Longer Newtonian Collimation Bolts - Has Anyone Tried Them?

[Benno85]

Hello Everyone,

It's been a long time since I've been on here, and what better way to make a return than to ask a silly question :)

Has anyone taken the path of using longer bolts to move the primary mirror of a Newtonian further up the tube? My reason for this is to achieve focus with a dslr. I know I can cut the tube but would like to try a less destructive approach first :)

I have the standard blue Skywatcher 8" f/5.

[erick]

Did this to a 6" Starwatcher with some success:-

http://www.iceinspace.com.au/forum/showthread.php?t=59447

In November will fit better and more stable long bolt mount options - I think it will work fine.

Your 8" mirror and cell will be heavier, but your replacement bolts will probably be thicker.

Just check that you can still see most, if not all, of your primary in the Secondary. You shouldn't lose much.

[Benno85]

Cheers mate, may I ask where you found the longer bolts and springs, and there is a particular specification I should try and find?

[erick]

It can be a "make good designs/plans", then "suck it and see" process, I think!

The six long bolts in Jen's scope (in photos in the thread) were bought from a hardware supplier in Ballarat (which, fortunately, had more range than Bunnings would have had). They had the right countersunk head, right thread - I just bought the longest they had and cut them to size.

The new spacers and springs will fit around three of the bolts (those that screw into the mirror cell) and three others (the "lock" bolts) can then be removed. The spacers were turned up by a fellow IISer, the three compression springs came from Bunnings. I have guessed the lengths, diameters or spacers and springs, and strength of spring. Crossing fingers they work as hoped. :P

One suggestion I would make is get stainless steel if you can - don't want anything rusting in the weather.

[bmitchell82]

if it where I, i would be cutting the tube and up sizing the secondary straight up because you will be loosing a bucket load of the light cone though vignetting in the focuser draw tube, but that is just me.

http://www.iceinspace.com.au/forum/showthread.php?t=59178

I go into quite a bit of depth on what i have done, and to be honest im actually thinking of moving to the next size up secondary which is the 88mm minor axis it would mean a 34% Center obstruction from the 70mm which is 27%, but well worth the extra light gathering capacity across the sensor! Andrews and Bintel both sell secondary mirrors at a pittance of cost (49 bucks plus 10 dollars P&H).

If you do decide the change of bolt length, be sure to pull the primary mirror out and drop some silicone blobs to hold the primary to the primary mirror cell and be sure to leave the mirror clips off the surface of the mirror.... they will cause big problems.!