Squaring the Focuser - Questions

[Astro_Baby]

I have never bothered squaring the focuser on my newt. Reason being theres a diverse opinion on whether

its worth it with some collimation experts saying not. I figured that given the stock focusers on

scopes are often not adjustable anyway (unless you mess about with plastic shims) there could be little

advantage.

Most people never seem to bother doing this it seems and this is a good place to apologise for what is

going to be a very long post even by my standards

Well this afternoon I was free and decided to mess about with the newt. One of the great benefits a

newt brings over a refractor is that you can constantly play with it to get collimation ever sweeter.

And, as you all know I am an incorrugible meddler.

Anyway I decided to square the focuser and this is where I have some problems. How do you square it ?

( yes yes I know the theory but bear with me a bit).........

These are the ways I have seen of doing it and the questions they raise in my mind. By the way I have

used basic terminology (up, down, left and right) with respect to the focuser rather than axial, lateral

etc to try and make it a bit simpler and clearer).

(1) LASERS

Remove the secondary and use a threaded rod thats the same size as the secondary retaining screw

through the secondary holder and then use a laser to identify whether your hitting the threaded pole.

It seems simple (assuming you can find a threaded pole the same size as the secondary screw and the

laser is perfectly collimated) but it seems a duff idea to me.

If the laser hits the threaded pole then it tells you the laser is correct in the up and down plane (ie

if you consider this from the position of looking down the focuser with the tube horizontal) the laser

can hit the pole by the focuser being adjusted in the up and down motion but it could still be out by a

mile with respect to left and right . ie the laser could be hitting the pole towards the primary or away from it.

So this method would seem to only work in one plane unless I am missing something. Am I ?

(2) RULES AND COMPASSES

Measuring the tube to find the point exactly opposite the centre of the focuser type method.

Now this method usually involves removing the focuser, measuring the diameter of the hole in the tube

to find its centre, calculating its centre from the front of the tube and then by dint of some simple math

calculatng where on the opposite side of the tube it should be. I know the math but there seems to be a

fault here in this process.

It assumes that the focuser does in fact sit perfectly central to the hole in the tube. My experience,

such as it is ,says this isnt nececcessarily true. When I consider my own tube it has a Moonlite fitted

and the consequence of that is the Moonlite fits into the original tube hole for the Skywatcher Crayford

with about 1/2" all round it spare. Because the fixing holes are not exact the Moonlite has the potential

to be at least 2mm off centre with respect to the hole in the tube. So the problem would be you could

work out the opposite point on the tube to the hole of the focuser cut out BUT the focuser could be out

by a few mil itself with respect to the hole.

As precision is everything HOW do you find the exact centre of your focuser ? - Again - enlighten me -

am I missing some crucual step here ?

(3) MEL DOES IT HER WAY

Unsatisfied with either of the above ways of doing it heres what I did......

I left the Moonlite mount plate attached to the tube but removed the focsuer unit itself. This left with

me a fixed round hole on the telecope tube albeit raised by 10mm or so away from the tube on the

outside but accessible (by removing the spider vanes) from the inside.

By messing about I made up a transparent template that gave these points on it.

(a) circumference of focuser plate hole

( centre point of focuser plate hole (worked out)

© distance from edge of the tube (the black front stifferenr/trim ring was removed)

(d) location of spider vane hole in line with focuser

(e) location of front stiffener/trim fixing hole in line with focuser

Once the template had been made up with respect to the focuser hole and the other fixing holes it was

simple enough to calculate the centre lines for all holes and the center point of the focuser hole in the

Moonlite base plate.

The template was now attached to the inside of the tube using the spider vane hole marker and the

front trim fitting marker OPPOSITE the focsuer and the edge of the tube was also used as a secondary check.

The template would now (in theory) identify the exact point opposite the centre of the focuser. Or at least as

accurately as it could be measured.

Before being so rash as to go drilling holes I wanted proof of this process. So using the same method I used

for spotting a mirror I applied a sticky hole reinforcer to the inside of the tube at the point indicated by the template.

