Aligning secondary mirror on Dobsonian; tightening the three screws

[Bionerd]

I use a laser Collimation. When adjusting the three knobs on my secondary mirror I can center the dot, but at least one knob is still loose. It doesn't take much to knock the alignment off. Is there a specific sequence I should be using so I can eventually have all three knobs tight. Should I loosen all knobs then try aligning the mirror with the center screw first?

[Peter Drew]

Hello Bionerd.  Welcome to this forum. The usual solution to your problem is to place an extra washer between the adjusting screws and the top of the diagonal holder. It is usually made from aluminium or plastic. This gives the screws something to bite into and stops the diagonal from rotating as you tighten the adjustment srews. In any event, careful and equal tightening in sequence helps.    

[Dannae]

Hi Bionerd,

The centre screw allows the secondary to rotate so that you can initially allow the full face of the secondary to coincide with the focuser tube. This is done first and then the screw can be tightened again. The three surrounding adjustment screws should allow the final tilt of the secondary to align the laser to the centre of the primary (as you are doing it seems). If one of the secondary screws seems loose I would try to wind down all adjustment screws first so they are snug against the mirror holder then try to align from there. There is the possibility that all of the adjusters are too far out (or in). What you describe doesn't sound right and sounds a mechanical issue.

Regards

Dannae

[Tinker1947]

Don't look to get them tight just enough to stop the mirror moving, your going to collimate quite often, it gets down to a minute or less after bit.....

[bambuko]

Don't look to get them tight just enough to stop the mirror moving, your going to collimate quite often, it gets down to a minute or less after bit.....

he is talking about secondary

unlikely to be adjusting it very often

your comment is more appropriate for the primary screws

the usual design of secondary adjustment is very poor

you could use extra washer either plastic or aluminium or slightly more fancy

this improves it somewhat and prevent screws digging in (which in turn makes it almost impossible to adjust it in future)

[Charic]

Hi Bionerd,

The centre screw allows the secondary to rotate so that you can initially allow the full face of the secondary to coincide with the focuser tube. This is done first and then the screw can be tightened again. The three surrounding adjustment screws should allow the final tilt of the secondary to align the laser to the centre of the primary (as you are doing it seems). If one of the secondary screws seems loose I would try to wind down all adjustment screws first so they are snug against the mirror holder then try to align from there. There is the possibility that all of the adjusters are too far out (or in). What you describe doesn't sound right and sounds a mechanical issue.

Regards

Dannae

The Screw surely allows the secondary to move to or away from the primary mirror ( up-down the tube or left right viewed though the focuser. Mine doesn't lock after any adjustment. If you screw clockwise till no further movement you  pull the secondary from the focuser, if you unscrew the mirror will fall out eventually.. I have never locked the screw itself after any rotation. Rotation by hand and tilt is achieved by adjustment of the Allen bolts on my telescope.

For adjusting my secondary (Skyliner) I fully undo the Allen locking bolts, then screw the centre bolt fully home. This ensures that the rear of the secondary is butted to the body of the adjuster. Everything should be flat and square prior to adjustment.  I then screw anti-clockwise a few turns on  the centre screw ( the spring providing tension to move the secondary towards the primary ) then tighten the Allen bolts until touching, I repeat this process until the mirror is central in the focuser tube. This should keep everything almost square between the  moving parts, until rotation and tilt is required, finished by gently tightening the Allen bolts. 

[Tinker1947]

The central screw in the secondary holder i believe is there to adjust the position of the secondary mirror to the focus tube, once this is achieved the 3 adjusting screws are used to centre the laser on the centre of the primary, using the central screw for anything else will throw the collimation out...i have set my 2 newts up like this and only tighten them to hold the laser in the doughnut in the centre of the primary....

[Jason D]

I use a laser Collimation. When adjusting the three knobs on my secondary mirror I can center the dot, but at least one knob is still loose. It doesn't take much to knock the alignment off. Is there a specific sequence I should be using so I can eventually have all three knobs tight. Should I loosen all knobs then try aligning the mirror with the center screw first?

Do not use the center bolt to adjust the laser beam reflection. Use only the following two adjustments:

1- Secondary mirror rotation

2- Secondary mirror 3 set screws

A- Loosen all 3 set screws then slightly tighten them -- enough to still maintain free rotational movement of the secondary mirror. It would be helpful to ensure all 3 set screws have similar heights. That is, ensure no set screw is noticeably over-tightened compared to the other two.

B- Rotate secondary mirror back-and-forth while watching the laser beam reflection on the primary mirror tracing an arc. Stop when the laser beam reflection is closest to the center spot. See attachment.

C- Center the laser on the primary mirror r by adjusting all the set screws. When done, ensure all 3 set screws are tight. It would be helpful to use plastic washers that are cut from a Milk plastic container. You can use 1 or 2 washers. No need to remove the secondary mirror. You can cut the washer radially and just slip it in. See attachments.

Jason

[Dave In Vermont]

While many guides to collimation are difficult to understand, there are a few out there that make things clear. Here's one of them:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

After you do this once, it will be like riding a bicycle.

Clear Skies & Don't Drop the Screwdriver! Aim the tube down!

Dave

[nicoscy]

Do not use the center bolt to adjust the laser beam reflection. Use only the following two adjustments:

1- Secondary mirror rotation

2- Secondary mirror 3 set screws

A- Loosen all 3 set screws then slightly tighten them -- enough to still maintain free rotational movement of the secondary mirror. It would be helpful to ensure all 3 set screws have similar heights. That is, ensure no set screw is noticeably over-tightened compared to the other two.

B- Rotate secondary mirror back-and-forth while watching the laser beam reflection on the primary mirror tracing an arc. Stop when the laser beam reflection is closest to the center spot. See attachment.

C- Center the laser on the primary mirror r by adjusting all the set screws. When done, ensure all 3 set screws are tight. It would be helpful to use plastic washers that are cut from a Milk plastic container. You can use 1 or 2 washers. No need to remove the secondary mirror. You can cut the washer radially and just slip it in. See attachments.

Jason

Another option is this if you don't want to do even the minimum DIY involved in Jason's post. 

If one of the screws is loose, despite following the excellent advice given above, I would recommend buying a screw that is longer than the others by 1mm to 2mm.

[Jason D]

Another option is this if you don't want to do even the minimum DIY involved in Jason's post. 

http://www.cloudynights.com/topic/418059-secondary-mirror-washer-question/?p=5381793  

[Bionerd]

Thanks for all the great advice. As soon as it's clear again I'll get on it