Testing out my system after 6 years

[alpal]

I wasn't able to use a V block - I stuck with the vice idea.

I spent well over 2 hours re-adjusting the laser.
I've got it much better - it's usable now.
I labelled each of the 3 adjusting screws so I could keep track of what I was doing -
labeled A, B,C.
e.g the first iteration was undo A & B by 1/2 a turn and tighten C - then retest -
that improved it.
A & B undo 1/8th turn C tighten - made it better.
C undo 1/16th B&A tighten etc etc.
If the laser got worse I would undo what I'd just done and try another combination.
I also found I didn't need to clamp it up tight with the adaptor set screw each time -
I got a feeling for how the spot was moving in a circle when I turned the laser.
I went through 25 iterations till I got it right.

See the picture.
The error is less than 5mm at 4.3 meter target distance which
for my 1016 mm telescope will represent less than a 1.25mm error.

cheers
Allan

[alpal]

Just checking different collimators -
it seems like the Howie Glatter is the best:


https://www.siderealtrading.com.au/p...2%80%B3-650nm/

Factory-aligned to 15 arc-seconds providing 0.1 inch accuracy at a distance of 20 feet

[alpal]

Originally Posted by alpal

Just checking different collimators -
it seems like the Howie Glatter is the best:

https://www.siderealtrading.com.au/p...2%80%B3-650nm/

15 arc seconds ?

Actually - that advert is wrong!
0.1 inches at 20 feet or 240 inches -
Tan-1 0.1/240 = 0.02387 degrees = 86 arc seconds.

Something to think about but that is the best you can get
and has been found to be perfect.

cheers
Allan

[alpal]

Talking of inaccuracies -
I just noticed that on my NEQ6 Pro mount that I bought 14 years ago -
I soon found out that the Latitude Graticule had a 1 degree error in it.
When I correctly aligned the angle -
I wrote 38.0 degrees where it actually was.

I am going to check a device called Polemaster out.
https://www.qhyccd.com/polemaster/

see pics.

cheers
Allan

[OzEclipse (Joe Cali)]

Originally Posted by alpal

Putting it into perspective:

The primary mirror moving around is a bugbear of just about every Newt.
What mirror cell can you buy that has any design in it that stops lateral movement?
People put stronger collimation springs in which helps but only masks the problem.
This should have been sorted out 40 years ago by the all those so called engineers
who designed the mirror cells.
Amateurs have had to do DIY modifications to make things work properly.
Consumer law says that a product sold should be fit for the purpose to which it is intended.
No one should have to re-collimate their Newt. every time they use it.
They should hold collimation for months or even years and certainly not change
collimation when the OTA is moved from vertical to horizontal.

The reason the so called engineers didn't sort it out 40 years ago is because it was sorted out a long time before that. Have a read of the ATM Porter Series of books written in the 1930's. Pretty sure there is a design in there in one of the first two volumes. My newt mirror cell is based on that hundred year old design and made in 1980. It has no springs, just 6 screws, 3 push, three pull, once set and locked, no droop or shift of the optical axis.

[alpal]

Originally Posted by OzEclipse

The reason the so called engineers didn't sort it out 40 years ago is because it was sorted out a long time before that. Have a read of the ATM Porter Series of books written in the 1930's. Pretty sure there is a design in there in one of the first two volumes. My newt mirror cell is based on that hundred year old design and made in 1980. It has no springs, just 6 screws, 3 push, three pull, once set and locked, no droop or shift of the optical axis.

Thanks Joe - I wasn't aware of that design.
I've never seen it before.
Do you have to undo one screw at the same time
as you do up the other screw in matching pairs?


If my modification doesn't work well I may
design a new mirror cell with that method.


cheers
Allan

[alpal]

I just finished getting the telescope ready.
I've cleaned the primary mirror and installed the anti-lateral movement springs.

I soaked the mirror for 5 minutes in warm water with a drop of dish washing liquid.
I gave it a rinse with demineralised water but there was still some dust on it.
I soaked it again and this time I used cotton wool balls
to dab on the mirror - NOT rubbing from side to side.
It still had some dust like spots but 99% of them had gone
so I didn't do any more to it.
I was too scared of damaging it - I can always return to that job
with new ideas bit it looks pretty good now.


I didn't adjust the laser through the exact center of the 45 degree window
so that I could take pictures of the red spot.
As can be seen from the 3rd picture there is still a little bit
of movement of the laser spot especially when moving
the telescope from horizontal LHS to RHS which simulates a meridian flip.
Therefore I can't consider my idea a 100% success.
I put more compression on the springs but it stayed about the same.
It is still much better than without the springs so I'll go with what I've got.
Remember - the Telescope is never horizontal in actual use -
it would go from say 45 degrees to 90 degrees vertical.
At least my laser is now orthogonal which helps.
Finally I collimated it using my Takahashi 2X Barlow
reflecting off the paper reinforcement at the center of the mirror - as per the pics.
The springs are nice as they don't interfere with
the 3 collimation knobs - it adjusts normally.

