Collimation, conflicting information & confusion

[Darth Wader (Wade)]

After having searched this forum I came across the following video link:
http://www.andysshotglass.com/Collimating.html
A very informative video indeed which put at ease my anxieties about collimating my 8" dob. So today I buy a Skywatcher cheshire combination tool and get ready to collimate. Having lost the link to the video, I again searched "cheshire collimation" on this forum and noticed another thread in which it is said that a cheshire will collimate the primary mirror only, and that a laser collimator will do both... http://www.iceinspace.com.au/forum/s...ation+cheshire

The video link from the first URL clearly shows you how to do it with a collimation cap - a primitive cheshire in itself. Who is right? Have I wasted my money buying the chesire combo tool? Really starting to wish I'd saved up for a nice apo instead of buying a reflector!

Cheers
Wade

[RAJAH235]

Hi Wade,
No. You haven't wasted your dough on the Cheshire.
Using it, will automatically set the "off-set" of the 2ndary.
I use the simple process of blue tacking a piece of white paper to the opposite
side of the focuser tube & collimating the 2ndary, first.
Once done, continue on with the primary.
Easy peasy.......

JIC....see > http://www.skyandtelescope.com/howto/diy/3306996.html
& > http://web.telia.com/~u41105032/kolli/kolli.html

[Starkler (Geoff)]

The Skywatcher cheshire combination tool which you bought is everything you need to properly collimate your scope. You dont need a laser and imo it isnt even preferable.

[Darth Wader (Wade)]

No worries, thanks guys!

[rmcpb (Rob)]

The only real advantage of a properly aligned laser is that it can be used for a quick check each session. The cheshire can do it all, it just takes a bit of time getting used to it.

[Darth Wader (Wade)]

Gave collimation a go last. What a pain in the backside.

Took me more than 2 hours total, and it's not even properly aligned. The diagrams in the instruction manual & on the net are totally different to the actual view & it is all very confusing. To add insult to injury, the locking hex bolts on the back of the primary don't just lock the mirror position - they move it around too! So just after I'd gotten it right, then locked the hex bolts - I'd have to do more adjusting. Finally I thought I'd gotten it as close as I could so I took it outside and pointed it at Jupiter. Terrible image with the planet wobbling around like a bowl of jelly (which I attributed to warm currents inside the OTA at the time). Brought it back inside to check the collimation and surprise surprise, it's out. Will attempt to correct this later today, patience permitting.

To all of you who suggested a refractor to my wife last Christmas when she came here asking which scope to buy as a first scope for me - I thank you from the bottom of my heart. If she'd gotten me a reflector and I had to do this back then, I'm pretty sure I would have given up.

[erick (Eric)]

I suggest you unscrew the "lock" screws and leave them loose, unless you plan to transport the scope. Most people report the same - they just push the scope back out of collimation - yeah, they shouldn't, but they often do.

Screw "in" the primary mirror collimating screws until those springs are well compressed. That is, pull the mirror back down towards the fan end of the tube. Then try collimating again?

At what elevation do you collimate? 45 deg elevation is a good compromise. Once you have achieved fairly good collimation at that elevation, check it out as you move the scope from horizontal to vertical. In the ideal world, it will not change. In the real world it probably will for these scopes! A mm or two won't kill your viewing. Often it is the primary mirror "sagging" on the springs, hence the recommendation of getting them under more compression or even replacing them with stronger springs.

But an 8" mirror isn't that heavy - are you sure your upper tube assembly is rigidly attached to the "mirror box" - all screws on the struts done up firmly? Any movement of the UTA relative to the mirror box will be a problem. You cannot feel any twist or tilt or strange behaviour in the UTA as you elevate the scope?

Also, better that you collimate it where you are going to use it. My 8" solid tube was great. I could bounce it for two hours to a viewing site and the collimation hardly changed from the last session - but a strut design, I would not be surprised if moving it around upsets the collimation. Do you collapse it to take it outside, then open it again?

Also you are right, from warm house to cold outdoors won't help your view.

[Darth Wader (Wade)]

Hi Eric!

Thankyou for the tips, very much appreciated.

Originally Posted by erick

But an 8" mirror isn't that heavy - are you sure your upper tube assembly is rigidly attached to the "mirror box" - all screws on the struts done up firmly? Any movement of the UTA relative to the mirror box will be a problem. You cannot feel any twist or tilt or strange behaviour in the UTA as you elevate the scope?

Nope, no movement at all, it's nice and sturdy.

I've done as you suggested re: the lock locking bolts & screws on the primary. I have been attempting to collimate at a 45 degree angle as it was suggested in a video I watched to avoid dropping things on the primary (good advice)! This afternoon's effort went fairly well as I collimated outside and there was more light than the previous night indoors. I only have one problem - the black dot from my cheshire is not lined up exactly with the annulus in the centre of the primary mirror. Not sure if this is a big problem or not but I physically could not get it any closer without overtightening one screw, much to my frustration.

Cheers
Wade

[Batfink (Peter)]

Hi Wade, I too was having a hard time trying to collimate my 8" dob, what I found was, you may not have the same problem but worth a check, is that the springs on the primary weren't strong enough to push the mirror cell out when I loosened the adjusting screws.

So to get around the problem, until I find stronger springs, is to screw in all the adjusting screws then just tighten the locking screws so they apply slight pressure, now as you start to loosen the adjusting screws slightly tighten the locking screws to keep pressure on the mirror cell, so they actually are replacing the job of the springs, you can get quite fine adjustments useing this method.

Peter

[Darth Wader (Wade)]

Thanks Peter - will definitely give this a go.
Cheers
Wade