Odd artifacts on star donuts when collimating

[TrickySpot]

Hi Guys,

Any ideas what would cause the odd lines/semi-circles in my unfocused star images?? When totally focused, the stars appear slightly oblong, especially after 5 or more minute exposures.

I thought it was a light leak, so built an extra long dew shield which did nothing, covered my entire focuser and camera in black cloth which did nothing - the only thing that helped was to cover the top half of the OTA in front, but if I look in to see what is behind what was covered, there is nothing that is shiny or bright!? Totally baffling me!! 

Attached are two images of either side of in focus - any help would be most appreciated!

Thanks!

[John]

Currents of warm air within the tube I reckon. They are remarkably persistent in sealed scopes such as mak-newtonians and tend to stick to the tube walls.

[Tim]

Is it always the same patterns that appear?

This is on your mn190 I guess? Hate to say it but you are going to have to tweak that collimation some more there :\

If the patterns vary night to night then as John says, tube currents could be to blame, you need to give that scope a good hour or two to reach thermal equilibrium.

If the pattern is always the same, or similar, you might be seeing the bright edge of one of the mirrors, skywatcher mirrors sometimes have a ground edge on the front, which if your collimation isn't near perfect, especially if your secondary is rotated, which is easy to do in a mak newt, and not easy to spot, (also indicated in oblong stars if you can discount tracking errors) might just be picking that reflection up. You can blacken the ground edge with a marker, paint it with blackboard paint, or make a mask for the edge of the mirror.

With any luck it will just be tube currents though

Tim

[Peter Drew]

+1 for internal air currents.

[Langy]

Oblong starts especially at 5 minutes could normally be put down to tracking errors, the rest of what has been discussed is well over my head.

Have you also tried some shorter exposure times to rule out tracking?

Like I say I may just be totally way off and showing I'm way out of my depth on the initial question, but a different thought may also help.

[TrickySpot]

I know its not tracking :-) The two images at the top are 3 second exposures, but the brighter area of the doughnut when focused causes the stars to have a little 'diffraction spike' on one corner and after 5 minutes of guiding (with about 0.5 oscillation in PHD), the stars turn out longer. Its more tear drop shaped really than oblong.

If hot air can be the issue (in the scope, not me), that makes sense - the scope usually lives outside, but I had it inside for a strip down of the focuser to make sure it was orthogonal, so I did forget that the few hours before I went out, it was both open at one end and inside a warm house.....didn't think about waiting for it to cool as usually I don't need to :-)

Its been back outside in its usual home over nights, so hopefully I can do another star test tonight.

Thanks for all the help guys!

[RT65CB-SWL]

I only use 'cat's' and a refractor, but I too suspect the lines are 'thermal' related, and the semi-circles on the de-focussed rings look like a reflection of the secondary. Hope this helps.

[TrickySpot]

BTW, its not the lines in the background, that's just normal noise from my QHY5, its only the areas on the actual doughnuts :-)

[Tim]

BTW, a cats eye Cheshire and auto collimator make for near perfect collimation in the mn190

[TrickySpot]

BTW, a cats eye Cheshire and auto collimator make for near perfect collimation in the mn190

I just just bought a hotech laser collimator from FLO......I read they were very good! I should have got the 2" version though, bit stupid of me, hope I can get the 2" adapter separately! I guess its still a bit futile at the moment though, my focuser is also not sure which way it wants to point......

I took the whole secondary apart today again and started square from the corrector and worked inwards slowly, I did see a glint of the side of the secondary disappear (thanks Tim!), and managed a single 15 min exposure with good round stars, but because of the cloud my test star was very bright, so I'm just guessing between the cool down and the rotation of the secondary, it sorted itself out!

But having said all that, the resolution is terrible, I got virtually no detail at all from M31 - I know my secondary isn't giving 100% performance because of a chip/crack which is masked off, and I must still figure out what happened to the primary. Just waiting to get some news before I start stripping the rear cell down and see if I can get it seated properly and/or centered. 

Thanks for all the help guys!