Centre Spotting Discussions

[davidpretorius]

it is easy to centre spot mate!!! it is hard to not get nervous first time though!

[iceman]

Someone write a how-to for centre-spotting, please! :) the question comes up often and would be great to have a local article to point them to.

[netwolf]

Yeah i am a bit nervious about opening her up.. But if i do i want to do it right and properly. Is there some particular textar or marker that should be used for centre spoting. i have head that using a ring type marker is better, but where do you get these.

[johnno]

You can get the ring type markers at any newsagency,they use them in ring binders for strengthening the paper holes so they dont tear.
Regards.John

[davidpretorius]

you will be right netwolf,

do it on the carpet at home,
undo the 4 or 5 screws on the side at the bottom of the outside of the tube.
lift the tube up and out of the way.
there are then three vertical screws that hold the mirror down via a little rubber angle. undo these and put to one side.
you can then lift the mirror out of the base and onto the carpet, face up!!!
i centre spotted mine with rulers, but the best i have heard of from this forum is to get a piece of paper that is bigger than the mirror. put it on the a hard surface, say a book on the carpet.
then place the mirror on it face up and then trace the circle around the mirror.
remove mirror and then cut out the circle.
fold circle in half and in half again and again and again.
unfold and you should have a very accurate indication of what is the centre of the circle.
place over the mirror and align edges
with a small marker, press down in the centre of the paper. this sould leave a mark on the mirror underneath.
Take paper away
either make spot bigger with marker or place the sticky ring binder around the mark.
I used the marker and made a spot. I do not see the spot at all, unless it is daytime and i am way out of focus.

Good luck

(mike i will try and put pictures to this one and submit as an article) can anyone else please proof read these words and comments for changes?

[square_peg114GT]

Well, I center spot a little different than most, but I'm happy with the results.

This is how I do it with my Hardin (GSO). To start, place the tube on a table covered with a soft towel. Remove the mirror from your scope and from the mirror cell. This is easier than you'd think, just remove the 6 screws around the end of the scope and the whole rear mirror assembly comes off. The GSO's are rather a tight fit and, at least the first time, you may need to tap the lower ring off with a piece of wood and a mallet. I've found Synta's to be a little looser.

Once the rear assembly is off, remove the 3 mirror clips (details in the mirror clips forum - lol). Lift the mirror off and place it. face up, on a clean sheet of paper. Trace the mirror and cut out your pattern. Fold the circle in half, then in half again. Snip about 1mm off the point of the folded circle. Unfold it and you have a very small, perfectly centered hole. Lay it on your mirror and place a dot on the center with a fine point dry erase marker!

Remove the paper and draw 4 lines away from the center dot forming an 'X'. Be sure to leave about a 1mm gap bewteen the dot and your lines so the dot stands alone. Make the 'X' lines about 4mm long. If you make a mistake, just wipe it off with a damp Q-tip. That's the beauty of marking your mirror with a dry erase marker instead of a paper reinforcing ring. Fear not! The center of your mirror is behind the secondary and doesn't contribute to the image. If you prefer the paper reinforcing ring install it during this step instead of making the 'X'.

When re-installing the mirror, be careful not to over-tighten the clips. The slightest pressure applied by the mirror clips can change the shape of the mirror!! Some people say to leave enough room for a credit card between the mirror and the clip. I don't go that loose, but I'm careful that the clips do no more than barely touch the mirror. I've never seen any mirror distortion in star tests.

Before reinstalling the rear assembly, align your secondary!!! Use a chesire/sight tube. It's very easy to do with the mirror removed. No confusing reflected images. Rack the focuser in so you can see all of the opening at the rear of the tube. Just center the the rear end of the tube in the secondary. Don't forget to check the radial alignment of the secondary, too.

Now reinstall your primary and collimate normally, starting by double checking your secondary alignment with the mirror in place. I collimate with only the chesire/sight tube, but lasers still work with the dry erase marker mark on the center of the mirror. Rotate your chesire so that the cross-hairs are turned 45 degrees from your new 'X' lines. You'll get a very neat pattern to align to align by.

I remark the mirror after each cleaning, which for me is about twice a year.

Cheers mates.

:cheers:

[netwolf]

Pegster, what do you clean the mirror with?

