Jump to content

Collimation Problem - Coulter Odyssey 13.1"

Atty-FAM

Recommended Posts

Atty-FAM

Atty-FAM

Posted
Posted

I recently acquired an old Coulter Odyssey 13.1" Dobsonian and have been trying to collimate it.  Using a Cheshire collimating tool, I see that the image of the secondary mirror is off center, to the right of the center line of the collimator.  When I reach around and squeeze the cast iron holder of the secondary mirror (which is V-shaped), the image of the secondary mirror moves toward the center.  So, I infer that I need to make such a squeeze permanent.  The problem is that the cast iron holder has a high degree of elasticity and immediately snaps back to the original position.  I don't want to risk any damage to the secondary mirror.  How do I achieve this modification without risk to the mirror?  Thank you. 

Link to comment
Share on other sites
LeeB

LeeB

Posted
Posted

is there no adjustment on the secondary holder, if its fixed which i believe did appear on some coulter otas, I would make a new secondary holder, an easy one can be made from four thin threaded bars and a wooden holder with three adjustment screws. That way both centralisation of the vanes and tilt can be adjusted.

Link to comment
Share on other sites
NGC 1502

NGC 1502

Posted
Posted

I recently acquired an old Coulter Odyssey 13.1" Dobsonian and have been trying to collimate it.  Using a Cheshire collimating tool, I see that the image of the secondary mirror is off center, to the right of the center line of the collimator.  When I reach around and squeeze the cast iron holder of the secondary mirror (which is V-shaped), the image of the secondary mirror moves toward the center.  So, I infer that I need to make such a squeeze permanent.  The problem is that the cast iron holder has a high degree of elasticity and immediately snaps back to the original position.  I don't want to risk any damage to the secondary mirror.  How do I achieve this modification without risk to the mirror?  Thank you. 

Hi and welcome to SGL.

On most reflectors, the secondary mirror can be adjusted in several ways.

The supporting spider arms can usually be adjusted along the tube, or the secondary holder may adjust relative to the supporting spider.  Also, the spider arms are normally adjustable for length.

These adjustments should enable the secondary mirror to be centred under the focuser.

There's also usually a method of tipping/tilting the secondary so that it is pointing at the primary mirror, often three screws offer this adjustment.

I've seen some scopes where the above may not apply, necessitating a bit of bending or other tweaking to sort things out.

Some pics of the relevant parts would help those of us at SGL offer more advice  :smiley:

HTH, Ed.

Link to comment
Share on other sites
John

John

Posted
Posted

Hello and welcome to the forum  :smiley:

The Coulter Odyssey is a vintage dobsonian design which may not be familiar to a lot of members. Here is the secondary arrangement on the scope which might help work out the issues here. I hope the design of yours matches this:

post-118-0-88823000-1405883611_thumb.jpg

Link to comment
Share on other sites
Atty-FAM

Atty-FAM

Posted
  • Author
Posted

Reply to iamjulian. 

Thank you.  You comment that it may be normal that way.  That is interesting.  With everything collimated other than the off-center secondary, I have very good focus.  The rings of Saturn are sharp and I can even see two or three of its moons. 

Reply to John.

Thank you.  I will download the guide. 

Reply to LeeB and NGC 1502.

Thank you.  It is an older Coulter, probably from the early 1980s.  I see no way of adjusting the secondary except by rotation around the screw, which is irrelevant here.  The screw holes do not provide for sliding the holder in any direction.   There are no spider arms.  A cast iron bar runs from one side to another, and the cast iron holder is scrcewed to the middle of it.   Global Moderator has posted a correct picture of what it looks like. 

Reply to John - photo of secondary.

That is it exactly.  The center dot of the primary is centered on the image of the secondary which is offset from the center of the collimator when looking through the Cheshire. 

Reply to Moonshane,

There is only one bolt hole in the vane and one in the holder.  I suppose I could expand the size of the hole on the holder, but I am not sure that would give the same result as squeezing the angle of the holder itself. 

Link to comment
Share on other sites
Atty-FAM

Atty-FAM

Posted
  • Author
Posted

I should note that I am attempting to maximize the collimation before removing the focuser (which is a slide focuser and is driving me crazy) and enlaring the focuser hole to accept a 2" JMI NGF DX1 focuser. 

Link to comment
Share on other sites
NGC 1502

NGC 1502

Posted
Posted

Lots of good stuff already posted, but of course if it ain't broke, don't fix it :smiley: 

However, the pic in post # 6, shows lots of potential adjustments.  

Where the full diameter bar is bolted to the tube, if the holes are oversize, that will allow adjustment up and down the tube, and rotational adjustment as well.

Where the secondary mount is bolted to the bar, if that hole is oversize, lots of potential for adjustment there too.

Collimation will be trial & error, but once sorted, should stay that way, unless the scope takes a bump. Dobs are best suited to DIY types, but only basic tools needed here.

Replacing the sliding focuser sounds a good move.

Hope you sort it, Ed.

Link to comment
Share on other sites
Atty-FAM

Atty-FAM

Posted
  • Author
Posted

Success!   I acted on the suggestions that there may be room for movement in the bolt hole, and there was - and it was just enough to bring the image of the secondary to the place shown in the Coulter Odyssey manual to which John referred me.  Then I was able to recollimate the primary mirror.  As a consequence, the adjustment screws of the primary are now much more in aligned than they had been to account for the misalignment of the secondary. 

I hope that tonight will remain clear enough to check it out on the planets and stars. 

Thank you all for your guidance in resolving this problem. 

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.