I know - this sounds very complicated in text - in reality its simpler.

Can anyone see a fault in my thinking or approach ? I can - it assumes the machining of the scope tube is

acurate enough with respect to the spider vane holes and the tube edges BUT I wonder if thats any less accurate

than the methods (1) and (2) stated above which also rely on the manufacturer getting it very close as far as the cut

out for the focuser is concerned.

SQUARING THE FOCUSER (cue soft music - Moolight Becomes Her perhaps - hum along chaps )

Anyway I then used the paper circle as a target for my trusty Cheshire and messed about with the focuser

collimation screws (very slow process) until the focsuer was cinched down tight to its mounting AND showing

the Cheshire crosshairs as on target (now that bits hard work). The focuser wasnt far off true it seems as

the adjustment needed to target the crosshairs to the circle opposite was only about 1 - 1.5mm on one adjuster.

COLLIMATION RESULTS - after all this palava I went through my own collimation process as stated in my guide

and all went well. In fact collimation is now sweeter. How do I know that ? Well in the past there has always

been a slightly off edge to the secondary (I think gurus would call it axial error ).Its elipse shape in the Cheshire

was always slightly skewed to one side by a fraction. It was never massively noticeable and most people would have

ignored it or gone mad trying to eradicate it.

I never could get rid of it and just ignored it. Star testing always showed the scope as bang on. BUT post this process

its gone and I believe that to be in part because of the finer squaring of the focuser.

So - collimation gurus step forward (that includes you Jason ) and tell me if you can see any flaws in my

squaring process and whether you believe it could have been responsible for a slightly offline elipse in the Cheshire pattern.

Mel

ps - No I am not writing a guide on this one its too much like hardwork

[Rusty Strings]

I used the "rules and compasses" method after determining that my moonlite was sitting centrally in the focuser hole and it seemed to work fine for me. Has it made a difference? I think so, like Mel, I always found the collimation was near enough but it I felt it could be better, so either I've got better at it, or squaring the focuser made the difference.

[stafford_stargazer]

I went through this with my 200p,my focusser was not square,i sussed this out pretty soon and rectified it.

I always too thought the secondary was slightly off,i knew this wasnt right as common sense tell me this.

After my squaring i got the collimation spot on,using laser and star test.

Im a sucker for things being right and i noticed this when i first got it,well pretty much.

Surprising how much easier it is to collimate once things point where they should

G

[TopHouse]

<cough>

http://stargazerslounge.com/equipment-discussion/99623-centering-focuser-tutorial-focuser.html

[stafford_stargazer]

Would you like a tablet for that cough sir!

[TopHouse]

Don't get me started on fishermans friends!

[Astro_Baby]

Yes TH - saw it but my process was a bit different but the question (cos I dont know what the answer is) is given that the tube is probably inaccurately made, the focuser hole may not be the true centre for the focuser etc does the process I used seem sound ?

Given that an F5 scope has a very tiny sweet spot and collimation is critical (at least in theory) does the process of templating based on a hole which may or may not be accurate with regards to the tube work well enough.

I had to do mine somewhat differently and base it on the underside of the Moonlite mounting plate because the Moonlite doesnt fit exactly central to the original hole in the tube.

I guess I needed a sanity check on whether what I am doing makes sense.

[TopHouse]

Well, (he says doing his best Lloyd Grossman) It seems to me that if when using a sight tube you could see a slight flaw in secondary centering, and that flaw was gone when you centered it, I'd say you did right. Especially if the direction you had to alter the focusser was the 'opposite' direction to the apparent 'skew' because guess what? I had that too, pics about half way down here :-

http://stargazerslounge.com/equipment-discussion/99090-collimating-video-camera-tutorial-photos-2.html

[Astro_Baby]

(huffs and puffs and sounds like Janet Street Porter)

Yes me too - I assume I am a genius and its all ok now BUT whats weird is although that secondary flaw has been there ages it hasnt ALWAYS been there. I can be sure of that because the pics taken for my collimation guide were on file - I only thought to look at those about half an hour ago. DOH !!

