Dob mods

[koputai]

Guys,
I'm just doing a few mods to my 8" Dob. I've cut a hole in the back and installed a cooling fan (old model with solid back plate), and am now putting adhesive weather seal baffles in the tube.
Everyone has told me to flock the inside of the tube oposite the focusser, which I will do, but nobody has mentioned baffling the focusser tube.
The focusser tube is 60mm inside diameter, and from my calculations the light cone entering the focusser is only 26.8mm. This is based on the secondary mirror being 47mm minor diameter and the focusser being 120mm from the secondary. So, I've now baffled the inside of the focusser entry with 9.5mm high weather strip, cutting the apperture down to 41mm. I now can't see nearly as much of the eyepiece holder from outside the scope on the opposite side.
Thoughts?

Cheers,
Jason.

[Night Owl]

Ok, where do you get enough flock to flock a 12" Dobs? What is the difference between good flock and bad flock? And how do you install flock?:thumbsup:

[koputai]

I haven't done the flocking yet, and won't do the whole tube, just the side opposite the focusser. The best stuff I've found so far was bought at Lincraft, and is 'Sticko Felt'. It comes as an adhesive backed sheet, 23 x 30.5 cm for $4. They had other felt there, but it was way too shiny.

Cheers,
Jason.

[goober]

That's the stuff I used on my GSO dob. You had to be careful, because when it sticks, it sticks!

[koputai]

Hahahaha, tell me about it. I practiced for a while before taking the backing off, then had a go. A sticky mess eventuated, so I salvaged what I could and used it for small sections forward of and around the spider. I'll be off to Lincraft to buy another couple of sheets this arvo.
It would be a lot easier if I took the spider out of the tube, but I'm really scared of having to do a full collimation from the start.

Cheers,
Jason.

[goober]

Been there, done that! I took the primary and spider out and did the whole tube (12"). I quickly worked out I'm not 12" in diameter :)

In hindsight, I should have cut the A4 sheets down into more managable sizes (i.e. A5), as it was really tricky getting an A4 down on the inside of the OTA smoothly.

[erick]

Have no fear, rip that spider and secondary out. Just undo the external "nuts" and ease it out and place down gently - if your collimation screws are firm, the secondary should not move relative to the spider. When all done, reverse the process. Didn't seem to cause any major collimation issues in my 8". I guess centering the spider again might be an issue. In that case, count the exact number of turns for each "nut" to come off and replace them with exactly the same number of turns. I didn't do that.