Testing out my system after 6 years

[alpal]

Hi All,
I have just completed my indoor testing of my 10" f4 telescope system
which hadn’t been fired up since January 2017.
I’m very pleased as I was never sure that everything would still work.
I finished the maintenance so far –

mostly outside cleaning and then I assembled it all.
All electrical contacts on the UPS were cleaned.

All batteries were replaced with new batteries.
The UPS needed 2 new lead acid gel batteries 18A/Hrs each.

I made a collage of some of the pictures of it.
see here:
https://i.ibb.co/tL3vHn3/telescope-system-testing-28-1-23.jpg

Everything is still working:
Main camera QHY-9M using EZYCap software.
Filter wheel – 5 position for LRGB Ha.
Guide camera - Lodestar using PHD2 guiding software,
Electric focuser with hand controller,
Mount with its SynScan hand controller.
Power supply - like a UPS - with its 3 off 12 Volt outputs,
Charger,
Both DC to AC inverters.
The laptop works and controls both cameras and filter wheel and mount with its software.
The Telrad is working with a new battery and needs to be mounted.


The only thing I found was the main mirror and secondary mirror could do with a clean.
That’s a big job but I have plenty of time and it will be cloudy for a while.
I look forward to testing it under the stars.


cheers
Allan

[alpal]

I'm getting ready to pull out the main mirror and secondary mirror and clean them.
I'll take some pictures as I go and post them here.


Talking of the main mirror:

Back in 2015 TS -Teleskop Service in Germany

- were offering certificates for their ONTC mirrors.
I'm not sure if they do now?
Here is my certificate from 2015 for my 10" f4 ONTC mirror completed by the famous Wolfgang Rohr.
You can see that the Strehl ratio is 0.922.
Peak to valley 1/6.8 waves.
RMS 1/22.1 waves.

here:

https://i.ibb.co/G5TSG9S/10-inch-f4-mirror-test-report-complete.jpg
using Google translate on the German text it says:

TS Mirror – TSHS250f4
D=258mm
Mirrored surface=254mm
SN=A_TSHS250f4- 04/09/2015
Slight overcorrection (about 1/6 lambda) in the middle.
However, this area is covered by the secondary mirror, so that the mirror is significantly better in practical operation than 1/6.8 wave.


cheers
Allan

[alpal]

I am still checking out the Newt.


I haven't cleaned the mirrors yet.
I'm investigating the collimation and anything that might affect it.
See the pics attached.
I put my laser into the focuser and and centered the spot inside
the paper reinforcement at the center of the main mirror
by adjusting the 3 screws on the spider -
I then adjusted the 3 collimation screws on the primary mirror cell
but I noticed that the spot changes position slightly if I move the telescope from a horizontal to a vertical position.
This is not helped by the 50 mm extensions I made - years ago -
in order to achieve focus when the coma corrector
was just starting to enter the main tube -
the ideal position for it.
It was either make 3 extensions or cut the valuable carbon fiber tube
to fit the mirror cell end piece over the end of the tube.
The cantilever effect of that extra 50mm makes the lateral movement
of the primary mirror worse than stock standard.
I notice that the Newt. designs I've seen all have a lateral movement
problem as there is nothing to stop that from happening -
the collimation screws work at right angles to the sideways/lateral movement so don't help much in that regard.


I found the collimation would not hold steady from other things too:
At first I traced it to a loose grub screw in the focuser - see pic.
Tightening that made a big difference.
As per the other pics it's very difficult to know that the true center
of the focuser laser aiming is correct even though I have a
brass compression ring on the 2" to 1.25" convertor - see pic.
which should in theory hold everything perfectly in alignment.
So at the moment I'm thinking about what to do as I don't want
to be like a dog chasing its tail over problems that don't exist.
When I last used the telescope it worked very well with nice round stars
right out to all 4 corners of the KAF8300 sensor.
Any thoughts would be appreciated?


cheers
Allan

[alpal]

Hi Guys,
I've got a simple idea that could apply to any Newt. mirror cell to prevent sideways lateral movement
so that collimation is maintained from the vertical to the horizontal position of the telescope tube.

