this is the part I really need to work on.

[alacant]

6 hours ago, spillage said:

then I would use the the laser

Hi.

The laser gets you close, but never nailed it for me.

How about a Cheshire sight tube equipped with Cross hairs?

Working at f3.9, it's the only reliable way I've found for both primary and secondary. 

HTH

**EDIT: of course, never forget the all comers collimation myths

Edited August 12, 2019 by alacant

[Ships and Stars]

7 hours ago, Adam J said:

Should that not be Cheshire for primary and laser for secondary??

I think it's Chesire for secondary and laser for primary once secondary is centered - at least according to one guide I read. After reading your post, I checked another site and they use the cheshire twice! I personally go with cheshire for secondary, then laser for primary, then do an airy disk test on a suitable star... seems ok, perhaps I could squeeze a bit more out of my scopes?

[Adam J]

17 hours ago, Ships and Stars said:

I think it's Chesire for secondary and laser for primary once secondary is centered - at least according to one guide I read. After reading your post, I checked another site and they use the cheshire twice! I personally go with cheshire for secondary, then laser for primary, then do an airy disk test on a suitable star... seems ok, perhaps I could squeeze a bit more out of my scopes?

I think its the other way around, normally you place the laser spot in the centre of the primary spot by adjusting the secondary position then in my case i change to a barlowed laser to project the image of the primary mirror spot back into the centre of the laser colimator, the Barlow takes out the effects of tilt. 

Of course if you dont already have the secondary centred in the focuser then you will need to adjust that with the Cheshire to centre it but that is a slightly different process to adjusting its tilt. 

Edited August 13, 2019 by Adam J

[spillage]

36 minutes ago, Adam J said:

I think its the other way around, normally you place the laser spot in the centre of the primary spot by adjusting the secondary position then in my case i change to a barlowed laser to project the image of the primary mirror spot back into the centre of the laser colimator, the Barlow takes out the effects of tilt. 

I also would agree with this. I do have a catseye collimator but only have a doughnut sticker on the primary not a triangle so its good but not as good. I am tempted to get a moonlite for the 8" as I feel this should reduce any tilt. 

[Ships and Stars]

3 hours ago, Adam J said:

I think its the other way around, normally you place the laser spot in the centre of the primary spot by adjusting the secondary position then in my case i change to a barlowed laser to project the image of the primary mirror spot back into the centre of the laser colimator, the Barlow takes out the effects of tilt. 

Of course if you dont already have the secondary centred in the focuser then you will need to adjust that with the Cheshire to centre it but that is a slightly different process to adjusting its tilt. 

Ah right, I see what you're saying. I had to take off my secondary when I was first setting it up, so that's when I think I used the cheshire in the first stages of collimation, otherwise I haven't had to bother the secondary since. I'll recheck it on my dobs though, bet they're not spot on. Thanks Adam.