Cure for Jumpy Secondary Syndrome

[acey]

My 12" flextube is only a couple of months old but already it's showing signs of "jumpy secondary syndrome". You see your laser dot close to the primary centre ring, give a tiny tweak to one of the secondary collimation screws expecting the dot to land dead centre - and instead it leaps erratically to the side. Very frustrating.

The reason is that over time the secondary collimation screws scratch and bite their way into the secondary holder causing dimples that they keep wanting to slide back into. The problem afflicts many brands of scope and I found an interesting discussion on the Cloudy Nights forum:

Telescope Reviews: Squirrely secondary mirror adjustments

from which I learned that the standard remedy is a washer (plastic or metal) inserted on top of the secondary holder, giving the screws a better surface to push against.

I decided to have a go, opting for the simplest solution: a washer cut out of a plastic milk bottle. And to make things even simpler, I cut a slit in the washer, from rim to centre, so that I could slide it in place without even needing to remove the secondary. Just a case of loosening the collimation screws and sliding in the washer, giving it a little twist so the secondary centre bolt can get through the slit.

And voila - as soon as I tried collimating I found that the motion is now smooth and predictable, I can get that laser dot bang on target in a jiffy with no hair torn out. This little fix is going to save me ten minutes of frustration every time I set up the scope.

If your secondary requires regular tuning, this is one mod that really is worth doing!

Andrew

[Doc]

This is very interesting Andrew. My secondry seems to wander a tad so I'll have a look.

[Keithp]

Must admit, although I think I understand how the screws work they never seem to alter the beam in the expected direction. More a case of the beam moves when you slacken them off rather than on the one you tighten.

Sounds like a neat idea Andrew. Did you loosen all the screws to get the secondary away from it's base. I assume you must have done this with the scope horizontal, just in case the secondary parted company with it's base?

Any chance of a pic so I/we can see exactly what the washer looks like?

Regards

All

[acey]

One photo shows the "washer" - very low tech! The other shows it in place in the secondary mount, where I've also replaced the original collimation screws with Bob's Knobs. Not a great pic but you can see from the white patch where the washer is sitting.

This is indeed a job to do with the tube lying horizontal - not because anything needs to become unattached, but because there's always a chance it might.

The secondary assembly has two parts. Top one (let's call it the "mount") has a large bolt going through the centre, surrounded by three small screws. Lower part (the "holder") consists of a cylinder to which the secondary mirror is fixed. The centre bolt goes through the mount into the top of the holder, while the three screws go through the mount but simply push against the holder. If you take out all three screws then the holder won't fall off but will go floppy, because there's a lot of play in the centre bolt. There needs to be, because by adjusting the screws you change the orientation of the holder. The weakness of the system is the lack of tension (supplied in the primary by springs), which would make things work so much better.

To insert the washer, simply loosen the three collimating screws just enough that you can slide in the washer. Don't unscrew the centre bolt! I suggest you start by slightly loosening just two screws, which will enable you to do most of the manouevering before sliding the washer under the final screw.

Even if you make the slit wider than the centre bolt, there should be no danger of the washer falling out, because the collimating screws hold it in place.

Andrew

[irishbloke]

I bought a set of bob's knobs for my skyliner 250px but fouind when I installed them the secondary was very jumpy. I'm going to give this a go to see if I can get them to work smoothly.

Thanks for this.

Brian

[Keithp]

Many thanks for the pictures Andrew, also the explanation. Going to do this myself. What about if the 'washer' was made from thin sheet steel or aluminium? So long as I make the hole and slit big enough to slide in.. Actually... thinking about it, it might be worthwhile removing the spider complete, dis-assembling the secondary and unscrewing it completely to fit a full thin sheet washer.

Anybody else think of any reason not to? seeing as it seems proven to improve the alignment responses from the screws.

Regards

Keithp

[irishbloke]

It's a pain to get it back together and aligned properly again??!!!

I'm not as adventurous. I'll stick with the slit.

[acey]

Looking at the Cloudy Nights discussion, the standard approach is a metal washer, fitted by first removing the secondary. Many people glue the washer to the top of the secondary holder.

I was originally going to remove the secondary in order to fit the milk-bottle washer, then realised I could save myself some effort. Also didn't want to have to do it all over again if the washer made no improvement and I wanted to remove it.

With a metal washer you would certainly be better removing the secondary. Removing and replacing a secondary is no hassle - I did it plenty of times with my former dob, which had a similar mount. Getting it square under the focusser is the only extra step in aligning and it's a simple a one-off job.

What gets me is that the manufacturer hasn't thought to design the secondary mount just a little better. It's like they don't want you to adjust it - the Flextube ships with hex screws but without the necessary Allen key.

Andrew

[acey]

PS You don't need to remove the whole spider to remove the secondary. Just unscrew the centre bolt and the holder will detach. Make sure you're holding the secondary holder while loosening the bolt! Also I would suggest that you have the tube lying on the floor rather than sitting horizontally on the mount, because removal of the secondary may unbalance the scope and cause it to swing up at the crucial moment.