The laser spot from the focuser should not move when the tube is moved.
2 off L brackets are used for each each section.
You could have 3 or even 4 L bracket systems each consisting of 2 brackets connected by a spring.
Each one is separated by a strong spring.
There is a wide hole in the bottom base plate L bracket so
that the system won't bind with different tilts of the mirror.
Now there is a strong spring resisting lateral movement which works only in compression -
but the 3 collimation knobs can be adjusted as per normal.
I did a quick drawing.
What do you all think?

cheers
Allan

[alpal]

Talking of lateral primary mirror movement.
Dan from stargazers lounge designed and built a mirror cell
device that prevents lateral movement here:
https://stargazerslounge.com/topic/394734-200800-newtonian-astrograph-rebuilt-with-custom-mirror-cell/


I attach a pic of it here:

[alpal]

I often think of amateur astronomers who have won many awards including APODs
and have large libraries of beautiful pictures without using high end scopes and spending megabucks.
In this case Carlos Taylor is using only
a SkyWatcher Quattro-10S (250/1000) Newtonian reflector
which retails for under 1,000 Euros:
https://teleskop-austria.com/SWN25010i_SkyWatcher-Quattro-10S-250-1000-Newtonian-reflector

and has a 2" focuser.
he just won another APOD here:
https://apod.nasa.gov/apod/ap230119.html

and he has an astrobin library of amazing pictures here:
https://www.astrobin.com/users/CAPastrophotography/

[alpal]

Putting it into perspective:

if you want to use a full frame camera as per many imagers then it starts to get expensive -
you need a 3" focuser and corrector which would cost much more than Carlos Taylor's entire Newt. -
in fact 3 times the price or more.
Then a full frame CMOS camera would cost 4 times the price of his Newt.

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.

[rmuhlack]

A pertinent thread for me, as I have also just this week fired up astro gear (that had been out in the observatory shed) after a similar period of inactivity (about six years for me too). The PC that had been left out there is struggling, but the mount and scope are okay, and I actually managed to collect some respectable shots (some easy lunar to get my feet wet).

[alpal]

Nice going Richard,
the pictures look great -
it must be a good feeling to finally fire everything up.
Are you still able to take long exposures while guiding with PHD?
Will that be your next step?

I kept all my stuff indoors in mostly locked cupboards.

cheers
Allan

[rmuhlack]

long exposures while guiding with PHD is the plan for the other scope (VC200L) and mount (NEQ6). However before I get too carried away i'm going to have to take everything out of the astro sheds to give them a good clean. SO MANY BUGS !!

[alpal]

Yes - it's worth doing.


In the meantime I bought some hardware from Bunnings
to help me work out how to install 3 lateral loading springs for my mirror cell.
I think I know how to do it. see pic

[alpal]

I finally made the first of 3 lateral axis spring devices for my telescope.
The first picture shows the device placed under the primary mirror but
not bolted on with the wing nuts yet.
The next 3 pictures give different views of the mechanism.
I had to make many parts - including the L brackets with 2 holes -
one hole is a slot type to allow tension adjustment of the spring
when all 3 are finally fitted.
The wooden block is made out of very hard wood.
I had to use calipers to draw a radius on some paper and transfer that to the rounded end
to match the inside radius of the carbon fiber tube.
It fits the tube perfectly and took ages to get the curve right.

A flat blade Speedbor was used to make an 18mm hole for the spring to fit.
A piece of copper tube and some cloth tape was used to make
a bushing for the end of the bolt closest to the L bracket.
A brass bush would have been nicer but only a machine shop
could make that and it's not critical.
A spring bucket would be nice inside the wooden end piece but
once again I don't have a lathe to make one and
I don't think it's critical either.

Now I have to make the other 2 and then drill 3 holes in the mirror cell to mount them.
I'm hoping this will be the answer for millions of people out there who have no lateral control
of the movement of their primary telescope mirror
which causes their collimation to change when the telescope is in different positions.



cheers
Allan

[Startrek]

Allan,
Great post and thread
Very ingenious design on the primary too
I’m a Newt man through and through since the start
Gee I’m glad I image at f5 not f4
One observation I made was that my carbon fibre newt holds collimation better than the metal tube newts ( obviously little or no thermal expansion along the tube from end to end ) Although that’s at f5 , f4 may be a different proposition

Cheers
Martin

PS: I’ve ordered another Klaus Helmerich Carbon fibre tube for my 8” f5 GSO metal tube newt , so will be retrofitting in March

[alpal]

Thanks Martin,
I like imaging at f4 - it's much faster.
I want to get a good image in less than one night and maybe
even get 2 good images in one night.
Yes - carbon fibre Newts hold collimation better than the metal tube Newts
and they also hold focus without changing for hours on end -
in fact they can hold focus all night.
Good luck with the new CF tube.