Now - I'm waiting for a clear night to do a full test of my system.
I will be especially interested in the performance of the NEQ6 pro mount.
Although it has hardly ever been used the grease in it is original
and might cause a problem?

cheers
Allan

[bojan]

Originally Posted by OzEclipse

.. It has no springs, just 6 screws, 3 push, three pull, once set and locked, no droop or shift of the optical axis.

Mine has springs in a form of spring washers (compressed between 2 flat washers and nuts).. which I tightened in the process of colimation couple of years ago when I last collimated the scope and never again since then. The only part left from original cell I bought from US back in '80-ies is the black aluminium cast.. the rest is my design & workmanship.

[alpal]

Originally Posted by bojan

Mine has springs in a form of spring washers (compressed between 2 flat washers and nuts).. which I tightened in the process of colimation couple of years ago when I last collimated the scope and never again since then. The only part left from original cell I bought from US back in '80-ies is the black aluminium cast.. the rest is my design & workmanship.

Thanks Bojan,
you also have those 3 very long bolts that go out in a radial pattern
and attach to the walls of the tube.
I assume they stop lateral movement?

How can they allow the mirror to tilt -
when you adjust the collimation screws?
How can you easily adjust the collimation?



cheers
Allan

[bojan]

Hi Alan,

I hope the drawing and marked image is illustrative enough to explain how this assembly works..

There are 3 pairs of push-pull assemblies.

Push action is performed by upper screw (blue).

Pull action is performed by spring washers on the lower screw (red)), and they are tightened with nuts once the colimation is achieved.

[alpal]

Originally Posted by bojan

Hi Alan,

I hope the drawing and marked image is illustrative enough to explain how this assembly works..

There are 3 pairs of push-pull assemblies.

Push action is performed by upper screw (blue).

Pull action is performed by spring washers on the lower screw (red)), and they are tightened with nuts once the colimation is achieved.

OK thanks.
There is a lot of leverage from those long bolts -
what stops the mirror going up or down when

the angle of the telescope is changed?
Surely the focus would change?

[bojan]

Long bolts are screwed into ring so bolt heads are in contact with tube.. and then tightened with nuts. So there is no movement at all when scope elevation is changed.
I made this assembly as it is back in early '80 of the last century

[alpal]

Originally Posted by bojan

Long bolts are screwed into ring so bolt heads are in contact with tube.. and then tightened with nuts. So there is no movement at all when scope elevation is changed.
I made this assembly as it is back in early '80 of the last century

OK thanks - I don't quite follow that but if it works then great.




Here is another mirror cell design that I can't follow either:
https://teleskop-austria.com/FN25010...#nav-fotoalbum

possibility to adjust the distance between main and secondary mirror individually
(in case you want to change the focus position for other accessories)
adjustable fit of the main mirror, no wobbling around or deformation due to pressure
complete absorption of forces and focus shift by swinging from zenith to horizon,

[bojan]

Originally Posted by alpal

...Here is another mirror cell design that I can't follow either:
https://teleskop-austria.com/FN25010...#nav-fotoalbum

I think it this design, colimation is done with push-pull screws beneath the mirror pads. Then the mirror is lightly pressed towards pads with top ring, tightened with two screws at each corner..

[alpal]

Originally Posted by bojan

I think it this design, colimation is done with push-pull screws beneath the mirror pads. Then the mirror is lightly pressed towards pads with top ring, tightened with two screws at each corner..

That could be right -it's not explained.
There are some weird designs.

[alpal]

I have been thinking about those springs I added.

It looks like other mirror cell designs that actually lock the mirror once adjusted are the best
but they are more difficult to collimate.
Maybe the springs could be replaced with a rod and a ball joint
to lock the mirror but still allow normal collimation adjustment? -
just lock it after setting the collimation?

The problem is that the whole telescope can be modelled as a set of springs:
the mirror holder is a spring,
the carbon fiber tube is a spring,
the L-brackets I added are springs.
the focuser and camera are springs -
so is the spider holding the secondary mirror.

I have added 3 strong springs which hold everything a little tighter
which has improved the performance but I would
prefer it if I could detect no red dot movement in
the laser 45 degree window when moving the OTA from horizontal
left to right or to vertical.

