Secondary mirror question.

[Valor]

Anyone know what size the secondary mirror is on a factory shipped Meade 10" SF EQ?

After much thought, I am going to change the whole spider vane and get a new mirror. Meades secondary mirror is almost impossible to collminate due to the placement of the adjustment screws. The vanes are in the way.

The curved vanes offer retro fit, but that puts me back at square one....

Any ideas?

[Valor]

What size mirror would i have to get if my primary is 10" F/4.5? This stuff is stumping me...

[umadog]

I would more or less ignore what size Meade have used. These manufacturers are not beyond under-sizing their secondaries (so not all the primary is being used) or making them larger than they really need to be. You can easily calculate the size it should be. See this article: Sizing Up the Newtonian Secondary | Gary Seronik.com and this calculator: Mel Bartels' Diagonal Calculator

If you look at the second figure (the schematic) you'll get a good feeling for how it all works. Basically, if you know the distance from the middle of the secondary stalk to the image plane then you can calculate everything. I draw your attention to a fact mentioned in the article: if the secondary minor axis is under about 20% of primary mirror diameter diameter then the performance of the telescope is indistinguishable from an unobstructed instrument of the same aperture. People can become very obsessed about placing the smallest possible secondary into their scope: don't fall into that trap. Get a secondary that's a good compromise between field illumination and obstruction size.

It may be possible to use your current spider if you rotate the spider by 45 degrees and drill 4 new holes. Maybe then the collimation screws will be more accessible? Otherwise, where are you planning on sourcing the spider? If you've got the budget, I'd suggest one from Protostar or Astrosystems. The reason is that the mirror is held inside a closed holder and so a secondary heater can readily be used to keep away dew. Using a secondary heater on secondary with an exposed back is a nightmare: it creates a huge heat plume and does a poor job of warming the secondary.

[Valor]

I thought about the rotation of the spider vanes, but a picture will best describe my dilemma. I will provide one for you soon. I am currently on the road for work and will not be home till Thurs night.

Thanks for the reply, I will get back to you...