lonebod Posted September 8, 2015 Share Posted September 8, 2015 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. Link to comment Share on other sites More sharing options...
Davey-T Posted September 8, 2015 Share Posted September 8, 2015 Which SCT ?Dave Link to comment Share on other sites More sharing options...
rwilkey Posted September 8, 2015 Share Posted September 8, 2015 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. Link to comment Share on other sites More sharing options...
cjdawson Posted September 8, 2015 Share Posted September 8, 2015 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. Link to comment Share on other sites More sharing options...
michael.h.f.wilkinson Posted September 8, 2015 Share Posted September 8, 2015 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. Link to comment Share on other sites More sharing options...
Gazabone Posted September 8, 2015 Share Posted September 8, 2015 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. Link to comment Share on other sites More sharing options...
lonebod Posted September 8, 2015 Author Share Posted September 8, 2015 Hi guys thanks for the replies. I have a Celestron 9.25" Evolution.Sent from my SM-G925F using Tapatalk Link to comment Share on other sites More sharing options...
Davey-T Posted September 8, 2015 Share Posted September 8, 2015 As mentioned may just need more cool down time, gets quite warm indoors, I leave mine out in obsy permanently.Dave Link to comment Share on other sites More sharing options...
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