Collimation error or tube currents?

[matt_man21]

Hey Folks,

I'm just having an episode of cognitive flatulance, as i dont seem to be able to tell the difference between collimtion error and thermal currents... the scope (8" LX-10 SCT) has been cooling off in the garden for the past hour and This artifact arround a defocused star seems to be staying put and is very stable... Would somebody be so kind as to put my mind to rest?

I can provide other images if they are required...

Any help would be very welcome!

Matt

[John]

I think thats a tube current. More cooling time needed perhaps. SCT's can take quite a time to cool in the winter if they are coming from a warm house. You should still be OK to use it a low / medium power on some deep sky objects though.

The collimation looks quite good to me - the stars airey disk looks pretty central in the diffraction rings, despite the column of warm air still in the tube.

[matt_man21]

Cheers for the reassurance John, I've been trying to nail the collimation on my sct for the past few days and think that I'm starting to get the hang of it... Least now i wont be trying to correct a thermal current!!!!

I never realised how critical Collimation is on an SCT, It wasn't till my first iteration a few nights ago that i actually got to see surface detail on Mars for the first time.... It's like getting a brand new telescope!

Lesson learned... love your scope and it will love you back!

[Freddie]

Looks like tube currents to me too. Exactly what I see in my CPC when not cooled enough.

Collimation looks close but maybe a little more space between rings on the left than right. It is always hard to tell from a single image though. If you don't have MetaGuide, download a copy (free) as it has a really nice collimation routine.

[Peter Drew]

It's tube current distortion due to black body radiation, particularly noticeable in SCT's where it also stems from the shade tube. We used to know it as the "Finlow effect" after it was raised to our attention by a client Gerald Finlow some 30 years ago.