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NorthernLight
23-11-2014, 03:45 PM
Hi There,

I am hoping that someone has the time to read through my short report and can point me in the right direction. I have read pretty much all forum entries and the suggested links like Astrobabys guide to collimation.
With my Celestron 6"/F5 Netwon I have not had the same issues.

Please Help! I`d be glad for any helpful advice.

After cleaning both mirrors of my GSO 12"/F5 Netwon (Dobson mounted) I have re-assembled all parts but cannot get the secondary`s position right.

I use a laser and a pinhole. This is what I did so far:

1) I bring the secondary to the right height, so I see it as a circle through the focuser.

2) I adjust the secondaries tilt so that the main mirrors reflection appears centered in the field of view.

What I observe is that the quadrants that are formed by the spider vanes are not symetrical. The lower left quadrant is the largest, the top right one the smallest. I can do whatever I want, no change in tilt or height adjustment centers the view and brings all quadrants to the same size.
I also observe that the center spot on the primary is not aligning with the center of the focuser drawtube (my pinhole).

A member of my local astro club recommended using a laser for the secondary alignment. This is the procedure:

I) insert collimation laser (Orion LaserMate Deluxe) into focuser drawtube.

II) adjust secondary until the laser beam is centered in the primaries center spot.

When I then use the pinhole again, I cannot see the full disc of the primary through it. It is somewhat shifted to the lower left.

I dis- and re-assembled the whole system twice but faced the same issues. It appears as if the focuser drawtube, if it were extended, does not meet the center of the telescope tube.

I know it all sounds a bit confusing, if not bizarre. But I really need some help here. Even if that is an address to some shop that can fix matters.

Many Thanks

glend
23-11-2014, 05:11 PM
When you removed the secondary did you remove or alter the spider vanes? Your secondary may have had a little offset originally, did you take a photo down the focuser tube before you began? Is your primary at the same height as before disassembly?

NorthernLight
24-11-2014, 02:07 PM
Hi Glen, no I didn`t remove the vanes but as a means to overcome my issue I adjusted the screws that hold the spider vanes but centered them again as I noticed that it had virtually no effect.

The height of the primary is adjusted so that it sits halfway between min. and max. although I am not sure whether that was the height it had before.

No, unfortunately I did not take images down the barrel before disassembly of the secondary- only for the primary I took photos of each step.

Jason D
24-11-2014, 06:03 PM
Refer to the last post on the following page
http://www.cloudynights.com/topic/214339-useful-info-about-secondary-mirror-alignment/page-17
Follow the steps I outlined in the above post and see if the end result looks better.
Jason

mental4astro
24-11-2014, 07:57 PM
Hi Max,

By the sounds of it, you are just not getting the position of the secondary right because you are just eyeballing the secondary through the focuser. This is not accurate enough, and a laser doesn't collimate the secondary. The laser ONLY works for the primary mirror. It took me a long time to understand this.

Eyeballing the secondary through the focuser, you have just too much room for error. The simplest tool for this is the humble cheshire eyepiece.

173415

1: The reason for the long shaft is that you can then slide it in and out of the focuser's draw tube until you have the secondary nicely inside the rim of the Cheshire. You then have a great deal of control over the position of the secondary. You can accurately determine how far left & right, up and down. Once the secondary is centred, only then do you look at the reflectio of the primary in the secondary. The crosshairs of the Cheshire are then used to get the centre dot of the primary all lined up by ONLY using the collimation screws of the secondary.

2: Now you can use your laser. You should just need the slightest tweak of the secondary's collimation knobs to centre the laser into the primary's centre spot. Then all that's left to do is tweak the primary's collimation knobs.

That's it.

The main reason you haven't been able to deal with the secondary effectively is you haven't been able to control the position of the secondary well enough. There are several different 'cheshire' type of tools, and they all do the same thing - deal with the secondary first. The most basic of these is one of the old Kodak plastic film canisters with a hole in the lid and another in the base so you have the secondary mirror sit within the view of the lid's hole. Only once the secondary is set do you begin to consider the primary.

I hope this helps. Let us know how you go.

Mental.

NorthernLight
30-11-2014, 05:57 PM
Hello Alexander and Jason,

thanks a lot for your detailed descriptions. I now have a much better understanding of the physics behind it. Without your advise my frustrations about this seemingly impossible task seemed to have displaced my conviction for this hobby. I feel much better now :)

I will go ahead and procure a Cheshire. Are there many differences? I saw that the Celestron version is $20 more then the Skywatcher although both look identical.

Thanks again for your time,
Clear Skies!