Originally Posted by Eternal
You could have also purchased a low profile Bintel 10:1 crayford focuser to solve the problem.
If you had looked at the information I have given. I did purchase a Bintel (GSO) so called low profile crayford. which has a racked in height of 3 inches
to the 2" drawtube face. These are called lo profile
, but at 3" they are not. Only Moonlite sells a low profile focuser which has a racking in height of 1.25
inches to the 2" drawtube face. Even the JMI lo profile focuser has a racked in height 2.5" and cost 3 X the price of a GSO
Remember also the original SW880 10" scope came with an R&P focuser which had a 4"
fully racked in profile and focusing at 15mm out with a 25mm plossl. This monstrosity was used to compensate for the short tube length, the distance from the Primary to the secondary being only 915mm - The radius of the tube 140mm The total fl being 1200mm (I will leave you to work out the rest of the dimensions needed to achieve this)
This has been a long standing problem with dobs and camera interfacing (see other posts and stickies by the site administrator Iceman on this very subject) as the camera has, from the body face a focal length of 35mm to the sensor. By applying the formula in determining the focal point on a 10" F5 scope this means that the Canon 350D focuses with the focuser fully racked in and the camera face directly on the drawtube. Not allowing for T Ring and 2" adapter. (which adds 25mm) You will see how the figure of 60mm compensation is needed.
Changing from a 4" R&P to a Bintel GSO 3 only reduces the focal length by 25.5mm
The method I have used has lowered the focal point for prime focus
by a total of 60mm This allows the camera with T ring and 2 " adapter to focus at a racked out position of 10-15mm above the focuser body.
I had a similar problem with an 8" dob I constructed and can assure you that other than moving the primary mirror up by 60mm. This is the only feasable alternative. It also requires that you have to extend the focal length by 60mm to bring the focal point back for conventional EPs'
I would also repeat that this is for Prime Focus Photography and not Afocal, which presents no problem at all. The solution was worked out mathematically, experimentally and in practise and it works I am sure you will appreciate that this has been thoroughly examined and confirmed by more knowledeable people than my humble self
Did you check my website for detailed photos and comment?
Have a nice day