[Doc]

Very good advice Andrew. God I have visions of broken primary mirrors all over Britain

[calibos]

Putting aside the issue of hex or philips head collimation screws (seriously how much would thumbscrews add to the cost of the scopes Mr Manufacturer dudes )

The first issue with chinese scope mirror stalks is that the metal is generally aluminium which is a soft metal. This means the 3 collimation screws dig out dimples/divots in the stalk. This makes accurate secondary collimation at best a frustrating experience or at worst impossible. This is because once you have made divots the secondary collimation bolts keep sliding back into them in a manner of speaking.

The solution? To glue a hard steel metal washer onto the top of the stalk. Now the collimation screws can't dig out divots.

Even after doing that secondary collimation can be harder than it should be. This is because the flat heads of the 3 collimation screws bearing against the flat washer put torque on the stalk. While what we are using the screws to accomplish is to correctly set the tilt of the secondary, this twisting torque applied by the flat screw heads is trying to and succeeding in messing up the rotation of the secondary.

The Solution? take a metal file and round off the ends of the 3 collimation screws. Don't file them to a point. File them to a round head.

So you've now done all that. Secondary collimation is much easier. You've gotten quite good at and aren't afraid of the process anymore. You have decided you want to attempt 100% perfect collimation to get the best out of your scope. You hit the final obstacle. You know that just the tiniest twist of one of those collimation screws will get that secondary tilt perfect, but it just wont budge. You know you have to loosen the opposite collimation screw first to be able to tighten that first one just right. In the process you loosen the opposite screw a tad too much and you are back to square one and it takes a few iterations back and forth just to get the collimation back to 'close, but no cigar'.

This is where the Milk Jug washer secondary mod comes in. Its a low friction material so it doesn't let the screws impart any torque to the stalk so the screws are still just adjusting secondary tilt and not messing up rotation. And there is just enough 'give' in this thin layer of plastic to let you do that final tiny tweak without you having to loosen up the opposite collimation screw first.

I only did this mod last week after reading that CN thread and its a revelation. My advice for anyone when they get a newt is first add Bobs Knobs or equivelent, then add a metal washer, round off their screws and add the milk jug washer before they ever start learing to collimate.

[Keithp]

Hey, just done it and what a difference. :hello2:Went down the local hardware store and got a washer, quite thin, about 1/1.5 mm thick, about 30mm dia. with a 10 mm centre hole. I laid the OTA carefully on the floor and holding the secondary by the edges completely unscrewed the three adjusters. It stayed in place held by the centre screw. Next got a phillips screwdriver and again holding the secondary by the edges unscrewed the centre screw.

There is a spring between the holder and the spider that goes round the centre screw. It doesn't shoot out but as you release the screw slowly stays in place. Next I put the washer between the holder and spider and screwed it back up. First until it just nipped the holder back to the spider then backed off a couple of turns to allow an adjustment gap of about 3/4mm. Next had a look down the focuser and checked for orientation of the secondary. This was all quite useful actually as having read all I can about collimation I can now appreciate what most texts are going on about, also by how much the centre screw is screwed into the secondary holder.

Anyway turned the secondary so it was roughly in position and re-inserted the three adjusting screws untill they just touched the back of the washer on the secondary. From there adjusted the screws to centre up the secondary as per all of the collimation guides. Slipped the laser collimator in and switched it on...missed the doughnut by about the width of the doughnut ring, about 4/5mm. This time using the adjusting screws the dot went smack in the centre. Took me 1 minute at the most. Never had that before.

Back to the primary and one or two adjustments there put the little red dot behind the centre spot on the collimator.

Job done... Happy bunny!

Just got to wait for this latest weather system to move on through.

If I've missed anything feel free to advise.

Regards

All

Keithp

[albedo0.39]

I spent a couple of weeks modding my new 16" LB and this was one of the first things I did.

1) Remove the washer and screws.

2) Using a disc sander held in a vice, carefully grind each screw to a gentle point.

3) Take the washer and mark out where the 3 screws would bear on the surface, with a pen.

4) With a slow speed drill of ~ 1mm mark these 3 points until a tiny dimple is to be seen (you are not trying to drill a hole, but just mark the area)

5) Take much larger drill, (5-6mm or larger) again using a low speed at first countersink on the small spots you have just created until you get a well defined dimple that accomodates the point of the screw (again, you are not trying to drill a hole, but mark the washer with a depression)

7) Repeat for the other 2 screws

8) Drop a spot of oil in each dimple and re-assemble the secondary.

Now when you got to collimate, it is so precise, it's like night and day. The turn of the screw is smooth and predictable and there will be no wandering at all because now, the screw are located in the depressions you just created.

Good luck

[Keithp]

Hey albedo0.39,

That sounds like a neat modification:icon_salut:, I'm going out to get another washer to try it!

Regards

All