Focuser hole positioning

[shaneaust]

I recently completed work on a 10" dob.
I have been using it (mainly for testing purposes initially) and have noticed a coma that I just can't get rid of, no matter what I do collimation-wise.
So - tonight, just for the heck of it, I recalculated where the centre of the focuser hole should be (43.75") and where I actually drilled it (44.50"). http://www.cloudynights.com/ubbthreads/images/graemlins/foreheadslap.gif
Before I go and re-drill another hole - is the additional 3/4" that I drilled in error likely to be a cause of my coma probs? Viewing stars thru the scope, they all appear to have "tails" at about 270º

Would be much obliged for any help on this! http://www.cloudynights.com/ubbthreads/images/graemlins/bow.gif

[asimov]

Hmmm, an interesting problem. I'd only be guessing if I were to say it's still a collimation problem, having the tails. Do you give your scope a cool down period? Ie: a couple of hours?

Sorry I couldn't be of much help here but there are others here that would be more suited to answer your initial question.

Welcome to IIS anyway.

[Merlin66]

From the dimesions you quote I'd have to assume the main mirror is about F4.8, if the mirror tests out OK then I'd have to guess that the mirror is not square to the tube and/ or the focusser is not square to the tube.

Make sure the reflection of the secondary appears central to the main mirror ( put a central dot on the main mirror to help) also check the focusser is looking central to the secondary ( a bit of white paper inside the tube opposite the focusser to outline the secondary may help) with a straight edge across the focusser along the telescope axis should show an equal gap top and bottom.
Finally use a ronchi screen to check focus and it will also show mis-alignment.

Let us know how you go.

[gbeal]

Welcome as well.
At first read I assumed you were wondering about the point to place focuser from a focus point of view, but I now think you are concerned with coma.
The way I see it, as long as the image is in focus then the focuser is correctly located.
BUT, and that's a BIG but. The focuser must be square to the tube, and the secondary directly (bearing in mind any offset you are looking to use) directly underneath the focuser.
In your post you have tried a few collimation exercises, so most of this will be old hat.
To me it sounds seriously like mis-collimation though. Given your location, is there any one locally that can help? Like Merlin said, let us know what you work out.

[shaneaust]

Thanks Gbeal:

I suspect that at least part of my problem is that my objective mirror is not installed dead centre in the tube. My scope is too big and heavy to move indoors when I'm finished with it at nite, so I have to remove both mirrors each nite, then re-install again the next nite, which is a major collimation hassle. To give an idea, my objective mirror weighs ~4 kg...then I have the tube (5 feet) and it's a heavy mother, too!
I may have to look at a box/struts construction for my scope, as the present setup is just too bulky.
Anyway, with all this collimation, I have a hard time getting views that are "stable" each nite.

Just out of curiosity - what sort of pilot are you?

[shaneaust]

Never mind, Gary - had a look at your website and saw that you're a helicopter pilot.

You have some good equipment there, and some awesome images!

How long did it take to complete your 10" dob? I was lucky and had a slack December, so I worked on it for about 3 weeks solid and finished it - bought the primary from a company here and had the secondary as a left-over from my old Tasco, all else was built. As you can see, though, I am still having some probs getting decent views of the sky!

Mick

[gbeal]

Helicopter, an AS350B3 (Squirrel). Every day I go to work I have this big smile on my dial.
Back to business though.
Why make it a chore, as this will quickly defeat the purpose of using your scope: FUN.
I too have a 10" newt, and it is a one piece tube, with a full thickness mirror, and a heavy primary mirror cell (University Optics cast).
OK, the tube isn't light, in fact from memory it hits about 11 kilos, but it makes for a good all round size, one which is easily handled, and in fact one which fits in the car.
Maybe re-think the tube. If it has to be able to be dismantled, maybe think of cutting it in half, and making a simple clip arrangement to joint the two halves together each time. My friend has a 16" like this.
So it is looking like the coma is collimation or lack of it perhaps.