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Dob Design Dilemma

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Posted by Mattsztanko on November 19, 2001 00:58:31 UTC

Hi everyone,

I have been reading through a lot of the postings in this forum and am impressed by how much time you all take to help each other out. I have learnt quite a bit from just reading through various postings and am posting this to see if anyone can help me with my dilemma.

I am trying to design and build a scope so I can use it for both viewing and photography with minimum adjustments required. My idea is to use a fairly long drawtube for viewing and when taking photos to change over to a shorter drawtube to allow for the adaptor length and distance to film surface. This means that the viewing focal plane distance extends out about 120mm from the scope tube. I have used a CAD programme to draw to scale where the light cone falls on the secondary and what size the cone will be. By drawing parallel lines to the edge of the primary reflecting down to the focal point, I am assuming this is the extent of light I need to worry about. Is this correct ?

The reason I chose this method is because I didn’t want to have to move the position of the primary when taking photos, and I thought this option was much easier (as I will be making a Crayford design focuser that will allow me to push back on the sprung loaded thumb screw assembly and just remove the draw tube). Also there would be no need for recollamation. I make it that the light cone ends up about 4mm from the edge of the secondary taking into account a 1.75mm offset. I wanted to calculate it this way so I could actually see what was happening and be sure everything will be in the right place. Also by doing this I could check to see if the drawtube for a 1" eyepiece would at any stage be obstructing the light path, and it seemed OK. I am using a 10" primary at 64" focal length and a 2" secondary.

As I don’t understand what things like % of fully illuminated view means, (please could someone explain this to me in laymans terms)I am just a bit worried that I may be missing something that may affect the scope’s performance. Please can somebody help by letting me know if I am on the right track or not so I can get on with the building rather than just planning !!

Thanks in advance,


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