Centring Secondary

[sbooder]

Hi All,

I should know this but I need to make sure. When centring the secondary in the focuser aperture, do you centre the apparent circle or do you centre the whole physical mirror?

Thanks.

[Astro_Baby]

The whole thing....take a look at my collimation guide and it will talk you through it. Do a google for astro baby for the site address.

[sbooder]

I have you in my favourites anyway so it will be no problem getting to the tutorial.

[dweller25]

The apparent circle should be centered under the focuser, ignore the non reflective edges of the secondary.

[sbooder]

Astro_baby,

I just looked at your site and I am now confused. You say above the whole thing needs to be centred but the images on your website say different, it looks like the apparent circle created by the mirror ellipse being a 45% is what is centred.

Maybe i used the wrong words, but I think David has cottoned on to my bad English.

[Jason D]

Hi All,

I should know this but I need to make sure. When centring the secondary in the focuser aperture, do you centre the apparent circle or do you centre the whole physical mirror?

Thanks.

You need to center the secondary mirror edge and the primary mirror reflection. Do NOT center the secondary mirror shadow. Do NOT worry about the spider vanes reflections. See attachments.

Jason

[sbooder]

I am going to make a simple sight tube to centre my secondary, and reading on the net it seems that the correct length is 1.25" x focal ratio, which for my 250mm f4.8 newt would make it 6”, dose that sound right?

[russ]

I am going to make a simple sight tube to centre my secondary, and reading on the net it seems that the correct length is 1.25" x focal ratio, which for my 250mm f4.8 newt would make it 6”, dose that sound right?

Crikey that will be a whopping sight tube for an f8 newt....10"

[sbooder]

It is my fault, I misread the info...Dough! It is just the tube should be close to ratio, although that would still make an f8 sight tube on an 8" newt, mirror damaging. I think 5" will do for my purposes.

[Rob]

Jason those are great examples there.. nice one

Rob

[sixela]

Crikey that will be a whopping sight tube for an f8 newt....10"

You can also move Muhammad to the mountain, you know .

All that matters is the aspect ratio of the cone from the pupil to the opening on the far end. Making that more narrow can be achieved by lengthening the tube but also by narrowing the opening.

For such long focal ratio scopes, you usually make the far edge of the tool a lot more narrow than 1.25" rather than making the tube longer.

And just if Jason's post was somehow unclear to anyone: you centre the entire aluminised face of the secondary under the focuser (but not its unaluminised portions), looking from the vantage point that makes the primary reflection just smaller.

Jason's figures are --for clarity's sake-- taken with something that is not an optimally narrow sight tube, in which you'd see this:

[sbooder]

That used to work fine with my 1.25" focuser but with the 2" focuser even with the 1.25" adapter in I can not position it so I can see only a small amount of space around the mirror face even with my pinhole collimator in, it just widens out to soon. I need to get a sight tube closer to the mirror to narrow the field of view so I can be more precise than I am at the moment.

[sixela]

I need to get a sight tube closer to the mirror to narrow the field of view so I can be more precise than I am at the moment.

My point is that an annular baffle at the end of a short sight tube (or even the focuser draw tube and a collimation cap) will mimick a long full width sight tube without all the unpleasant mechanical properties.

[sbooder]

True. But I have now made a sight tube that I think will work. It just so happens that the thin steel tubing from those cheap garage solar garden lights you stick in the ground, is a perfect fit so I cut a 5" length of that and fitted a pinhole top to it. I will let you know how it works.

[sbooder]

Thanks for all the advice and images for guidance all. I have one more question about the focuser.

Are there any benefits from insulating the focuser from the OTA, e.g. placing a foam or rubber gasket between them?

[Jason D]

Are there any benefits from insulating the focuser from the OTA, e.g. placing a foam or rubber gasket between them?

I can't think of any benefits

Jason