I just finished my 2nd wooden block as per the pics.
I'm doing it slowly at my own pace.

One thing that worries me about my design is that it's so
simple why didn't someone else do it before?
is there a problem with it? - e.g -
will it affect the focus or the guiding?
A 2 axis closed loop system as per PHD2 guiding may not work
as well with unknown spring constants causing a change in the dynamic damping -
if anyone knows - please chime in?
In any case I'll soon find out - in fact it's a good scientific experiment.



cheers
Allan

[alpal]

Latest news:

I finished all 3 assemblies for the anti-lateral movement springs.
In order to treat the wood I soaked it in raw Linseed oil -
then wiped clean -
it's already bone dry after one day - might need another coat?
I changed the washer setup as well.
It's ready to be installed.

I'm thinking about whether I should install the mirror fan as well ? -
I was told that it makes little difference for a 10" mirror.

cheers
Allan

[strongmanmike]

Ok, I'm confused....the problem is not normally the primary mirror holder that moves or "flops", as the scope tracks across the sky but rather the actual primary mirror itself..?

What am I missing here..?:question:

Mike

[alpal]

Hi Mike,
not the mirror itself but the mirror cell or mirror holder.
Look at most telescope mirror cells -
or at least the ones I've seen.
They have 3 collimation adjustment screws underneath the mirror cell.
What keeps the mirror cell and mirror in place when you move
the OTA from horizontal to vertical?
The 3 screws don't have tight threads - there will be some movement sideways.
That will affect the collimation when the OTA is at different positions.



cheers
Allan

[alpal]

Update:
I was wondering about my laser and if it was suitable for precise collimation?
I found out that it wasn't orthogonal.

After some internet research I discovered that the laser is easily tested
by clamping it in a vice and aiming it at a graph on the wall.
When first tested the error was over 8cm at my target 4.3 meters away.
Luckily the laser has adjustment Allen head grub screws.
They were sealed with silicone so I removed that and preceded to adjust it.
The least error I could get after 1 hour of tinkering was a 4.2 cm or 42 mm. see pics.

My testing involved rotating and re-tightening the clamping screw
in the 2" to 1.25" adapter- - which has a compression ring.
The fact that I could rotate the laser a full 360 degrees back to zero degrees and be on the same spot gives me confidence that my testing method is sound.
A 42 mm error at 4.3 meters away equals
inverse Tan of 42/4300 = 0.56 degrees.
My 10" f4 telescope has a FL of 1016mm
so the error in collimation will be 42/4300 x 1016 = 9.9mm
when projecting the laser spot on to the paper reinforcement circle.
That to me seems hopeless to use as a collimating too?
see pics below.
can anyone please comment?


cheers
Allan

[big-blue]

Hi Allan
I have had consistent results from rotating the laser in a v-block such as a piece of angle iron or aluminium.
A V-block minimises play in any radial direction so that you can re-set axial accuracy while rotating the laser. I reckon I get to within couple of mm of dead centre with careful adjustment, via target at a few meters as you have done.
The eyepiece holder such as you are using might still have residual radial L-R slop under the single thumbscrew. Just the act of clamping could shift the laser.
When I find a good eyepiece holder in my various bits, I set it aside to use for collimation in my Newt. Once I know the laser itself is collimated, I rotate this 'best' EP holder with laser fitted, to check its own wobble. Good luck.!
& btw, my own preferred method for collimation is via Cheshire eyepiece.

[alpal]

Thanks - I'll try that.
The eyepiece holder does have a compression ring which in theory should hold the laser or an eyepiece in exactly an orthogonal position.
It does return to zero precisely after a 360 degrees turn and locking.



cheers
Allan

[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/product/howie-glatter-single-deep-red-beam-laser-collimator-2%E2%80%B3-650nm/

[alpal]

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]

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]

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]

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]

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]

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]

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/FN25010c-new_250-1000-Lacerta-Photonewton-Carbontube-made-in-Hungary#nav-fotoalbum

[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..

[alpal]

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]

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

[alpal]

I will have to move the secondary mirror down by 5.5 + 9.4 mm = 15 mm
towards the primary mirror.

The spider was installed at the wrong rotation.
Mike Sidonio's Newt. has the spider offset by 45 degrees to the focuser:
https://pbase.com/strongmanmike2002/collimating_with_the_catseye
It's obvious now that it's a better spot for the spider as it's not
placing direct strain on the focuser which could cause tilt.
Why didn't TS in Germany think of that too?

Gee wizz - moving the focuser tube up 15 mm could be a problem.

