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Marke
I have stayed out of this thread up to now, but I will make a cuple of comments.
I recently purchased a 2nd hand Dob which is a 20" f5 which is a similar design to the Astrosystems one in that it has a closed back. The reason the original builder made this decision was that where he was living at the time suffered from considerable issues with exhausts, dirt and dust and he was concerned to full protect the mirror from damage. As such the mirror box seals completely with a cover even over the fan.
I have used the scope for 6 months now and while it can take a while to cool down fully a couple of action help mitigate this. I store the scope in an unheated (and in summer an uncooled room) which is as close to the outside temp as possible. I try to always setup well before dark. This makes setup quicker, collimation easier and gives more time for mirror to cool.
If there is a sharp drop in temp I do notice some deteriation in image quality due to some thermal effects, however the sheer light gathering power of the big mirror more than outweighs this in comparison to other scopes I have used. At the most recent SV camp I set up necxt to Paddy's open truss 16" (similar design to an Obsession UC) which would have cooled much quicker than my scope. The images in the 20" still easily beat the 16".
Obviously, I would get better images quicker and more consistently with an open back but it still produces great images. Sure I could have gone the SDM/Obsession route and ended up paying 3 times what I did, but overall, I am happy with it.
All this is not to disparage what John and Mark have said as I have complete respect for their knowledge and experience but to encourage you in this venture.
Look forward to hearing how your scope performs
Malcolm
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