SCT Focus

[lonebod]

Hi guys,

This could be a simple question; i certainly hope so.

I been a refactor user for a wee while now and have recently got an SCT. The collimation seems to be quite good, but when i look as starts i see what seem to be diffraction spike at higher magnification.

Would you suggests this is just atmospheric conditions causing this at high mag?

I use the same EP's and filters on my refactor and get perfect round stars, but don;t tend to use as high mag.

I know the atmospheric limitation tends to be around 250 - 300x on a good day and I am at times getting close to this. 

Just want to make sure I'm not missing something in my setup.

Thanks,

G.

[Davey-T]

Which SCT ?

Dave

[rwilkey]

Hi Lonebod, you are probably using just too much magnification and have not quite got it in focus.  Remember, an SCT has a much longer focal length than a refractor, so using the same eyepieces that you used in the refractor might not always be possible or practical.  For many SCT's, a 12mm ep is often the highest power you might use.  As Dave says above, which SCT, and we can work out the powers by the focal length along with a note of what ep's you are using.

[cjdawson]

Also don't forget that an SCT is a closed tube, which takes longer to cool down.  When out of focus, I've seen what looks like a difraction, but rather than being a spike, it's a black line eating into the out of focus donut.  This does tend to get better as the scope cools, and it does clear, but takes a while.

[michael.h.f.wilkinson]

If you look at the out-of-focus doughnut of bright stars, you can often see a dark trail rising from the shadow of the secondary. That is a caused by a thermal current, and indicates the scope isn't properly cooled. I have also seen "diffraction spikes" when using my 2" Amici prism at high magnification (normally I only use that for wide field), especially when focus isn't perfect.

[Gazabone]

Simple way to check the collimation; defocus a star to make it into a "donut" and if the "hole" in the middle is dead centre, your collimation is good. If it's not adjust the secondary mirror screws just like the secondary in a newt until the hole is centred.

[lonebod]

Hi guys thanks for the replies. I have a Celestron 9.25" Evolution.

Sent from my SM-G925F using Tapatalk

[Davey-T]

As mentioned may just need more cool down time, gets quite warm indoors, I leave mine out in obsy permanently.

Dave