Help

[supernova]

MY 16in Dob has 2 mounting points for the primary mirror cell.

I have contacted Orion direct, but they say the installation in the Manuel is correct.

I am not sure, because the collimation is never acceptable.

What would be the correct measurment from the primery to the secondary?

Tube diameter 434mm

Tube length 1580mm

Focal Length 1600mm

Focal ratio f4

The standard mount for the cell is at the BOTTOM. [in the pic]

Here is a pic of the mirror cell.

If more info is needed then please pm me.

[barkis]

Bernie, the measurement from the Mirror to the secondary would only be relevant if you were having a focusing issue, not collimation.

What exactly is the problem you are having with your Dob.

Ron.

[ringz]

Bernie,

Those mounting holes are just to change the position of the focal point within your focuser.

If you fix the cell into the tube with the top holes the focus will be such that you have to rack the focuser further IN. If you choose the bottom holes, then you will have to rack you focuser OUTwards to reach focus.

As long as you choose top or bottom set of holes consistently around the tube when mounting the cell, collimation won't be affected.

--

Martyn

Edited to correct errors that Astronut noticed (mentioned further down the message list)

[ringz]

Bernie,

Looking at your cell picture, those springs don't seem, to me, to be very beefy. I'm not too sure that they are up to controlling a 16" mirror. Perhaps they are the reason your mirror hasn't got a very stable collimation.

What does anyone else think?

--

Martyn

[daz]

They do look a bit wimpy!

As the others have said Bernie, this position won't affect collimation. The springs would though!!

[supernova]

OK so the position of the cell is ok.

And the springs need looking at.

That could be a problem with replacement ?

I have no probs with stripping the cell down.

But where do I get the springs :scratch:

[ringz]

Bernie,

Perhaps someone like Ambermile is more qualified than me to answer what sort of springs you need and where to get them.

It might be worth an email to orion optics with that picture attached to check that they have fitted the correct ones for a 16".

--

Martyn

[supernova]

Thinking about it I can see the point about the position as regards focus. That makes sense.

So the cell needs beefing up to hold the mirror.

I can do that.

I have found that the mirror was a bit loose thanks to Astronut and

Wobbly Bob at Salisbury.

I need to Finnish this referb and tell Orion how to make the thing better as regards Enginering.

That I will do.

It took long enough to make it.

[supernova]

Bernie,

Perhaps someone like Ambermile is more qualified than me to answer what sort of springs you need and where to get them.

It might be worth an email to orion optics with that picture attached to check that they have fitted the correct ones for a 16".

--

Martyn

OK Martin thanks I will look into it.

[Casus belli]

I seem to remember a stateside company that does replacement doohickies. Bobs Knobs

http://www.bobsknobs.com/

[Astronut]

Bernie,

Those mounting holes are just to change the position of the focal point within your focuser.

If you fix the cell into the tube with the top holes the focus will be such that you have to rack the focuser further out. If you choose the bottom holes, then you will have to rack you focuser inwards to reach focus.

As long as you choose top or bottom set of holes consistently around the tube when mounting the cell, collimation won't be affected.

--

Martyn

Are you sure that's right? To my mind, it's the other way round. Fitting the cell to the tube using the top holes will put the mirror further away from the secondary requiring more inward travel for the focuser which would be my preferred method if the eyepieces don't have a problem reaching focus in either position. I believe for imaging purposes ie. camera attached instead of eyepiece, it might be better to have the focal point further out so in that case I'd use the bottom holes.

Bernie, you might benefit, ie. more consistent collimation, if those springs were more compressed before collimating. Regardless of their apparent weakness or otherwise, if they're as slack as they appear to be it's not going to hold collimation very well. If you compress them fully and then back off each screw by one turn or so before collimating they'll exert more pressure.

[supernova]

Hi Astronut.

The bottom holes are the ones that OO stated.

[ringz]

ummmmmmm, oh dear have I got things backwards again.

My excuse is that I've just started a 12 week stint as an intern with a Coventry charity and it's the first weeks work I've done in 10 years In other words I'm I'm ready for bed.

Typical isn't it, the sky here in Coventry is crystal clear and I'm too tired to enjoy it.

--

Martyn

I've edited out the errors in my previous message Astronut. Thanks for catching them.

[Astronut]

ummmmmmm, oh dear have I got things backwards again.

My excuse is that I've just started a 12 week stint as an intern with a Coventry charity and it's the first weeks work I've done in 10 years In other words I'm I'm ready for bed.

Typical isn't it, the sky here in Coventry is crystal clear and I'm too tired to enjoy it.

--

Martyn

Pity we haven't got star trek style transporters, I'd beam stright down. Guess what, it's cloudy again in NE Lancs but this time with a few little holes giving tantalising glimpses. I'm fully rested up and off work for another week too so I've actually got time to get out and forego sleep Someone up there is havin' a laugh

[supernova]

Bernie,

Those mounting holes are just to change the position of the focal point within your focuser.

If you fix the cell into the tube with the top holes the focus will be such that you have to rack the focuser further out. If you choose the bottom holes, then you will have to rack you focuser inwards to reach focus.

As long as you choose top or bottom set of holes consistently around the tube when mounting the cell, collimation won't be affected.

--

Martyn

Are you sure that's right? To my mind, it's the other way round. Fitting the cell to the tube using the top holes will put the mirror further away from the secondary requiring more inward travel for the focuser which would be my preferred method if the eyepieces don't have a problem reaching focus in either position. I believe for imaging purposes ie. camera attached instead of eyepiece, it might be better to have the focal point further out so in that case I'd use the bottom holes.

Bernie, you might benefit, ie. more consistent collimation, if those springs were more compressed before collimating. Regardless of their apparent weakness or otherwise, if they're as slack as they appear to be it's not going to hold collimation very well. If you compress them fully and then back off each screw by one turn or so before collimating they'll exert more pressure.

That makes a very good point the springs need looking at.

I am doing a compleat rework.

Bernie.

[supernova]

If I ever get my astro store/obsy finnished the rework will be done

I thank you all for your help and ideas

[Wobbly Bob]

Hiya Bernie, looking at those springs, I would agree with Martyn. They just don't look man enough IMHO, bear in mind that Mirror wieghs a fair old bit. I reckon if you get a couple of sets of different strength Stainless springs and do some trial and error tests.