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Originally Posted by xelasnave
Had a look at your site, love the set up.. I have the same bintel 12 inch and long for the day I can mount it with my two current guide scopes. I like where the drive motor is placed for RA that seems sensible and I dont know why others dont follow this design. I thought that there was a lot of light bouncing around in those tubes the thing that surprised me with the 300mm was the relatively poor contrast. I use a "light tube" extention on my scopes. A tube in the case of the 6 inch scope it is a tube 8 inches in diameter about 2.5 feet long with 8 baffles, this goes as an extention to the scope and dramatically improves contrast for me at my site. I came up with the idea to eliminate stray star light getting into the tube. I also noticed the focuser will not travell in far enough for fotos.. It will if you use a 3x barlow but on a dob mount only Moon shots are achievable. Great site great work great equipment.
alex
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Yes, everything you say is correct, the OTA off the shelf does not have good contrast. I corrected this by installing 6 light baffles on the inside of the OTA. If you look at the 'scope here
http://www.skylab.com.au/pmsa/equip.html you will notice that you can see the first light baffle infront of the secondary spider vanes. The contrast improvement was simply astonishing after installing the baffles.
Yes, the OTA will not come to focus, that's correct! What I had to do is move the optics 41.5mm closer together to put the new focus on the ccd plane such that the focuser was mid-position. It took a while to work out that 41.5mm was the number.
The most simple method to move the optics closer together was to shorten the tube length. Another mirror mount may have did the job, but the mirror mount that comes with the OTA is very good and well worth keeping.
So now I have focus at the camera sensor plane, but I can not use the 'scope visually as the eyepieces no longer reach focus. I need to find myself a 2-inch extension barrel to extend the eyepiece focus, but the 'scope is always setup for imaging so this doesnt bother me.