[square_peg114GT]

I clean my mirror like this.
http://www.wwnorton.com/astro21/sandt/optics.html

I cleaned up a scope that was donated to our club the same way. That scope had been stored open in a garage for several years. Lots of dust, car exhaust and cat hair on the mirror. An hour of soaking in warm soapy water and a ton of cotton balls eventually brought it back to like new condition.

[janoskiss]

That's a great link Pegster!
I've attached a pictorial Howto on centre spotting.

[asimov]

Yep. Nice link there peg. Very nice how-to Steve!

[netwolf]

I have an idea and wonder if it will work. Use some freeware cad program, use it to draw a cross hair then draw 6", 8", 10" and 14" circles using the cross hair as the common center. Print this onto a Transperency Sheet, use a hobby knife to cut litle slits in the cross. This template can then be set ontop of the primary mirror taped to the edges, or taped to the little clamps. You then use a non-perminent text to draw the lines off the cross hair through the slits.

It should not be difficult to make slits for diffrent patterns, but i am guessing the cross is the best to align using a cheshire.

What do you guys think?? Could these be our DIY guide.

[davidpretorius]

good idea!

[netwolf]

Anyone know where we can get exact dimensions (aprox would do) for the GS mirrors.
I remember seeing an site in the US that rebadged and sold GS who gave all the specs, anyone have a link to it as i dont have it anymore.

[square_peg114GT]

I don't know how much variance there is in GSO mirrors, but of the 2 - 6"ers I've measured, they varied by more than 1/16" in diameter. One was slightly under 6", the other was barely 5-7/8". Best just to trace or measure your own mirror, IMO.

[netwolf]

Pegster, you are correct it is probably best to trace your own, but for a howto i think we can genralise a bit. What you could do for a custome size is trace it, use a ruler to make a cross and photocopy onto a transperency.

I did a quick Visio drawing and printed it to a PDF file, see attached. Please review.

By the way the idea is not mine i got it from a document i read on the net on

Weasner LXD tips and tricks site.
http://www.weasner.com/lxd/

On this site there is a tips and tricks 1 document that contains a similar template for Schimdt-Newtonians.
http://www.optcorp.com/pdf/meadepdf/TipsTricks1.pdf

I just borowed the idea for our use. But i belive in giving credit.

Regards

[iceman]

NW, sounds like you're well on your way to creating the how-to guide for the site - a combination of all the great pics and advice given in this thread would be a fantastic how-to guide.

[square_peg114GT]

:thumbsup:

[netwolf]

I examined my scope today in preperation of the primary cell on either this or next weekend. I was mainly looking at the screws that would need to be removed at the base to pop the mirror cell out at the back. I noticed two things.

1. on the back of the mirror cell on there is a small hole in the outter ring of the cell. Is this normal? I also noted that it can be seen when looking down in the tube, at the edge of the mirror. I am guessing this is by design as it seems to be a machined hole.

2. I ran my hand over the tube as i was examning the back and noted a slight dent above the altitude bearing, i then examined the other side and it was smooth. I am thinking this is a defect. Looking into the tube i could note if positioned correctly with a torch that there is a slight dent there. I am not certain if this was there when i got the scope or something that has happned since due to my noob handling. I noted also that the plastic alt bearing on that side was a bit groved, now this was not there when i got it. I think perhaps the teflon on the rocker is groving it..

Does the dent pose any effect on my viewing? It does not protruede over the mirror so i am not sure it will have any effect. What do you guys think. I am more nervious no w about removing the mirror given the possible dent i may have inadvertenly caused. Will removing the mirror void warranty? Does warranty cover dents and groves?

Regards

[iceman]

Can you post a picture? If it's what I'm thinking, it helps you to align the mirror cell when putting it back in the OTA. That's normal.


Dings and dents are a normal part of handling a tube like that, you should see mine! It even has little bits of surface rust from neglect and very very (once!) irregular cleaning :)
And the alt bearing will get some grooves in it from the teflon pads, that's normal too. I guess you can just try to keep it clean from dust and grit to keep the alt motion as smooth as possible.

There's quite a gap between the inside of the OTA and the mirror, about an inch on each side usually. So unless it's quite a severe dint that protrudes in an inch, you won't notice it and it will have zero effect on your viewing.
I have flocking on the inside of my tube that is peeling off in one spot and actually does protrude slightly into the light path, and I can see when I have a bright object defocussed all the way out - but when things are in focus, I can't see it at all.