To move the primary mirror down 15 mm would mean having to make new
extension spacers for the mirror cell.
I have only just added anti-lateral movement springs which work
very well to stabilise the mirror cell.
The gap left over would be wide enough for those wooden blocks
as they could be placed partially over the metal ring.
This is starting to get very complicated.
see pictures.

cheers
Allan

[croweater]

Hi Alpal, I may be wrong but I think you may be overthinking the situation. In a newt if the spider is properly centred and you use a sight tube to centre your secondary mirror in the focuser and adjust your tilt to be able to just see all your primary mirror clips your offset should be automatically set. If you cant see all the primary it may be too close to the secondary and need to be moved down the tube.
I guess I may be over simplifying it but I like the KISS method. I may or may not have been any help here. :)
Cheers and regards, Richard

[alpal]

see here:
https://www.bbastrodesigns.com/NewtDesigner.html


click on "diagonal"
then click on "Diagonal Offset study".

A Newtonian telescope's diagonal must be offset in order to center the cone of illumination on the field of view. Because of geometry, there it is not possible to center the cone of illumination on the center of the field of view while also satisfying the desire for even illumination at the edge of the field. The diagonal is offset in relation to the focuser, so either the diagonal can be moved or the focuser can be moved. I assume a fixed focuser. Let's see why an offset is desired. etc

[croweater]

No worries alpal. I understand there needs to be offset. Im just saying if you use a sight tube and collimate as normal procedure this will happen as a matter of course. If you look at astrobaby's newtonion collimation page at the bottom this is demonstrated in picture and explainer of offset you will see in fast newt. I see this in my f5 newt and my star tests good. When I look through sight tube at secondary I see all mirror clips so must be getting whole light cone. Hope I make sense and if I'm wrong I apologize.
Cheers Richard

[alpal]

Thanks Richard,
it's here:
https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

At the moment I use my laser for collimation.
I may have to buy a Cheshire or some other device to get it perfect.

cheers
Allan.

[rmuhlack]

Rather than moving the primary or focuser to fix the offset, can't you just install a spacer of some sort between the spider and the secondary holder to move the secondary closer to the primary?

[croweater]

Hi Richard. Yeah I guess it depends what sort of secondary holder you have. In my Parks 8 inch it's on a threaded rod so I have a fair bit of adjustment. My 5 inch its on a stalk, so
no adjustment. Happily it's in the right place :)
Cheers mate

[croweater]

I have the orion cheshire/sight tube (from Bintel) and find it easy to use and gets me pretty close. If it needs to be any better than that I can star test it but I'm visual and not that bothered usually. I've got a laser but haven't used it for years.
Cheers to you mate, Richard

[alpal]

Hi Richard,

Apart from introducing cantilever changes in angle causing instability -
moving the secondary mirror down towards the primary will
change the point at which focus is found.
That point is now setup so that focus in my camera
is achieved just as the coma corrector enters the inside of the tube.
I am experimenting with different designs here:
https://stellafane.org/tm/newt-web/newt-web.html


cheers
Allan

[croweater]

Thanks for putting that link up Allan. Haven't seen that site before.Good stuff on there.
Cheers Richard (Croweater)

[rmuhlack]

well moving the primary would also change the focus point. If the offset is wrong and you want to fix it then you'll have to change something. if you don't want to cut new holes in your tube for a new focuser position, and you don't want to change the position of the primary or extend the secondary down because that will alter the focal plane, then what is left...? :shrug: You could get a new tube I guess, or the nuclear option of a whole new scope...

[alpal]

Hi Richard,
Moving the primary down 15mm and also the secondary down 15mm will
keep the focal point the same -
it will only fix the incorrect offset -
which is what I want.
Note: this telescope was taking pretty good pictures
even though it was set up incorrectly.
see here:
https://www.flickr.com/photos/24719437@N03/31782635104/in/photostream

cheers
Allan

[alpal]

Here is a good explanation of the offset and a picture too.
It's from here:
https://skyandtelescope.org/astronomy-equipment/offsetting-your-secondary-mirror/

[alpal]

A guy called Mark helped me out with a measuring method on another forum.
How to measure the offset away from the focuser?

