Cooling down of SCT

[resonator77]

can anyone help to understand the effects of cooling down in SCT?

what would happen if one uses the SCT scope without letting it to cool down?

How can I find out if the scope is cool enough to use so to speak ?

Many thanks for any help in advance.

[Merlin66]

I see tube currents and "lumpy" images... after 30-45 mins cool down these disappear and the image looks smooth and regular...

[resonator77]

Merlin,

I have 8" SCT view through which is dim and cloudy at the beginning when looking at Jupiter and Mars and next to it 5" SCT which shows clearer and sharper all those objects from the first minute of observation.

Is this an effect of cooling down process or better optics?

[Merlin66]

Could be cooling down mixed with the seeing conditions.

If the seeing is average then the smaller aperture may appear initially more crisper but as the larger aperture cools and hopefully the seeing improves the larger aperture will win out.

[brianb]

what would happen if one uses the SCT scope without letting it to cool down?

How can I find out if the scope is cool enough to use so to speak

You get tube currents, which can look like "smearing" of the image and/or excessive "swimming pool" turbulence. This will only affect the view at moderately high power or above.

If the view appears good, the scope is well enough cooled down.

Unless you're attempting to use very high powers, or doing high resolution imaging of the planets, don't let cooling worry you - just use the scope, you won't do it any harm!

[resonator77]

I noticed the background sky is not dark enough for Jupiter to stand out when viewing through 8" SCT. I did not have chance to wait for an hour or so yet to check if it is due to cooling down:confused:

[mcrossley]

And don't forget that accurate collimation of SCTs is essential to get the best out of them. Are you sure the 8" is as well collimated as the 5"?

[resonator77]

I believe it is collimated by the looks of planet and star shape. It is just dimmer and blander.

[barkis]

Whilst the light from any object you are observing with your SCT is focused down through the baffle tube, and hence to your focuser and eyepiece, The mirror is also open to any sky glow that might be present, and therefore contrast might suffer a bit, depending on how bad the glow is. Not a lot can be done other than a LPR filter.

Ideally, you need good dark skies, and no stray light. Trying to observe Jupiter for example, whilst the moon is in the vicinity, would erode contrast. I know the topic is cooling, but I thought I'd chuck that in too..

Ron.

[John]

I believe it is collimated by the looks of planet and star shape. It is just dimmer and blander.

How does a star image look when it's just out of focus ? - a collimated scope will show something looking a bit like a polo mint - the "hole" (actually the dark shadow of the secondary mirror) should be right in the centre - a little offset indicates that the collimation needs adjustment - contrast is the 1st thing to suffer in SCT's if the collimation is even a bit off as this thread discusses:

http://stargazerslounge.com/equipment-discussion/90408-importance-good-collimation.html

John