Odd feature in SCT diffraction pattern

[yuklop]

Hi All,

Has anyone seen this before. In the out of focus star pattern on my Lx200 classic, there is an area that is dark, almost like there is an obstruction in the light path somewhere.

I have since collimated a bit better, and checked there is no obstruction and made doubly sure there are no reflecting surfaces inside the tube (screw ends etc).

It now looks similar but less extreme, like an arrowhead pointing into the pattern.

Any ideas from the experts?

Thanks very much.

[John]

Could be a heat plume ?

If you go through sharp focus and defocus on the other side, the position of the "spike" should reverse it's position.

Here is another example of the same thing from another SCT owner:

 

 

Edited September 29, 2021 by John

[andrew s]

Looks like a thermal plume.  Does it change as the scope cools? Regards Andrew 

Edited September 29, 2021 by andrew s

[yuklop]

It could well be. Scope is permanently mounted in an obsy, so I thought I’d be fairly immune to that. But perhaps not. It might just be clear tonight, so I’ll check it out! Thanks for the idea.

[doublevodka]

47 minutes ago, yuklop said:

It could well be. Scope is permanently mounted in an obsy, so I thought I’d be fairly immune to that. But perhaps not. It might just be clear tonight, so I’ll check it out! Thanks for the idea.

Try having the roof and door open for an hour before observing, that should help dissipate any heat, certainly looks heat related, make sure the caps are off the scope too 👍

[Craney]

Maybe have the focuser end "high" on the mount and remove the end cap to allow any hot air to leave the scope  ??

[yuklop]

I think you were all spot on. I observed the feature last night and it does look like an air current. It does reverse side as you move through focus.

It was still very visible after 1 hour outside and the night wasn't that cold, although I suppose the difference in day temp to night temp was rather high last night.

Still, it was partially cloudy, seeing was awful, and it made a perfect chance to muck around with watching and learning about heat currents inside the tube. Oh, and of course fiddle with the collimation, which I am never sure I have quite as good as it could be! A pointless exercise when the seeing is like last night though!

I find this 12'' SCT, when its good its very good, but I have only seen that one summer night in the past couple of years. Unfortunately, in the winter (and last night), since I must look over houses, seeing is always poor, and this scope really enhances any bad seeing effects!