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Old 01-07-2020, 07:15 PM
Spaceballs1978 (Glenn)
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Back focus issue with 16 inch dob goto

Hi all, I’ve been into astronomy for some years at a very basic level with a Celestron 90mm refractor. A couple of months ago I stepped up my game in a big way and purchased a skywatcher 16” truss tube dob goto. I picked everything up like a fish to water and have already spent many nights viewing all of the planets apart from mercury. I have had great joy in seeing brilliant globular clusters and some wonderful nebula. Lastweek I stepped up my game again and upgraded my standard Plössl 10 and 25mm eyepieces to baader Morpheus 17.5 and 9mm and also a Hyperion 31mm. The 31mm works great focusing in the 2inch Crayford. My issue is with both Morpheus pieces they won’t focus on 2inch mode they try to come to focus with the focuser all the way extended but seem to need another 10mm To achieve focus. If I use them in 1 1/4 inch mode they focus within half way of the draw tube extend very well. Then to complicate matters more if I add my televue 2.5 powermate To both Morpheus they again won’t come into focus when the focus draw tube is all the way out, appear to be gaining focus but fall short again by about 10mm. They don’t focus at all inward. Collimation is checked and adjusted perfectly for each session and I have allowed over 2 hours cool down time each time I use the scope. They skywatcher Plössl EPs focus no worries at all. Any and all advice appreciated.
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Old 01-07-2020, 07:50 PM
gaseous (Patrick)
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I had a 16" skywatcher for a few years - great scope. A lot of better quality eyepieces don't have enough "out travel" to reach focus. I'd suggest getting a 35mm extension tube from Bintel or somewhere similar.
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Old 01-07-2020, 08:19 PM
Spaceballs1978 (Glenn)
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Quote:
Originally Posted by gaseous View Post
I had a 16" skywatcher for a few years - great scope. A lot of better quality eyepieces don't have enough "out travel" to reach focus. I'd suggest getting a 35mm extension tube from Bintel or somewhere similar.
Hi gaseous, thanks for the reply I was looking at them today on bintel and was a bit unsure which of the 3 available lengths would be best suited. 35 is the smaller of the three, I might grab one and give it a go. I can only think of an extender myself as an option. Being reasonably new to the upper end eyepieces I am greatful to get confirmation That it’s an easy fix. . I will let you know how I get along with it.
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Old 01-07-2020, 10:05 PM
gaseous (Patrick)
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Good luck Glenn. I've also got a 50mm extension tube that I've had for many years, but I seem to recall that it was too long, and there was not enough "in travel" when using this. Hopefully the 35mm is the right length - my memory isn't what it was. The other option is to have the eyepiece not fully inserted into the focusser, but you need to be careful that it's carefully and securely held in place if you're having it sitting out a bit further than usual.
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Old 13-07-2020, 08:22 PM
Spaceballs1978 (Glenn)
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Got my 35mm adapter and it did the trick thanks again
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