I am glad I didn't disassemble my Newt before measuring it.
I used 2 rolled up pieces paper with masking tape around them and kept trimming them until
they would sit by themselves between the secondary and the other side of the tube.
I was expecting to find that the offset was away from the focuser by 5.5 mm
but instead the focuser side was shorter by 1.3 mm.
The offset is wrong in value and it's wrong in direction as set up by TS in Germany.
Now I'm going to have to work out how I will do that offset myself when the spider is reinstalled.
see pic.

cheers
Allan

[croweater]

Allan if you use method C as described in the "sky and telescope" article, using a sight tube (which is what I've been pushing I guess) you dont have to worry. Just centre your spider and secondary mirror, use a sight tube to set up your secondary(then the cheshire to do primary) and you are done. I get good collimation with that method. The amount of tilt introduced is negligable and undetectable as far as I know. Though if you want to go for method B thats fine too. Let us know how you get on.
Cheers mate, Richard :thumbsup:

[alpal]

Thanks Richard,
method C does seem to take care of the offset away from the focuser.
I don't have a sight tube or a Cheshire.
I think I need to get those devices.

It seems that the offset away from the focuser -
is built into the design of the spider hub on some telescopes?
It seems that most spider hubs don't have that - why?
This is becoming a huge mystery.

cheers
Allan

[croweater]

I don't know why either Allan. I certainly don't claim to be an expert. I'm just doing what I've always done and the stars test good so that's good enough to me. I wonder if its imaging scopes or high end maybe that do it that way so there is no tilt in the system that would show up under very close scrutiny. Hopefully someone better qualified than me will chime in.
Cheers to you, Richard

[alpal]

Thanks Richard,
you have been very helpful.

This link sort of answers it too:
https://catseyecollimation.com/mccluney.html

Offset Away from the Focuser -
Unless the spider assembly or secondary mirror holder is specifically designed to include it, offsetting in this direction can be difficult. For a typical 4-vane spider the mounting holes may be drilled slightly shifted in the tube wall in order to accommodate the offset. It may also be possible to offset the spider using the spider leg mounting hardware by loosening the leg(s) nearest the focuser and tightening the farthest leg(s). This solution is less desirable as it may tend to increase the width of diffraction spikes seen around bright objects, or even make each existing spike 'branch' into two spikes. The primary reason for offsetting the diagonal away from the focuser is to keep the optical centerline and the telescope tube centerline coincident and prevent vignetting at the front entrance of the telescope. If this could be a problem, or if the most perfectly possible aligned system is desired, then include this offset dimension. When offset in this direction is not included, the optical centerline will be reflected by the secondary mirror by slightly more than 90 degrees. This will be compensated for by primary and secondary mirror tilt with no detriment to the telescope's performance.




And here as Richard pointed out:

And here where there is no offset outwards from the focuser so it's done via tilt:

https://skyandtelesc...condary-mirror/ (https://skyandtelescope.org/astronomy-equipment/offsetting-your-secondary-mirror/)

But what if you want the fully illuminated field centered in the eyepiece,
but must leave the secondary mirror centered in the telescope tube?
It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.

[croweater]

Thanks Allan. It seems to me its almost an overkill for perfectionists but thats ok. This hobby does tend to harbour many people of that ilk. Look how we pay exorbitant amounts for a few degrees extra fov or round stars right at the edge of the field. I used to chase that too once. When showing my wife some globulars with a Nagler t6 I once had she said" so you payed hundreds extra to see a bit more black around it" I laughed a bit sheepishly :lol: I now have more modest eyepieces and I'm quite happy with 60 degree fields. :)
Anyway I find the Orion combination cheshire/sight tube very good and I think the Celestron one the same but a bit cheaper. My laser makes a very good cat toy and he goes beserk running around after the dot. :)
Cheers, Richard

[alpal]

I'm like that.
if someone posts a picture I go straight to the 4 corners and start pixel peeping.
Can I find any out of round stars or stars with comet tails on them? :)

I've looked through Carl Zeiss eyepieces and there was a large difference
compared to ordinary eyepieces especially when looking at planets.
Even when looking at Orion - you could see more nebulosity.

cheers
Allan

[alpal]

I made my own Cheshire eyepiece by drilling a 1.5 mm hole in the center of a 1.25" focuser end plug.
I post the picture of what I found.
I can see the 3 mirror holding clips so you'd think it would be OK as is?
How important is offset and collimation?


cheers
Allan

[croweater]

Yeah your right Alan, You do get what you pay for. My wife saw my post and said " Richard tell the truth. You're just too tight to pay for the good stuff"

[croweater]

Looks good to me Alan. Just need a cheshire or well collimated laser to check the cross hairs or laser dot are centred on the primary spot and you're all good I reckon, though I cant see that well but looks like your secondary holder not quite centred in focuser draw tube so could do better but probably not crucial
Cheers, Richard