I took a a couple of extra images after I installed the Telrad.

cheers
Allan

[raymo]

Over a number of years back in the 70's I had three Newts I imported from the U.K, 6", 8.5" and 10". All had the primary mirror permanently attached
to the rear cover plate with adhesive. So to inspect or clean the mirror I just had to undo the 3 [6" and 8.5"] or 4 [10"] retaining screws and remove the rear cover complete with mirror. As the cover plate retaining screws were a very snug fit, the only collimation adjustment possible was the secondary mirror. I used these scopes for years for public outreach, and rarely had to make any adjustments. From memory the permanent attachment didn't seem to introduce any aberrations.
raymo

[alpal]

Originally Posted by raymo

Over a number of years back in the 70's I had three Newts I imported from the U.K, 6", 8.5" and 10". All had the primary mirror permanently attached
to the rear cover plate with adhesive. So to inspect or clean the mirror I just had to undo the 3 [6" and 8.5"] or 4 [10"] retaining screws and remove the rear cover complete with mirror. As the cover plate retaining screws were a very snug fit, the only collimation adjustment possible was the secondary mirror. I used these scopes for years for public outreach, and rarely had to make any adjustments. From memory the permanent attachment didn't seem to introduce any aberrations.
raymo

Thanks Raymo,
at f4 focal ratio the collimation is very critical.
I have had the time to investigate my collimation more closely
and to look for a solution which would allow me
to use the 3 collimation screws normally and to easily adjust it.
I already found that my laser wasn't orthogonal and I fixed that.
The 3 strong anti-lateral movement springs I chose have worked quite well but only
a night out under stars will confirm it -
the bench tests still confirms some small movement -
however I am now looking at ball joint rods to replace the springs
which would more effectively lock the primary mirror in place
but allow normal collimation adjustment.
https://www.ebay.com.au/itm/165602750441\

I'm not sure how to implement it yet.

see pic

cheers
Allan

[alpal]

I now publish some measurements.

It looks like the springs have made the mirror holder very stable.
First of all - see first picture -
just the mirror cell when placed flat on a table without the mirror.
First picture – the mirror was surprisingly central when checked with the mirror lying flat in its holder on the bench.
The outside metal ring fits into the carbon fibre OTA tube with a proper groove.
Using a set square and my digital vernier I was able to make accurate and precise measurements.
The result was that 3 measurements agreed to better than 0.7 mm.
So in this state the mirror is “exactly” centralised in the OTA tube.
I am happy about that.

Next - see 2nd picture.
with the telescope placed so the mirror end was over the edge of my table
so I could use my digital vernier in all rotations -
I measured between the central marked cross of the mirror and
the outside of the metal ring -
with the springs labelled A, B, C - in line with the springs.
I did it twice so 2 full 360 degree rotations
and recorded the results.

Note:
for my laser - after adjusting -
the error is less than 5mm at 4.3 meter target distance which
for my 1016 mm telescope will represent less than a 1.25mm error at the mirror with different rotations in the focuser.

My mirror centralising error is at most 1.7 mm.
The mirror will never be horizontal as per the test -
that is an extreme case.
It won't usually be below 45 degrees so the actual error
will be less than 1.7 mm in actual use.
Part of that could be my measurement error.
Therefore I think I can say that my strong springs have worked.

Any error with that laser in different positions is not coming from the mirror holder movement.
Maybe there is a movement of the secondary or the focuser
which caused the laser spot to move slightly upon rotation?
see my post:
24-02-2023, 01:41 AM

Any thoughts would be appreciated.

cheers
Allan


Edit -
correction factor.
I measured between the central marked cross of the mirror and
the outside of the metal ring -
that is at an angle so the actual measurement is less.
The depth of the mirror from the back of the metal ring is 84.00 mm.
Therefore a measurement of 163.0 mm on the hypotenuse is at an angle of Sin 84/163 = 31.0 degrees.
The actual distance of the center of the mirror to the outside of the ring is Cos 31.0 x 163 = 139.7 mm.
The correction factor is 139.7/ 163.0 = 0.857 = 85.7 %.
So a stated maximum error above of 1.7 mm is actually 1.7 x 0.857 = 1.5 mm.

[alpal]

Embarrassing problem as I should have noticed this before.


I am putting my 10" f4 Newt back into service after 6 years and this is where I'm up to:
I hadn't cleaned the secondary mirror yet because I wanted to check it for offset -
just in case it might have a small error in it.
When I used my digital vernier – after about an hour of making difficult measurements -
I came up with a result that I thought must be wrong –
It tells me that the secondary mirror is offset in the wrong direction by 9.4 mm.
An online calculator says the offset should be 5.5 mm towards the primary mirror –

offset is approximated by: M/(4*F) where M is the minor axis of the secondary mirror and F is the F ratio.
So for my 10 inch f/4 with secondary minor axis diameter of 88mm, the offset will be 88/(4*4) = 5.5mm.

but it’s offset the other way by 9.4 mm towards the front opening of the telescope!
Then I looked in the focuser and sure enough I can see that it’s wrong
without even measuring it. See pic attached.
( I placed a red shirt on the opposite side of the tube to make a photo more obvious
and I focused on the edge of the secondary mirror using my DSLR on a tripod. )

I don’t know how this could have happened.
It was bought from TS in Germany in 2015 and they set up the position of the secondary mirror by
installing the spider and focuser.
How could professionals who charge a lot of money and make 100s of telescopes make such an error?
How did my telescope even work at all like that?
There is also the question of the other offset which would
be away from the focuser by the same amount 5.5mm.
The center of the spider is centralised so it may not have any offset
away from the focuser.
I'm not sure how to measure that.
Any comments?


cheers
Allan