How to centre spot a Dob?

[dugnsuz]

Hi all,
Thanks for all the helpful posts re my $1000 question.
After some soul-searching I realised that we're all in it for the "WOW" in some way.
So for me perhaps my wow will be a large Dob.
The cheapest, least collimated(!!) option may be the go.
But, how do I safely centre spot a 10 or 12 inch Dob?
Any links? My basic searches have drawn a blank.
Cheers again

Doug :prey:

[iceman]

I've been waiting for someone to write a how-to for a while on this subject. Hopefully someone can put something together in the short term.

[davidpretorius]

this link from the Projects and DIY section top left is where i worked out how to centre spot mine

http://www.iceinspace.com.au/index.php?id=63,206,0,0,1,0

very easy to do!

[TidaLpHasE]

This link might also be helpful for dob users, it explains it the best i have been able to find with google.

http://skyandtelescope.com/printable/howto/scopes/article_790.asp

I am waiting for my 12" to arrive in the mail and will be needing to learn this as well, i'm not too keen on stripping my scope down and risking marking the mirror though.

Dave, is it as easy as you say to take apart and put back together ?

[davidpretorius]

Yes, i have read that one before and from memory, i had my computer on this link ass i was actually pulling mine apart.

Apart from being very very nervous, (like your first intimate session), looking back a few weeks, it was very easy. There were 5 or 6 screws around the side at the bottom of the tube. taking those outs whilst the tube was laying vertically on a towel or the carpet. then lifting the tube up and putting to one side. picking up the mirror and base and then putting on a table or leaving on the ground if your are still nervous. removing the 4 odd screws that hold the mirror and then the mirror lifts out.

i then used the rulers and marked the centre with a black texta.

remember that light from the stars is coming into the tube virtually parrallel, so that practically no starlight will actually hit the spot because the secondary mirror at the top is in the way. you can see the spot when out of focus, but at focus, the spot is not there, so do not worry!

once done, place mirror back in holder, do not tighten screws too much as it will pinch the mirror. before putting tube back on, have a look at the bottom of the base to get a feel for the setting screws and adjusting screws for when you collimate.

[janoskiss]

Doug, the scope should already be centre spotted when you get it. I think I forgot to mention: make sure you get one of the new Dobs, with the right-angle finder and maybe Rigel quickfinder.

[RapidEye]

OK, I learned this centerspotting trick from Robert Bruce Thompson - author of the new book "Astronomy Hacks" available from O'Reilly.

I'm working from memory and don't have the book in front of me, but its what I did on my GS scope:

Pull the cell from the OTA
Wearing powder free latex gloves, pull the mirror from the cell
Place the mirror upside down on a large piece of paper - bigger than the mirror
Carefully trace the outline of the mirror.
Safely set the mirror aside
Carefully cut the circle you just drew out of the paper
Fold the paper in half, then in half again so that it looks like a triangle with the short edge being curved.
Take a pair of scissors and cut a very small piece off of the tip of the triangle
Unfold your paper and work it so that it lays flat - you now have a perfect trace of your mirror with a hole in the exact middle.
Wearing your gloves again, place the paper over the mirror again, carefully aligning the edges.
Take a small tipped marker and very carefully make a dot on the mirror right in the middle of your hole
Remove the paper
Then use the dot as a guide to place your paper circle
The entire process should take about 10 - 15 minutes once you get the mirror pulled from the cell! :D

Hope that makes sense! If not, get Robert's book - it goes over it in more detail and also has pictures.

[RapidEye]

Actually, NONE of the light will hit the center spot - its completely in the shadow of the secondary.

That spot you are referring to when out of focus is the shadow of the secondary.

[davidpretorius]

i was going to put "none", but then i thought that the physics guys would have said "what about diffraction patterns thru the 4 slits, you can't have none!!!".

i took an easy way out and put practically, knowing there was no where to go without getting caught!

[davidpretorius]

that is a great technique, rapideye, jamie oliver does the same thing for working out how to get a cicular shape to fit a cake container.

[janoskiss]

I'd prefer not to touch the mirror surface (even at the edges; with paper or otherwise). Here is how I did it:

I laid the mirror on a flat surface; Placed blocks of wood around it that are just a little thicker than the mirror, so that I could lay two metal rulers at right angles over the top of it without the rulers touching each other or the mirror. Then I carefully looked directly down at the centre mirror and using the mm markings on the two rulers I visually figured out where the centre is and marked it with a fine tipped permanent marker. Then I stuck a triangle shaped marker around the spot. I cut it out of dark blue strong cloth-reinforced tape. It stays on no probs when I wash the mirror.

If you have dandruff, get a good anti-dandruff shampoo and use it before attempting this. :D

[RAJAH235]

I simply cut a 'thin cardstock disk', similar to Rapid's, & used that. There's no problems associated with centre spotting in this way, as long as due care is taken. Remember, it's overcoated, & this method can be done very safely, easily & quickly. :shrug:
Just the facts, m'am. :D L.

[RapidEye]

Yeah, I was suprised how easy it was and how well it worked!
The only hiccup I had was trying to mark the hole without moving the paper with my hand :doh:

I wound up doing something similar to what janoskiss talks about: I put a large book next to the mirror so I could rest my hand on that and not touch the paper.

I didn't list that in my original post because its really going to make a difference how big the mirror is (mine was 10") and how steady your hands are (not going there).

[RAJAH235]

It helps to have a *helper* to hold the paper/cardboard disk, for the second it takes to mark the centre. :D L.

[dugnsuz]

Thanks for all the tips
All really helpful.
Is it only Bintel who spot their Dobs or do some come from GSO already done?
Cheers
Doug

[xrekcor]

Jim's (RapidEye) is the easiest and quite accurate way to go about it, an is the way I would go about it. As long as the paper is clean and fresh you should have no problem with it _resting_ on the surface of the mirror and dropping grime, look for something lint free. Greaseproof paper is good for this. Also you can easily align the mirror with the traced outline and snipped out center through the greasepoof paper.


regards