does this matter?

[wils07]

I have just collaminated my scope and it has ended up being like the 1st picture but because i have a f5 scope it should be like the 2nd but does it matter?

[brantuk]

A star test should confirm the collimation for you and the manual should indicate how you want it to look (as per your pics above). I'm not sure why you would want it offset though

[Jason D]

Wils07,

Your view should look like the right diagram in my attachment -- not like the left one.

Jason

[wils07]

how do i go about offsetting the secondary mirror because at the moment its perfect with even gaps?

[barkis]

how do i go about offsetting the secondary mirror because at the moment its perfect with even gaps?

Don't do anything yet, Focus and de focus a star, and watch the circle of light. You should see diffraction rings If they appear to have any distortion, or any unusual edge brightness, then you will need to apply the offset. If not, just leave it.

Chances are your secondary is a bit oversize, and is intercepting the whole of the light returning from the main mirror.

Ron.

[Jason D]

Secondary offset has nothing to do with star testing.

Offseting the secondary mirror away from the focuser is unnecessary.

Offsetting the secondary mirror towards the primary mirror will be handled automatically when the secondary mirror is centered/rounded under the focuser.

[wils07]

Im confused as i believe my secondary mirror to be centred under the focuser and in the tube assembley so how do i off set it like the picture. And what happens if i leave it as is would it distort the image?

[Astro_Baby]

IT wont be like the firts picture (although a quick look may make it appear so) because with an F5 you will either have the secondary mirror round and tru under the focuser but with an offset inner circle (Like Jasons green picture) or you will have a conmcentric inner circle and an offset from under the focuser tube as per Jasons red picture.

The real key is get the secondary mirror under the focuser to be central and true to a circle - that will take care of any offsets.

The rest will just follow. Before you start pulling haior out though I would sugget a star test before you do much else.

[The Warthog]

The required offset is very small, a matter of millimetres or fractions of millimetres. If you ignore it entirely it will have very little effect on your scope's imaging. The reason for the offset is that the edge of the cone of light coming from your mirror is slightly closer to the focuser at the top of the secondary than it is at the bottom. So, the secondary should be offset away from the focuser. You can achieve this by tightening the screw on the spider away from the focuser, and loosening the one toward the focuser. It is the least of your worries in collimation.

[The Warthog]

Oh, you can google tables that will tell you what the offsset should be. Such as: Secondary Mirror Offset Calculator . However, you will also start worrying yourself to death needlessly because they specify a centre dot offset.

[wils07]

I think a trip outside is in order as the moon and jupiter are close tonight so a couple of hours in the garden it is and i will report back.

Thanks Paul

[barkis]

Wils, of all the Newtonians out there, there are very few working with perfect collimation. You will be your own judge, by using the scope. Don't let the desire for perfection spoil your observing.

Ron.

[perrin6]

Offsetting is to do with the 'fully-illuminated field of view' which can be a bit of a struggle to understand. Offsetting the secondary does not effect collimation, it just ensures that the fully-illuminated field is centred. Low to medium magnifications generally have a fall-off in brightness of objects towards the edge of the field of view, usually this is not noticeable except in newtonians designed with an under-sized secondary mirror (to maximize contrast for planetary viewing) or in very low power, wide field views.

Offsetting the secondary is aided by placing a black dot on it at a certain distance from the centre (towards the top of the telescope) and then adjusting the small mirror so that the dot aligns with the centre of a cheshire collimator. However this would be very small for a 130mm scope (it is only about 2.5mm in my 13.2 inch) and probably not worth bothering about,

[perrin6]

I obtained the above info from 'The dobsonian telescope' by Kriege & Berry.

[Jason D]

The required offset is very small, a matter of millimetres or fractions of millimetres. If you ignore it entirely it will have very little effect on your scope's imaging. The reason for the offset is that the edge of the cone of light coming from your mirror is slightly closer to the focuser at the top of the secondary than it is at the bottom. So, the secondary should be offset away from the focuser. You can achieve this by tightening the screw on the spider away from the focuser, and loosening the one toward the focuser. It is the least of your worries in collimation.

Not entirely true...

There are 2 main reasons to why the secondary mirror needs to be offsetted away from the focuser:

1- Improve the accuracy of DSC by coinciding the optical axis of the primary mirror with the OTA mechanical axis.

and/or

2- Avoid front OTA/UTA aperture vignetting for certain scopes.

Secondary offset away from the focuser has little impact on imaging quality

Jason

[Jason D]

Offsetting is to do with the 'fully-illuminated field of view' which can be a bit of a struggle to understand. Offsetting the secondary does not effect collimation, it just ensures that the fully-illuminated field is centred.

You can center the 100% illumination field with and without an offset along the focuser axis (away from the focuser offset)

Offsetting the secondary is aided by placing a black dot on it at a certain distance from the centre

This is not recommended. In theory it should work but in practice it could cause more headache

Jason

[Jason D]

TOP: With away from focuser offset (classic offset)

BOTTON: Without away from focuser offset (new mdoel)

[wils07]

After a couple of hour outside i got good clear images of jupiter and the moon. I also did the star test and all was well so im gonna leave as is for now as this was the 2nd time i have ever collaminated and got it as close as i can.

One thing i did change was the 3 grub screws for the secondary tilt adjustment.

I used three m4 35mm hex bolts so adjustment is a little easier .

[mdstuart]

Very nice...just go out and split the double double in Lyra which is still up early in the evening and you will know your scope is pretty much perfect in its collimation.

Mark

[sixela]

I have just collaminated my scope and it has ended up being like the 1st picture

That's only possible if the perspective effects vanish, which means you have a scope with infinite f/ratio. Must be very impractical .

The point is not to bother with it: make sure that the secondary itself (NOT the primary reflection in it!) is centred under the focuser and looks circular, ideally from the vantage point that makes the primary reflection almost as large as the secondary itself. Then make sure the centre spot is also centred under the focuser, and that the Cheshire ring is, and you're done.

But in an f/5 scope you automagically end up with picture #2 if you do so. If you think you end up with #1, then either your Cheshire reading is actually incorrect (bad) or the secondary itself really isn't centred under the focuser (not that bad, but the fully illuminated field isnt centred). In the latter case, you're probably confusing the edge of the secondary (which is usually black-on-black) with the edge of the primary reflection in it.

[wils07]

Here is a pic of it the left hand side it the top or where the focuser is.

[sixela]

That's actually not good at all, given that it's through a centred pupil. It's also too far from the focal plane to see the entire primary reflection in the secondary, so you're missing some cues you'd have if you racked the focuser in more.

But the secondary is no centred under the focuser (it's much too much to the lower left), it's slightly misrotated (it points a bit "up"), the focuser axis doesn't point to the primary's centre (and the reflection of the cross-hairs and the cross-hairs don't overlap, so axial collimation can't be good even for the current position of the secondary).

In other words, the only feature this shares with Jason's "figure one" is that the secondary outline is concentric with what I guess is the Cheshire ring. But it's closer to his "INCORRECT" secondary placement figure, except that figure refers to a scope with correct axial collimation, something not achieved here either.

[wils07]

Should the crosshairs on the cheshire overlap the reflected ones in the mirror then?

[wils07]

Here is my 2nd go and i moved the secondary mirror forward.

[The Warthog]

To me, that looks as close as dammit. Sixela is correct in his analysis of your previous pic, but you seem to have taken care of his suggestions. I always keep the French proverb in mind, "Le mieux est souvent l'ennemi du bien" or "you can drive yourself crazy striving for perfection in areas where the incremental benefit is less than the work required to achieve it." French is such a compact language.