I am moderately sure my focuser is squared and I am happy with the process but it does seem a rather innaccurate thing to do.

I mean its ok measuring the circumference of a tube BUT the stupid tubes on most scopes arent perfectly round. The Russian ones are from TAL cos its a machined up bit of pipe but a rolled and seamed tube will never be perfectly round. So even that process means you introduce errors in the measurements. When you add in the lack of precsion to the focuser fittings it all makes for a very ropey basis for believeing you are exactly opposite the centre of the focuser.

I dont know maybe I want too much perfection.

I'm agonising about whther to buy a Catseye system at the moment - got that collimatio monkey on me back

ps I must have entirely missed your thread on collimation with a video camera which is weird cos I generally look at any threads about collimation cos its my passion - it has to be I never get to use the scope for anything else

[TopHouse]

Well we are the founder members of the SGL collimation OCD support group!

Were the tutorial pics taken pre moonlight?

Has it had a knock at some point?

[Astro_Baby]

No the pics were taken post Moonlite. I shall test the lil Mod out quite soon.

Its always taking knocks cos it has to be transporeted 40 miles for each use.

[TopHouse]

oooo bummer! Get yourself oop north, only a couple of miles up the road from me is the moors and it gets black as coal up there.

[Astro_Baby]

I wish I am stuck in light pollution hell.

[Jason D]

Mel,

When I squared my Moonlite focuser years ago, I removed the secondary mirror, marked the OTA opposite to the focuser using simple measurement techniques, then inserted a laser and adjusted the focuser. I have a Moonlite focuser with easy four adjustment set screws located at each corner.

Precise squaring is not important. Reasonable squaring is desirable.

Collimation is about aligning optics. It is not about mechanical alignment (unless you have DSC).

Check the attachment. Figure “A” shows collimated optics. Figure “B” is the same as “A” but with the OTA and focuser wrapped around the optics. Figure “C” has the same aligned optics as “A” but the OTA it tilted. Finally, figure “D” is exactly the same as “C” but the whole thing has been rotated counter-clockwise.

Even though the optics alignment is the same for all figures, note how figure “B” has a squared focuser and figure “D” does not.

The points here is that no one should be obsessed by the mechanical alignment of the focuser. As long as the mechanical alignment of the secondary mirror assembly and the focuser is reasonable, we can always adjust the secondary and primary mirrors to achieve excellent collimation.

By the way, the up/down alignment is more important than left/right alignment of the focuser. That is why the laser technique you have described is a good one. Just refer again to figure “D”. The focuser is pointing upward with respect to the OTA yet a perfect collimation was achieved. You would raise or lower the secondary mirror to compensate for any small errors along the left/right direction.

Jason

[ollypenrice]

(huffs and puffs and sounds like Janet Street Porter)

As long as you don't look like Janet Street Porter...

Olly

[Astro_Baby]

Jason - would you believe that a slight 'not round' view of the sceondary that cannot be corrected by the secondary adjustment would be cause by a misaligned focuser.

The result I was getting prior to squaring the focuser showed the dark elipse of the secondary slightly rotated in the Cheshire. Its a hard one to explain and I will try and bash out a picture to give you a representation.

In a nut sheel though I was finding that the secondary was always slighly (very slightly) not completely circular under the focuser. It really only showed though in the refelection of the secondary as I imagine the focal length doubling increased the error.

[Jason D]

Mel, see attachment

Left figure is for a perfect alignment. The red circle represents the focuser opening.

Let us misalign the focuser and shift it as shown in the middle figure

Now we will need to tilt the secondary mirror; however, we will also need to rotate the secondary to realign the focuser/primary axis. That last rotation what causes the secondary mirror to look elliptical.

My point in the previous post is that axial alignment is the most critical step in collimation which can be achieved perfectly even if the secondary mirror and the focuser are not in perfect alignment. Having somewhat an elliptical secondary will hardly impact the image for visual observation.

I would do my best to square the focuser within reason then I would spend more time fine adjusting the spider vanes using the thumb knobs to align the secondary mirror assembly with the focuser.

Jason