[alpal]

That's funny. LOL - :lol:

[alpal]

That picture was taken when the telescope was a bit out of alignment.
I should have re-collimated it first.
I don't see how I can get a proper collimation without offsetting the
secondary mirror downwards.


cheers
Allan

[croweater]

Do you have enough movement of your secondary holder(by using threaded rod that holds it, swivel movement and tilt screws to center the holder in the draw tube? If you can do that first. then adjust your tilt to center the primary in the secondary mirror. Sorry if I am teaching you to suck eggs Alan :)

[alpal]

Hi Richard,
I don't know yet.
I haven't dared to touch it until experts had advised me first.
The information contained in the faulty setup is valuable to know what happened.
It also helps others.

cheers
Allan

[croweater]

No worries mate. Good luck with it all. All said and done if all else fails you cant go wrong if you follow astrobaby's excellent instructions.

[alpal]

Latest:

I re-collimated the scope and -
took a clearer image using my mobile phone through a Cheshire cap.
and a person on another forum made this image and agrees that there is an offset problem.
he says -
The current secondary mirror (dashed green circle) needs to be moved to the solid green circle
(about 1/6 of the secondary mirror diameter, so ~15mm).

I am about to pull the secondary out for a closer look.

cheers
Allan

[alpal]

I have some pics after I pulled the hub and spider out.
I think I have enough to work it out now.


cheers
Allan

[alpal]

What should I do about the dimple plate for the 3 adjustment screws?
So - the dimples didn't line up so they just reversed the plate -
or is that the normal setup?

[croweater]

Doesn't look normal to me Al. my plate has no dimples so never seen that before, but you'd think it was the wrong way around. But the dimples too widely spaced to line up with the screws so I don't know what they're for. Maybe try just centering the plate under the three screws and see where you end up.
Your secondary looks miles out. Should be centered under focuser drawtube as the other forum guy indicates.
Also, just a suggestion maybe start new thread with name like "secondary mirror collimation offset problem, help needed" as then this will alert many of the experienced guys on this site of your problem. (your current title maybe not asking for the help you want)
Cheers, Richard

[alpal]

Thanks Richard,
I'm OK - I'm getting plenty of help.

It looks like that dimple plate is for another telescope secondary mirror hub.
The screws should have been rounded to fit into the dimples
but when it didn't line up they just reversed it and didn't care.

I am making a 15mm thick round spacer for the 3 adjustment screws -
and I'll get a longer 6mm thread - M6 Allen head bolt for the center part.

cheers
Allan

[croweater]

Sounds like you're onto it Allan. Good luck.

[alpal]

Latest news,
I fitted a 15 mm spacer and used a longer central socket head bolt -
I also rounded the 3 screw ends a little bit.
After a lot of collimation help from another forum I achieved a better result.


cheers
Allan

[RobF]

Hi Allan

Once you get it all working, would also suggest reviewing software - NINA has great support for native QHY9M drivers now, and the download speeds much faster than I ever could get from ASCOM support. Rapidly developing into a very powerful free solution.

[alpal]

Hi Rob,
thanks for your reply.
I will need to review software at some stage.

Beware of the high speed download on a QHY9-M.
See the picture which shows dark frames.
The high speed readout has a noise level 4 times higher than
the low speed download which is about equivalent to the sky noise
in Melbourne - an average of 4,225 bits out of 65K bits.
High speed is the default readout so it needs to be unticked.

cheers
Allan

[alpal]

Re: the 15 mm offset spacer -

For the record I attach the before and after image via
the Cheshire eyepiece with added circles.
I am now ready to take some pictures outside when the weather is clear.

cheers
Allan

[alpal]

I have done a safety tether string modification.


see pic.
I used part of a new shoe lace.
It's glued on at 2 spots just for extra security.

The lace could not be glued to the paint on the back of the mirror as
the glue would attach to the paint and silicone remnants and be easily ripped off.
It had to be glued directly to the glass.
I gently scraped off the paint with a fine chisel and used metho but
finished off with Acetone for a super clean surface.
I used 5 minute epoxy resin.

When I touched up some areas on the inside of the tube using a tiny brush -
with Krylon 1602 Ultra Flat Black they show up against the
TS "Black" anti-reflective paint.
Which is better?
The Krylon looks darker.
The next time the spider is out I might need to spray paint
as much of the inside of the tube as possible especially opposite the focuser.

cheers
Allan