Don't be nervous - you've done nothing wrong. The small dent will cause no issues. I don't think removing the mirror will void the warranty, it's normal maintenance when you own a reflector. But I also don't know what you'd possibly claim warranty on unless the mirror suddenly cracked or the focuser snapped in two?
Warranty most certainly wouldn't cover dents - that's part of normal handling. They're not china dolls, they can take a bit of a bashing. Just be careful with the optics.

HTH

[Starkler]

Check the staples that hold down the nylon pads on the base that the plastic alt hub sit on. You might find the staples havent been punched down properly and are gouging the plastic.

[netwolf]

Thanks iceman, i feel better now. I was not to phased by the hole because it looks like it was ment to be there. The ding was alarming. But i gues this is going to happen sooner or later. I am going to take photos of the whole process when i center spot my mirror. So i will take a shot of it the hole then also.

Does anyone know if Trannsperency's can cause harm to the mirror. I dont intent to let it touch but i am not sure i can stop it from touching. I hope to have it taped to the edge so its stretched over the mirror.

[netwolf]

Thanks Starkler, i will check that out, i think you are right on the money with the stapple. Has anyone modified these bearings or replaced them with something else?

Can anyone also reccomend a Texta i should get for this?

[RAJAH235]

I used a permanent marker. It's in the 'blind' spot along with the reinforcing ring.
The teflon pads are a must. Do not replace with anything but teflon.
The transparency will not damage the mirror, but if poss., get someone to hold it for the second it takes you to 'make your mark'!
ps. If people are having trouble with the balance of their tubes, experiment with the position of the alt teflon pads. Move them up in the 'D' to tighten, & down the 'D' to free up the movement. (about 1/4" to 1/2" is all you need). You might just be able to get rid of your dreaded 'Springs'!
HTH. :D L.

[netwolf]

Thanks Rajah. I have seen posts from people recommending non-perminent ones so you can wipe them off later if need arise. I also considerd looking at the Catseye (http://www.catseyecollimation.com/) type method were you make get the binder reinforcers. But a bit of tape over the cross hole in the transperency, attach the non adhesive side of the reinforcer rings to the tape and then remove the adhesive covering. Press down the transperency remove tape and presto ring on mirror. But this method as simple as a marker.

One thing i had a problem with making the template was deciding on the Cross hair size. Am not sure what the smalles shadowed area is. I made the bold cross with a 10-12mm diameter. Not sure if this is within the shadow area of the secondary. Idealy I wanted this to be about 5mm less than the smallest secondary shadow for a 6".

Does anyone know what the shadow diameter would be for the various GSO dobs?

[square_peg114GT]

Should be the same as the secondary's diameter. As seen from infinity (where astronomical objects are) the 4 feet of tube you've got are insignificant. Secondary on a 6" GSO is 1-5/16".

[slice of heaven]

Tried this option Laurie but the alt bearing surface is too slippery to make repositioning the pads worthwhile. I dare say the choice of surface is because of the pressure the springs apply.
There are ways to modify the alt bearing though to make the springs redundant.

[janoskiss]

Laser collimator? :poke:

[slice of heaven]

I was thinking the same :)
Seems the 'Title' is a bit off topic :lol:

[iceman]

I've split the thread, the centre spot discussions are now in this thread.

[netwolf]

thanks iceman

[netwolf]

I went into Bintel today and bought a 2" GSO Barlow and 1.25" Orion Combination Cheshiere/Sgiht tube. Mike was very helpfull and gave me some advise on center spoting. Another gentelmen there i forget his name now (with specs and a pony tail) said the best way wos some brown paper, a compas and a texta. He gave me a print out that comes with the laser collimator that had some similar instructions on it. They guys at Bintel are very helpful and friendly and they know there stuff. Also they gave me a discount, not that i asked but i did mention iceinspace and thanked them for there sposorship of the comp.

So I am all set .. 1 Center Spot DIY coming up soon, I hope.

[iceman]

Excellent work, don't forget to take pictures and document as you go!

Nice one! They are good blokes.

[netwolf]

where can i get these reinforcement rings, if thats what they are called?

[asimov]

At the news agency.